Introduction générale

Depuis sa création, la composition de l'atmosphère de la Terre n'a cessé d'évoluer.

L'atmosphère primitive était composée d'eau, de dioxyde de carbone (CO 2 ) et de diazote (N 2 ), mais il n'y avait pas de dioxygène (O 2 ) [START_REF] Zahnle | Earth's earliest atmosphere Elements[END_REF]. Vinrent ensuite les atmosphères primaire et secondaire, avec les mêmes constituants mais une évolution de leurs proportions.

Puis, apparurent les cyanobactéries qui en réalisant la photosynthèse ont produit du dioxygène. La présence d'oxygène dans l'atmosphère a conduit à la création de la couche d'ozone dans la stratosphère. Le dernier évènement majeur dans l'atmosphère terrestre est apparu il y a seulement 150 ans, avec la révolution industrielle et la libération de gaz en grandes quantités, notamment de CO 2 . En effet, depuis le début de cette nouvelle ère, les activités anthropiques s'accélèrent, avec à titre d'exemple, l'intensification de l'agriculture, des activités industrielles, du transport, de la déforestation et de la production d'énergie et de déchets.

De nos jours, plusieurs sujets environnementaux soulèvent des inquiétudes, notamment du fait des changements dans la composition de l'atmosphère. Ainsi la destruction de la couche d'ozone et le réchauffement climatique, qui modifient notre environnement et les conditions de vie sur Terre, sont directement liées aux émissions gazeuses anthropiques.

Parmi les gaz qui contribuent au changement climatique ou à la destruction de la couche d'ozone, on peut citer le CO 2 , le méthane (CH 4 ) et le protoxyde d'azote (N 2 O).

Des scientifiques du monde entier travaillent actuellement à alerter la population et les politiques sur ces problèmes environnementaux afin d'en enrayer les conséquences. Des conférences et conventions telles que Genève (1979), Vienne (1985), Kyoto (1997), et les différentes conférences des Parties (COP) sont dédiées à ces thématiques. Les scientifiques doivent travailler sur les processus à l'origine des émissions de gaz à effet de serre pour développer des méthodes d'estimation de leurs émissions les plus précises possibles ainsi que pour mettre en place des stratégies d'atténuation.

Objectifs de la thèse

Concernant les émissions de N 2 O par les sols, les études présentées dans la littérature peuvent se classer en 3 catégories:

-L'étude du déterminisme des émissions de N 2 O et des facteurs de contrôle qui peuvent mener à des émissions [START_REF] Breuer | Temperature and moisture effects on nitrification rates in tropical rain-forest soils[END_REF][START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF][START_REF] Kurganova | Effect of the temperature and moisture on the N2O emission from some arable soils[END_REF][START_REF] Stanford | EFFECT OF TEMPERATURE ON DENITRIFICATION RATE IN[END_REF][START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]).

-Les méthodes de mesure directes in situ, sur des surfaces variées (cultures, prairies, forêts), pendant des périodes spécifiques, s'appuyant sur différents principes et équipements de mesure [START_REF] Beauchamp | Nitrous oxide emission from agricultural soils[END_REF][START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF][START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Phillips | Nitrous oxide flux measurements from an intensively managed irrigated pasture using micrometeorological techniques[END_REF][START_REF] Stehfest | N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions[END_REF]).

-La modélisation, à partir de données collectées au champ ou au laboratoire, combinant des éléments issus des mesures et des études de déterminisme. Les modèles peuvent avoir des applications prédictives ou d'identification des sources [START_REF] Gilhespy | First 20 years of DNDC (DeNitrification DeComposition)[END_REF][START_REF] Gu | Simulating denitrification and nitrous oxide emissions from subtropical maize-winter wheat rotations in Southwestern China using NOEv2 model Agriculture Ecosystems &amp[END_REF][START_REF] Langensiepen | Validating CERES-wheat under North-German environmental conditions[END_REF][START_REF] Riley | NLOSS: A mechanistic model of denitrified N2O and N-2 evolution from soil[END_REF][START_REF] Wolf | Modeling N2O emissions from steppe in Inner Mongolia, China, with consideration of spring thaw and grazing intensity[END_REF].

Ces 3 types d'études sont liés. L'objectif de la thèse a été de travailler sur chacune de ces approches complémentaires, en étudiant l'effet de la température sur les émissions de N 2 O (déterminisme), en mesurant les émissions de N 2 O à l'aide de différentes méthodes (mesures) mais aussi en développement des méthodes d'attribution des flux, c'est-à-dire pour retrouver les sources d'émissions, et ainsi reconstituer la variabilité spatiale des émissions de N 2 O (modélisation).

Par ailleurs, le N 2 O est produit à l'échelle microscopique mais ses effets se manifestent à l'échelle globale. Le changement d'échelle (Figure 1) peut s'appréhender de façon graduelle. Les 3 approches d'étude proposées peuvent se développer aux différentes échelles : (1) les études de déterminisme plus spécifiquement de la micro-échelle à l'échelle du paysage, (2) les mesures de flux in situ, plus spécifiquement de l'échelle de la parcelle et du petit paysage et enfin à l'échelle globale, (3) la modélisation parfois à micro-échelle mais permettant surtout le changement d'échelle vers celles régionale, nationale et globale. Nous avons combiné les expérimentations en laboratoire, les expérimentations sur le terrain, et la modélisation pour progresser sur la quantification des flux de N 2 O, à l'échelle de la parcelle et du paysage. Les objectifs de notre travail étaient (i) de progresser sur le déterminisme des émissions de N 2 O et d'améliorer les outils de quantification indirecte par les modèles, avec un travail spécifique sur l'effet de la variation de température sur les émissions de N 2 O, pour être intégré dans les modèles, (ii) de progresser sur la réalisation de mesures directes à l'échelle de la parcelle et du paysage, avec l'application de techniques de mesure intégratives dans le temps et le développement d'un dispositif de mesures intégratives dans le temps et l'espace, et, (iii) de progresser dans l'attribution des sources des émissions de N 2 O, par le développement d'approches d'attribution de flux à partir des données collectées pendant la campagne de mesures.

Plusieurs questions scientifiques ont été levées sur l'effet des variations de température sur les émissions de N 2 O, sur la campagne de mesures, et sur le développement d'une méthode d'attribution de flux.

Concernant l'effet de la température sur les émissions de N 2 O :

-Est-ce que les émissions de N 2 O sont plus fortes lorsque la température augmente au cours du temps ? -Qu'en est-il de la régulation par la température du processus de dénitrification et plus particulièrement du processus de réduction du N 2 O ? -Est-ce que les modèles prennent correctement en compte l'effet de la température ?

Concernant la campagne de mesures :

-Est-ce que les mesures faites à partir de 3 méthodes différentes et à différentes échelles peuvent fournir des valeurs comparables ? -Est-ce que la variabilité spatio-temporelle peut être totalement couverte ? -Quel type de culture et pratique culturale émet le plus de N 2 O ? Concernant le développement d'une méthode d'attribution de flux :

-Est-ce que les origines des flux de N 2 O peuvent être retrouvées à partir de mesures locales et intégratives ?

-Quelle méthode serait la meilleure pour cela et comment estimer sa validité et ses incertitudes ?

-Quelle est la variabilité spatiale des flux de N 2 O sur un paysage d'un rayon de 1 km, occupé par un mélange de cultures et de forêts ?

Les résultats de ce travail sont présentés en 6 parties :

-La première partie est une synthèse bibliographique dont le but est de rappeler les problématiques environnementale et scientifique de ce travail, de décrire les émissions de N 2 O, les processus impliqués ainsi que l'effet des conditions environnementales. De plus, cette partie introduit les différentes méthodes de quantification pour évaluer les émissions de N 2 O.

-La deuxième partie présente le site expérimental à partir duquel du sol a été prélevé pour une expérimentation en laboratoire et sur lequel une campagne de mesures de 2 mois a été réalisée. Elle présente également des études précédentes réalisées sur ce site expérimental.

-La troisième partie porte sur une expérimentation en laboratoire sur l'effet du changement journalier de température sur les émissions de N 2 O par un sol agricole.

-La quatrième partie présente la campagne de mesures qui a eu lieu pendant 2 mois au printemps 2015, avec 3 types de mesures réalisés à l'échelle de la parcelle et du paysage, avec l'utilisation de chambres automatiques, d'une chambre manuelle, et d'un système d'eddy covariance.

-La cinquième partie présente 2 approches d'attribution de flux de N 2 O, développées à partir des résultats trouvés pendant la campagne de mesures par les différentes méthodes, et leur validation.

-Enfin, la dernière partie présente les conclusions de cette thèse et les perspectives de ce travail.

General introduction

Since the creation of Earth, its atmospheric composition never stopped to evolve. It began with the primitive atmosphere made of water (H 2 O), carbon dioxide (CO 2 ), dinitrogen (N 2 ) but no dioxygen (O 2 ) [START_REF] Zahnle | Earth's earliest atmosphere Elements[END_REF]). There were then the primary and secondary atmospheres with the same constituents but with an evolution of the proportion. After that, came the first cyanobacteria which realized photosynthesis and consequently produced dioxygen. The presence of oxygen in the atmosphere leads to the creation of the ozone layer in the stratosphere. The last big event in the terrestrial atmosphere occurred 150 years ago, with the human industrial revolution and the liberation of huge amount of gases, especially CO 2 . Indeed, since the beginning of this new era, anthropogenic activities never stopped to increase with for example the intensification of agriculture, industrial activities, transportation, deforestation, production of energy and waste.

Nowadays, several environmental subjects are under concern, and many of them are due to the change of the terrestrial atmosphere. Indeed, the ozone layer depletion and the global warming, which are threatening our environment, our lives, are directly caused by the anthropogenic emissions. Amongst gases which contribute the most to the climate change and the ozone layer depletion, there are CO 2 , methane (CH 4 ), and nitrous oxide (N 2 O). Scientists all over the world are now working on warning people and politics about these environmental problems to limit consequences. Conferences and conventions such as in Geneva (1979), Vienna (1985), Kyoto (1997), and the different COP (Conference Of Parties) took place to discuss these matters and find solutions. Research groups are asked to work on the processes leading to greenhouse gas emissions to develop estimation methods with uncertainties as low as possible and then propose mitigation strategies.

Objectives of this PhD

When talking about N 2 O emissions by soils, 3 types of study are presented in the literature:

-The study of the N 2 O emissions determinism and the control factors which can lead or not to emissions [START_REF] Breuer | Temperature and moisture effects on nitrification rates in tropical rain-forest soils[END_REF][START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF][START_REF] Kurganova | Effect of the temperature and moisture on the N2O emission from some arable soils[END_REF][START_REF] Stanford | EFFECT OF TEMPERATURE ON DENITRIFICATION RATE IN[END_REF][START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]).

-Direct N 2 O emission measurements on the field, on a specific area (arable land, grassland, forest), during a specific period. These measurements can be done using different types of method [START_REF] Beauchamp | Nitrous oxide emission from agricultural soils[END_REF][START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF][START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Phillips | Nitrous oxide flux measurements from an intensively managed irrigated pasture using micrometeorological techniques[END_REF][START_REF] Stehfest | N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions[END_REF].

-Modeling, done from data collected on the field or in a laboratory and combining both determinism and measurements. Models will then allow predicting N 2 O emissions or retrieving sources of emissions [START_REF] Gilhespy | First 20 years of DNDC (DeNitrification DeComposition)[END_REF][START_REF] Gu | Simulating denitrification and nitrous oxide emissions from subtropical maize-winter wheat rotations in Southwestern China using NOEv2 model Agriculture Ecosystems &amp[END_REF][START_REF] Langensiepen | Validating CERES-wheat under North-German environmental conditions[END_REF][START_REF] Riley | NLOSS: A mechanistic model of denitrified N2O and N-2 evolution from soil[END_REF][START_REF] Wolf | Modeling N2O emissions from steppe in Inner Mongolia, China, with consideration of spring thaw and grazing intensity[END_REF].

All of these types of study are linked. The aim of this PhD was to develop specific studies on each of these 3 complementary types, by studying the effect of temperature on N 2 O emissions (determinism), by measuring N 2 O emissions on several sites using different methods (measurements), and at last, by developing 2 flux attribution methods, i.e. to retrieve emission sources and then recreate the spatial variability of emissions at the landscape scale (modeling). N 2 O is produced at the microscale, but its effects are visible at the global scale. The up-scaling (Figure 1) is gradual. Each of the 3 types of study proposed can be developed at different scales: (1) determinism studies more specifically at the microscale to the landscape scale, (2) field flux measurements more specifically at the plot and landscape scale but also at the global scale, (3) the modeling, sometimes at the microscale but allowing the up-scaling to regional, national and global scales. We combined laboratory experiments, field experiments and modeling to progress on the quantification of N 2 O fluxes, at the plot and landscape scale. The objectives of our work were (i) to progress on the determinism of N 2 O emissions and to improve indirect quantification tools with modeling, with a specific work about the effect of a varying temperature on N 2 O emissions to be integrated in models, (ii) to progress on the realization of direct measurements at the plot and landscape scale with the application of integrative measurement techniques over time and the development of integrative measurement layout in time and space, and, (iii) to progress on the source attributions of N 2 O emissions with the development of flux attribution approaches from data collected during a measurement campaign.

Several scientific questions were raised about the effect of temperature variations on N 2 O emissions, the measurement campaign and the development of a flux attribution method.

Concerning the effect of temperature on N 2 O emissions:

-Are N 2 O emissions higher when temperature increases over time?

-What about the temperature regulation of the total denitrification processes and particularly on the N 2 O reduction process? -Do N 2 O emission models correctly take into account the effect of temperature?

Concerning the measurement campaign:

-Can measurements made with 3 different methods and at different scale provide comparable values?

-Can the spatial and temporal variability be totally covered? -Which type of crop and cultural practices emitted the most?

Concerning the development of a flux attribution method:

-Can the origin of the N 2 O fluxes be retrieved from the integrated and local measurements? -Which method would be the best for that, and how to estimate its validity and its uncertainty?

-What is the spatial variability of the N 2 O flux over a 1 km radius landscape occupied by a mix of croplands and forests?

Results of this work are presented in 6 parts:

-The first part is a bibliographic synthesis whose goal is to remind the environmental and scientific issues of this work, to describe N 2 O emissions, which processes are involved and what are the effects of the environmental properties on them. Also it introduces the different quantification methods to assess N 2 O emissions.

-The second part presents the experimental site where soil has been sampled for a laboratory experiment and where a 2-month measurement campaign has been carried out. It also presents previous studies performed on this experimental site.

-The third part presents a laboratory experiment on the effect of daily temperature variations on N 2 O emitted by arable soils.

-The fourth part presents the measurement campaign that has taken place during 2 months on the spring of 2015, with 3 types of measurements made at the plot and landscape scale, using automatic chambers, a manual chamber, and an eddy covariance system.

-The fifth part presents 2 flux attribution approaches of N 2 O emissions, developed from results found during the measurement campaign by the different methods, and their validation.

-Finally, the last part presents the conclusions of this PhD and the overviews of this work. The sun emits radiations due to its surface temperature of about 6000°K. These radiations wavelength are comprised between 0.2 µm (Ultraviolet (UV)) and 4 µm (Infrared (IR)). The mean solar flux reaching the high layers of the atmosphere is currently equal to 342 W m -2 .

Because of the Earth mean albedo (the reflecting power), around 30% of those radiations are directly reflected by clouds, aerosols and the Earth surface. The remaining radiations are either absorbed by the Earth-Atmosphere system as heat (Figure 2) [START_REF] Ipcc | Adaptation, and Vulnerability. Part A: global and sectoral aspects. Contribution of working group II to the fifth assessment report of the intergovernmental panel[END_REF] or reach the space. The radiative balance of the Earth-atmosphere system is ensured by the energy exchange between the atmosphere, the Earth and space. The Earth mean surface temperature being 290 K, a part of the received energy is emitted as IR radiation. The remaining energy is absorbed by the water vapor and the greenhouse gases and then are reemitted toward space and the Earth surface (Figure 3). This reemission leads to an increase of the Earth surface temperature of 33°C. This is the greenhouse effect. Without this greenhouse effect, the mean Earth surface temperature would be of -18°C instead of the current +15°C. A change in the intensity of the incoming solar radiation, in the terrestrial albedo, or in the capacity of the atmosphere to partially absorb the terrestrial IR radiation, is a natural or anthropogenic radiative forcing of the Earth system. This radiative forcing can be negative if it leads to a decrease of the Earth surface and atmosphere temperatures, or positive if it leads to an increase. Among the anthropogenic radiative forcing, the anthropogenic emissions leading to an increase of the greenhouse gases leads to a positive forcing.

N 2 O and other greenhouse gases

Some atmospheric constituents are defined as greenhouse gases because they absorb IR radiations emitted by soil. The water vapor is the first gas responsible of the natural greenhouse effect. Among the other gases, many are influenced by anthropogenic activities.

According to the last report of the IPCC (2014), around 60% of the anthropogenic radiative forcing comes from CO 2 . With an increase of 0.4% of its concentration per year, CO 2 is one of the main actors of the radiative forcing. The rest of the radiative forcing comes from other gases such as methane (CH 4 ), nitrous oxide (N 2 O), chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), carbon tetrafluoride (CF 4 ) and other trace gases.

Several parameters are to be taken into account to assess the environmental impact of a greenhouse gas: the annual increase of its concentration, its molar radiative power (capacity to heat the atmosphere up), its lifetime in the atmosphere, and its interaction with other gases.

Calculation of the global warming potential is an indication that takes these parameters into account. The global warming potential is calculated by evaluating the contribution of 1 kg of a gas compared to the same quantity of CO 2 over 100 years. For N 2 O, its global warming potential is around 300 times higher than CO 2 [START_REF] Ipcc | Adaptation, and Vulnerability. Part A: global and sectoral aspects. Contribution of working group II to the fifth assessment report of the intergovernmental panel[END_REF].

The current mean concentration of N 2 O in the atmosphere is 329 ppb (https://www.esrl.noaa.gov/gmd/aggi/aggi.html) with an annual increase of 0.73 ± 0.03 ppb y - the global scale. It is a chemically unreactive gas with a mean lifetime in the atmosphere of about 120 years. Its long lifetime implies that first, N 2 O emitted accumulates in the atmosphere for decades and thus the major part of the N 2 O naturally or anthropogenically emitted is still in the atmosphere. Secondly, even if we stop the N 2 O emissions right away, it will take more than a century to completely eliminate the anthropogenic N 2 O. The increase of N 2 O emissions since decades has an impact on the environment. Indeed, this increase is one of the causes for the increase of greenhouse gas effect in the atmosphere, thus the climate perturbation, mainly due to its very high global warming potential. Currently, N 2 O is estimated to contribute to 6.2% of the additional greenhouse effect at the global scale (IPCC, 2014).

N 2 O and the ozone layer depletion

Ozone layer depletion

The ozone layer depletion is a phenomenon observed since the late 1970s. It is a decline of the total amount of ozone in Earth's stratosphere. During spring when temperatures are really low in the stratosphere around polar regions, large holes in the ozone layer are observed. The ozone layer depletion is caused by different compounds emitted at the surface and transported into the stratosphere by wind. These compounds are mainly anthropogenic (HCFC, CFC), or at least anthropogenic influenced (CO 2 , CH 4 , N 2 O).

CFCs and other contributory substances are referred to as ozone-depleting substances.

Since the ozone layer prevents most harmful UV B wavelengths (280-315 nm) of UV light from passing through the Earth's atmosphere, observed and projected decreases in ozone generated worldwide concern, leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is demonstrated that a variety of biological consequences such as increases in sunburn, skin cancer, cataract, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased of UV exposure due to ozone depletion.

Mechanism of ozone layer depletion by N 2 O

Ozone is produced in the stratosphere by photolysis of dioxygen to give 2 oxygen (O) atoms, followed by the reaction of an O 2 molecule with an O atom. Inert in the troposphere, N 2 O represents the main source of nitric oxides (NO x ) in the stratosphere, making it the first gas responsible of the ozone layer depletion since the prohibition of the CFC [START_REF] Ravishankara | Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st[END_REF]).

In the stratosphere, O is produced by the photolysis of O 3 and O 2 , due to the light radiation Human activities have amplified N 2 O emissions, for example, by increasing the amount of reactive nitrogen through the use of synthetic fertilizers [START_REF] Driscoll | Nitrogen pollution in the northeastern United States: Sources, effects, and management options[END_REF].

There are several existing N 2 O sources, but agriculture appears by far to be the main anthropogenic N 2 O emission source (Figure 5). 16.3 Tg N-N 2 O is assessed to be emitted each year. A third comes from anthropogenic sources. 77% of the N 2 O anthropogenic emissions are estimated to come from nitrogen present in mineral fertilizers and manure, and from other agricultural sources (UNEP 2013). Other important anthropogenic sources are industry and fossil fuel combustion, biomass burning, aquaculture and wastewater. Sources are numerous, but sinks are limited. The final budget between sources and sinks is positive and represents an accumulation of N 2 O in the atmosphere. However, high uncertainties remain on the estimation of N 2 O emissions. That is why it is important to improve the quantification of sources and sinks in order to define economic, agronomical and political strategy to reduce N 2 O emissions.

2. Mechanisms and processes of N 2 O production.

N 2 O emissions

N is transformed by chemical and biological process, in the water-soil-atmosphere system. In its natural cycle, N goes through these 3 compartments (Figure 6). N is present in the soil in organic and inorganic forms. Indeed, the extra N is lost as N 2 O emissions in the atmosphere or nitrate leaching in groundwater for example. N 2 O emissions from soils are the result of microbial activities, which have complex interactions with soil conditions, climatic conditions and land uses [START_REF] Skiba | The control of nitrous oxide emissions from agricultural and natural soils[END_REF][START_REF] Weitz | N(2)O emissions from humid tropical agricultural soils: effects of soil moisture, texture and nitrogen availability Soil Biology &amp[END_REF]. The variety of pedoclimatic contexts and combinations of soil agricultural practices make their impacts on N 2 O emissions difficult to study in croplands [START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF]). Moreover, with the increase in food demand and environmental concerns, agriculture is currently facing new challenges (Matson 1997;Foley, Crosson et al. 2011) which is requiring the development of more sustainable cropping systems. The intensity of the N 2 O emissions by soils depends on the environmental physicochemical conditions. Micro-organisms responsible for the N 2 O production are sensitive to numerous parameters as the water content in soil, the oxygen quantity, the carbon and organic matter availability, the mineral nitrogen availability, and of course the pH and the temperature. These parameters influence the structure and the composition of the microbial communities responsible of the nitrification and the denitrification, the energetic microbial metabolisms (i.e aerobic respiration vs denitrification), and these processes themselves.

Denitrification

The denitrification is an anaerobic microbial respiratory process in which the oxygen of nitrogen oxides NO 3 -and NO Denitrification capacity concern more than 60 types of microorganisms, but mainly bacteria, of which Pseudomonas appears to be the most important [START_REF] Williams | NOx and N2O emissions from soils[END_REF]). In natural soils, denitrifying bacteria are mainly heterotrophic in aerobic or anaerobic conditions.

Indeed, these microorganisms can respire using the oxygen in NO 3 -, NO 2 -or N 2 O, when the level of O 2 is low. Some organisms are also capable of producing N 2 O and NO by denitrification in aerobic conditions [START_REF] Colliver | Production of nitrogen oxide and dinitrogen oxide by autotrophic nitrifiers[END_REF]. The quantity of N 2 O emitted by denitrification depends on the denitrification reaction rate but also on the rate of N 2 O reduction. This rate depends on soils and environmental properties, such as oxygen availability, organic matter content, gas diffusion, pH, temperature, mineral nitrogen content (Figure 8) [START_REF] Robertson | Nitrification and denitrification in humid tropical ecosystems: potential controls on nitrogen retention Mineral nutrients in tropical forest and savanna ecosystems[END_REF]). These parameters have an influence on the activity of the 2 main enzymes implicated on the production and consumption of N 2 O by denitrification in soils, NO 3 -reductase and NO 2 -reductase [START_REF] Dendooven | DYNAMICS OF REDUCTION ENZYMES INVOLVED IN THE DENITRIFICATION PROCESS IN PASTURE SOIL Soil Biology &amp[END_REF][START_REF] Letey | EFFECT OF AIR-FILLED POROSITY, NITRATE CONCENTRATION, AND TIME ON THE RATIO OF N2O-N2 EVOLUTION DURING DENITRIFICATION[END_REF].

Nitrification

The nitrification is an aerobic process, heterotrophic or autotrophic, leading to the [START_REF] Bremner | Sources of nitrous oxide in soils Nutrient Cycling in[END_REF]. Autotrophic nitrification is the main process in arable soils while heterotrophic nitrification is the main process on acid soils in subtropical forests [START_REF] Zhang | N2O production pathways in the subtropical acid forest soils in China[END_REF]. Nitrification also depends on soil and environmental properties, such as NH 4 + content, oxygen availability, water content, pH, and temperature (Figure 9) [START_REF] Robertson | Nitrification and denitrification in humid tropical ecosystems: potential controls on nitrogen retention Mineral nutrients in tropical forest and savanna ecosystems[END_REF].

Environmental factors influencing N 2 O emissions

N 2 O emissions by soils generally present a high temporal and spatial variability (Mathieu et al. 2006b). In the case of cultivated soils, this variability will be even greater since those environmental parameters are often altered by agricultural practices (eg tillage, fertilization, cover crops and pesticides) [START_REF] Smith | Impacts of land management on fluxes of trace greenhouse gases[END_REF][START_REF] Vilain | Effect of slope position and land use on nitrous oxide (N2O) emissions (Seine Basin, France)[END_REF]. [START_REF] Robertson | Nitrification and denitrification in humid tropical ecosystems: potential controls on nitrogen retention Mineral nutrients in tropical forest and savanna ecosystems[END_REF] proposed that the factors that influence the N 2 O emissions depend on the study scale. Indeed, control factors can be proximal or distal. Proximal factors are local and control the N 2 O production at the micro-scale. On the other hand, distal factors are non-local and control N 2 O production at a larger scale [START_REF] Beauchamp | Nitrous oxide emission from agricultural soils[END_REF][START_REF] Bouwman | Direct emission of nitrous oxide from agricultural soils Nutrient Cycling in[END_REF][START_REF] Mosier | Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation[END_REF]. Some factors appear to be favorable to the biomass bacterial activity leading to N 2 O emissions.

Proximal factors

Stehfest and [START_REF] Stehfest | N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions[END_REF] compiled and analyzed emissions data, and they concluded that N 2 O emissions will depend on the presence or absence of oxygen. The presence of oxygen is related to the water content of the environment. Indeed, the more the soils are waterlogged, the less there is oxygen available (diffusion coefficient of O 2 in water is inferior to the one in air, [START_REF] Rabot | Temporal Variability of Nitrous Oxide Emissions by Soils as Affected by Hydric History Soil[END_REF])) and therefore denitrification as respiratory process is privileged in comparison to aerobic respiration. However, when water content is low, there is a lot of free space for gases, and therefore nitrification can prevail as a source of N 2 O production due to the oxygen availability, in comparison with denitrification. There is a threshold for the WFPS at 60 %. Under this value the nitrification will be the main source of N 2 O production, above this value it will be the denitrification (Linn and Doran 1984) (Figure 10). Thus, during rain events, there are generally peaks of N 2 O emissions from soils, as rain led to the formation of favorable sites for anaerobic denitrification processes. In temperate climates, the annual highest emission peaks coincide with the spring rains which arrive at the same time that the use of fertilizers in cultivated land. Moreover, in a dry soil, N 2 O emissions can be very limited [START_REF] Bateman | Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space[END_REF]. Another factor that has an impact on the denitrification regulation is the organic carbon availability. The soil organic matter plays a direct and indirect role on denitrification as most of the denitrifying bacteria are heterotrophic. It first plays the role of the electrons donor during denitrification [START_REF] Groffman | Challenges to incorporating spatially and temporally explicit phenomena (hotspots and hot moments) in denitrification models[END_REF]. Moreover, organic matter globally stimulates the bacterial activity and increases the anaerobic sites formation thanks to the organisms respiration [START_REF] Parkin | SOIL MICROSITES AS A SOURCE OF DENITRIFICATION[END_REF]. N 2 O fluxes increase with the organic carbon availability [START_REF] Harrison-Kirk | Soil organic matter and texture affect responses to dry/wet cycles: Effects on carbon dioxide and nitrous oxide emissions Soil Biology &amp[END_REF].

Another factor on the nitrification/denitrification regulation is the mineral nitrogen availability, as NH 4 + and NO 3 -. More important N 2 O emissions are observed when nitrogen fertilizers are added [START_REF] Avrahami | N2O emission rates in a California meadow soil are influenced by fertilizer level, soil moisture and the community structure of ammoniaoxidizing bacteria[END_REF]Kavdir et al. 2008). Input of crop residue can also stimulate microbial activity and lead to N 2 O emissions [START_REF] Chen | Soil nitrous oxide emissions following crop residue addition: a meta-analysis[END_REF].

pH regulates both nitrification and denitrification. As many microbial activities, the ones that lead to the N 2 O emissions are sensitive to soil pH [START_REF] Baath | GROWTH-RATE AND RESPONSE OF BACTERIAL COMMUNITIES TO PH IN LIMED AND ASH TREATED FOREST SOILS Soil Biology &amp[END_REF]. Soil

pH favorable to bacterial activity are neutral or slightly basic [START_REF] Chen | Soil nitrous oxide emissions following crop residue addition: a meta-analysis[END_REF][START_REF] Simek | The influence of soil pH on denitrification: progress towards the understanding of this interaction over the last 50 years[END_REF]. Soil pH can be artificially maintained to a favorable value with liming to reduce N 2 O emissions.

Temperature positively affects the functioning of microbial communities [START_REF] Avrahami | Response of Nitrosospira sp strain AF-Like ammonia oxidizers to changes in temperature, soil moisture content, and fertilizer concentration[END_REF][START_REF] Pietikainen | Comparison of temperature effects on soil respiration and bacterial and fungal growth rates[END_REF], and affects N 2 O emissions. There is a positive correlation between the increase of temperature and the increase of N 2 O emissions, in the range of temperature measured on the field (Smith et al 2003). Every microbial activity varies with temperature, it is negligible under a threshold temperature and increase until it reaches an optimal temperature. There are optimal temperatures for both nitrification and denitrification processes and a threshold at 11°C was proposed by [START_REF] Stanford | EFFECT OF TEMPERATURE ON DENITRIFICATION RATE IN[END_REF] under which denitrification rate sharply decrease. The denitrifying activity increase with temperature comprised between 4 and 37°C [START_REF] Braker | Influence of temperature on the composition and activity of denitrifying soil communities[END_REF] while the nitrification decrease with the decrease of the temperature from 20°C to 5°C [START_REF] Russell | Effect of temperature and nitrogen source on nitrification in a sandy soil Communications in[END_REF]. However, microorganisms are able to adapt to their environment, and so it remains difficult to define a universal temperature function [START_REF] Farquharson | Concepts in modelling N2O emissions from land use[END_REF]. Furthermore, it will be possible to observe significant emissions of N 2 O in cold environments and during freezing and thawing periods [START_REF] Morkved | N2O emissions and product ratios of nitrification and denitrification as affected by freezing and thawing Soil Biology &amp[END_REF][START_REF] Teepe | Nitrous oxide emissions from soil during freezing and thawing periods Soil Biology &amp[END_REF]).

Distal factors

Landscapes represent an indirect controlling factor on N 2 O emissions due to their influence on the soil mineralogy, its texture and structure, on the distribution of the water, carbon and nitrogen content.

The soil use and the management practices have an impact on the N 2 O production.

Many studies measured N 2 O emissions from the different types of soil use, and found differences explained by the effect of the soil use on humidity, carbon and N content [START_REF] Abdalla | Nitrous oxide fluxes and denitrification sensitivity to temperature in Irish pasture soils[END_REF][START_REF] Beauchamp | Nitrous oxide emission from agricultural soils[END_REF][START_REF] Breuer | N2O emission from tropical forest soils of Australia[END_REF][START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]. Management practices (type and quantity of fertilization, tillage or ploughing, straw removal, irrigation) have an impact on N 2 O emissions [START_REF] Vermue | High N2O variations induced by agricultural practices in integrated weed management systems[END_REF]. The application of N fertilizer have to be controlled, otherwise it can lead to huge amount of N 2 O emissions. [START_REF] Van Groenigen | Towards an agronomic assessment of N2O emissions: a case study for arable crops[END_REF], showed the impact of N surplus and N 2 O emissions (Figure 11).

Texture and soil structure define the porous system therefore the WFPS. The pore size, connectivity and tortuosity are factors controlling the period of oxygen restriction and so denitrification.

Seasonal variations of precipitations and temperature control the water content via inputs and evapotranspiration, and then control soil humidity, and the carbon and N availability [START_REF] Groffman | Evaluating annual nitrous oxide fluxes at the ecosystem scale[END_REF]. Local change in precipitation can trigger change on N 2 O emissions due to an increase of the soil water content.

III. Quantification methods of N 2 O emissions

Despite the importance of N 2 O as a gas responsible for the global warming and the ozone layer depletion, its budget at the global scale is still not well understood and not well quantified. The difficulty to quantify N 2 O emissions comes from their high temporal and spatial variability [START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF]). However, several investigation methods have been developed in order to study N 2 O emissions by soils. Some have been developed to measure N 2 O emission in a laboratory, some others directly on the field. In a laboratory, measurements are generally done in soil cylinder sampled on the field and brought back to the laboratory [START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF][START_REF] Kurganova | Effect of the temperature and moisture on the N2O emission from some arable soils[END_REF][START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]. In situ, the 2 main methods to quantify N 2 O emissions are chambers and micrometeorological methods [START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF][START_REF] Laville | Effect of management, climate and soil conditions on N2O and NO emissions from an arable crop rotation using high temporal resolution measurements[END_REF][START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF][START_REF] Pattey | Tools for quantifying N2O emissions from agroecosystems[END_REF]). These 2 methods do not measure at the same spatial scale but they can be used together and combined [START_REF] Griffis | Reconciling the differences between top-down and bottom-up estimates of nitrous oxide emissions for the US Corn Belt[END_REF][START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF]. Both methods have their own advantages and disadvantages [START_REF] Denmead | Approaches to measuring fluxes of methane and nitrous oxide between landscapes and the atmosphere[END_REF]. Analyzer to quantify N 2 O emissions are generally gas chromatographs, cascade quantum lasers (QCL) or Infra-Red spectrometers.

Soil cylinder method

The soil cylinder method consists in taking soil samples on the field and to bring them back to the laboratory. Samples can be destructured, meaning that cylinders have been recreated afterward with a mix of the soil samples [START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]). This technique is easier but it does not represent exactly the field conditions. Cylinders can also be sampled non-destructured [START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF][START_REF] Tiedje | PERSPECTIVES ON MEASUREMENT OF DENITRIFICATION IN THE FIELD INCLUDING RECOMMENDED PROTOCOLS FOR ACETYLENE[END_REF]). To do so, cylinders have to be implanted directly on the field before brought back to the laboratory. This technique permits to be in more realistic conditions respecting the field soil structure. Once in the laboratory, all kind of treatment can be applied to the samples, such as a change in temperature, water-filled pore space, nitrate content, organic matter content, and pH, so that factors controlling N 2 O emissions can be studied. Soil cylinders are placed on closed volumes (e.g. columns, jars) with a lid placed at the top and gas samples are taken from it to be analyzed. N 2 O emitted from the soil cylinders accumulates on these volumes and the increase of the N 2 O concentration is measured. From this increase of gas concentration, a flux can be calculated.

Chamber method

The chamber method consists on placing a box on the ground in the field, enclosing a given area, and to measure the accumulation of gas inside it. This method can be used for any other gases such as CO 2 or CH 4 . The flux is proportional to the rate of concentration change in the chamber over time. 2 kinds of chambers can be distinguished:

(i) The closed system for which there is no air replacement in the chamber volume during measurements. The flux of gas is calculated as:

= Eq. (1.9)
where F g is the flux of gas at the surface (kg m -2 s -1 ), V is the volume of the head space (m 3 ),

A is the surface area covered by the chamber (m -2 ), ρ g is the gas concentration in the chamber (kg m -3 ), t is the time (s).

(

ii) The open system for which a constant air flux is imposed inside the chamber. The flux of gas is calculated as:

= ( , , )
Eq. (1.10)

where F g is the flux of gas at the surface (kg m -2 s -1 ), v is the volume flow rate (m 3 s -1 ), ρ g,0 is the gas concentration in the air leaving the chamber (kg m -3 ), ρ g,i is the gas concentration in the air entering the chamber (kg m -3 ), A is the suface area covered by the chamber (m -2 ).

Closed system chambers are used more often than the open system chambers because they are easier to use and concentration changes are easier to detect [START_REF] Denmead | Approaches to measuring fluxes of methane and nitrous oxide between landscapes and the atmosphere[END_REF].

Chambers can be manual meaning that measurements are done punctually [START_REF] Clayton | MEASUREMENT OF NITROUS-OXIDE EMISSIONS FROM FERTILIZED GRASSLAND USING CLOSED CHAMBERS[END_REF][START_REF] Henault | Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions[END_REF][START_REF] Laville | Field comparison of nitrous oxide emission measurements using micrometeorological and chamber methods[END_REF][START_REF] Mosier | Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation[END_REF]. Chambers are placed on pre-installed frames on the field and gas accumulates during 30 min to 2 hours. Gases are sampled through a septum with a syringe and placed into vacuumed vials, then brought back to the laboratory for analysis with a gas chromatograph. Chambers always stay at the same place and several spatial repetitions are needed. Manual chambers are the most used technique to measure soil gas emissions. It is the simplest and cheapest chamber method but on the other hand, manual chambers need someone to manipulate them and so it only can be used punctually, therefore the temporal variability will not be studied very thoroughly.

The fast-box technique is a manual chamber directly connected to an analyzer on the field [START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF][START_REF] Hensen | Dairy farm CH4 and N2O emissions, from one square metre to the full farm scale Agriculture Ecosystems &amp[END_REF]. Measurements are done for a couple of minutes. The advantages are principally that measurements are done faster, only one fast-box is needed to do all the measurements, measurements can be done anywhere without any frame pre-installed on the ground, and many sites can be sampled. However, this technique needs an analyzer that can be reliable on the field.

Chambers can also be automated, meaning that they are placed on the ground and connected to an analyzer directly on the field, and are programmed to open and close [START_REF] Breuer | N2O emission from tropical forest soils of Australia[END_REF][START_REF] Butterbachbahl | Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions Nutrient Cycling in[END_REF][START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF][START_REF] Neftel | N2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements[END_REF].

Measurements are done automatically in a closed system and the whole system can run as long as it is needed. The advantage is that no one is needed to do the measurements and they can be done all the time, during nights and weekends, so that a temporal dynamic can really be observed. On the other hand, once they are placed on a specific place, they cannot be moved to another place, and so the spatial variability cannot be studied. Furthermore, they need power to run and so they must be placed near an electrical supplier or used with a generator and the analyzer must be stable and accurate. It is the most expensive chamber method.

Plants might need to be cut in certain case, if the chamber cannot be perfectly hermetic. This can lead to an alteration of the environment and so biases measurements. The size of the chamber is important. If the chamber is too high, the gas emissions could not be detected or misdetected due to a bad mix of the air inside the chamber. On the other hand, if chambers are too small, environmental conditions such as temperature, humidity, gas concentration, inside the chamber can be impacted and lead to a biased measurement [START_REF] Hutchinson | Vents and seals in non-steady-state chambers used for measuring gas exchange between soil and the atmosphere[END_REF]. Also chambers have to be well inserted on the soil to limit lateral gas diffusion. The deployment time also has an impact on the air and soil temperature and on humidity inside the chamber. It also impacts gas leaks, with for consequence, errors on flux estimation. That is why it is highly recommended to avoid deployment time higher than 60 minutes [START_REF] Parkin | Sampling Protocols. Chapter 3. Chamber-based trace gas flux measurements[END_REF] and to prefer deployment time lower than 40 minutes [START_REF] Rochette | Chamber measurements of soil nitrous oxide flux: Are absolute values reliable?[END_REF].

Fluxes obtained with the chamber method (automated and manual) are generally associated with very large uncertainties because of the very small surface area investigated coupled to underlying various artefacts leading to alteration of the natural concentration gradient within the soil [START_REF] Davidson | Minimizing artifacts and biases in chamber-based measurements of soil respiration[END_REF] and non-linear phenomena, chamber design and flux calculations [START_REF] Pihlatie | Comparison of static chambers to measure CH4 emissions from soils[END_REF].

Micrometeorological methods

Micrometeorological methods consist on estimating fluxes from wind turbulence and from gas concentrations from a mast or a tower located in the studied area. Micrometeorological methods are the most appropriate for estimating gas fluxes in situations representative of ecosystems. It is assumed that fluxes are almost constant with height and that concentrations change vertically but not horizontally [START_REF] Denmead | Approaches to measuring fluxes of methane and nitrous oxide between landscapes and the atmosphere[END_REF]. The flux measured at a height z is the result of many sources upwind. The use of a micrometeorological mast/tower provides information on temporal variability over a wide spatial area defined by the height of the mast/tower (plot, landscape). Flux footprint is the area "seen" by the instrument at the tower.

In other words, it is an area upwind from the tower, such that fluxes generated in this area are registered by the tower instruments. Another frequently used term, fetch, usually refers to the distance from the tower to the maximal upwind distance that is contribution to the flux measured by the tower. The fetch of the tower depends mainly on the height of the mast/tower. It also depends on the roughness of the field and the thermal stability (Burba and Anderson 2010).

These methods permit non-intrusive quantification of trace gases by not disturbing gas exchange between the soil and the atmosphere, they integrate fluxes over larger scale (plot, landscape) and provide continuous measurements. Fluxes obtained by micrometeorological methods are dependent on wind conditions during the measurement period. Therefore they are wind-dependent and cannot be considered as absolute values. Also the installation of such method requires a very high technical level and is quite expensive [START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF]. Gases are analyzed by IR spectroscopy: Fourier transform IR spectroscopy (Hashmonay 2001), QCL or tunable diode laser absorption spectroscopy (De [START_REF] Klein | Measurement of nitrous oxide emissions from grassland soil using photo-acoustic infra-red spectroscopy, long-path infrared spectroscopy, gas chromatography, and continuous flow isotope-ratio mass spectrometry[END_REF].

The eddy Covariance is the most direct method to measure vertical turbulent fluxes of gas in the atmospheric boundary layer [START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF][START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Laville | Nitrous oxide fluxes from a fertilised maize crop using micrometeorological and chamber methods[END_REF][START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF]. Flux is calculated as the covariance between the fluctuations (denoted by prime and equal to the instantaneous value minus the mean value) of gas density (ρ g in g m -3 ) and vertical wind speed (w in m s -1 ), and the dry mole fraction of the gas in the air (s in kg kg -1 of dry air) (Burba 2013). Eq. (1.11) This method requires state-of-the-art instruments measuring generally at frequencies of 10

F = ' ' ′
Hz. The vertical wind speed w is measured with a 3-D sonic anemometer as well as the wind direction.

IV. Modeling of N 2 O emissions

Estimations of direct N 2 O emissions have really high uncertainties, because of their high spatial and temporal variability [START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF] The goal is to estimate emissions for a higher number of sites using simpler parameters such as temperature, humidity or nitrogen content instead of realizing direct measurements. Several models have been developed in order to simplify its use, or to integrate a new parameter compared to another model. Some are multiplicative models like NLOSS [START_REF] Riley | NLOSS: A mechanistic model of denitrified N2O and N-2 evolution from soil[END_REF] or NOE [START_REF] Henault | Predicting in situ soil N(2)O emission using NOE algorithm and soil database[END_REF]. They use simple mathematical formulas to express temperature, nitrate and water content effects to simulate N 2 O fluxes. Formulas are calculated from field measurements. There are also ecosystem models such as DNDC [START_REF] Li | A MODEL OF NITROUS-OXIDE EVOLUTION FROM SOIL DRIVEN BY RAINFALL EVENTS .1[END_REF], DAYCENT [START_REF] Parton | Generalized model for NOx and N2O emissions from soils[END_REF] or CERES-EGC [START_REF] Gabrielle | Simulation of nitrous oxide emissions from wheat-cropped soils using[END_REF]. They do not simulate only N 2 O emissions but the evolution of numerous parameters in the environment.

The IPCC approach

The IPCC approach (1997) has for objective to estimate annual anthropic N 2 O emissions at the country scale. This approach introduces emission factors (EF) which are statistically evaluated from data collected on different situations and which give the quantity of N 2 O emitted for a quantity of nitrogen fertilizer input. EF are defined as:

= ! " # $% ! " # ! "
Eq. (1.12)

with N N2O fertilized the N 2 O emissions of a fertilized plot (kg N ha -1 y -1 ), N N2O not fertilized the N 2 O emissions of a non-fertilized plot (kg N ha -1 y -1 ), and N fertilizer the quantity of fertilizer input (kg N ha -1 y -1 ). This approach makes the simple assumption that the nitrogen input is the only regulation factor of anthropic N 2 O emissions. with F SN the total quantity of artificial fertilizer used (kg N ha y -1 ), F AW the nitrogen quantity from manure used as fertilizer (kg N y -1 ), F CR the nitrogen quantity from crop residues which go back to soils (kg N y -1 ), F BN the nitrogen quantity coming from the crops with nitrogen fixation (kg N y -1 ), and EF 1 = 0.01 kg N-N 2 O kg -1 of N input².

However this method has an important uncertainty, but the purpose is to give a simple method to estimate N 2 O emissions without experimentations.

NOE

NOE for Nitrous Oxide Emission [START_REF] Henault | Predicting in situ soil N(2)O emission using NOE algorithm and soil database[END_REF] The basic principle of the NOE model is presented Figure 12. Specific parametrization at each studied site permit to simulate realistic fluxes over different pedoclimatic contexts [START_REF] Gabrielle | Simulation of nitrous oxide emissions from wheat-cropped soils using[END_REF][START_REF] Gu | Simulating denitrification and nitrous oxide emissions from subtropical maize-winter wheat rotations in Southwestern China using NOEv2 model Agriculture Ecosystems &amp[END_REF][START_REF] Hergoualc'h K | The utility of process-based models for simulating N2O emissions from soils: A case study based on Costa Rican coffee plantations Soil Biology &amp[END_REF].

Ecosystem models

Ecosystem models, such as DNDC [START_REF] Li | A MODEL OF NITROUS-OXIDE EVOLUTION FROM SOIL DRIVEN BY RAINFALL EVENTS .1[END_REF] or CERES-EGC [START_REF] Gabrielle | Simulation of nitrous oxide emissions from wheat-cropped soils using[END_REF], do not only simulate N 2 O emissions but everything in the ecosystem. These models need input data measured on the field and will then simulate evolution of these parameters at a daily time step. Input data are (i) physical soil properties (depth, texture, density), (ii) chemical soil properties (pH, organic carbon and nitrogen, total nitrogen), (iii) hydric soil properties (hydraulic conductivity, field capacity, wilting point, retention curves, infiltration rate), (iv) site characteristics (mean temperature, thermal amplitude, albedo, latitude), (v) microbiological soil properties, (vi) meteorological variables (air temperature and humidity, precipitation, wind speed), and (vii) agronomic variables (land use, tillage, type of fertilizer).

From these input data, several variables will be simulated such as fluxes and concentrations of nitrogen compound (NH 3 , NO x , N 2 O, NO 3 -, N tot ), the plant growth, the crop yield, the water and nitrogen content as well as the temperature in the different layers of the soil. There is a specific module for each parameter and so for N 2 O emissions. This module is NOE for CERES-EGC for example. DNDC was used in many countries to simulate N 2 O emissions by arable soils and grasslands at the plot and landscape scales [START_REF] Brown | Development and application of a mechanistic model to estimate emission of nitrous oxide from UK agriculture[END_REF][START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF][START_REF] Saggar | Chambers, micrometeorological measurements, and the New Zealand Denitrification-Decomposition model for nitrous oxide emission estimates from an irrigated dairy-grazed pasture[END_REF], as well as CERES [START_REF] Dufosse | Using Agroecosystem Modeling to Improve the Estimates of N2O Emissions in the Life-Cycle Assessment of Biofuels Waste Biomass[END_REF][START_REF] Langensiepen | Validating CERES-wheat under North-German environmental conditions[END_REF][START_REF] Xiong | Evaluation of CERES-Wheat simulation of Wheat Production in[END_REF]) (Figure 13).

Geographical situation

The OS² site is located on the edge of the small natural regions of Beauce Chartraine

Pedology and geology

The OS² site mainly presents silty soils: degraded Luvisols (hydromorphic soils) that are more or less deep and stony, and Colluviosols and Fluviosols down the slope near the creek and the Loir (Figure 16). These soils are representative of the soils in France. The soils are silts, developed on a tertiary formation of impermeable flint clay which can reach thirty meters of thickness.

Topography

The highest point of the site is 214 m above sea level and the lowest is 168 m (Figure 17).

Climate

The site is under degraded oceanic climate with annual rainfall of 598 mm, an average annual temperature of 10.6 ° C and an evapotranspiration potential of 740 mm. The long-term monthly average rainfall shows a moderately uniform distribution throughout the year.

Hydrology

Natural soils are rapidly clogged due to the impermeable substratum. They require proper artificial drainage to be cultivated. The clay layer that is found in depth limits the vertical transfers, so the transfers to the chalk sheet (170 m of altitude) are negligible. The Loir river and creeks are mainly fed by agricultural drainage waters.

Crop management

The site is in a region of cereal crops and offers 1% meadow (Figure 18). The main rotations encountered are rapeseed-wheat-wheat or rapeseed-wheat-barley, with nearly 50% of the area seeded with wheat (Figure 18). The main rotation head is rapeseed with about 30% of the surface. Winter barley is the third cereal the most seeded with 15%. Most farmers practice plowing and the straw is mostly buried, except that in general there is no plowing before seeding wheat following rapeseed. The plots are limed regularly about every 5 years in summer. Inputs of exogenous organic matter are regularly applied.

II. N 2 O emission measurements

Both direct and indirect -water emitting N 2 O due to the lixiviation of the soil NO 3 --N 2 O flux measurements were conducted since 2009 [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF][START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF][START_REF] Gu | A regional experiment suggests that soil texture is a major control of N2O emissions from tile-drained winter wheat fields during the fertilization period Soil Biology &amp[END_REF][START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF]. N 2 O emissions were measured with the chamber method, using non steady-state chambers designed and operated as described in Rochette and Bertrand (2008), or one fast-box at weekly to monthly time step. Measurements were done on different sites at different dates: (i) at shoulder and foot-slope positions along three sloping sites from late February to April 2009 and late February to May 2010 [START_REF] Gu | A regional experiment suggests that soil texture is a major control of N2O emissions from tile-drained winter wheat fields during the fertilization period Soil Biology &amp[END_REF][START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF]),

(ii) in a barley/wheat field on hydromorphous soils, on March and April 2011 and March 2012 [START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF], and (iii) on 4 different plots -2 undrained plots and 2 drained plots -during 2 growing seasons, from seeding to harvest (November to July), in 2010[START_REF] Guimbaud | A portable infrared laser spectrometer for flux measurements of trace gases at the geosphere-atmosphere interface Measurement Science &amp[END_REF]and in 2012-2013[START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF]. In all cases, the highest N 2 O emissions were measured during spring after fertilization.

The mean N 2 O fluxes reported in [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF] were 71 µg of N-N 2 O m -² h -1 and were larger in the undrained plots than in the drained plots, in both growing seasons, with pulse emissions comprised between 400 and 800 µg of N-N 2 O m -² h -1 on undrained soils. In [START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF], measured fluxes were comprised between 0 and 1500 µg of N-N 2 O m -² h -1 . For drained soil cropped with wheat, lower pulses of N 2 O in the range of 0-120 µg of N-N 2 O m -² h -1 have been observed previously by [START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF] Indirect N 2 O fluxes were also measured on the Loir stream, with a floating chamber connected to an analyzer [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF]. Indirect N 2 O emissions were estimated to contribute to the total N 2 O emissions to 1.6%, with a mean value of 190 µg of N-N 2 O m -2 h -1 .

However this result needs more investigation in different sites due to the complexity of the measurement from non-permanent streams.

III. N 2 O emissions determinism

Results of these measurement campaigns made on different sites on different conditions enable to study the determinism of N 2 O emissions. Last, the influence of distal factors such as topography has been studied. In [START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF], landscape positions had a significant effect on N 2 O fluxes with larger emission in the foot-slope at only one of the 3 investigated drained sites. Topography did not affect N 2 O emissions mostly because the topographic effects on soil hydrology were partly offset by tiledrainage. To further investigate the influence of artificial drainage on N 2 O emission, measurements were made on drained and undrained plots [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF]. The undrained soils showed significantly larger emissions than drained soils during both dry and wet years. The net effect of artificial drainage may be a large decrease in the direct N 2 O emissions. Drainage was the main factor explaining the spatial variability of the N 2 O emissions within the studied soils, and its effect was dominant over other permanent soil variables. This strongly suggests that drainage must be taken into account for N 2 O emission inventory.

IV. Simulations

Simulations of N 2 O emissions were also performed, using data collected on the OS² site. 2 approaches have been tested so far, using NOE and DNDC models. [START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF] evaluated the possibility to simulate N 2 O fluxes at scales finer than the plot, because of the very high variability at this scale, with hot spots of emissions. The frequency distributions are asymmetrical and it is not taken into account in models. [START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF] Mots-clés: Gaz à effet de serre, protoxyde d'azote, effet de la température, dénitrification, Q 10

I. Introduction

Nitrous oxide is naturally present in trace quantities in the terrestrial atmosphere, but its concentration increased since the industrialization period by approximatively 15% (UNEP 2013). Human activities have amplified N 2 O emissions, for example, by increasing the amount of reactive nitrogen in the biosphere through the use of synthetic fertilizers [START_REF] Driscoll | Nitrogen pollution in the northeastern United States: Sources, effects, and management options[END_REF]. There are several existing anthropogenic N 2 O sources, with agricultural soils being the dominating N 2 O emission source. 77% of the N 2 O anthropogenic emissions are estimated to come from nitrogen present in mineral fertilizers and manure and applied to soils, and from other agricultural sources (UNEP 2013). The increase of the global population leads to an increase of food demands. To satisfy this demand, the agricultural production is expected to increase with a rise of the use of N fertilizers [START_REF] Galloway | Transformation of the nitrogen cycle: Recent trends, questions, and potential solutions[END_REF], and thus, also with increasing N 2 O emissions. This greenhouse gas has a warming potential of about 298 times the one of CO 2 (IPCC 2014), and it is also the most important trace gas driving the stratospheric ozone layer depletion [START_REF] Ravishankara | Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st[END_REF]. N 2 O is estimated to contribute to approximatively 8% of the global anthropogenic warming, with mean global temperature being expected to rise between 1 and 4°C by 2100 [START_REF] Ipcc | Adaptation, and Vulnerability. Part A: global and sectoral aspects. Contribution of working group II to the fifth assessment report of the intergovernmental panel[END_REF][START_REF] Ipcc | Adaptation, and Vulnerability. Part A: global and sectoral aspects. Contribution of working group II to the fifth assessment report of the intergovernmental panel[END_REF][START_REF] Rustad | A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming[END_REF]. Reducing agricultural N 2 O emissions has thus become a key focus in national mitigation strategies targeting the agricultural sector. at the soil-atmosphere interface is the net result of simultaneously occurring production and consumption processes [START_REF] Conrad | Soil microorganisms as controllers of atmospheric trace gases (H-2, CO, CH4, OCS, N2O, and NO[END_REF]. Furthermore, aerobic and anaerobic sites can exist in close vicinity in soils, so that the attribution of N 2 O emissions to specific microbial of physico-chemical source and sink processes remains difficult [START_REF] Butterbach-Bahl | Nitrous oxide emissions from soils: how well do we understand the processes and their controls?[END_REF].

Moreover, the contribution of the main source process of denitrification to observed N 2 O emissions depends on environmental conditions (Mathieu et al. 2006a) such as water content and oxygen availability, soil nitrate, organic matter availability, pH and temperature.

Temperature stimulates the metabolism and the functioning of microbial communities [START_REF] Avrahami | Response of Nitrosospira sp strain AF-Like ammonia oxidizers to changes in temperature, soil moisture content, and fertilizer concentration[END_REF][START_REF] Pietikainen | Comparison of temperature effects on soil respiration and bacterial and fungal growth rates[END_REF], and, thus, stimulates organic matter mineralization, and soil C and N cycling as e.g., indicated by increased rates of N 2 O emissions or soil respiration [START_REF] Luo | Decadal variability of soil CO2, NO, N2O, and CH4 fluxes at the Hoglwald Forest[END_REF]). As a consequence, a positive correlation between the increase of temperature and the increase of N 2 O emissions is often observed in field studies (Smith et al 2003). There are optimal temperatures for both nitrification and denitrification processes and a threshold at 11°C was proposed by [START_REF] Stanford | EFFECT OF TEMPERATURE ON DENITRIFICATION RATE IN[END_REF] under which rates of denitrification sharply decrease. The optimum temperature for denitrification was found to be around 37°C [START_REF] Braker | Influence of temperature on the composition and activity of denitrifying soil communities[END_REF]). Also, nitrification has been found to be temperature sensitive [START_REF] Breuer | Temperature and moisture effects on nitrification rates in tropical rain-forest soils[END_REF]) and to decrease with the decrease of the temperature from 20°C to 5°C [START_REF] Russell | Effect of temperature and nitrogen source on nitrification in a sandy soil Communications in[END_REF]. However, microorganisms are able to adapt to their environment, and so it remains difficult to define a universal temperature function [START_REF] Breuer | Local temperature optimum of N2O production rates in tropical rain forest soils of Australia[END_REF][START_REF] Farquharson | Concepts in modelling N2O emissions from land use[END_REF]. Furthermore, significant emissions of N 2 O also occur during following soil freezing and thawing in response to increased substrate availability, tight coupling of oxidative and reductive processes and increased anaerobic soil conditions in such periods [START_REF] De Bruijn | Model evaluation of different mechanisms driving freeze-thaw N2O emissions Agriculture Ecosystems &amp[END_REF][START_REF] Morkved | N2O emissions and product ratios of nitrification and denitrification as affected by freezing and thawing Soil Biology &amp[END_REF][START_REF] Teepe | Nitrous oxide emissions from soil during freezing and thawing periods Soil Biology &amp[END_REF]).

The study of denitrification process alone can be done in anaerobic conditions to promote denitrification over nitrification. Nevertheless, as N 2 cannot be measured easily, measurements of denitrification activity in soils often relies on the use of high concentrations of acetylene (approximatively 10% in volume), in the headspace of incubation vessels.

Acetylene blocks the N 2 O reductase of the denitrification chain, so that, the end product of denitrification is the more easily measurable N 2 O and not N 2 [START_REF] Yoshinari | ACETYLENE INHIBITION OF NITROUS-OXIDE REDUCTION AND MEASUREMENT OF DENITRIFICATION AND NITROGEN-FIXATION IN SOIL Soil Biology &amp[END_REF].

However, several problems can appear with the use of acetylene such as an underestimation [START_REF] Groffman | Methods for measuring denitrification: Diverse approaches to a difficult problem[END_REF]. However, at least the scavenging effect can be avoided if the acetylene blockage technique is used under anaerobic conditions.

There are few publications that quantify the temperature impact on N 2 O emissions.

They are either based on laboratory experiments [START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF][START_REF] Kurganova | Effect of the temperature and moisture on the N2O emission from some arable soils[END_REF][START_REF] Luo | Effects of soil temperature and moisture on methane uptake and nitrous oxide emissions across three different ecosystem types[END_REF][START_REF] Stanford | EFFECT OF TEMPERATURE ON DENITRIFICATION RATE IN[END_REF][START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF] or on field measurements [START_REF] Luo | Effects of soil temperature and moisture on methane uptake and nitrous oxide emissions across three different ecosystem types[END_REF][START_REF] Papen | A 3-year continuous record of nitrogen trace gas fluxes from untreated and limed soil of a N-saturated spruce and beech forest ecosystem in Germany -1. N2O emissions[END_REF]. They showed that soil N 2 O emissions generally increase with increasing temperatures, though a second strong optimum can be found around 0°C due to the above mentioned freeze-thaw effects on soil N 2 O production, and that this effect depends on soil texture, soil organic carbon concentrations and pH. However, previous studies were not all performed at the same temperature, at the same percentage of total water content and they also used different type of soils, but all were done under static conditions with no temperature variations. The change in emission rate is characterized by the Q 10 , which is the emission rate at (T + 10°C) / emission rate at T°C. In this study, we propose to investigate the effect of a varying temperature on N 2 O emissions by soils, and so, be closer to the real field conditions. Indeed, we measured N 2 O emissions in a range of temperature that is common in April in France, when fertilization occurs and N 2 O emissions are high [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF][START_REF] Gu | A regional experiment suggests that soil texture is a major control of N2O emissions from tile-drained winter wheat fields during the fertilization period Soil Biology &amp[END_REF]. No studies have been done in these realistic conditions so far. Q 10 are important because they can be used in N 2 O emissions models such as NOE [START_REF] Henault | Predicting in situ soil N(2)O emission using NOE algorithm and soil database[END_REF].

The objective of this paper is to study the effect of temperature on both denitrification and total N 2 O emissions by soil, at a constant temperature and at an increasing temperature, and to compare their rates.

II. Materials and methods

Site description

Soil samples were collected from the experimental site OS² ("Observatoire Spatialisé Orléanais des Sols") located in the valley of Loir in Faux Perche (48°23'N, 1°11'E, elevation 202 m above sea level), about 30 km southwest of Chartres, France. The climate records in Chartres (28070001, 4°27'N, 1°30'E, elevation 155 m above sea level) show a mean annual temperature of 10.6 °C, precipitation of 598 mm and potential evaporation of 740 mm. Monthly precipitation is evenly distributed throughout the year. The sampling field was cultivated with wheat.

Soil sampling

36 non-destructured soil samples were collected on the 19 th of February 2015

following the sampling plan presented in Figure 19, allowing the constitution of 6 groups of samples, each containing a sample from each subplot. Sampling was done using 6.5 cm high and 7 cm diameter stainless steel cylinders, representing a volume of 250 cm 3 . Soil samples were collected from the surface after removal of the litter and aboveground layers. Samples

were then placed at 4°C. Cylinders of soils were placed in 750 cm 3 glass jars (Figure 20). On the following day, 20 mL of a KNO 3 solution (0.02 M) was added to each sample to simulate field fertilization (20 mg N kg -1 of soil).

In order to define experimental conditions for conducting our experiment, we used monitored temperature dynamics of spring 2013 at the experimental site where soil samples were collected. A temperature variation from 4 to 16°C over a 12h-period was selected, as it was observed on April 2013 at 5 cm deep (Figure 21). Glass jars were separated in three groups. Twelve (2 groups of 6) of them were placed at 16°C in an air-conditioned laboratory, twelve of them at 4°C in a cold chamber, and twelve of them also at 4°C in a cold chamber before the experiment and then in a climatic chamber where temperature increased from 4°C to 16°C within 12 hours (Figure 22).

Experiment

The experiment started 4 days after sampling. Glass jars were hermetically closed and the headspace of the jars was flushed for 5 minute with pure N 2 to remove all of the N 2 O and O 2 . Following headspace N 2 flushing, 1 mL of Krypton (Kr) was added to each sample.

Finally, in half of the jars of each group, 50 mL of acetylene was injected in the headspace to block the N 2 O reductase, enzyme of denitrification under anaerobic conditions. At each temperature, N 2 O production and total denitrification were measured in samples without and with acetylene. For this, 20 mL of headspace gas was sampled through a septum with a syringe. The gas sample was injected into vacuumed glass vials (t 0 ) (Figure 23). In the following, gas samples were taken every 2 hours for twelve hours during the four days of the experiment. The lower the ratio, the higher is the reduction of N 2 O to N 2 in relation to N 2 O production.

N 2 O reduction ratio

Gases analyzes and flux calculation

Gas samples were analyzed with a gas chromatograph (GC Thermo GC (ECD detector, Ar/CH 4 as vector gas, filled porapak Q column)) for the determination of N 2 O concentrations in the headspace gas samples (Figure 23

Q 10

Q 10 were calculated for each day using samples at 4°C and samples at 16°C. Q 10 were calculated following this equation:

D EF = G HI°K G 7°K ( H HIL7 ) Eq. (3.2)
With R being the rate of the reaction of N 2 O emission production M = N '8O)-+ ) ,

Auxiliary measurements

Treatment specific analysis of the soil nitrogen contents was realized at the end of the experiment with 3 times 8 g of soil. The remaining soil samples were then weighted and dried into the oven at 105°C for 24h to calculate the density, porosity and water-filled pore space.

Statistical analysis

Statistical analyses were performed using XLSTAT® for testing linearity of kinetics (linear regression tests), correlations between activity rates and soil factors (ANOVA) and mean comparisons (t-tests). Statistical significance for all tests was set at p < 0.05.

III. Results

Incubation condition characterization

Both Kr (0.1 %) and O 2 concentration (5%) were constant over time, revealing the absence of leaks during incubations and partial anaerobic conditions all along the experiment.

At the end of the 4 days of incubation, soil WFPS was as high as expected (> 80 %).

Nevertheless, WFPS of soils incubated at 16°C were in tendency lower than at the other temperatures probably due to condensation of water at the wall of the glass vessels (Table 1).

CO 2 production

For all treatments, the CO 2 concentration always increased linearly over the entire 12hours incubation period, i.e., the hypothesis of a linear increase could not be rejected (p < 0.05). CO 2 production rates were higher at 16°C than at 4°C and 4 to 16°C incubations. In presence of acetylene, CO 2 production rates were going from 90 ± 6 µg C-CO 2 kg -1 of soil (day 4) to 165 ± 18 C-CO 2 kg -1 of soil (day 1) at 16°C, from 50 ± 3 µg C-CO 2 kg -1 of soil (day 4) to 77 ± 5 C-CO 2 kg -1 of soil (day 1) at 4-16°C and from 38 ± 5 µg C-CO 2 kg -1 of soil (day 4) to 54 ± 7 C-CO 2 kg -1 of soil (day 1) at 4°C (Figure 24). Without acetylene addition, CO 2 production rates were going from 74 ± 8 µg C-CO 2 kg -1 of soil (day 4) to 143 ± 29 C-CO 2 kg -1 of soil (day 1) at 16°C, from 53 ± 5 C-CO 2 kg -1 of soil (day 4) to 88 ± 13 C-CO 2 kg -1 of soil (day 1) at 4-16°C and from 49 ± 4 C-CO 2 kg -1 of soil (day 4) to 89 ± 11 C-CO 2 kg -1 of soil (day 1) at°4C. CO 2 production decreased with the length of the experiment, for samples with and without acetylene, and for each temperature (Figure 25). Whatever the day and temperature incubation, acetylene did not affect the CO 2 production rate. Rates of denitrification (incubations with acetylene) over the 12 hours incubation period varied from 42 ± 12 ng N-N 2 O g -1 of soil (observed on day 4) to 81 ± 14 ng N-N 2 O g -1 of soil (observed on day 2) at 16°C, from 15 ± 3 ng N-N 2 O g -1 of soil (observed on day 1) to 48 ± 7 ng N-N 2 O g -1 of soil (observed on day 3) during incubation with increasing temperature from 4 to 16°C and from 6 ± 2 ng N-N 2 O g -1 of soil (observed on day 1) to 23 ± 5 ng N-N 2 O g -1 of soil (observed on day 3) at 4°C (Table 3). N 2 O emissions (incubations without addition of acetylene) were varying from 1 ± 0 ng N-N 2 O g -1 of soil (observed on day 4) to 9 ± 3 ng N-N 2 O g -1 of soil (observed on day 1) at 16°C, from 2 ± 1 ng N-N 2 O g -1 of soil (observed on day 4) to 11 ± 3 ng N-N 2 O g -1 of soil (observed on day 2) during incubation with increasing temperature, and from 2 ± 1 ng N-N 2 O g -1 of soil (observed on day 1) to 3 ± 1 ng N-N 2 O g -1 of soil (observed on day 2) at 4°C.

Mean N 2 O flux (ng N-N 2 O g -1 of soil) D1 D2 D3 D4 Without C 2 H 2 16°C 9 ± 3 4 ± 1 1 ± 1 1 ± 0 4-16°C 5 ± 1 11 ± 3 6 ± 2 2 ± 1 4°C 2 ± 1 3 ± 1 3 ± 1 2 ±
Whatever the temperature of incubation, N 2 O production of soils with acetylene was always higher as of soils without acetylene, demonstrating both the efficiency of the N 2 O reduction during the experiment and of its inhibition by acetylene. Surprisingly, N 2 O emissions at day 3 and 4 were not higher at 16°C than at 4°C or 4-16°C while denitrification rates were always higher at 16°C than at 4°C and higher than at 4-16°C unless on day 4.

The mean N 2 O production at each sampling time (0, 2, 4, 6, 8, 10, 12 h) over the four incubation days, calculated for each temperature reveals that N 2 O emissions from soils incubated under increasing temperature from 4 to 16 °C were higher as compared to soils constantly incubated at 16°C (Figure 27). In contrast, the total denitrification and C mineralization were the highest for soils incubated at 16°C. While denitrification rates and N 2 O production were either constant or slowly decreasing over the 12 hours incubations at constant temperatures, denitrification rates and soil N 2 O production were increasing with time in incubation with increasing temperature. It should be noted that an apparent acceleration occurred after 8 hours of increasing temperatures, which corresponds to an incubation temperature of 12°C.

The ratio of N 2 O released to the headspace during denitrification was dependent on the incubation temperature (Figure 28). During the 1 st day of experiment and for the treatments 4 and 4-16°C, it was around 0.3, i.e., 30% of the total denitrification rate (N 2 O + N 2 ) was emitted as N 2 O. This value decreased to 0.1 for the 4°C treatment and to 0.05 for the 4-16°C treatment after 4 days, i.e., the end of the experiment. The N 2 O production rate during denitrification for the 16°C treatment was 0.17 during the first day of incubation and < 0.015 in the following days.

IV. Discussion

The originality of our experiment was to perform soil incubations at varying temperatures. These conditions were defined to be relevant with field conditions and were associated with more classical conditions for laboratories experiment, i.e. incubations at constant temperature, framing the varying temperature.

Results obtained for CO 2 production are very classical: observed CO 2 production rates were quite low but comparable to observed rates of C mineralization in agricultural soils in other studies [START_REF] Buscot | Microorganisms in soils: Roles in genesis and functions[END_REF]. The Q 10 value of respiration in our study was around Our study treatments with acetylene showed that the ratio of N 2 O emission to total denitrification was lower for the 16°C treatment compared to the 4°C or 4-16°C treatment.

This difference in the denitrification efficiency, or formation of the end product N 2 in favor of N 2 O, might also explain that N 2 O emissions did not increase with temperature in our experiment. Indeed N 2 O reduction appears less efficient at low temperature and varying temperature than the first steps of denitrification (more sensitive at low temperature) which leads to at least a stabilization and probably a decrease of N 2 O emissions with increasing temperatures.

Incubations at increasing temperature reveal an acceleration of the denitrification rates at 12°C, which is consistent with observations of [START_REF] Stanford | EFFECT OF TEMPERATURE ON DENITRIFICATION RATE IN[END_REF] currently taken into account in the NOE model [START_REF] Henault | Predicting in situ soil N(2)O emission using NOE algorithm and soil database[END_REF].

The biodiversity of soil microorganisms able to mineralize organic C has been found to be much larger than the ones able to denitrify and reduce N et al. 2004). While a temperature sensitivity of microbial processes with a Q 10 of 2 could be admitted as the norm, we observed that the C mineralization was not affected by the different temperature treatments (constant 4°C or 16°C, or a 12 h-increase from 4 to 16°C), but that denitrification and moreover N 2 O reduction decreased with time.

This study also questions current approaches for simulating effects of temperature on soil N 2 O emissions due to denitrification as our study indicates controls at 2 levels. First, Q 10 values observed for CO 2 production were about constant over the 4 days of our study, while the Q 10 for denitrification rates declined from values as high as 6.47 at day 1 to finally reach 1.72 at day 4. Therefore, the current assumption in denitrification models that the temperature sensitivity of denitrification remains constant with time is too simplistic. Secondly, this study also suggests that the temperature dependencies of denitrification varies for the different enzymatic steps, i.e., as in our study, that the temperature dependency of N 2 O production markedly differs from that of the N 2 O reduction to N 2 . Finally, our experiment also confirmed that N 2 O reduction to N 2 is the final process determining soil N 2 O emissions.

V. Conclusion

This study has clearly demonstrated that the temperature and the temperature variations significantly affect soil denitrification, N 2 O production and reduction by denitrification. Surprisingly, at our highest incubation temperature of 16°C, soil N 2 O emissions were, in tendency, lower than for the treatments at 4°C or 4-16°C, while total denitrification was however higher at higher temperature. This observation appears to be due to a relative lower ratio of N 2 O reduction to N 2 O production at lower soil temperatures. It remains unclear however, if this result can be generalized for other soils in the region or elsewhere, as such experimental approach, as realized in this study, has not been realized before. Our results suggest that a dynamic parametrization of the temperature dependency of the different enzymatic steps might be required to realistically simulate N 2 O production and emissions from soils.

Après avoir travaillé à l'échelle du cylindre de sol dans le chapitre 3 sur l'effet de la température du sol sur les émissions de N 2 O, et ce sur un sol prélevé sur le site expérimental présenté au chapitre 2, nous allons maintenant présenter des résultats de mesures directes is produced at the microscale mainly by microbial processes, but the effects of its increased atmospheric concentration are visible at the global scale. N 2 O emissions can be studied at the aggregate, field, landscape, regional, national and global scales. Regardless of the scale, the very high spatial and temporal variability of N 2 O emissions from soil make its quantification difficult [START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF]. Furthermore, N 2 O emissions depend on numerous factors such as crop type, fertilizer type, and N application rate, as well as soil properties, such as soil organic carbon content, humidity, pH, and texture [START_REF] Stehfest | N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions[END_REF].

Several measurement methods have been developed to monitor N 2 O emissions, with increasing accuracy, providing flux estimates at different scales. Chambers, which provide a gas-tight enclosure of a certain soil surface area while measuring the changes in gas concentrations in the chamber headspace, provide flux estimates at the plot scale. They can be used to assess (i) the spatial variability of N 2 O fluxes if they are mobile [START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Grossel | Simulating the spatial variability of nitrous oxide emission from cropped soils at the within-field scale using the NOE model[END_REF]) and (ii) the temporal variability if they are static, i.e., remain at a fixed position [START_REF] Henault | Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions[END_REF][START_REF] Laville | Field comparison of nitrous oxide emission measurements using micrometeorological and chamber methods[END_REF]. High-frequency measurements can be obtained using automatic chambers [START_REF] Butterbachbahl | Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions Nutrient Cycling in[END_REF][START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF][START_REF] Neftel | N2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements[END_REF]. The use of a micrometeorological mast/tower -to estimate the fluxes from meteorological conditions and gas concentrations -based either on eddy covariance (EC) [START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF][START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Laville | Nitrous oxide fluxes from a fertilised maize crop using micrometeorological and chamber methods[END_REF][START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF], relaxed eddy accumulation [START_REF] Baker | Conditional sampling revisited[END_REF][START_REF] Mcinnes | Relaxed eddy accumulation Micrometeorology in agricultural systems[END_REF], or the flux gradient method [START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF][START_REF] Wagnerriddle | Estimates of nitrous oxide emissions from agricultural fields over 28 months[END_REF][START_REF] Zhang | Quantifying nitrous oxide fluxes on multiple spatial scales in the Upper Midwest[END_REF] provides information on the temporal variability over a wide spatial area, the so-called footprint, which is defined mainly by the height of the mast (plot, landscape and regional scale). Using aircraft [START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF][START_REF] Pattey | Towards standards for measuring greenhouse gas fluxes from agricultural fields using instrumented towers[END_REF]) to measure N 2 O in the atmosphere provides information at an even larger scale (regional). These methods have been used to measure the biosphere-atmosphere exchange of trace gases for different ecosystems, including grassland [START_REF] Clayton | MEASUREMENT OF NITROUS-OXIDE EMISSIONS FROM FERTILIZED GRASSLAND USING CLOSED CHAMBERS[END_REF][START_REF] Neftel | N2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements[END_REF][START_REF] Smith | MICROMETEOROLOGICAL AND CHAMBER METHODS FOR MEASUREMENT OF NITROUS-OXIDE FLUXES BETWEEN SOILS AND THE ATMOSPHERE -OVERVIEW AND CONCLUSIONS[END_REF], forests [START_REF] Breuer | N2O emission from tropical forest soils of Australia[END_REF][START_REF] Butterbachbahl | Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions Nutrient Cycling in[END_REF], wheat stubble [START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF], corn [START_REF] Laville | Nitrous oxide fluxes from a fertilised maize crop using micrometeorological and chamber methods[END_REF], cotton [START_REF] Wang | Comparison between static chamber and tunable diode laser-based eddy covariance techniques for measuring nitrous oxide fluxes from a cotton field[END_REF], and mixed fields [START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF][START_REF] Flechard | Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe Agriculture Ecosystems &amp[END_REF][START_REF] Zhang | Quantifying nitrous oxide fluxes on multiple spatial scales in the Upper Midwest[END_REF], during different seasons: spring [START_REF] Clayton | MEASUREMENT OF NITROUS-OXIDE EMISSIONS FROM FERTILIZED GRASSLAND USING CLOSED CHAMBERS[END_REF][START_REF] Henault | Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions[END_REF][START_REF] Smith | MICROMETEOROLOGICAL AND CHAMBER METHODS FOR MEASUREMENT OF NITROUS-OXIDE FLUXES BETWEEN SOILS AND THE ATMOSPHERE -OVERVIEW AND CONCLUSIONS[END_REF], summer [START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF][START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF][START_REF] Neftel | N2O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements[END_REF]) and autumn [START_REF] Laville | Field comparison of nitrous oxide emission measurements using micrometeorological and chamber methods[END_REF], and for different durations, ranging from 4 days [START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF]) to 4 years [START_REF] Desjardins | Multiscale estimates of N2O emissions from agricultural lands[END_REF].

Each of the many methods used to measure N 2 O emissions from soils has specific advantages and limitations [START_REF] Denmead | Approaches to measuring fluxes of methane and nitrous oxide between landscapes and the atmosphere[END_REF]. The fluxes obtained by the chamber method are generally associated with very large uncertainties because (i) the very small surface area covered by the chamber is not representative of the ecosystem and (ii) the gas-tight closure of the chamber affects natural gas diffusion gradients across the soil-atmosphere-interface [START_REF] Davidson | Minimizing artifacts and biases in chamber-based measurements of soil respiration[END_REF], which is a non-linear phenomenon and should be considered when calculating the flux [START_REF] Pihlatie | Comparison of static chambers to measure CH4 emissions from soils[END_REF]). The fluxes obtained by micrometeorological methods are dependent on the wind conditions during the measurement period, therefore, they do not represent a static area over time. Moreover, micrometeorological flux measurements are most suitable for flat terrain and provide only an integrated flux over a varying footprint area. Their installation, maintenance and data evaluation require a very high technical level and are expensive in terms of instrumentation and scientific man-power [START_REF] Henault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF].

Schematically, micrometeorological methods are the most appropriate for estimating gas fluxes in situations representative of ecosystems while chamber methods are the most appropriate for comparing emissions related to different agricultural practices.

For micrometeorological methods, the footprint area depends on the measurement height, so higher masts can be used to survey entire landscapes. Nevertheless, these methods rely on assumptions of horizontal homogeneity and non-advective conditions; data collected in complex terrain are difficult to analyze and the choices of sites are biased towards flat and homogeneous areas [START_REF] Novick | Inferring the contribution of advection to total ecosystem scalar fluxes over a tall forest in complex terrain[END_REF]. While studies have addressed these problems for ecosystem CO 2 fluxes [START_REF] Gockede | A combination of quality assessment tools for eddy covariance measurements with footprint modelling for the characterisation of complex sites[END_REF]), this has so far not be done for N [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF][START_REF] Gu | A regional experiment suggests that soil texture is a major control of N2O emissions from tile-drained winter wheat fields during the fertilization period Soil Biology &amp[END_REF][START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF]). The 30-years mean temperature at this site is 10.6°C, while the mean annual rain fall is 600 mm. A 15-m tall mast was erected and emission measurements were focused on an area within a radius of 1 km around this mast (Fig. 30).Within this area, different types of crops and land uses are found: wheat (21 fields, surface 112 ha, representing 29% of the area), rapeseed (14 fields, 93 ha, 24%), winter barley (4 fields, 63 ha, 16%), catch crop -pea combination (1 field, 8 ha, 2%), catch crop -corn combination (1 field, 18 ha, 5%), fallow (4 fields, 8 ha, 2%) and forest (5 forests, 87 ha, 22%). The wheat, rapeseed and winter barley plots were fertilized during the experiment. Generally on wheat fields, a fertilization was brought shortly before the campaign begins (12 to 27 March, median amount 78 kg N ha -1 , liquid form) and a third one during the campaign (25 April to 12 May, median amount 62 kg N ha -1 , liquid or solid form). On rapeseed fields, a fertilizer input was brought earlier than for wheat (7 to 25 March, median amount 80 kg N ha -1 , liquid or solid form). Barley fields also received a fertilizer input shortly before the campaign (20 to 25 March, median amount 69 kg N ha -1 ). Corn was seeded on the 13 April (in green manure freshly ground in the plot) during the campaign and received 20 kg N ha -1 at seeding and 100 kg N ha -1 on the 10 May. Last, the pea field was seeded on the 10 March, also in a green manure freshly ground.

Experimental design

To assess both the spatial and temporal variability of N 2 O flux from the plot to landscape scale, 3 measurement methods were applied simultaneously: (1) automatic chambers placed on a wheat field next to the mast for continuous plot-scale measurements to study the local temporal variability; (2) fast-box measurements inside a 1-km radius from the mast at various plots to study the spatial variability, and (3) a 15-m tall EC system to provide continuous spatially integrated N 2 O fluxes at the landscape scale.

Automatic chamber measurements

8 automatic chambers were deployed in the plot next to the EC mast from 21 March to 12 May 2015. The chambers (50 cm x 50 cm x 15 cm; total volume of 37.5 L) consisted of an aluminum frame in which Plexiglas panels were fixed. Frames were inserted 2 cm deep into the soil. Chambers were connected to a gas chromatograph equipped with an electron capture detector (GC-ECD) placed in a truck (Figure 31). The N 2 O concentrations were measured in 2 groups of 4 closed chambers. These 2 groups were randomly selected, and placed as far as possible from the wheel traces. A measurement cycle for the 2 groups took 2 hours (30 min for each chamber, 12 cycles per day). In case of rain, the chambers were opened and the measurements were stopped to limit experimental biases; otherwise, measurements were performed continuously up to 12 times per day over the entire observation period. The whole system was described in a previous paper [START_REF] Butterbachbahl | Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions Nutrient Cycling in[END_REF].

Fast-box measurements

Eight snap-shot campaigns were performed during the experimental period using a fast-box system coupled to a lab-built QCL (Quantum Cascade Laser) spectrometer called SPIRIT [START_REF] Guimbaud | A portable infrared laser spectrometer for flux measurements of trace gases at the geosphere-atmosphere interface Measurement Science &amp[END_REF][START_REF] Guimbaud | A quantum cascade laser infrared spectrometer for CO2 stable isotope analysis: Field implementation at a hydrocarbon contaminated site under bio-remediation[END_REF]). The fast-box was a 50 cm x 50 cm x 15 cm chamber pressed directly onto the soil surface without the pre-installation of frames (Figure 32). Soft rubber foam (5 cm wide) on the chamber edges ensured tight sealing of the chamber headspace, and any leakage was assessed by on-line CO 2 analysis. The SPIRIT analyzer was equipped with a QCL emitting at 4.5 µm, to measure the N 2 O dry mixing ratio (vmr) at 0.7 Hz. The spectrometer was placed in a van to move among sites within the mast footprint. Sixteen sampling locations were selected based on their accessibility and their representativeness of soil occupation and soil type in the investigated area. We were able to investigate 7 to 9 of the 16 sites per measurement day, with 4 measurements per site. During each of the 8 snap-shot campaigns, downwind sites were systematically sampled and incremented by other sites chosen to optimize the number of measurements (distance, accessibility). The fast-box sites and the types of crop in the mast footprint are presented in Figure 30.

Eddy covariance measurements

The EC system was placed on the top of the 15-m tall mast (Figure 33). Wind was measured using a 3D ultrasonic anemometer (R3-50, Gill, UK). Air was sampled through a

Additional measurements

The automatic chamber system was complemented by temperature (TC Direct, UK) and humidity probes (TDR CS616, Campbell Scientific) placed in the soil at depths of 5, 10 and 20 cm (3 replicates per depth) which measured at a 1-h time step, and by a pluviometer (ARG100, Campbell Scientific). Furthermore, during each fast-box snap-shot campaign, soil temperature and soil humidity were measured with a probe placed 10 cm deep in the soil, for the sites investigated daily. Moreover, soil samples were collected from these sites at a depth of 0-20 cm and brought to the laboratory to measure mineral nitrogen (N), including both ammonium (NH 4 + ) and nitrate (NO 3 -) by colorimetry after extraction in K 2 SO 4 0.5M according to [START_REF] Cataldo | RAPID COLORIMETRIC DETERMINATION OF NITRATE IN PLANT-TISSUE BY NITRATION OF SALICYLIC-ACID Communications in[END_REF] and pH using a glass electrode in a soil suspension diluted in a potassium chloride solution (1:5 ratio, NF ISO 10390). In each site, 3 undisturbed soil cores using 9 cm diameter cylinders (0.5 L) were taken in April to measure bulk density in the 5-15 cm layer, as a trade-off between available experimental devices and representativeness of the tilled layer. Soil samples were also taken in the 0-20 cm layer, dried at room temperature and sieved at 2 mm mesh to analyze soil texture, total nitrogen and carbon content at LAS (Soil Laboratory Analysis, France). Lincoln, NE. LI-COR). Briefly, first 2D-rotation was applied on the wind components; then de-spiking was applied. Spectral corrections for low-pass and high-pass filtering effects were applied according to Moncrieff's studies [START_REF] Moncrieff | Averaging, Detrending, and Filtering of Eddy Covariance Time Series[END_REF][START_REF] Moncrieff | A system to measure surface fluxes of momentum, sensible heat, water vapour and carbon dioxide[END_REF].

Flux calculation methods

For

Moreover, a recursive running mean filter was applied on the raw N 2 O signal to remove artificial fluctuations due to temperature variation. These fluctuations were due to offset drift on the raw N 2 O signal, probably caused by temperature variations in the analyzer box. The time constant required for the filter (50 s) was determined using Allan variance analysis [START_REF] Mammarella | A case study of eddy covariance flux of N2O measured within forest ecosystems: quality control and flux error analysis[END_REF]. The lag time between the N 2 O mixing ratio and w was automatically adjusted in a 1.5-s window around a median value of 2.3 s. The total random uncertainty was calculated according to Finkelstein and Sims [START_REF] Finkelstein | Sampling error in eddy correlation flux measurements[END_REF].

The EC calculations also provided the wind direction (WD) and wind speed at mast height z, P Q(4), as well as the friction velocity (u*), sensible heat flux (H), Obukhov length (L), and the standard deviation of the horizontal cross-wind component of the wind velocity σ V , which were used by the footprint models.

Footprint defined by micrometeorological approaches

The contribution of each surface area (landscape element) to the N 2 O flux measured by the EC system is by definition the footprint of this surface (Wilson, 1991). The footprint was determined inside a 1 km radius around the mast at all times by both the Kormann and Meixner model (K&M) [START_REF] Kormann | An analytical footprint model for non-neutral stratification Boundary-Layer[END_REF] and the FIDES [START_REF] Loubet | An inverse model to estimate ammonia emissions from fields[END_REF][START_REF] Loubet | Advection of NH3 over a pasture field and its effect on gradient flux measurements[END_REF][START_REF] Loubet | Investigation of the interaction between sources and sinks of atmospheric ammonia in an upland landscape using a simplified dispersion-exchange model[END_REF] footprint models, the latter being similar to the K&M model, but with a parameterized lateral dispersion and a different way to include the reference height. The input data of these models are mast height, field coordinates, WD, u*, L, σ V , as well as the dynamic roughness length z 0 estimated in neutral conditions using the definition of the logarithmic wind profile in the boundary layer (Kaimal and Finnigan, 1994):

P Q(4) = ; * S ln ( V V ) Eq. (4.1)
where k is the Von Karman constant (k = 0.414) and d is the displacement height which, in a simplified approach was assumed to be constant for all wind directions and equal to d = 0.0469 m. z 0 theoretically depends on the land use (crop type, canopy height, forest and presence of buildings). In our approach, z 0 was calculated from all the P Q(4) measurements and averaged over four directions in order to separate the main forests area (at north and southwest) from the main crop areas.

By definition of the footprint, the N 2 O flux at the mast at time t F N2O,mast,t is:

W'X,YZ[\,\ = ∑ ^FW'X,+, × A +, a +bE Eq. (4.2)
Where i is a landscape element, A i,t is the footprint of this area at time t, and F N2O,i,t is the N 2 O flux emitted from the given area. Bottom: wind direction measured by the EC system..

Statistics

Statistical

N 2 O fluxes and fast-box measurements

The fluxes measured by the fast-box method are shown in Fig. 35. N 2 O accumulation into chambers were non-linear in about 30% of cases justifying the use of the HM model [START_REF] Pedersen | A comprehensive approach to soil-atmosphere trace-gas flux estimation with static chambers[END_REF]Brümmer et al., 2017). The mean fluxes measured during each snapshot campaign ranged from 2 ± 2 to 101 ± 19 µg of N-N 2 O m -2 h -1 . A maximum was observed on 8 April with 295 ± 69 µg of N-N 2 O m -2 h -1 , which is consistent with the automatic chamber observations. The maxima for the different crops varied by date (Table 6). Following the emissions peaks at the beginning of April, the fluxes decreased towards the end of the measuring period. all the values are of the same order of magnitude, the variability is consistent with the wellknown spatial and temporal variability. The combination of the different measurement techniques reveals that the wheat field investigated using the automatic chambers emitted higher fluxes than most of the fields within the investigated area.

Table 7: Mean and standard error of global fluxes measured with different techniques

considering either all data or only periods when all data were measured simultaneously.

Mean (µg N-N 2 O m -2 h -1 ) Standard error (µg N-N 2 O m -2 h -1 )

Eddy covariance mast

All data 45.7 7.5

Only when both automatic systems were measuring simultaneously 45.2 7.5

Automatic chambers

All data 71.5 8.1

Only when both automatic systems were measuring simultaneously 53.0 6.0

Fast-box All data 37.0 6.2

Footprints

As the main wind directions during the experimental period were north (April) and south (May), the forest on the north and the fields directly south of the EC mast, where the automatic chambers were placed, were the main contributors to the footprint of the EC flux measurements during the campaign (Figure 36). Both footprint models predicted a larger contribution of the fields near the EC mast than fields further away. FIDES assessed a larger contribution of the fields in the south -north corridor, leading to a larger contribution of fields cultivated with wheat, rapeseed, and winter barley. In contrast, K&M included contributions of fluxes from the western part of the experimental site in its footprint prediction, leading to a larger contribution of fields cultivated with catch crop-pea, catch cropcorn, fallow and forest. Moreover, FIDES assumed a larger contribution of the wheat plot to the total N 2 O flux, i.e., where the automatic chambers were placed, than the K&M model.

Nevertheless, the plot contributions to the footprint obtained with both micrometeorological models were significantly correlated (p < 0.001), (Figure 37). Neither

IV. Discussion

Comparisons between methods

In this study, we compiled a database of continuous N 2 O fluxes using different measurement techniques at different scales. The results obtained directly by the EC system and the fast-box located inside the footprint were generally consistent with low values on 15, 16, 24 and 28 April. On 2 April, both high and low values from the fast-box revealed strong spatial variability, and the emission values from the mast were included in this variability.

Nevertheless, the fast-box results were all low, whereas high values were observed by the mast on 8 and 27 April. This means that the overall spatial variability was not completely covered by the fast-box system and that there were other N 2 O sources inside the footprint at these times. Fast-box measurements are snapshots of fluxes for a given time and location, so "hot moments" might have been missed and "cold spots" could have been surveyed.

The EC fluxes had the same dynamics as those observed by the automatic chambers when both systems were working and when the automatic chambers were upwind from the mast. There was an increase in the flux at the beginning of April, followed by an asymptotic decrease and steady values, with a significant pulse at the end of the month and at the beginning of May. These pulses can be explained by the anaerobic soil conditions following precipitation events that occurred immediately before the emission pulses were observed. The highest emission pulse observed by the automatic chambers on 4 April was not detected by the EC system because the wind blew from the north (Figure 34) and the chambers, located south of the mast, were not in the mast footprint at this time. Nevertheless, during periods when the wind came from the south, the results of the mast and the chambers were in agreement, with background emissions and small pulses with similar values for both systems.

Similar results were observed over a carrot and a wheat stubble field, with similar values measured by both systems with dependence on wind [START_REF] Christensen | Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques[END_REF].

For the automatic chambers, the gaps in the N 2 O emissions measurements were caused by the interruption of N 2 O measurements due to the detection of precipitation. The field was fertilized on the 24 March, followed by a rain event on 29 March, triggering a high emission pulse on 30 March. N 2 O pulse emissions showed a delay of several hours or days following rain events. The mean value of the total campaign for the automatic chamber (71 µg of N-

N 2 O m -² h -1
) was almost twice as high as the mean value obtained from the fast-box (37 µg of

N-N 2 O m -² h -1
) and the EC system (42.5 µg of N-N 2 O m -² h -1 ). This can be explained by the fact that for the automatic chambers, only one location was measured, whereas for the fastbox, the mean value was measured across different crops (e.g., wheat, rapeseed, forest) and locations, and for the EC system an integrated mean value was obtained from different fields and crops in the footprint.

Spatial variability inside the 1-km radius area

N 2 O emissions were very dependent on the location and type of crop. Such variability was observed by [START_REF] Hargreaves | Measurement of nitrous oxide emission from agricultural land using micrometeorological methods[END_REF] when comparing N 2 O fluxes measured over an arable cropland with an EC system at a height of 5 m for carrot, wheat stubble, potatoes, spinach, onions and wheat and by [START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF], who compared measurements with an EC system at 3.5 m for corn and alfalfa fields. On average, rapeseed and wheat showed the same order of magnitude of N 2 O emissions, forest and winter barley appeared to emit very small amounts of N 2 O and the catch crop-pea and corn combination emitted more than wheat and rapeseed. Differences between these emissions might be due to the different types of soil, agricultural practices, soil properties, or a combination of these factors. The results obtained for winter barley were probably biased by the dates of the measurements, which were all performed during periods of low emissions. Regarding the high N 2 O emissions by the catch crop -pea plot, the pea had just been seeded, and the high emissions were probably due to catch crop decomposition, although the mineral N content measured in soil was not especially

high. An increase in N 2 O emissions of approximatively 0.1 kg N ha -1 y -1 globally has been attributed to catch crop [START_REF] Pellerin | Contribution of Bacteria to Initial Input and Cycling of Nitrogen in[END_REF], and our results were probably influenced by a corresponding event. Correlation tests indicated an influence of soil humidity, WFPS and

V. Conclusion

I. Introduction

In order to quantify N 2 O emissions at the landscape scale, combining EC measurements with chamber measurements has been proven useful as it provides additional information on spatial variability as well as the temporal variability of fluxes for defined plots [START_REF] Eugster | Eddy covariance for quantifying trace gas fluxes from soils[END_REF]. 2 types of approach are currently available to link micrometeorological measurements with chamber measurements. The first is the bottom-up approach, which involves: (i) direct measurements at the soil surface and (ii) a method for extrapolating local results to larger scales of time and space, using basic extrapolation or ecosystem models such as Landscape-DNDC [START_REF] Haas | LandscapeDNDC: a process model for simulation of biosphereatmosphere-hydrosphere exchange processes at site and regional scale Landscape[END_REF] or CERES-NOE [START_REF] Gabrielle | Simulation of nitrous oxide emissions from wheat-cropped soils using[END_REF]). The second is the top-down approach, which involves (i) integrative micrometeorological measurements and (ii) atmospheric transport modeling. At the landscape scale, the Kormann and Meixner model (K&M) [START_REF] Kormann | An analytical footprint model for non-neutral stratification Boundary-Layer[END_REF] and the FIDES model [START_REF] Loubet | An inverse model to estimate ammonia emissions from fields[END_REF][START_REF] Loubet | Advection of NH3 over a pasture field and its effect on gradient flux measurements[END_REF][START_REF] Loubet | Investigation of the interaction between sources and sinks of atmospheric ammonia in an upland landscape using a simplified dispersion-exchange model[END_REF] This first method is based on the basic assumption that only the type of land use affects the soil N 2 O emissions [START_REF] Freibauer | Controls and models for estimating direct nitrous oxide emissions from temperate and sub-boreal agricultural mineral soils in Europe[END_REF]. This hypothesis means that the N 2 O flux of a given land use lu at a given time t (F N2O,lu,t ) is homogeneous in the landscape. It is useful to determine the ratio α lu,t .

∝ :;, = d ,!e, d
,fg h , Eq. (5.1)

Wheat was chosen as the reference crop because it covered the largest area of the landscape.

Here, F N2O,lu,t is the mean N 2 O flux measured for land use lu at time t. Combining Eq. (4.2)

and Eq. ( 5.1) and considering the sum over each land use instead of the sum over each crop leads to:

W'X,YZ[\,\ = '8,>?). , ∑ ^α:;, × A +, a :;

Eq. (5.2)

Further assuming that the variation of α :;, with time is small ((α :;, ~〈α :; 〉), leads to:

W'X,YZ[\,\ = '8,>?). , ∑ ^〈α :; 〉 × A :;, a :;

Eq. ( 5. Eq. (5.5) α lu,t was calculated on each date t and averaged over the 8 fast-box sampling dates to calculate the average 〈α lu 〉. Moreover, data were taken only when at least 50% of the footprint calculated by both the FIDES and K&M models was within the 1-km radius. We supposed that N 2 O was emitted homogenously between the 1-km radius and beyond. The standard error v w !e was also calculated. The overall uncertainty of F '8,:;, was calculated by error propagation, accounting for the uncertainty of the EC measurements ( W'X,YZ[\,\ ) estimated as the total random uncertainty and the uncertainty on 〈α x 〉 estimated with the standard error The N 2 O fluxes from denitrification are commonly described by empirical multiplicative models based on soil variables [START_REF] Henault | Predicting in situ soil N(2)O emission using NOE algorithm and soil database[END_REF][START_REF] Schmidt | Using a boundary line approach to analyze N2O flux data from agricultural soils[END_REF].

( v w x ). <) y ,!e,
Applying such a model over the landscape with crop type i and further considering that denitrification is dominant, in line with previous studies in the same region [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF], leads to the daily N 2 O flux defined as:

W'X,YZ[\,\ = ∑ ~∑ ^N&( • O +, × NF +, × A +, a
+ +* :;

• :;

Eq. (5.7)

where i is the index for each field and N&( • O +, is the soil NO 3 -content of field i at time t.

Introducing the assumption that NF +, is only a function of land use lu leads to W'X,YZ[\,\ = ∑ ~NF ,:; ∑ ^N&( • O +, × A +, a + +* :;

• :;

Eq. (5.8)

Again, taking wheat as the reference land use, we define the ratio based on normalized fluxes:

β :;, = Wd ,!e,

Wd

,fg h , Eq. (5.9)

We can demonstrate that β :;, is constant (see supplementary material) and noticing that NF ,:; = 〈ƒ :; 〉 • NF ,>?). and that NF ,>?). can be taken out of the summing operator (constant) leads to:

NF ,>?). = m n o,pqrs,s ∑ …〈 † !e 〉 ∑ ^N 8 ‡ L O , ×u , a $ !e !e
Eq. (5.10)

N&( • O +, = NNO • O Š.AS =@;* ,+ + NO • 9)= ,+ * ~1 -• + (Ž -Ž 9)= ,+ )• Eq. (5.12)
where • + is an empirical function fitted to field measurements, increasing from 0 at t = t fert to 1 at t end , f i (t-Ž 9)= )=1 for t < t fert or t > t end . For each plot, the exact date and amount of fertilizer were recorded. An input of 0.5 kg ha -1 of NO 3 -was estimated for each kg ha -1 of N input as the main fertilizer applied was ammonium nitrate (50% N-NH 4 + and 50% N-NO 3 -).

〈ƒ :; 〉 was calculated from the fast-box measurements, on 8 sampling dates with

Wd ,!e
Wd ,fg h

. The same hypothesis of homogeneity beyond the 1-km radius applied for the LU attribution method was applied here. The standard error v • !e was also calculated. The overall uncertainty on '8,+, was calculated by error propagation, accounting for the uncertainty on the EC measurements (F mast , total random uncertainty) and the uncertainty on

〈β lu 〉 ( v • !e = standard error). <) y , d , ≤ <) • 〈 † 〉 + <) } pqrs,s m pqrs,s + ∑ <) • 〈 † 〉 A +, N&( • O +, + v † <N 8 ‡ L O , N 8 ‡ L O , A +, +
Eq. (5.13) Spatial and temporal dependencies on errors were not considered as their characterization is beyond the scope of this study.

Evaluation of the validity of the 2 flux attribution methods

As the fluxes measured with the automatic chambers were not used to estimate the spatial distribution of the N 2 O fluxes, these data could be used to validate our calculation for a specific plot using a top-down approach. The LU and NCLU flux attribution methods were used to calculate plot-specific N 2 O fluxes. Furthermore, the uncertainties were calculated for each value of N 2 O flux obtained using the flux attribution methods. Only values whose uncertainties were lower than the calculated flux (|∆F/F|<1) were considered, as done in a previous study [START_REF] Mammarella | A case study of eddy covariance flux of N2O measured within forest ecosystems: quality control and flux error analysis[END_REF] to remove unreliable fluxes with very large uncertainty, that were mostly large ones. The total N 2 O emitted from each plot in the 1-km radius area during the entire campaign was calculated. The retrieved flux was compared to measurements on the field in which the automatic chambers were placed. This wheat field was considered representative of every wheat field in the area because despite the well-known spatial variability of N 2 O fluxes, similar flux dynamics have been observed on 6 to 8 replicate sites over 2 years in this site in previous studies [START_REF] Gu | Modeling nitrous oxide emissions from tile-drained winter wheat fields in Central France Nutrient Cycling in[END_REF][START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF] . The precision of the model was tested by calculating the coefficient of determination R 2 and its accuracy by considering the root mean square error and the relative root mean square error.

Last, we investigated the effect of the hypothesis about emissions outside the 1-km radius area. We used a bottom-up approach using Eq. ( 2) and replaced the N 2 O flux F N2Oi with the previously calculated flux. If 100% of the footprint was within the radius, we will find again the results obtained with the LU and the NCLU approaches.

Statistics

ANOVA and the Newman-Keuls test were used for the fast-box and soil data to test the null hypothesis that the type of crop or land use did not affect soil N 2 O emissions.

Evaluation of the attribution methods

The performance of the attribution methods was tested at the plot scale using the data collected by the automatic chambers. The simulations obtained via the LU attribution methods were fairly consistent with the automatic chamber dynamics (Fig. 39), except for the first high pulse. However the values are always the same order of magnitude, and the mean values over the whole period were almost identical. However, for the NCLU attribution methods, the model over-estimated the fluxes on this plot, with high emission pulses throughout the campaign (Fig. 39). The mean values on this plot and the total N 2 O emitted during the campaign were calculated for each method (Table 9). The LU values were almost the same as those measured by the automatic chambers, while the values were higher for the NCLU approach. Nevertheless, the best model precision was obtained with the NCLU method combined to the K&M footprint model (R 2 =0.50). The RMSE is of the same order of magnitude than the mean flux for both approaches. The rRMSE ranged from 0.63 to 0.77.

Spatial variability of N 2 O emissions

The apparent N 2 O emissions recorded by the EC system amounted to 0.31 kg N-N 2 O ha -1 month -1 throughout the experimental period, and total emissions were estimated at 0.30 and 0.29 kg N-N 2 O ha -1 month -1 using the LU flux attribution approach combined with FIDES and K&M, respectively, and 0.39 and 0.44 N-N 2 O kg ha -1 month -1 using the NCLU flux attribution approach combined with FIDES and K&M, respectively. The N 2 O emissions estimated using the attribution methods at the landscape scale were consistently higher for the Total N 2 O emitted (kg) 4.7 ± 0.5 5.2 ± 2.6 5.3 ± 2.9 7.8 ± 1.9 6.7 ± 1.9

NCLU approach than those calculated from the results obtained by the EC system alone, which partially cover the total area of 1-km radius. On the other hand, the LU approach produces very similar results (Table 10).

The bottom-up approach results gave total emissions of 0.18, 0.19, 0.23 and 0.29 kg ha -1 month -1 N for LU + FIDES, LU + KM, NCLU + FIDES and NCLU + KM respectively, corresponding to 58% to 65% of the fluxes calculated using the LU and NCLU flux attribution approaches. Therefore, 58% to 65% of the N 2 O fluxes were from the 1-km radius area.

The emission maps of the entire landscape, estimated from the LU and NCLU flux attribution techniques combined with the footprint models, show a clear pattern with crop cultivations as the main driver of N 2 O emissions (Fig. 40). The catch crop -pea and corn combinations, wheat, and rapeseed fields dominated the emissions, while very low emissions were attributed to forests, winter barley and fallow fields. Correlation tests showed that the 2 flux attribution approaches were correlated (Table 11).

Top-down

EC system LU + FIDES LU + K&M NCLU + FIDES NCLU + K&M Mean flux (µg N-N 2 O m -2 h -1 )

LU LU NCLU NCLU

IV. Discussion

Methods to assess the spatial variability

The main novelty of this study is the definition of different methods for estimating and mapping soil N 2 O emissions at the landscape scale from a single temporal N 2 O emission dynamics measured by an EC system. Combinations of different measurement methods to estimate N 2 O emissions have been deployed previously, including chambers and EC measurements in combination with both top-down and bottom-up approaches using the use of footprint models [START_REF] Griffis | Reconciling the differences between top-down and bottom-up estimates of nitrous oxide emissions for the US Corn Belt[END_REF][START_REF] Molodovskaya | Nitrous Oxide from Heterogeneous Agricultural Landscapes: Source Contribution Analysis by Eddy Covariance and Chambers[END_REF]. None of these studies used measurements at this spatial scale with a 15-m mast or considered continuous observations over a 2-month period. The results obtained with the top-down approach resulted in N 2 O emissions in the same range of magnitude as the N 2 O emissions measured by the EC system.

Both methods and both footprints models resulted in similar model precision and accuracy when comparing to the flux dynamics observed with automatic chambers in one field (Table 9). The best precision was obtained with the K&M model and the NCLU approach (R 2 =0.50) probably because the first pulse is better reproduced with this approach; however, considering the mean flux over the campaign in this field, the LU flux attribution approach produced results closer to the measured results than those of the NCLU approach. This can probably be explained by the pulse at mid-April simulated by the NCLU approach when no pulse was actually measured. Simulation of N 2 O fluxes by more complex models such as semi-empirical ones or crop models often result in moderate R 2 and RMSE on the same order than mean flux or larger (e.g. [START_REF] Gu | Modeling nitrous oxide emissions from tile-drained winter wheat fields in Central France Nutrient Cycling in[END_REF][START_REF] Gabrielle | Simulation of nitrous oxide emissions from wheat-cropped soils using[END_REF]. This outlines the present difficulty to obtain good prediction of N 2 O fluxes; in this view, the present results are satisfactory and both attribution approaches can be considered as consistent.

Variables

Automatic chambers measurements were added to the protocol to propose a validation method for the flux attribution approaches. The results obtained by the LU approach were, in term of mean value, closer to the measured values than those obtained with the NCLU approach (Fig. 39 and Table 9). Consequently, the addition of the NO 3 -function in the LU flux attribution approach does not significantly improve the model, probably because the empirical NO 3 -function is not sufficiently accurate. Indeed, we supposed that all the added NO 3 -was fully available one day after input, although NO 3 -is actually consumed by other processes such as plant uptake or leaching, and no consideration was taken about the time of diffusion and the type of input (liquid or solid form). On the other hand, the NCLU method was able to predict the absence of emission pulse at the third fertilization (11 May) when denitrification was limited despite large NO 3 -content because of soil dryness. This is also the reason why there was no correlation between N 2 O emissions and NO 3 -on the field with automatic chambers.

Integrated fluxes and spatial variability over the 1-km radius area

Levels of N 2 O emissions

The share of N 2 O emissions from the N added to the fertilized zones during the fertilization period resulted in direct emission factors between 0.24 and 0.37%. These values are high in relation to the duration of the experiment, and if projected to an entire year, could be higher than the general IPCC N [START_REF] Grossel | The effect of tile-drainage on nitrous oxide emissions from soils and drainage streams in a cropped landscape in Central France Agriculture Ecosystems &amp[END_REF][START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF][START_REF] Henault | Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions[END_REF]. The magnitude of the N 2 O fluxes reported for the wheat plots in this study are in general agreement with previous work, e.g., Grossel et al.

(2016) (mean: 71 µg of N-N 2 O m -² h -1 , pulse emissions: 400-800 µg of N-N 2 O m -² h -1 , undrained soils). For drained soil cropped with wheat, lower pulses of N 2 O in the range of 0-120 µg of N-N 2 O m -² h -1 have been observed previously [START_REF] Gu | Effect of topography on nitrous oxide emissions from winter wheat fields in Central France[END_REF]. Another study in France measuring N 2 O emissions from wheat and rapeseed plots reported fertilizationinduced pulse emissions in the range of 2 to 292 µg N-N 2 O m -² h -1 in April [START_REF] Henault | Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions[END_REF].

Spatial variability of N 2 O emissions

The spatial variability predicted by the developed approach is expected to mirror the basic hypotheses about the chosen footprint model and chosen control factors for the attribution method (LU or NCLU). The 2 emission maps generated by the LU model are similar in terms of quantity and spatial distribution. The same observation is valid for the NCLU model. Hence, it does not matter whether the K&M or FIDES footprint model is used.

On the other hand, the use of LU and NCLU led to different results on total emission, with the LU model producing results similar to the direct measurements by EC and slightly larger results by the NCLU model (Table 10). In the LU method, the area covered by wheat/rapeseed (53% of the total area) emitted 72% of the total emissions; the forest/winter barley (40% of the total area) emitted 2% and the catch crop -pea/corn (7% of the total area) emitted 26%.

Uncertainties of the methods and perspectives

The quality and uncertainty of the emission map obtained depend on the design and schedule of the measurements with the fast-box system, although they are fairly small overall. The application of flux attribution method is becoming more feasible as the employed instrumentation is becoming more common, with prices expected to drop in the future [START_REF] Eugster | Eddy covariance for quantifying trace gas fluxes from soils[END_REF]. As EC systems for monitoring N 2 O emissions are becoming an integral part of the ICOS network (Integrated Carbon Observation System), which aims at quantify greenhouse gas fluxes over longer time periods for various ecosystem types, the developed approach for flux attribution is likely to become interesting for other groups. obtained by the automatic chamber method (10% difference on the mean flux). Therefore these method were applied on the whole site resulting in original maps of N 2 O emissions at the landscape scale. Our results suggest that prior to the use of the fast-box method for flux monitoring, the spatial variability of the N 2 O flux should be evaluated carefully. This would enable targeting measuring points that are representative of a wider area.

V. Conclusion

Objectifs

Le To do so, a laboratory experiment has been carried out on the effect of daily temperature variations on N 2 O emissions. Furthermore, a measurement campaign on the field has taken place in spring 2015, using different measurement techniques at both the plot and landscape scale, to measure N 2 O emissions, and to develop flux attribution methods at the landscape scale. Furthermore, we tried to answer to several scientific questions raised.

II. Perspectives

Effect of the temperature variations on N 2 O emissions

Several scientific questions were raised about the effect of temperature variations on N 2 O emissions:

-Are N 2 O emissions higher when temperature increases over time?

-What about denitrification and N 2 O reduction processes?

-Do N 2 O emission models correctly take into account the effect of temperature?

The experimental study was conducted to assess the effect of daily temperature variations on N 2 O emissions by soils. Both total N 2 O emissions and denitrification were studied. Results

showed that the temperature affected processes involved in N 2 O emissions by arable soils in anaerobic conditions. Both denitrification and N 2 O reduction were higher at 16°C than at 4°C and 4-16°C, but surprisingly not N 2 O emissions. In comparison with CO 2 mineralization, this study reveals the very complex control of N 2 O emission by temperature with different responses for the production and reduction processes, also varying over time. Incubations made at changing temperature, relevant to field conditions, suggest that a dynamic parametrization of models need to be investigated to better take into account the effect of temperature.

N 2 O emission measurements at the plot and landscape scale, using 3 different methods

Several scientific questions were raised about the measurement campaign:

- This study demonstrates the potential of this methodology to represent spatial variability at the landscape scale and to reduce uncertainties on soil N 2 O emission at the landscape scale. The LU flux attribution method seems more appropriate when agricultural practices are similar at the landscape scale while NCLU flux attribution method seems more appropriate when differ markedly across the landscape.

General conclusions

From a scientific point of view, this applied research study answers to an ecological problematic on causes and mechanisms responsible for the climatic change, by bringing complementary data on N 2 O emissions in our territory.

Studies were developed at different scales, from the soil cylinder scale for the temperature experiment, to the plot and landscape scale for the measurement campaign and the flux attribution method. Regarding the temperature experiment, the originality of our experiment was to perform soil incubations at varying temperatures. These conditions were defined to be relevant with field conditions and were associated with more classical conditions for laboratories experiment. This experiment made it possible to obtain information on the behavior of emissions during a daily increase in temperature. No studies have been done on such conditions before. Concerning the measurement campaign and the flux attribution methods, the novelty was to use simultaneously 3 types of measurements methods and to combine them to create original maps, showing the spatial variability of the N 2 O fluxes over a 1-km radius landscape occupied by a mix of croplands and forests.

II. Overviews

Concerning the temperature experiment, it would be interesting to validate these results by doing this experiment again with more samples, and also during a longer time to see if dynamics observed during our experiment will continue. To get closer to the real conditions again, the same experiment with a decreasing temperature, representing the second part of the day should be done. An entire daily temperature dynamic would then be reproduced.

Moreover, this experiment could also be realized with soils coming from different climate.

Also, this study reveals points of improvement in the modeling of N 2 O flux. Indeed, models take the temperature effect into account but this effect does not evolve over time. It would be interesting to apply a temperature function varying over time on models and see if it fits better with the real measurements. Moreover, we saw that N 2 O emissions were higher for an increasing temperature than for a constant higher one. This difference, due to a higher N 2 O reduction into N 2 at a higher temperature, should also be taken into account in models. A dynamic parametrization of the temperature dependency of the different enzymatic steps might be required for N 2 O simulations.

Regarding the measurement campaign, the flux attribution methods developed from field measurements show encouraging results. New measurement campaigns like this one should be carried out, over the same period with more fast-box measurements which are input data for the flux attribution methods. The higher the number of measurements, the more precise the flux attributions will be and we will see if these methods can really be applied.

Moreover, with more fast-box measurements and with a better dispersion of the measurement sites, the spatial variability will be better covered, as well as the wind directions, types of soil Initially, we planned to make DNDC and CERES-NOE models run with data from the campaign but we ran out of time. Nevertheless, all these results are now available for developing ecosystem models. Both these ecosystems models and the methodologies hereby proposed for upscaling N 2 O emissions will help in soil N 2 O emission quantification at large scales, relevant to the inventories and mitigation strategies.

Figures

Figures

Figures

  Figures .......................................................................................................

Figure 1 :

 1 Figure 1: Différentes échelles dans lesquelles le N 2 O peut être étudié.

Figure 1 :

 1 Figure 1: Different scale in which N 2 O can be studied.

Figure 2 :

 2 Figure 2: Global energy gain and loss in the Earth system (W m -2 ) (IPCC 2007).

Figure 3 :

 3 Figure 3: Emission spectrum intensity and wavelength of the Sun and the Earth, and

  produced by the reaction of N 2 O with an excited oxygen atom O*: Nitrous oxide is naturally present in trace quantities in the terrestrial atmosphere, but its concentration increased since the industrialization period with the same dynamic as CO 2 (UNEP 2013) (Figure4).

Figure 4 :

 4 Figure 4: Increase of the N 2 O concentration in the atmosphere through the age following

Figure 5 :

 5 Figure 5: Main N 2 O emission sources and their quantities emitted in Tg N-N 2 O y -1 , and

  N can enter the cycle by different ways, e.g. by the fixation of the atmospheric N by plants, or by N input with the use of fertilizers. Most of the plants can use N only in nitric form (Nor coming from the input of fertilizer on arable soils, can be assimilated by plants, immobilized by microorganisms, of leached in the drainage water or be denitrified. Ammonia (NH 4 + ) can be assimilated by plants, immobilized by microorganisms, or be nitrified. In agriculture, plants need to absorb N to grow. However, when inputs overpass plant needs, the extra N leads to losses that have an impact on the environment.

Figure 6 :

 6 Figure 6: Nitrogen cycle in soils (Robertson and Groffman 2007).

Figure 7 :

 7 Figure 7: Biotic and abiotic processes of N 2 O (Butterbach-Bahl et al. 2013).

Figure 8 :

 8 Figure 8: Major factors controlling denitrification (Robertson 1989).

Figure 9 :

 9 Figure 9: Major factors controlling nitrification (Robertson 1989).

Figure 10 :

 10 Figure 10: Major factors controlling nitrification (Henault et al. 2011)

Figure 11 :

 11 Figure 11: Impact of N surplus on N 2 O emissions (Van Groenigen et al. 2010).

Figure 12 :

 12 Figure 12: Basic principle of the NOE model (Grossel et al. 2014).

Figure 13 :

 13 Figure 13: Basic principle of the DNDC model (Smith et al. 2002).

and

  Faux-Perche, near the Loir river source. OS² is located 120 km southwest of Paris, at the north of the city of Illiers-Combray, in the Eure-et-Loir department, Center region, France (Figure14). 87% of the cultivated plots are tile-drained (Figure15). Creeks that run through the site are mainly fed by agricultural drainage water. Creeks are discharged entirely in the Loir river. The creeks and the Loir river, are temporary streams, which no longer flow in dry periods.

Figure 14 :

 14 Figure 14: Localization of the OS² site

Figure 16 :

 16 Figure 16: Types of soil in the OS² site and localization of the studies done in this site

Figure 17 :

 17 Figure 17: Topography of the OS² site

Figure 18 :

 18 Figure 18: Crop management of the OS² site, averaged from 2009 to 2015

First, a link

  was systematically observed between N2O emissions and well-known local factors such as soil water and nitrogen content. WFPS influenced the effect of soil inorganic N contents on N 2 O fluxes. Differences in N 2 O fluxes, between shoulder and footslope, correlated linearly with differences in WFPS. Spatial variations in N 2 O emissions were regulated by the influence of hydrological processes on soil aeration intensity (Gu et al. 2011). Secondly, in drained soils, soil texture (clay or silt content), pH and exchangeable magnesium (Mg) related significantly (p < 0.05) to N 2 O emission factors, i.e. ratio between N 2 O fluxes and N added. Suggestions were made that (1) soil clay content decreased gas diffusivity and promoted N 2 O reduction thereby controlling N 2 O emissions across the region, and (2) the effects of soil pH and Mg on N 2 O emissions indirectly reflected the effect of soil texture due to the interactions of soil properties (Gu et al. 2013). Thus, soil texture may partly regulate the spatial variability of N 2 O emissions in drained landscapes.

  used NOE for their simulations. The model was tested deterministically in order to predict the flux dynamic and to reproduce the high emission points. Then the denitrification part of the model was tested stochastically to simulate the flux distributions. To do so, they used Monte Carlo simulation and randomly generated input variables from the measured frequency distributions.The deterministic prediction of flux dynamic provided a good agreement with measurements in 1 studied case out of 6. The denitrification process was considered to be the main source of N 2 O in 5 cases out of 6. Moreover, the model acceptably simulated frequency distributions in 4 cases out of 5. As a result, this study proposed that simple process models like NOE, in association with Monte Carlo simulations, can be used to improve simulation of the biased frequency distributions of N 2 O emissions. This study also provides valuable information about the range of spatial variations in N 2 O fluxes.[START_REF] Gu | Modeling nitrous oxide emissions from tile-drained winter wheat fields in Central France Nutrient Cycling in[END_REF] evaluated the possibility to simulate the inter-plot variability at the landscape scale using an ecosystem model.[START_REF] Gu | Modeling nitrous oxide emissions from tile-drained winter wheat fields in Central France Nutrient Cycling in[END_REF] used the DNDC model and modified non-linear empirical NOE model from data collected from a previous study[START_REF] Gu | A regional experiment suggests that soil texture is a major control of N2O emissions from tile-drained winter wheat fields during the fertilization period Soil Biology &amp[END_REF]. Necessary modifications of the model on the optimum crop production and both the field capacity and wilting point were done for DNDC to have a better agreement respectively with crop biomass yields and soil water content. In NOE, multiple effects of varying soil water and N contents on the fraction of N 2 O emitted through the denitrification process were added. DNDC and NOE predicted with success, the background N 2 O emissions and the pulses of emission due to the addition of fertilizer, in all the sites, during the experimental period but they respectively overestimated the daily fluxes on the sampling dates on average by 54 and 25 %. Cumulative emissions were a bit overestimated by DNDC and underestimated by NOE, respectively by 4% and 15%. Differences between the 2 models assessments indicate that low frequency measurements led to uncertainty in model validation, for daily and cumulative emissions. Nevertheless, DNDC represented correctly the effect of tile drainage on soil hydrology, as suggested by the validations for soil water content with daily resolution. Soil NH 4 + and NO 3 -contents were overestimated by the model, mostly due to incorrect N partitioning when the solution of urea ammonium nitrate was applied. La synthèse bibliographique (chapitre 1) a permis de resituer le contexte et l'étude et de montrer que de nombreux paramètres peuvent affecter les émissions de N 2 O. L'étude du déterminisme des émissions de N 2 O est une étape clé pour réaliser des changements d'échelle en permettant de développer des outils de quantification indirecte qui pourront être utilisés dans les modèles. Dans ce chapitre on s'intéresse aux effets de la variation journalière de température sur les émissions. Ces expérimentations ont été réalisées à partir de sols issus du site expérimental présenté chapitre 2. Résumé Une étude expérimentale a été conduite pour évaluer l'effet des variations journalières de température du sol sur les émissions de N 2 O par les sols. Des échantillons de sols nondéstructurés ont été collectés sur un champ de blé et placés à différentes températures : 4°C, 16°C, et à une température variant de 4°C à 16°C en 12 h. Le but étant de recréer les conditions de terrain. Les échantillons ont été placés en conditions anaérobies et de l'acétylène a été ajouté dans la moitié des échantillons pour bloquer la réduction du N 2 O en N 2 et ainsi étudier à la fois les émissions totales de N 2 O et la dénitrification. Les concentrations en N 2 O mesurées pour les échantillons avec acétylène étaient les plus fortes à 16°C, mais pour les échantillons sans acétylène, les concentrations en N 2 O étaient plus élevées lorsque la température variait de 4 à 16°C. A 16°C, la réduction du N 2 O est plus importante qu'à 4-16°C. Les résultats démontrent l'importance de prendre en compte les variations journalières de température du sol pour les modèles.

N 2 O

 2 is emitted by soils by different biotic processes. Nitrification and denitrification, which are respectively aerobic and anaerobic processes, are the main mechanism responsible for the N 2 O production (Butterbach-Bahl et al. 2013; Syakila and Kroeze 2011). Moreover, N 2 O can be consumed in soils, most likely by denitrification, so that the flux of N 2 O observed

  of the denitrification rates due to scavenging of the denitrification intermediate NO as promoted by acetylene and O 2 , slow diffusion of acetylene to sites of actual denitrification in soils, or decomposition of acetylene degrading microbes (Butterbach-Bahl et al. 2013;

Figure 19 :

 19 Figure 19: Sampling plan for the soil collection of the 36 cylinders on a wheat field.

Figure 20 :

 20 Figure 20: Cylinder of soil placed into a glass jars with inlet and outlet tube on the top.

Figure 21 :

 21 Figure 21: Evolution of the soil temperature, in 5 cm soil depth, in April 2015, on the

Figure 22 :

 22 Figure 22: Temporal variation of the incubation temperature for the 4-16°C treatment.

  Without acetylene, N 2 O emissions are the result of the combination of both N 2 O production and N 2 O reduction during denitrification. With acetylene, N 2 O reduction to N 2 is blocked and the measured N 2 O emissions reflect total rates of denitrification i.e., N 2 O + N 2 forms. To calculate the ratio r of the reduction of N 2 O into N 2 , the following equation was used:, = 8 9:;/ -).<;=) >+ ?@; .A) B:)*) 8 9:;/ -).<;=) >+ ? .A) B:)*)

  ). A micro gas chromatograph (µGC SRA GC (TCD detector, He as vector gas, porapak Q capillary column for CO 2 , O 2 molecular sieve)) was used for measuring concentrations of O 2 , CO 2 and Kr. Determination of Kr allowed to check for leaks of the incubation vessels, while determination of O 2 concentration allowed us to ensure that incubation conditions were indeed anaerobic. From the measurements of CO 2 concentrations, carbon (C) mineralization was calculated.With the N 2 O and CO 2 concentration measured in ppm, the volume of the jar and cylinder, the pressure and the temperature, fluxes of N 2 O in g N-N 2 O ha -1 d -1 were calculated.Flux values were averaged across the 6 treatment replicates. Variability of N 2 O emission rates are reported as standard errors.

Figure 23 :

 23 Figure 23: Photo of a vacuumed vial used for gas sampling and the gas chromatograph

Figure 24 :

 24 Figure 24: Evolution of the CO 2 production for samples placed at 16°C, 4-16°C and 4°C,

Figure 25 :

 25 Figure 25: CO 2 mean fluxes measured after 12h along the days, averaged on 6 samples,

Figure 26 :

 26 Figure 26: Evolution of the N 2 O production for samples placed at 16°C, 4-16°C and 4°C,

Figure 27 :

 27 Figure 27: Evolution of the N 2 O production for samples placed at different temperature,

Figure 28 :

 28 Figure 28: Evolution of the ratio of N 2 O emission to total denitrification for the different

Figure 29 :

 29 Figure 29: Q 10 calculated from N 2 O fluxes at 4 and 16°C for the plus acetylene

  d'émissions de N 2 O, réalisées sur le site dont était issu le sol du chapitre 3. Dans ce chapitre, le regard que nous allons porter sur les émissions de N 2 O se situe à l'échelle de la parcelle mais également à celle du paysage. Ce regard sera orienté vers la mesure directe et le déterminisme multifactoriel. Résumé La variabilité spatiale des flux de protoxyde d'azote (N 2 O) par les sols est large, quelle que soit l'échelle d'étude, rendant très importantes les incertitudes sur les émissions de N 2 O par les sols. L'objectif de cette étude était d'estimer les flux de N 2 O à l'échelle du paysage en combinant les résultats de mesures faites à différentes échelles. Pendant une campagne de 2 mois (mi-mars à mi-mai 2015), les flux de N 2 O ont été mesurés sur une petite surface agricole (~km²) (i) continuellement à l'échelle de la parcelle avec des chambres automatiques sur un champ de blé, (ii) ponctuellement sur un groupe de 16 parcelles incluant différents types de sols et cultures en utilisant une chambre mobile (appelée fast-box), (3) continuellement à l'échelle du paysage avec eddy covariance en utilisant un mât de 15 m en association avec 2 modèles d'advection-dispersion (les modèles FIDES et Kormann et Meixner) pour déterminer les empreintes des mesures d'eddy covariance. De plus, les propriétés des sols ont été mesurées sur tous les sites pour fournir une meilleure compréhension des facteurs contrôlant la variabilité des flux de N 2 O. Les flux de N 2 O mesurés par les différentes méthodes ont montré un bon accord en terme de magnitude et de dynamique temporelle, surtout lorsque les chambres automatiques étaient dans l'empreinte du mât d'eddy covariance. Les moyennes des émissions de N 2 O ont été de 53 ± 6 µg N-N 2 O m -2 h -1 pour les chambres automatiques, 45 ± 7 N-N 2 O m -2 h -1 pour le système d'eddy covariance et 37 ± 9 N-N 2 O m -2 h -1 pour les mesures fast-box, lorsque l'on regarde uniquement les périodes où les systèmes de mesures automatiques fonctionnaient. Les flux de N 2 O mesurés par les chambres automatiques and par la fast-box étaient positivement corrélés à l'humidité du sol (p < 0.01), la teneur en eau (p < 0.01) et la teneur en nitrate du sol (p < 0.01). Les combinaisons champs de culture intermédiaire -pois/maïs émettaient plus de N 2 O que les champs de blé et de colza, et bien plus que les forêts. Mots-clés : gaz à effet de serre, mesure par chambres, eddy covariance Abstract The spatial variability of soil nitrous oxide (N 2 O) fluxes is large -regardless of the study scale -resulting in very large uncertainties in soil N 2 O emission assessments. The objectives of this study were to assess the N 2 O flux at the landscape scale by coupling the results of measurements performed at different scales and to propose a method for obtaining emission maps based on these results. During a 2-month campaign (mid-March to mid-May 2015), N 2 O fluxes were measured in a small cropland area (~km²) (i) continuously at the plot scale using automatic chambers in a wheat field, (ii) punctually on a group of 16 plots including different types of soils and crops using a mobile chamber (fast-box), and (iii) continuously at the landscape scale by eddy covariance using a 15-m height mast and 2 advection-dispersion models (the FIDES and Kormann and Meixner models) to determine the eddy covariance measurement footprints. The soil properties were measured at all sites to provide a better understanding of the factors controlling the variability of the N 2 O flux. The N 2 O fluxes, measured by the different methods, showed good agreement in magnitude and temporal dynamics, especially when the automatic chambers were in the eddy covariance mast footprint. Overall, the mean measured N 2 O emission was 53 ± 6 µg N-N 2 O m -2 h -1 for the automatic chambers, 45 ± 7 N-N 2 O m -2 h -1 for the eddy covariance system and 37 ± 9 N-N 2 O m -2 h -1 for the fast-box, for periods when both automatic measurement systems were functioning. The N 2 O fluxes measured by the automatic chambers and the fast-box were positively correlated with soil humidity (p < 0.01), water-filled pore space (p < 0.01) and nitrate soil content (p < 0.05). Catch crop-pea and catch crop-corn fields emitted more N 2 O than wheat and rapeseed fields, and much more than forests. Keywords: greenhouse gas, chamber measurements, eddy covariance I. Introduction N 2 O has been the subject of concern due to its impact on global warming and ozone layer depletion (UNEP 2013). There are several N 2 O sources, but agriculture is by far the main anthropogenic N 2 O emission source. Indeed, 77% of N 2 O emissions due to anthropogenic activity are estimated to come from mineral and organic fertilizer application to agricultural land and other agricultural sources (UNEP 2013). Monitoring N 2 O emissions from agricultural fields is a key step to better constraining N 2 O sources and determining the underlying processes driving emissions in order to develop mitigation recommendations. N 2 O

  2 O fluxes. N 2 O fluxes are more difficult to measure due to atmospheric concentrations in the ppb range and because the pulses of N 2 O fluxes are often sporadic. Additionally, high spatial variability needs to be considered. Fertilized croplands have been shown to be important sources of N 2 O emissions, but crop type, fertilization and field management across a landscape is mosaic, challenging the assumption of the homogeneity of EC measurements. In addition to the EC measurements, footprint models can be used. Footprint models, such as the Kormann and Meixner model (K&M) (Kormann and Meixner 2001) and the FIDES model (Loubet et al. 2010; Loubet et al. 2009; Loubet et al. 2001), calculate from the wind speed and direction, the origin of the air flux coming. Thereby, the N 2 O flux source areas can be known. Furthermore, footprint models will allow a better comparison between the different N 2 O measurement methods. During a N 2 O emission measurements campaign, the objectives were (i) to know if N 2 O fluxes measured with several methods covering several scales of the landscape are comparable, (ii) to see if the spatial and temporal variability can be covered, and (iii) to determine which type of crop emit the most. N 2 O emission measurements was performed from 16 March 2015 to 19 May 2015 at the OS² ("Observatoire Spatialisé Orléanais des Sols") experimental site, located 120 km southwest of Paris. Previous experiments conducted on this site with manual chambers since 2009 have shown N 2 O emission pulses following spring fertilization in March-April

Figure 30 :

 30 Figure 30: Experimental site with the location of the EC mast, the automatic chambers

Figure 31 :

 31 Figure 31: (a) One automatic chamber to measure N 2 O emissions, (b) the gas

Figure 32 :

 32 Figure 32: (a) the fast-box to measure N 2 O emission, (b) the van with the SPIRIT inside

  40-m long (3/8 inch internal diameter) heated and insulated PFA tube at a flow rate of 55 L min -1 using a pump (SV-1010-B, Busch, Switzerland) while ensuring the turbulent flow necessary to lower the tube low-path filtering on the N 2 O signal. Air was analyzed using a closed-path QCL spectrometer (Aerodyne Inc., USA) operating at 7.8 µm, to provide simultaneous measurements of the N 2 O, CH 4 and H 2 O dry vmr. Water vapor was not filtered, but a correction for water vapor was applied to the N 2 O measurements. Data were recorded at 10 Hz. Measurements were performed from 16 to 19 March and from 1 April to 19 May. There was no measurement from 19 March to 1 April due to a QCL failure.

Figure 33 :

 33 Figure 33: (a) The 15 meters mast, (b) The anemometer at the top of the mast and the

  the automatic chambers, fluxes were calculated from the linear change in 4 measurements of the chamber headspace gas concentrations. The overall N 2 O flux was calculated every 2 hours as the mean value of the 8 flux estimates based on the measurements performed during each 2-hour cycle. The standard error was calculated for each point value from the 8 chambers. For the fast-box system, N 2 O emissions were calculated based on the increase in headspace during 4 min, assuming a linear or non-linear increase with the Hutchinson and Mosier model as described by Pedersen (Pedersen et al. 2010). The emissions from each location were estimated based on the mean value of the 4 flux measurements performed on each sampling date. The standard error was also calculated. Moreover, the N 2 O emissions from each crop type were estimated as the average of the fast-box measurements for the specific crop during the whole period. The momentum, heat and N 2 O fluxes were calculated by the EC method based on the maximum covariance during 30-min intervals. The state of the art EC methodology was used in Eddypro software (EddyPro® Version 5, 2015,

  analyses based on regression techniques, correlations and tests of hypotheses were performed using XLSTAT® and MATLAB®. ANOVA and the Newman-Keuls post-hoc test (were used for the fast-box and soil data to test the null hypothesis that the type of crop or land use did not affect soil N 2 O emissions. The Newman-Keuls was chosen as post-hoc test because even if it is less conservative, it appears more powerful than pairwise comparisons and Tukey's range test. It is largely used in soil science[START_REF] Martin-Laurent | DNA extraction from soils: old bias for new microbial diversity analysis methods[END_REF][START_REF] Ouédraogo | Use of compost to improve soil properties and crop productivity under low input agricultural system in West Africa Agriculture[END_REF][START_REF] Ruser | Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting Soil[END_REF]. Linear regressions were performed for the soil N 2 O emissions against each of the ancillary variables. Statistical significance for all tests was set at p < 0.05.Several pulses of N 2 O emissions were measured by the automatic chambers (Figure34). During the first days of the experiment (March 20 th -24 th ), the N 2 O emissions were equal to 21 µg N-N 2 O m -² h -1 . N 2 O emissions pulses were observed on 25, 27 and 29 March, reaching followed by the highest pulse, reaching 708 µg N-N 2 O m -² h -1 on 30 March and an additional pulse on 4 April. After this period of high emission, the N 2 O emissions decreased asymptotically to reach the initial level. At the beginning of May, the N 2 O emissions increased for 4 days. Some diurnal fluctuations were observed throughout the experimental period. The total emissions measured on this plot during the experiment (53 days) were 909 ± 103 g N-N 2 O ha -1 , which was extrapolated to 8.0 ± 0.9 kg N-N 2 O for the 8.8 ha field.Regarding the measured ancillary variables, the N 2 O fluxes measured by the automatic chambers were correlated with soil moisture (p < 0.01) and soil temperature (p < 0.01).

Figure 34 :

 34 Figure 34: Top: N 2 O fluxes (µg N-N 2 O m -² h -1 ) measured by the EC system, the

Figure 35 :

 35 Figure 35: N 2 O fluxes (numbers in µg N-N 2 O m -² h -1 ) measured by the fast-box on the

  with the fast-box on every measurement day, with the mean flux, the maximum flux and the type of crop where the maximum flux was measured. Regarding the measured ancillary variables, the N 2 O fluxes measured with the fastbox were correlated with soil moisture (p < 0.01), water-filled pore space (WFPS) (p < 0.01) and NO 3 -soil content (p < 0.05). 1.3. N 2 O fluxes detected by the eddy covariance method and the associated footprint defined by micrometeorological approaches EC measurements showed N 2 O emissions throughout the experimental period (Figure 34). A high N 2 O emission pulse of 291 µg N-N 2 O m -² h -1 was reached on 3 April. After this pulse, the mean fluxes decreased from 10 to 25 April. On 26 April, a smaller pulse of N 2 O emissions was observed, with emissions increasing in the following days until 3 May, followed by a couple of days with emission pulses. The apparent total N 2 O emissions measured from the arable soils inside the 1-km radius footprint (389.29 ha) during the experimental period (48.6 days) were estimated at 194 ± 33 kg of N-N 2 O. 1.4. Comparison of N 2 O fluxes obtained by the different techniques Table 7 presents the mean and standard error of the global fluxes measured with the different techniques considering either all the data or only periods when all the data were measured simultaneously. The means vary from 37 ± 9 to 71 ± 8 µg of N-N 2 O m -2 h -1 . While

  model attributed any contribution (even a very weak contribution) of fluxes from the wheat field during the first days of the experiment to the total observed EC-measured N 2 O flux as the wind was blowing from other directions. This factor led to the very low contribution of this plot to the signal observed at the top of the EC mast, although high N 2 O fluxes were measured during this period by the automatic chamber system. In contrast, during the second part of the experiment, the magnitude of fluxes attributed to the wheat plot were consistent with those observed by the chambers.

Figure 36 :

 36 Figure 36: On the left: contribution of each crop to the total footprint of the eddy

Figure 37 :

 37 Figure 37: Contributions of the fields in the 1-km radius of the total footprint calculated

  on the N 2 O flux. High soil humidity and WFPS are favorable to denitrification (Butterbach-Bahl et al. 2013). The presence of oxygen decreases with water content (Rabot et al. 2014); therefore, denitrification is favored. The NO 3 -soil content also favors denitrification, potentially favoring both N 2 and N 2 O production (Firestone and Davidson 1989).

N 2 O

 2 fluxes measured using several methods covering different scales of the landscape gave consistent results. The mean measured N 2 O fluxes were 42.5 ± 7 µg N-N 2 O m -² h -1 for the EC mast and 37 ± 9 µg N-N 2 O m -² h -1 for the fast-box over a similar area, while the mean N 2 O flux measured by the automatic chambers over a fertilized wheat field was 71 ± 8 N µg N-N 2 O m -² h -1 . The N 2 O fluxes measured by the automatic chambers and the fast-box were positively correlated with soil humidity (p < 0.01), water-filled pore space (p < 0.01) and nitrate soil content (p < 0.05). Catch crop-pea and catch crop-corn fields emitted more N 2 O than wheat and rapeseed fields, and much more than forests. Dans ce chapitre, nous allons spécifiquement travailler sur le changement d'échelles entre celle de la parcelle et celle du paysage agricole. Les résultats des mesures présentées dans le chapitre 4 vont être utilisés pour tenter de préciser les sources des émissions de N 2 O. En effet, les mesures d'eddy covariance vont être combinées avec les mesures de fast-box pour attribuer des flux de N 2 O à une parcelle spécifique en suivant le principe d'une approche top-down. Les mesures réalisées avec les chambres automatiques vont quant à elles nous servir à valider la méthode d'attribution de flux. Résumé La variabilité spatiale des flux de protoxyde d'azote (N 2 O) est large, quelque soit l'échelle d'étude, rendant très importantes les incertitudes sur les estimations des émissions de N 2 O par les sols. Les objectifs de cette étude étaient d'estimer les flux de N 2 O à l'échelle du paysage en couplant les résultats de mesures effectuées à différentes échelles et de proposer une méthode pour cartographier les émissions de N 2 O basées sur ces résultats. Pour faire la carte des émissions de N 2 O sur la surface totale, nous avons évalué 2 méthodes d'attribution des flux permettant d'estimer les flux de N 2 O de chaque champ sur la période totale. Ces méthodes ont utilisé un modèle de footprint combiné avec les mesures fast-box sur chaque type de culture, pour déterminer la contribution de chaque champ au flux mesuré par le mât d'eddy covariance. Deux modèles de footprint ont été comparés (les modèles FIDES et Kormann et Meixner) et 2 hypothèses sur la dépendance des émissions de N 2 O par rapport à la distribution des cultures et sur la teneur en nitrate du sol ont été testées. Les chambres automatiques ont été utilisées pour évaluer les méthodes d'attribution. Sur toute la surface et pendant la période expérimentale de 2 mois, les flux de N 2 O ont variés de 0.18 à 0.44 kg N-N 2 O ha -1 mois -1 selon la surface. La plus simple méthode d'attribution des flux, prenant en compte seulement l'utilisation des terres, a montré de bonnes similarités avec les mesures aux champs fournies par les chambres automatiques (10%-13% de différence). Cette étude démontre le potentiel de cette méthodologie à représenter la variabilité spatiale des émissions de N 2 O à l'échelle du paysage et de réduire les incertitudes dans son évaluation. Mots-clés : gaz à effet de serre, mesures par chambres, eddy Covariance, méthode d'attribution des flux, effet de l'utilisation des terres Abstract The spatial variability of soil nitrous oxide (N 2 O) fluxes is large -regardless of the study scale -resulting in very large uncertainties in soil N 2 O emission assessments. The objectives of this study were to assess N 2 O fluxes at the landscape scale by coupling results of measurements performed at different scales and to propose a method to retrieve emission maps based on these results. To map the N 2 O emissions of the entire area, 2 flux attribution methods were evaluated which allowed estimating the N 2 O flux of each field during the whole period. These methods used a footprint model in combination with fast-box measurements over each crop type to determine the contribution of each field to the flux measured at the eddy covariance mast. Two footprint models were compared (the FIDES, and the Kormann and Meixner models) and 2 hypotheses on the dependency of N 2 O emissions on crop distribution and soil nitrate contents were tested. Automatic chambers were used to evaluate the attribution methods. Over the whole area during the 2-month experimental period, the N 2 O flux varied from 0.18 to 0.44 kg N-N 2 O ha -1 month -1 depending on the attribution method and footprint

  calculate the contribution of fields to the total footprint involved in N 2 O flux detection dependent on meteorological conditions. Although relevant, this information is insufficient for describing the spatial variability of N 2 O emissions. Understanding this spatial variability (especially at the landscape scale) remains a key concern for mitigating soil N 2 O emissions.In this study, several scientific issues were raised: (1) Can the origin of the N 2 O fluxes be retrieved from the integrated and local measurements?(2) What is the integrated N 2 O flux and the spatial variability of this flux over a 1-km radius landscape occupied by a mix of croplands and forests? Finally, (3) which method would be the best, and how can its validity and uncertainty be estimated?II. Materials and MethodsMethodswere developed to estimate the contribution of each field to the EC flux and to map the spatial variability of the soil N 2 O flux for the entire study area. These methods are referred to as the flux attribution methods, which are based on the combined results of the EC flux measurements and footprint analysis in consideration of the spatial information provided by the fast-box flux measurements.Two attribution methods were tested. The first one used the land use (lu) -defined here as either forest or crop type (wheat, rapeseed, barley, catch crop -corn, catch crop -pea) -as the main factor controlling N 2 O emissions (LU method), and the second used both the land use and soil nitrate (

  3) This enables the determination of the dynamics of the N 2 O flux in the reference crop (wheat): N 2 O flux in each land use lu can be estimated at each time t as: F '8,:;, = 〈α :; 〉 × '8,>?). , = 〈α :; 〉 × m n o,pqrs,s ∑ ^〈t !e 〉×u , a !e

Chapter 5 :

 5 Development of flux attribution methods for mapping N2O emissions by agricultural soils 121 Noticing that '8,+, = ƒ :;(+),=)9 • & ,=)9 • N&( • O +, , the flux in each crop can be calculated as: +, is mainly driven by the fertilization date and amount and is different for each field i.

2 O

 2 fluxes measured by manual chambers. The 〈' '" 〉 ratios are the ratios of the flux of the given land use to the flux of the reference crop (wheat). A, AB and B are the different groups determined statistically. (b) Nitrate concentration in each land use type. The 〈" '" 〉 ratios are the ratios of the N 2 O emission factors of the given land use to the emission factor of the reference crop (wheat). A, AB and B are the different groups determined statistically.

Figure 39 :

 39 Figure 39: Comparison of the daily averaged N 2 O flux (µg N-N 2 O m -² d -1 ) estimated on

  O flux measured by the EC system throughout the campaign with the N 2 O flux calculated at the mast location using both top-down and bottom-up methods, with the LU and NCLU attribution methods using either the FIDES or Kormann and Meixner (K&M) footprint models over a 1-km radius area.

Figure 40 :

 40 Figure 40: Averaged N 2 O emission maps (g N-N 2 O h -1 ) over the whole campaign using

  For example, N 2 O emissions by winter barley crops (representing 16% of the surface) were probably underestimated in the proposed emission map because N 2 O emissions were measured only twice for this type of crop and only during the second part of the experiment when N 2 O emissions were generally low. As fast-box measurements are labor consuming and depend on physical accessibility, a trade-off has to be defined between commitment and emission map quality before each measurement campaign. Nevertheless, the range of uncertainties obtained by our simulations remains low compared to the range of uncertainties obtained by applying the IPCC method: the confidence interval on the emission factor covers one order of magnitude (see previous paragraph), whereas the uncertainty calculated here on the predictions represents approximatively 50% of the total emissions and the difference between the proposed attribution methods for the total emissions of the area is approximatively a factor 2. Thus, the LU and NCLU attribution methods may be interesting tools to improve the quantification of N 2 O emissions by a mixed crop/forest area while attributing the contribution of each land area. The proposed flux attribution methods are based on the known effect of soil crop/land use and NO 3 -content on soil N 2 O emission. The methods proposed here are intended to be generic and should be tested in different areas over longer periods of time. Our hypothesis of controlling N 2 O emission by soil crop/land use and soil NO 3 -content was statistically validated. Our results with the NCLU method indicate that the relationship between soil NO 3 contents and N 2 O fluxes should be revisited, with an experimental campaign developed until the harvest. Including longer periods may enable to determine a temporal variability of the parameters α and β (i.e. different crop dynamics in response to climatic conditions). The NCLU model may be improved by keeping an empirical response of soil NO 3 -(without representing the underlying processes of consumption) considering different functions for different crops, different fertilizer types and different plant stages. More data would be needed for testing this. A better predictor of soil N 2 O flux may also be the soil excess total inorganic N rather than NO 3 -(Van Groenigen et al. 2010). Last, a more generic approach could be developed by improving the LU and NCLU flux attribution approaches using mechanistic models (e.g. Landscape-DNDC (Haas et al. 2013) or CERES-NOE (Gabrielle et al. 2006)) or based on the soil water and soil temperature dynamics and a mechanistic modeling of the NO 3 -dynamics, i.e. with consideration of the underlying processes of consumption, rather than the simple empirical function used in this study. Nevertheless the present objective was to provide a first approach based on simple empirical functions. Moreover, even with full crop or ecosystems models, it is still challenging to predict the dynamics of NO 3 -consumption in the upper soil layer (e.g. a case study with DNDC in Gu et al., 2014 and with CERES-EGC in Gabrielle et al., 2006, in the same region of France). This campaign lasted only 2 months. Without measurements covering an entire year, our results cannot be scaled to annual flux, even though the likely key period of high N 2 O emissions associated with fertilization was assessed. In view of present results, future campaigns may benefit of a more frequent fast-box sampling to help refining the α or β parameters. Such an approach could be applied in different situations, with an adaptation of factors to local conditions and scales. Application on other sites would probably need an assessment of local factors controlling the spatial variability of N 2 O emissions. For example, in the case of hydromorphous areas in footslopes, we suggest to introduce simulation units based on both elevation and fields, using a digital elevation model.

  During this study, 2 flux attribution methods were proposed to determine the spatial and temporal variability of N 2 O emissions, one based on land use (LU) and the other based on both land use and NO 3 -content (NCLU). This study has demonstrated the capability of the proposed flux attribution method to determine both the spatial and temporal variability of the N 2 O flux over a 1-km radius landscape. Some encouraging results were obtained: taking into account the fertilization with NCLU approach resulted in a better simulation of flux dynamics with the K&M model, but also over-estimated mean flux compared to the automatic chambers. On the other hand, the LU method showed close agreement with the results

2 .

 2 Effet de la variation de température sur les émissions de N 2 O Plusieurs questions scientifiques ont été soulevées sur l'effet de la variation de température sur les émissions de N 2 O: -Est-ce que les émissions de N 2 O sont plus fortes lorsque la température augmente au cours du temps ? -Qu'en est-il de la régulation par la température du processus de dénitrification et plus particulièrement du processus de réduction du N 2 O ? -Est-ce que les modèles prennent correctement en compte l'effet de la température ? L'étude expérimentale a été conduite pour évaluer l'effet des variations journalières de température sur les émissions de N 2 O par les sols. Les émissions totales de N 2 O et la dénitrification ont été étudiées. Les résultats montraient que la température affectait les processus impliqués dans les émissions de N 2 O par les sols cultivés en condition anaérobie. La dénitrification et la réduction du N 2 O étaient plus élevées à 16°C qu'à 4°C et à 4-16°C mais, plus étonnamment, pas les émissions de N 2 O. En comparaison avec la minéralisation du CO 2 , cette étude révèle le contrôle très complexe des émissions de N 2 O par la température avec des réponses différentes pour les processus de production et de réduction, qui varient également avec la température. Les incubations faites à une température changeante, plus en adéquation avec les conditions de terrain, suggèrent qu'une paramétrisation dynamique des modèles doit être étudiée pour mieux prendre en compte l'effet de la température. 3. Mesures des émissions de N 2 O à l'echelle de la parcelle et du paysage avec 3 méthodes différentes Plusieurs questions scientifiques ont été soulevées sur la campagne de mesures des émissions de N 2 O: -Est-ce que les mesures faites à partir de 3 méthodes différentes et à différentes échelles peuvent fournir des valeurs comparables ? -Est-ce que la variabilité spatio-temporelle peut être totalement couverte ? -Quel type de culture émet le plus de N 2 O ? Les objectifs de cette campagne étaient d'évaluer les flux de N 2 O à l'échelle du paysage en combinant les résultats des mesures réalisées à différentes échelles. De par leurs fréquences et leurs propres caractéristiques, ces méthodes ont fourni différentes informations sur les flux de N 2 O and sur leurs variabilités spatio-temporelles. Par conséquent, ce fut intéressant de pouvoir utiliser ces méthodes ensemble et de comparer les résultats. Les résultats ont montré que les différentes techniques mesurant à différentes échelles pouvaient être utilisées simultanément pour mesurer les flux de N 2 O et que ces valeurs obtenues étaient comparables lorsque les mesures étaient faites en même temps. Les résultats ont montré un bon accord en termes d'amplitude et de dynamiques temporelles. Le système EC et les chambres automatiques fonctionnaient en permanence, couvrant la variabilité temporelle du site. Cependant, les chambres automatiques couvraient seulement la variabilité temporelle d'un champ de blé, et le système EC d'une surface qui dépendait de la force et de la direction du vent. La variabilité spatiale a été couverte par les mesures fast-box réalisées sur 16 sites différents. Cependant, en comparant ces résultats avec ceux du système EC, on a remarqué que la variabilité spatiale n'a pas été parfaitement couverte. Pendant cette campagne de 2 mois, la parcelle piège à nitrates -pois et piège à nitrates -maïs ont été les cultures les plus émettrices de N 2 O. Ensuite vint les parcelles de blé et de colza, et enfin les forêts avec les plus faibles flux de N 2 O émis. Les flux de N 2 O, mesurés par les chambres automatiques et la fast-box, étaient positivement corrélés à l'humidité du sol (p < 0.01), la teneur en eau (p < 0.01), et la teneur du sol en nitrate ((p < 0.05). Cependant, la température n'est pas apparue comme un facteur de contrôle des émissions de N 2 O. A la vue des résultats obtenus lors de l'expérimentation en laboratoire sur l'effet de la température sur les émissions de N 2 O, ce n'est pas surprenant car la relation entre émissions de N 2 O et température n'était pas linéaire. D'autres études sont nécessaires pour tester la cohérence entre le terrain et cette expérimentation en laboratoire. 4. Développement de méthodes d'attribution de flux pour cartographier les émissions de N 2 O par les sols agricoles Plusieurs questions scientifiques ont été soulevées sur le développement de méthode d'attribution des flux -Est-ce que les origines des flux de N 2 O peuvent être retrouvées à partir de mesures locales et intégratives ? -Quelle méthode serait la meilleure pour cela et comment estimer sa validité et ses incertitudes ? -Quelle est la variabilité spatiale des flux de N 2 O sur un paysage d'un rayon de 1 km, occupé par un mélange de cultures et de forêts ? Au cours de cette étude, les données provenant de la campagne de mesures (mesures de fastbox et d'EC) ont été utilisées pour développer 2 méthodes d'attribution de flux pour trouver l'origine des flux de N 2 O. Les 2 méthodes d'attribution de flux proposaient de retrouver la variabilité spatiale et temporelle des émissions de N 2 O sur un paysage d'un rayon de 1 km, l'une basée sur l'utilisation du sol (LU), l'autre basée sur à la fois l'utilisation du sol et de la teneur en NO 3 -du sol (NCLU). Pour tester la validité de ces méthodes d'attribution de flux, les résultats trouvés pour le champ de blé où étaient placées les chambres automatiques, ont été comparés directement avec les mesures des chambres automatiques. Des résultats encourageants ont été trouvés, notamment pour la méthode LU, montrant un très bon accord avec les mesures obtenues avec les chambres automatiques (10% de différence). Finalement, des cartes originales ont été créées, représentant la variabilité spatiale des flux de N 2 O sur un paysage de 1 km de rayon occupé par un mélange de terres cultivées et de forêts. Cette étude démontre le potentiel de cette méthodologie pour représenter la variabilité spatiale à l'échelle du paysage et pour réduire les incertitudes sur les émissions de N 2 O par les sols à l'échelle du paysage. La méthode d'attribution de flux LU semble plus adaptée lorsque les pratiques agricoles sont similaires à l'échelle du paysage alors que la méthode NCLU semble plus adaptée lorsque les pratiques agricoles sont différentes 5. Conclusions générales D'un point de vue scientifique, cette recherché appliquée répond à une problématique écologique sur les causes et mécanismes responsables du changement climatique, en apportant des données complémentaires sur les émissions de N 2 O sur notre territoire. Des études ont été développées à différentes échelles, de l'échelle du cylindre pour l'expérimentation sur la température, à l'échelle de la parcelle et du paysage pour la campagne de mesures et la méthode d'attribution des flux. Concernant l'expérimentation en laboratoire, l'originalité de cette expérimentation vient du fait qu'elle a été réalisée avec des températures d'incubations changeantes au cours du temps. Ces conditions étaient en adéquation avec les conditions de terrain et ont été associées avec des conditions plus classiques pour des expérimentations en laboratoire. Cette expérimentation a permis d'obtenir des informations sur le comportement des émissions pendant une augmentation journalière de température. Aucune étude n'avait été menée dans de telles conditions. En ce qui concerne la campagne de mesure et le développement des méthodes d'attribution de flux, la nouveauté a été d'utiliser simultanément 3 types de méthodes de mesures et de les combiner pour créer des cartes originales représentant la variabilité spatiale des flux de N 2 O sur une surface de 1 km de rayon, occupée par un mélange de terres cultivées et de forêts.

  stratospheric ozone layer depletion are currently two of the major ecological problems the most frequently discussed. The increase of the greenhouse gas concentrations in the atmosphere is the main cause (IPCC 2014). Amongst these gases, N 2 O is particularly important because it is the 3 rd gas responsible for the global warming and now the 1 st gas responsible for the stratospheric ozone depletion. Agriculture is the main anthropogenic source of N 2 O emissions, due to the use of N fertilizers. Over the last decades, various studies have attempted to determine quantitatively and with increasing precision the different sources of N 2 O. The high uncertainties linked to the sources of emissions make that the N 2 O concern remains a current problem. The objectives of our work were (i) to progress on the determinism of N 2 O emissions and to improve indirect quantification tools with modeling with a specific work about the effect of a varying temperature on N 2 O emissions to be integrated in models, (ii) to progress on the realization of direct measurements at the plot and landscape scale with the application of integrative measurement techniques over time and the development of integrative measurement layout in time and space and (iii) to progress on the source attributions of N 2 O emissions with the development of flux attribution approaches from data collected during a measurement campaign.

  Can measurements made with 3 different methods and at different scale provide comparable values? -Can the spatial and temporal variability be totally covered? -Which type of crop emitted the most N 2 O?The objectives of this study were to assess N 2 O fluxes at the landscape scale by coupling results of measurements performed at different scales. Due to their frequencies and their specific characteristics, these methods provided different information on N 2 O fluxes and their spatial and temporal variability. It was therefore very interesting to be able to use these methods together and then to cross-check the results. Results showed that the different techniques measuring at different scale could be used simultaneously to measure N 2 O fluxes and that values obtained were comparable when running at the same time. Results showed good agreement in magnitude and temporal dynamics. Both the EC system and the automatic chambers were running continuously, covering the temporal variability of the site. However the automatic chambers covered only the temporal variability of a wheat field, and the EC system of an area depending of the wind direction and strength. The spatial variability was covered by the fast-box with measurements made on 16 different sites. However, when comparing both results from the fast-box and the EC mast, the spatial variability does not seem to have been perfectly covered. During the 2-month campaign, the catch crop-pea and the catch crop-corn fields were the crops emitted the highest amount of N 2 O. Then came the wheat and rapeseed fields, and finally the forest with the lowest N 2 O emitted.The N 2 O fluxes measured by the automatic chambers and the fast-box were positively correlated with soil humidity (p < 0.01), water-filled pore space (p < 0.01) and nitrate soil content (p < 0.05). However, temperature did not appear as a control factor of N 2 O emissions.In view of the results obtained during the laboratory experiment dealing with the effect of soil temperature on N 2 O emissions, it is not surprising as the relation between N 2 O emissions and temperature was not linear. Further investigations are required to test consistencies between field and this specific laboratory experiment.4. Development of flux attribution methods for mapping N 2 O emissions by agricultural soilsSeveral scientific questions were raised about the development of a flux attribution method:-Can the origin of the N 2 O fluxes be retrieved from the integrated and local measurements?-Which method would be the best for that, and how to estimate its validity and its uncertainty? -What is the spatial variability of the N 2 O flux over a 1 km radius landscape occupied by a mix of croplands and forests? During this study, data from the measurement campaign (fast-box and EC measurements) were used to develop 2 flux attribution methods to find the origin of the N 2 O fluxes. The 2 flux attribution methods proposed to retrieve both spatial and temporal variability of N 2 O emissions over a 1-km radius landscape, one based on land use (LU) and the other one based on both land use and NO 3 -content (NCLU). To test the validity of these flux attribution methods, results found for the wheat field where were placed the automatic chambers were compared directly with the automatic chamber measurements. Some encouraging results were obtained, especially with the LU method, showing a close agreement with the results obtained with the automatic chamber method (10% difference). Finally, original maps were created, showing the spatial variability of the N 2 O fluxes over a 1 km radius landscape occupied by a mix of croplands and forests.

-.

  and culture. However, fast-box measurements are time-consuming. It needs many people due to the number of repetition and the number of sites, and combined with the maintenance of the 2 automatic systems running at the same time, the soil sampling, and the treatment and analyses in the laboratory of the soil samples. Nevertheless, the protocol we have developed could be applied in different situations, e.g., across Europe, to develop generic flux attribution models. After development and validation of the flux attribution methods, N 2 O measurement networks, which already use EC mast/tower, could use these methods to better evaluate N 2 O emissions in the studied ecosystem. Furthermore, these methods could be developed and applied to evaluate other gas emissions such as CH 4 .Pea and corn were the 2 crops which emitted the most N 2 O during the campaign, although they were not really developed. These 2 crops followed a catch crop and this probably explains large fluxes. During their growing period, catch crops have consumed all the soil NO 3 When straws were buried, their decomposition probably induced, either directly or indirectly, N 2 O emissions. For the corn, a little amount of fertilizer was added after seeding, for pea nothing was added, but both emitted large amount of N 2 O. So catch crops need to be investigated as well as the mechanisms involved in the soil straw degradation, which obviously leads to high N 2 O emissions.

Figure 1 :

 1 Figure 1: Different scale in which N 2 O can be studied. .............................................................

Figure 2 :

 2 Figure 2: Global energy gain and loss in the Earth system (W m -2 ) (Mudakavi 2010). ..........

Figure 3 :

 3 Figure 3: Emission spectrum intensity and wavelength of the Sun and the Earth, and

Figure 4 :

 4 Figure 4: Increase of the N 2 O concentration in the atmosphere through the age following the

Figure 5 :

 5 Figure 5: Main N 2 O emission sources and their quantities emitted in Tg N-N 2 O y -1 , and the

Figure 6 :

 6 Figure 6: Nitrogen cycle in soils (Robertson and Groffman 2007). .........................................

Figure 7 :

 7 Figure 7: Biotic and abiotic processes of N 2 O (Butterbach-Bahl et al. 2013). ........................

Figure 8 :

 8 Figure 8: Major factors controlling denitrification (Robertson 1989). ....................................

Figure 9 :

 9 Figure 9: Major factors controlling nitrification (Robertson 1989). ........................................

Figure 10 :

 10 Figure 10: Major factors controlling nitrification (Henault et al. 2011) ..................................

Figure 11 :

 11 Figure 11: Impact of N surplus on N 2 O emissions (Van Groenigen et al. 2010). ....................

Figure 12 :

 12 Figure 12: Basic principle of the NOE model (Grossel et al. 2014). .......................................

Figure 13 :

 13 Figure 13: Basic principle of the DNDC model (Smith et al. 2002). .......................................

Figure 14 :

 14 Figure 14: Localization of the OS² site ....................................................................................

Figure 15 :

 15 Figure 15: OS² map representing the land use and the presence or not of drainage ................

Figures 162 Figure 16 :Figure 17 :Figure 18 :Figure 19 :Figure 22 :Figure 23 :Figure 26 :Figures 163 Figure 27 :Figure 28 :

 162161718192223261632728 Figures

Figures 164 RedFigure 36 :Figure 37 :Figure 38 :

 164363738 Figures

  the maximum flux and the type of crop where the maximum flux was measured. ............... 105 Table 7: Mean and standard error of global fluxes measured with different techniques considering either all data or only periods when all data were measured simultaneously. .... 106 Table 8: (a) N 2 O fluxes measured by manual chambers. The α•P ratios are the ratios of the flux of the given land use to the flux of the reference crop (wheat). A, AB and B are the different groups determined statistically. (b) Nitrate concentration in each land use type. The ƒ•P ratios are the ratios of the N 2 O emission factors of the given land use to the emission factor of the reference crop (wheat). A, AB and B are the different groups determined statistically. ............................................................................................................................. 124 Table 9: Comparison of the N 2 O flux measured by the automatic chambers and the LU and NCLU attribution methods on the wheat field where the automatic chambers were installed throughout the campaign. ....................................................................................................... 128 Table 10: Comparison of the N 2 O flux measured by the EC system throughout the campaign with the N 2 O flux calculated at the mast location using both top-down and bottom-up methods, with the LU and NCLU attribution methods using either the FIDES or Kormann and Meixner (K&M) footprint models over a 1-km radius area. .................................................. Table 11: Correlation matrix of N 2 O emissions calculated over the whole area using the flux attribution methods. ................................................................................................................Emissions de protoxyde d'azote par les sols agricoles : effet des dynamiques de température; mesures à l'échelle de la parcelle et du paysage Les sols agricoles sont la principale source du gaz à effet de serre N 2 O. Ces émissions sont caractérisées par une variabilité spatiale et temporelle considérable, ce qui rend très difficile leur quantification. L'UR SOLS étudie depuis 2008 les émissions de N 2 O dans une zone agricole du Centre de la France. Spécifiquement, nous avons étudié au laboratoire l'effet de la température sur ces émissions et développé une méthode permettant l'estimation des émissions de N 2 O à l'échelle du paysage. De façon surprenante, nous avons observé que les émissions de N 2 O n'augmentent pas systématiquement avec la température. L'indicateur Q 10 est apparu, pour les émissions de N 2 O, variable avec le temps. L'utilisation de l'acétylène, inhibiteur de la réduction de N 2 O, a révélé que les processus biologiques de production et de consommation de N 2 O répondent différemment à la température. Les émissions de N 2 O mesurées au champ à l'aide de différentes techniques ont permis d'obtenir des résultats cohérents, avec des moyennes de 43 µg N-N 2 O m -2 h -1 pour la méthode par eddy covariance, 37 µg N-N 2 O m -2 h -1 pour la méthode de fast-box et 71 µg N-N 2 O m -2 h -1 pour la méthode des chambres automatiques sur un blé fertilisé. Des méthodes d'attribution des flux ont été développées pour déterminer de façon exhaustive les variations spatiales et temporelles des émissions de N 2 O avec élaboration de cartes originales d'émissions à l'échelle du paysage. L'ensemble de ces résultats pourra être utilisé pour le développement de modèles de fonctionnement des écosystèmes. Ils vont contribuer à quantifier les émissions de N 2 O aux échelles adaptées pour les inventaires et les stratégies d'atténuation. Mots clés : N 2 O, sols agricoles, changement d'échelle, effet de la température, chambres, eddy covarianceNitrous oxide emissions by agricultural soils: effect of temperature dynamics; up-scaling measurements from the plot to the landscape The greenhouse gas N 2 O is mainly emitted by soils. Soil emissions are characterized by considerable spatial and temporal variabilities that make their quantification very difficult. While soil N 2 O emissions are studied on an agricultural area in the Central France by the UR SOLS since 2008, we specifically studied in the laboratory the effect of temperature on these emissions and also developed a method for upscaling N 2 O emissions from the plot to the landscape scales. Surprisingly, N 2 O emissions were observed not to increase with temperature. Q 10 values, describing N 2 O emission sensitivity to temperature, were observed to change over time. The use of acetylene for inhibiting N 2 O reduction has revealed that the biological processes involved in the N 2 O production and its consumption respond differently to temperature variations. N 2 O fluxes measured in the field using several methods covering different scales of the landscape gave consistent results. The mean measured N 2 O fluxes were 43 µg N-N 2 O m -2 h -1 for the eddy covariance mast, 37 µg N-N 2 O m -2 h -1 for the fast-box over a similar area, while it was 71 µg N-N 2 O m -2 h -1 by the automatic chambers over a fertilized wheat field. Flux attribution methods were developed to determine both the spatial and temporal variability of the N 2 O flux over a 1-km landscape, resulting in original maps of N 2 O emissions at the landscape scale. All these results could be further used for developing ecosystem models. Both these ecosystems models and the methodologies hereby proposed for upscaling N 2 O emissions will help in soil N 2 O emission quantification at large scales, relevant to the inventories and mitigation strategies. Key-words : N 2 O, agriculture soils, up-scaling, temperature effect, chambers, eddy covariance

Table 1 : N 2 O mean fluxes over a 4-days incubation period. Water-Filled Pore Space (WFPS), soil total mineral nitrogen and nitrates content were measured at the end of the experiment.

 1 Soil nitrate contents varied in the range of 3.7 and 7.2 mg N-NO 3 -kg -1 of soil, and were slightly lower in soils of the 16°C treatment as compared to soils of the other treatments.

	16°C	4-16°C	4°C

Table 2 : Correlation (p value) calculated between N 2 O fluxes, Water-Filled Pore Space (WFPS), total mineral nitrogen and nitrate content.

 2

Table 3 : Mean N 2 O fluxes in ng N-N 2 O g -1 of soil and their standard error, obtained for the different temperature and acetylene treatments over a 4-days period.

 3

	1

  models such as CENTURY or DAYCENT[START_REF] Grosso | Simulated interaction of carbon dynamics and nitrogen trace gas fluxes using the DAYCENT model Modeling Carbon and Nitrogen Dynamics for[END_REF][START_REF] Kelly | Intra-annual and interannual variability of ecosystem processes in shortgrass steppe[END_REF][START_REF] Parton | DAYCENT and its land surface submodel: description and testing[END_REF].If the 0°C anomaly is excluded, which is linked to soil freezing-thawing effects on microbial processes, soil N 2 O emissions are generally observed to increase with temperature[START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF][START_REF] Kurganova | Effect of the temperature and moisture on the N2O emission from some arable soils[END_REF].[START_REF] Dobbie | The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly drained gleysol[END_REF],Kurganova and de Gerenyu (2008), had observed Q 10 values comprised between 1.2 and 50 for N 2 O emissions (Table4) and models used for simulating N 2 O emissions assume as well a positive response of N 2 O production to increasing temperature[START_REF] Heinen | Simplified denitrification models: Overview and properties[END_REF]) (Table5). In contrast, our experiments did not show that N 2 O emissions for soils of the 16°C treatment were higher as those of the 4°C treatment or N 2 O emissions observed for the treatment where

		Stanford (1975) Dobbie & Smith (2001)	Kurganova & de Gerenyu (2010)	Zhang (2016)	This study (2017)
	Process studied	Denitrification	N 2 O emissions	N 2 O emissions	N 2 O emissions	N 2 O emissions and denitrification
	Type of soil		Arable and grassland	Arable	Forest and grassland	Arable
	Cylinder of soil	non-intact	intact	non-intact	non-intact	intact
	WFPS	no data	63 and 88%	60, 75, 90%	no data	75-97%
	Temperature	5, 10, 15, 25, 35, 45 °C	5, 12 and 18 °C	5, 10, 15, 20, 25 °C	8, 18 and 28°C	4-16°C
	Days	2	11	14	50	4
			after 2.5h and every
	Gas sampling	after 2, 4, 6, 8, 12, 24 and 48h	day at the same hour during 4 days, then at the 7th and the 11th	at day 1, 2, 4, 7, 10 and 12	Every 24h	Every 2 h during 12 h on each day
			day
		3 mg of glucose
	Addition	in 20 mL of water + 60 ppm	water + 10 g N.m -2	80 mg N.kg -1 of NH 4 NO 3	no data	20 mL of a KNO 3 solution (0.02 M)
		of NO 3 -
	Q 10	89 pour T < 11°C	Arable 5-12°C : 50	WFPS 60% + 5-15°C : 1.2	8-18°C: 0,82-1.06 N 2 O emissions: 3.85-0.33
		2 pour T > 11°C Arable 12-18°C : 8,9 60% + 10-20°C : 2.2	18-28°C: 0.91-1.22	Denitrification: 6.47-1.72
			Grassland 5-12°C : 3,7 60% + 15-25°C : 1.4
			Grassland 12-18°C : 2,3 75% + 5-15°C : 2.8
				75% + 10-20°C : 4.2
				75% + 15-25°C : 3.4
				90% + 5-15°C : 5.4
				90% + 10-20°C : 20
				90% + 15-25°C : 33

2, i.e., a value previously observed in other studies

[START_REF] Song | Effects of soil moisture, temperature, and nitrogen fertilization on soil respiration and nitrous oxide emission during maize growth period in northeast China[END_REF][START_REF] Uvarov | Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil Soil Biology &amp[END_REF][START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]

. A Q 10 of 2 is also used to describe the temperature response of C mineralization in temperature increases from 4 to 16°C over a 12h-period. In tendency and close to be significantly different, soil N 2 O emissions at 16°C were even lower as observed for the other treatments. Recently

[START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]

had also observed slightly higher soil N 2 O emissions at 8°C both in pine and meadow ecosystems in China as compared to those measured at 18°C or even at 28°C (only for the meadow soil). Accordingly, they calculated Q 10 values less than 1, which is different from those reported by other researchers in a temperate forest ecosystem

[START_REF] Bagherzadeh | Temperature dependence of carbon mineralization and nitrous oxide emissions in a temperate forest ecosystem[END_REF]

and in a maize field

[START_REF] Song | Effects of soil moisture, temperature, and nitrogen fertilization on soil respiration and nitrous oxide emission during maize growth period in northeast China[END_REF]

. Based on their observations,

[START_REF] Zhang | Temperature sensitivity of soil carbon dioxide and nitrous oxide emissions in mountain forest and meadow ecosystems in China[END_REF]

suggested that the effect of climate warming on N 2 O would differ across regions.

Table 4 : Different laboratory conditions and results on studies about the effect of temperature on N 2 O emissions and denitrification Table 5: Q 10 used by different models simulating N 2 O emissions (Heinen 2006).

 4

Table 6 : Fluxes measured

 6

	Type of crop with
	the highest flux

Table 8 : (a) N

 8

	Crop		Wheat (reference)	Rapeseed Forest Barley	Catch crop -corn	Catch crop -pea
	Number of measurements	n	15	15	8	2	3	5
		Mean	53.19 AB	17.07 AB 5.70 B 0.93 B	26.05 AB	94.21 A
		Standard error	75.86	21.39	6.88	0.00	9.22	111.42
	N 2 O fluxes (µg N-N 2 O m -2 h -1 )	min	-2,08	1.78	0.42	0.93	15.95	8,64
		median	16.29	7.99	3.22	0.94	28.19	44.30
		max	295.15	84.60	21.24	0.94	34.01	270.19
	α lu	Ratios	1.00	1.00	0.04	0.04	1.00	3.8
	Crop		Wheat (reference)	Rapeseed Forest Barley	Catch crop -corn	Catch crop -pea
	Number of measurements	n	19	20	8	2	3	5
		Mean	7.31 AB	3.88 AB 2.58 B 1.65 B	7.59 AB	4.48 A
	[N-NO 3 (mg kg -1 ) -]	Standard error min	8.17 1.00	3.51 0.28	1.10 1.31	0.48 1.31	1.50 5.97	1.78 2.02
		median	4.78	2.38	2.67	1.65	7.89	5.45
		max	35.49	11.78	4.64	1.99	8.93	6.00
	β lu	Ratios	1.00	1.00	0.18	0.18	1.00	2.11

Table 10 : Comparison of the N 2

 10

		42.7	41.3	40.1	53.9	59.8
	Mean N 2 O flux (kg ha -1 month -1 )	0.31	0.30	0.29	0.39	0.44
	Total N 2 O emitted
	during the campaign	193.6	187.3	182.0	244.4	271.1
	(kg)
				Bottom-up
		EC system	LU + FIDES	LU + K&M	NCLU + FIDES	NCLU + K&M
	Mean N 2 O flux (µg N-N 2 O m -2 h -1 )	42.7	25.2	26.5	31.7	39.2
	Mean N 2 O flux (kg ha -1 month -1 )	0.31	0.18	0.19	0.23	0.29
	Total N 2 O emitted
	during the campaign	193.6	114.3	120.1	143.9	177.7
	(kg)

Table 11 : Correlation matrix of N 2 O emissions calculated over the whole area using the flux attribution methods.

 11

  2 O estimation of 1% [0.3-3%] (IPCC 2006). N 2 O emission observations in this area, typical of intensive agriculture in France, should therefore be monitored across entire years. The difference may also be due to the period of the experiment because the weeks following spring fertilization generally present the highest emissions, and previous studies have shown that high N 2 O pulse emissions occur in April

  changement climatique et la destruction de la couche d'ozone stratosphérique sont actuellement 2 des problèmes écologiques majeurs le plus fréquemment discutés. L'augmentation de la concentration des gaz à effet de serre dans la stratosphère en est la principale cause (IPCC 2014). Parmi ces gaz, le N 2 O est particulièrement important car il est le 3 ème gaz responsable du réchauffement climatique et le 1 er gaz responsable de la destruction de la couche d'ozone. L'agriculture est la principale source anthropique des émissions de N 2 O, dû à l'utilisation de fertilisants azotés. Au cours des dernières décennies, de nombreuses études ont tenté de déterminé quantitativement et avec une précision croissante, les différentes sources de N 2 O. Les fortes incertitudes liées aux sources d'émissions, font que le N 2 O reste un problème actuel. Pour atteindre nos objectifs, une expérimentation en laboratoire a été réalisée sur l'effet de la variation journalière de température sur les émissions de N 2 O. De plus, une campagne de mesure sur le terrain a eu lieu au printemps 2015, avec différentes techniques de mesures, à l'échelle de la parcelle et du paysage, pour mesures les émissions de N 2 O, et pour développer 2 méthodes d'attribution de flux à l'échelle du paysage. Nous avons également essayé de répondre aux questions scientifiques soulevées par cette thèse.

	Les objectifs de notre travail étaient (i) de progresser sur le déterminisme des
	émissions de N 2 O et d'améliorer les outils de quantification indirecte par les modèles, avec un
	travail spécifique sur l'effet de la variation de température sur les émissions de N 2 O, pour être
	intégré dans les modèles, (ii) de progresser sur la réalisation de mesures directes à l'échelle de
	la parcelle et du paysage, avec l'application de techniques de mesures intégratives dans le
	temps et le développement d'un dispositif de mesures intégratives dans le temps et l'espace,
	et, (iii) de progresser dans l'attribution des sources des émissions de N 2 O, par le
	développement d'approches d'attribution de flux à partir des données collectées pendant la
	campagne de mesures.

  Concernant l'expérimentation sur la température, il serait intéressant de valider ces résultats en refaisant la même expérimentation avec plus d'échantillons, et aussi pendant un temps plus important et ainsi vérifier que les dynamiques observées pendant notre expérimentation se prolongent dans le temps. Pour être encore plus proche des conditions de terrains, la même expérimentation devrait être réalisée, avec cette fois ci une température qui diminue au cours du temps et qui représenterait la seconde partie de la journée. Ainsi, une dynamique journalière de température serait reproduite entièrement. De plus, cette expérimentation pourrait également être réalisée avec des sols provenant de différents climats. De plus, cette étude révèle des points d'amélioration dans la modélisation des flux de N 2 O. En effet, les modèles prennent l'effet de la température en compte mais cet effet n'évolue pas au cours du temps. Il serait donc intéressant d'appliquer une fonction de température qui varie au cours du temps dans les modèles pour voir si les simulations sont plus proches des mesures réelles. Par ailleurs, nous avons vu que les émissions de N 2 O étaient plus importantes pour une température qui augmente au fil du temps que pour une température plus élevée et constante. Cette différence, due à une plus forte réduction du N 2 O en N 2 à la température la plus élevée, doit également être prise en compte dans les modèles. Un paramétrage dynamique de la dépendance à la température des différentes étapes enzymatiques pourrait être nécessaire pour les simulations N 2 O. Cependant, les mesures de fast-box demandent beaucoup de temps. Elles demandent beaucoup de personnels techniques, du fait des nombreux sites et répétitions, de la combinaison avec la maintenance des 2 systèmes automatiques fonctionnant en même temps, les prélèvements de sols, et le traitement et les analyses en laboratoire des échantillons de sol. de N 2 O, qui utilisent déjà des mâts/tours d'eddy covariance, pourraient utiliser ces méthodes pour mieux évaluer les émissions de N 2 O dans un écosystème étudié. De plus, ces méthodes pourraient être développées et appliquées pour évaluer les autres émissions de gaz tel que le CH 4 . Les cultures de pois et de maïs ont été les 2 types de cultures qui ont le plus émis durant la campagne de mesures. Ces 2 cultures suivaient une culture de piège à nitrates cela explique sûrement ces larges flux. Pendant leur période de croissance, les cultures de piège à nitrates avait consommé tout le NO 3 -du sol. Lorsque les pailles ont été enfouies, leur décomposition a probablement entrainé des émissions de N 2 O, qu'elles aient été directes ou indirectes. Pour le maïs, un faible apport de fertilisant a été appliqué après le semis, mais rien pour le pois, mais ces 2 cultures ont émis de fortes quantités de N 2 O. Les cultures de piège à nitrates doivent être étudiées, tout comme les mécanismes impliqués dans la dégradation des pailles, qui mène visiblement à de fortes émissions de N 2 O.

	Initialement, nous avions également prévu au cours de cette thèse, de faire tourner les
	modèles DNDC et CERES-NOE, à partir des données de la campagne de mesures, mais nous
	n'avons pas eu le temps. Néanmoins, l'ensemble de ces résultats pourra être utilisé pour le
	développement de modèles de fonctionnement des écosystèmes. Ils vont contribuer à
	quantifier les émissions de N 2 O aux échelles adaptées pour les inventaires et les stratégies
	d'atténuation.
	.
	Cependant, le protocole que nous avons développé pourrait être appliqué dans des situations

En ce qui concerne la campagne de mesures, les méthodes d'attribution de flux développées à partir des mesures réelles ont montré des résultats encourageants. De nouvelles campagnes de mesures, comme celle présentée dans cette thèse, devraient être réalisées, sur la même période, avec plus de mesures fast-box qui sont les données d'entrée pour les méthodes d'attribution des flux. Plus il y aura de mesures, plus les attributions de flux seront précises et nous verrons alors si ces méthodes peuvent être réellement appliquées. Par ailleurs, avec plus de mesures fast-box et avec une meilleure dispersion des sites de mesures, la variabilité spatiale sera mieux couverte, tout comme les directions du vent et les types de sols et de cultures. différentes, e.g., à travers l'Europe, pour développer des modèles générique d'attribution de flux. Après le développement et la validation des méthodes d'attribution de flux, les réseaux de mesures

Table 1 :

 1 N 2 O mean fluxes over a 4-days incubation period. Water-Filled Pore Space (WFPS), soil total mineral nitrogen and nitrates content were measured at the end of the experiment. 68

Table 2 :

 2 Correlation (p value) calculated between N 2 O fluxes, Water-Filled Pore Space (WFPS), total mineral nitrogen and nitrate content. ................................................................ 69

Table 3 :

 3 Mean N 2 O fluxes in ng N-N 2 O g -1 of soil and their standard error, obtained for the different temperature and acetylene treatments over a 4-days period. ..................................... 74

Table 4 :

 4 Different laboratory conditions and results on studies about the effect of temperature on N 2 O emissions and denitrification ....................................................................................... 80

Table 5 :

 5 Q 10 used by different models simulating N 2 O emissions (Heinen 2006)................... 81

Table 6 :

 6 Fluxes measured with the fast-box on every measurement day, with the mean flux,

over the last three decades (IPCC

2014). Its concentration is uniform in the atmosphere at

Acknowledgement

This work was financed by the labex Voltaire (ANR-10-LABX-100-01) and by the KIT IMK-IFU (n° 272) -INRA (32020587) agreement. Thanks to Pierre Courtemanche for the experimental design, Didier Laloua for the sampling and the preparation of the experiment and Adeline Ayzac for the analyses. Thanks to the farmers Aurélien and Jean-Pierre Berger who agreed to let us use their field for the sampling. Acknowledgement This work was financed by the FP7 InGOS project (n° 284274), the Egide PROCOPE (n°33067TG), the KIT IMK-IFU (n° 272) -INRA (32020587) agreement and by the labex Voltaire (ANR-10-LABX-100-01). Thanks to Olivier Zurfluh, Jean-Christophe Gueudet, Brigitte Durand, Patrick Jacquet, Claude Robert, Guillaume Giot, Adeline Ayzac, Pierre Courtemanche, Christian Le Lay, Catherine Pasquier, Didier Laloua and Lionel Cottenot. Thanks to the farmers Aurélien and Jean-Pierre Berger who agreed to let us use their garden and fields for this campaign. Acknowledgement This work was financed by the FP7 InGOS project (n° 284274), the Egide PROCOPE (n°33067TG), the KIT IMK-IFU (n° 272) -INRA (32020587) agreement and by the labex Voltaire (ANR-10-LABX-100-01). Thanks to Olivier Zurfluh, Jean-Christophe Gueudet, Brigitte Durand, Patrick Jacquet, Claude Robert, Guillaume Giot, Adeline Ayzac, Pierre Courtemanche, Christian Le Lay, Catherine Pasquier, Didier Laloua and Lionel Cottenot. Thanks to the farmers Aurélien and Jean-Pierre Berger who agreed to let us use their garden and fields for this campaign.

approaches. The α lu and β lu ratios were calculated for the three groups defined from the Newman-Keuls test (Table 8). The NO 3 -content was estimated for each plot from the equation obtained from the slope of NO 3 -consumption derived from the soil NO 3 measurements (Fig. 38).

Conclusions and Overviews

Tables