Her Majesty

Bertrand Guenet
email: bertrand.guenet@lsce.ipsl.fr

Gabrielle Benoit

Claire Chenu

Dominique Arrouays

Jérôme Balesdent

Elisa Bruni

Jean-Pierre Caliman

Rémi Cardinael

Songchao Chen

Philippe Ciais

Dominique Desbois

Julien Fouche

Stefan Frank

Catherine Henault

Emanuele Lugato

Victoria Naipal

Thomas Nesme

Michael Obersteiner

Sylvain Pellerin

David S Powlson

Daniel Rasse

Frédéric Rees

Jean- François Soussana

Yang Su

Hanqin Tian

Hugo Valin

Feng Zhou

Can N 2 O emissions offset the benefits from soil organic carbon storage?

To respect the Paris agreement targeting a limitation of global warming below 2°C by 2100, and possibly below 1.5 °C, drastic reductions of greenhouse gas emissions are mandatory but not sufficient. Large-scale deployment of other climate mitigation strategies are also necessary. Among these, increasing soil organic carbon (SOC) stocks is an important lever because carbon in soils can be stored for long periods and land management options to achieve this already exist and have been widely tested. However, agricultural soils are also an important source of nitrous oxide, (N 2 O) a powerful greenhouse gas, and increasing SOC may influence N 2 O emissions, likely causing an increase in many cases, thus tending to offset the climate change benefit from increased SOC storage. Here, we review the main agricultural management options for increasing SOC stocks. We evaluate the amount of SOC that can be stored as well as resulting changes in N 2 O emissions to better estimate the climate benefits of these management options. We conclude that the climate mitigation induced by increased SOC storage is generally overestimated if associated N 2 O emissions are not considered, but is never fully offset. Some options (e.g, biochar or non-pyrogenic amendment application) may even decrease N 2 O emissions.

Key points:

• Carbon and nitrogen cycles in soil interact in numerous and complex ways and an impact of a land management change on one cycle will generally influence the other.

• Several land management options designed to increase soil organic carbon stocks exist and have been widely evaluated.

• Land management options to increase soil organic carbon also tend to increase nitrous oxide emissions in some cases.

• We conclude that when increased N 2 O emissions are taken into account, they partially offset the climate benefits of increased organic carbon storage, but never negate them completely.

INTRODUCTION

The United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement adopted in 2015 aimed at keeping global warming below 2°C by 2100, and at possibly further limiting the temperature increase to 1.5 °C. The Paris Agreement specified that the balance of anthropogenic greenhouse gas (GHG) emissions and sinks should be attained by the second half of this century. This requires not only drastic reductions in GHG emissions in the near future, but also net negative emission technologies because not all emissions will be reducible to zero within this time scale 1,[START_REF] Rogelj | Energy system transformations for limiting end-of-century warming to below 1.5 °C[END_REF] . To a large extent, these negative emissions imply land-based mitigation strategies [START_REF]Summary for Policymakers SPM[END_REF] , mostly involving the production of organic matter by plant photosynthesis coupled with carbon storage in living biomass and / or soil organic matter [START_REF] Paustian | Climate-smart soils[END_REF] . A pathway frequently discussed known as bioenergy with carbon capture and storage (BECCS) comprises generating energy using biomass, capturing the CO 2 evolved from this process and storing it in geological reservoirs. The deployment of BECCS faces both technical challenges and most likely limitations due to high costs and adverse environmental impacts [START_REF] Smith | Biophysical and economic limits to negative CO2emissions[END_REF][START_REF] Obersteiner | How to spend a dwindling greenhouse gas budget[END_REF] . On the other hand, the net removal of atmospheric CO 2 taken up by plants in agricultural soils (i.e., carbon sequestration) has recently come under sharp focus as a more affordable and practical alternative, potentially associated with positive economic outcomes and possibly applicable at large scale in managed lands [START_REF] Minasny | Soil carbon 4 per mille[END_REF][START_REF] Hepburn | The technological and economic prospects for CO2 utilization and removal[END_REF] . The role of soils as a key component of the global carbon cycle is now recognized by the scientific community and also by policymakers [START_REF] Smith | Biophysical and economic limits to negative CO2emissions[END_REF][START_REF] Obersteiner | How to spend a dwindling greenhouse gas budget[END_REF] . Soils have never been harnessed at large scale for the purpose of sequestering carbon, although they currently make up the largest reservoir of organic carbon in the terrestrial biosphere, with a size of 1,500 Pg C to a depth of one meter [START_REF] Jobbágy | the Vertical Distribution of Soil Organic Carbon and Its relation to climate and vegetation[END_REF] . However, the ecosystems which contain the largest stocks of soil organic carbon (SOC) are unmanaged (comprising boreal forests, permafrost soils and wetlands), whereas only soils from managed ecosystems, in particular agricultural soils, may be managed to increase SOC stocks (i.e., carbon sequestration) Agriculture is also a key target sector for the reduction of methane (CH 4 ) and nitrous oxide (N 2 O) emissions [START_REF] Tian | The terrestrial biosphere as a net source of greenhouse gases to the atmosphere[END_REF][START_REF] Wollenberg | Reducing emissions from agriculture to meet the 2 °C target[END_REF][START_REF]Climate Change and Land An IPCC Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems WG[END_REF] . Yet, few countries have included agriculture in their nationally determined contributions -a roadmap volunteered by national governments as part of the Paris Agreement to express their efforts to reduce GHG emissions -because of potential trade-offs with food production and uncertainties on achievable potentials [START_REF] Frank | Reducing greenhouse gas emissions in agriculture without compromising food security?[END_REF] .

Recent emphasis on promoting SOC storage has resulted in international initiatives such as the "4 per mil" initiative launched by France during the UNFCCC conference of the parties (COP) 21 [START_REF] Minasny | Soil carbon 4 per mille[END_REF][START_REF] Soussana | Matching policy and science: Rationale for the '4 per 1000 -soils for food security and climate' initiative[END_REF] . It relies on the concept that even a very small relative increase in SOC pools worldwide could offset a significant fraction of CO 2 emissions. Preliminary evaluation indicated that increasing global agricultural SOC stocks at an annual rate of 4‰ would result in a C sequestration potential of 2-3 Pg C yr -17 . This may contribute significantly to the objectives of the land sector to achieve the Paris agreement target [START_REF] Roe | Contribution of the land sector to a 1.5 °C world[END_REF] . Nevertheless, several studies have discussed and criticized the feasibility of enriching soils at a rate of 4‰ over a sustained period of years [START_REF] Van Groenigen | Sequestering Soil Organic Carbon: A Nitrogen Dilemma[END_REF][START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF][START_REF] Chabbi | Aligning agriculture and climate policy[END_REF] because: (i) it requires large amounts of new organic matter inputs, (ii) it requires large amounts of nutrients, (iii) it is difficult to achieve this target rate in all agricultural systems, and (iv) it may be hampered by the climate change-induced enhancement of SOC decomposition. Moreover, altered management practices may impact farmers' income and imply trade-offs with food production [START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF] . Data from long-term experiments show that it is very difficult to achieve the 4 per mil rate in temperate arable systems without drastic changes in management [START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF][START_REF] Batjes | Technologically achievable soil organic carbon sequestration in world croplands and grasslands[END_REF] . Finally, the annual rate of SOC increase generally levels off over time as the SOC pool increases and approaches a new equilibrium level [START_REF] Stewart | Soil carbon saturation: Concept, evidence and evaluation[END_REF] .

Nevertheless, concrete management options exist to increase SOC stocks such as cover crops, tillage management, crop rotations, organic amendments, agroforestry and biochar amendments with effects depending on local conditions [START_REF] Dignac | Increasing soil carbon storage: mechanisms, effects of agricultural practices and proxies. A review[END_REF][22][START_REF] Chenu | Increasing organic stocks in agricultural soils: Knowledge gaps and potential innovations[END_REF] . It must be noted that organic amendments may, in some case such as manures, be a transfer of carbon from one terrestrial location to another rather than a net removal of carbon from the atmosphere [START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF] . Nevertheless, well spatially distributed, organic amendments contribute to significant increase the SOC pools at regional scale [START_REF]Stocker du carbone dans les sols français : quel potentiel au regard de l'objectif 4 pour 1000 et à quel cout ?[END_REF] . These options have socio-economic impacts on farmers and land managers and indirect effects on ecosystem services, through changes in crop yields, water consumption, nitrate leaching, and CH 4 and N 2 O emissions which have to be considered when evaluating the feasibility and the relevance of implementing SOC storage options. The SOC storage potential of the various practices has been extensively assessed in the recent scientific literature [START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF][START_REF] Chenu | Increasing organic stocks in agricultural soils: Knowledge gaps and potential innovations[END_REF][START_REF] Ogle | Climate and Soil Characteristics Determine Where No-Till Management Can Store Carbon in Soils and Mitigate Greenhouse Gas Emissions[END_REF][START_REF] Cardinael | Revisiting IPCC Tier 1 coefficients for soil organic and biomass carbon storage in agroforestry systems[END_REF][START_REF] Fujisaki | Soil carbon stock changes in tropical croplands are mainly driven by carbon inputs: A synthesis[END_REF] , and recently revised by IPCC in its 2019 report on climate change and land [START_REF]Climate Change and Land An IPCC Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems WG[END_REF] . However, implications for the N cycle (in particular N 2 O emissions), and other biogeochemical cycles or crop yields have not been thoroughly documented so far [START_REF] Lugato | Mitigation potential of soil carbon management overestimated by neglecting N2O emissions[END_REF][START_REF] Oldfield | Global meta-analysis of the relationship between soil organic matter and crop yields[END_REF][START_REF] Bossio | The role of soil carbon in natural climate solutions[END_REF] . Neither have been the consequences of large-scale deployment of these measures, and constraints related to the nitrogen (N), phosphorous (P) and potassium (K) cycles. These aspects are important because they determine the overall GHG abatement efficiency of mitigation measures and set limits on their potential deployment. C and N cycles are strongly interlinked (box. 1) in particular in soils and we assume that the deployment of land based mitigation options to increase SOC may impact the N cycle and the associated N 2 O emissions. A recent modelling study suggests that measure to increase SOC sequestration might be offset by increased N 2 O, depending on the crop rotation and on the duration of the land management practices [START_REF] Lugato | Mitigation potential of soil carbon management overestimated by neglecting N2O emissions[END_REF] . A better understanding of such interactions is necessary to evaluate the benefits of different management practices aimed at increasing SOC storage and to predict the full GHG balance of each practice.

Here, we focus on the interactions between soil C and nutrient dynamics, and in particular on N dynamics and N 2 O emissions. The aims of the paper are to i) describe the mechanisms linking the C and N cycles in soils, ii) assess how N 2 O emissions may be affected by increased SOC pools as a land based mitigation option, iii) review our knowledge on the other impacts of these practices.

INTERWINED SOIL CARBON AND NITROGEN CYCLES

Because C and N cycles are tightly coupled in soils, altering one will affect the other as shown in Fig. 1.

In soils the C and the N cycles are sometimes totally interdependent, in particular when both are in organic forms but are sometimes uncoupled when C or N are present as minerals. Nevertheless they may still interact with each other. This section summarizes the main mechanisms explaining how changes in the soil C cycle and SOC sequestration interact with N cycle processes, and in particular N 2 O emissions (Fig. 1, Box 1). The first reason why soil C and N dynamics are interdependent is that both elements are stored predominantly as organic forms in the soil, sometimes within the same compounds (amino acids, proteins, etc.), thus mineralisation generally affects both. Moreover, the availability of mineral N in the soil controls a number of processes in both cycles and vice versa. For instance, mineral N transformations depend on carbon availability and plant dry matter production is limited by N availability. Nitrogen is needed to sustain photosynthesis and other physiological processes [START_REF] Engels | Mineral Nutrition, Yield and Source-Sink Relationships[END_REF] ; therefore higher N availability would likely lead to greater primary productivity and inputs of plantderived organic matter to the soil [START_REF] Glendining | The effects of long-term applications of inorganic nitrogen fertilizer on soil nitrogen in the Broadbalk Wheat Experiment[END_REF] . On the other hand, higher N availability also tends to lower the allocation of photosynthates to the root system [START_REF] Pausch | Carbon input by roots into the soil: Quantification of rhizodeposition from root to ecosystem scale[END_REF] . As root-derived C inputs contribute at least 2-3 times more than shootderived C inputs to SOC storage [START_REF] Rasse | Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation[END_REF][START_REF] Kätterer | Roots contribute more to refractory soil organic matter than above-ground crop residues, as revealed by a long-term field experiment[END_REF] , a high soil N availability could theoretically increase the plant biomass but the plant biomass produced might not be transformed into SOC as efficiently because of a reduced amount of root-derived C entering the soil [START_REF] Han | Changes in soil organic carbon in croplands subjected to fertilizer management: A global meta-analysis[END_REF] . Soil organic matter (SOM) turnover (i.e., rate of mineralization and transformation of SOM) also depends on the N availability for microorganisms. While a low mineral N availability may limit the mineralization rate of plant residues and amendments [START_REF] Recous | Soil inorganic N availibility: effect on maize residue decomposition[END_REF][START_REF] Fang | Nutrient supply enhanced wheat residue-carbon mineralization, microbial growth, and microbial carbon-use efficiency when residues were supplied at high rate in contrasting soils[END_REF] , the combination of regular inputs of fresh organic C with a low soil N availability can lead to positive priming effect, i.e. a higher rate of SOM mineralization, and a lower SOC storage potential [START_REF] Fontaine | Carbon input to soil may decrease soil carbon content[END_REF][START_REF] Chen | Soil C and N availability determine the priming effect: Microbial N mining and stoichiometric decomposition theories[END_REF] . Moreover, because of the relatively narrow range of C:N ratios of SOM in mineral layers [START_REF] Van Groenigen | Sequestering Soil Organic Carbon: A Nitrogen Dilemma[END_REF] and because of the importance of soil microbial processing for building up stable SOM [START_REF] Cotrufo | The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: do labile plant inputs form stable soil organic matter?[END_REF] in some ecosystems, large amounts of N are inevitably required to stabilize large amounts of SOC [START_REF] Bertrand | Stoichiometry constraints challenge the potential of agroecological practices for the soil C storage. A review[END_REF][START_REF] Cotrufo | Soil carbon storage informed by particulate and mineral-associated organic matter[END_REF] . Because of this stoichiometric requirement, it might seem acceptable to maintain a high availability of N in the soil by applying large amounts of mineral fertilizers. Such a strategy would however lead to potential N losses, e.g. as N 2 O emissions or nitrate leaching from soil, and further increase GHG emissions during fertilizer production. Thus the modest increases in SOC resulting from N fertilizer applications up to sensible agronomic rates are welcome in the context of C sequestration, but it would be counter-productive and inappropriate to recommend higher rates of N application aimed at promoting an additional increase in soil C.

Input of N to terrestrial ecosystems by biological N 2 fixation is another example of a close link between C and N resources. Root-associated or free-living N 2 -fixing bacteria depend on the availability of organic C resources for sustaining their heterotrophic needs, which may explain why N 2 fixation is only triggered when the amount of soil mineral N is low. In particular, the energy cost of N 2 fixation represents between 5% and 23% of daily photoassimilated C 44 . The associated CO 2 losses by respiration may therefore decrease the amount of plant C entering the soil. However, the consequence of this on the potential of SOC storage remains unclear. For example, the presence of leguminous plants can result in lower belowground C inputs compared to gramineous plants, leading to lower SOC concentrations [START_REF] Bessler | Aboveground overyielding in grassland mixtures is associated with reduced biomass partitioning to belowground organs[END_REF][START_REF] Lange | Plant diversity increases soil microbial activity and soil carbon storage[END_REF] . However, the net inputs of N to soils by leguminous plants have been shown to correlate with a net accumulation of SOC, by providing the organic N required to stabilize an additional amount of SOC in soils [START_REF] Jensen | Legumes for mitigation of climate change and the provision of feedstock for biofuels and biorefineries. A review[END_REF] . Similarly, crop rotations that include leguminous plants appear store more SOC than conventional monocultures [START_REF] Drinkwater | Legume-based cropping systems have reduced carbon and nitrogen losses[END_REF] , although this effect may be mainly due to longer periods of plant cover, and to the presence of deeper root systems than to biological N 2 fixation itself [START_REF] Jensen | Legumes for mitigation of climate change and the provision of feedstock for biofuels and biorefineries. A review[END_REF] . These feedbacks also depend on which non-leguminous plants are associated 49 to the N 2 -fixing plant, and may lead to contrasting results in terms of SOC storage [START_REF]Stocker du carbone dans les sols français : quel potentiel au regard de l'objectif 4 pour 1000 et à quel cout ?[END_REF] . Of course, obtaining N from legumes, where this is practicable, rather than from N fertilizer does eliminate the GHG emissions associated with N fertilizer manufacture.

Fresh C inputs to the soil through root exudates or amendments may temporarily decrease or increase soil pH, affecting the magnitude of N 2 O emissions. Their consumption by microorganisms may also decrease the local concentration of oxygen, leading to anaerobic conditions which are favourable to denitrification and N 2 O emissions [START_REF] Chen | Soil nitrous oxide emissions following crop residue addition: a metaanalysis[END_REF] . Furthermore, because organic materials generally act as electron donors in the denitrification process and because soil organic matter content may lower the redox potential of the soil [START_REF] Quin | Lowering N2O emissions from soils using eucalypt biochar: The importance of redox reactions[END_REF] , increasing the amount of soil organic matter may also increase the activity of denitrifiers and therefore increase N 2 O emissions [START_REF] Brettar | Nitrate elimination by denitrification in hardwood forest soils of the Upper Rhine floodplain -Correlation with redox potential and organic matter[END_REF][START_REF] Li | Carbon sequestration in arable soils is likely to increase nitrous oxide emissions, offsetting reductions in climate radiative forcing[END_REF] . These mechanisms likely explain why higher SOC contents in soils have indeed been shown to correlate with larger N 2 O emissions [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][START_REF] Hénault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF] . N 2 O emissions represent a particular case that illustrates how the soil N cycle may be influenced by the C cycle. As a rule, net N 2 O emissions from the soil at a given soil water-filled pore space (WFPS) will usually be lower when the soil mineral N content is low and when soil pH is alkaline or when C availability is reduced. Furthermore, because a low soil redox potential (< 400 mV) [START_REF] Brettar | Nitrate elimination by denitrification in hardwood forest soils of the Upper Rhine floodplain -Correlation with redox potential and organic matter[END_REF] is required for denitrification, N 2 O emissions have been suggested to have their optimum at 70-80% WFPS, while prolonged waterlogging conditions may result in complete nitrate reduction to N 2 instead of N 2 O 56 . Several mechanisms can therefore explain why attempts to modify the soil C cycle may also affect N 2 O emissions. On a longer time scale, the build-up of SOC by various strategies may be expected to increase the retention of water and fertilizer-N in the rooting zone through improved soil properties (eg.. water holding capacity, porosity, hydrophilicity) , in a manner favourable for the denitrification to occur. This might trigger a higher primary production and enhancing further SOC storage, but also increase the risk of N 2 O emissions because of the increase in N sources and the shift to soil environmental conditions more favourable to N 2 O emissions. In the remainder of this paper we consider possible interactions between increased SOC and changes in N 2 O emission for a range of management practices designed to increase SOC (Table 1).

HOW SOC STORING PRACTICES AFFECT N 2 O EMISSIONS

Balancing the Nitrogen inputs

Since mineral N availability drives N 2 O emissions as well as crop productivity and C inputs into the soil [START_REF] Wang | Data-driven estimates of global nitrous oxide emissions from croplands[END_REF][START_REF] Ladha | Role of Nitrogen Fertilization in Sustaining Organic Matter in Cultivated Soils[END_REF] , N fertilization should be carefully managed. A balance should be obtained between N inputs (including fertilizers, manures and biological nitrogen fixation through symbiosis between N 2 -fixing bacteria and some plant species) and N exported in harvested products in order to reduce a N surplus that can be source of N 2 O, but without a major negative effect on crop productivity. This N surplus should ideally be zero, but it is actually large and positive in many regions of the world, having intensive agriculture (e.g. parts of China, India, Europe, North America), and negative in other regions (e.g. Africa) [START_REF] Gruber | An Earth-system perspective of the global nitrogen cycle[END_REF] . Excess N associated with a positive surplus is a major cause of N 2 O emissions on farms, but also of nitrate leaching losses, part of which contributes to indirect N 2 O emissions if nitrate is denitrified within surface waters. Overall, N surplus is a strong driver of N 2 O emissions, especially when considering that the rate of emission is no longer linear for high N input [START_REF] Shcherbak | Global metaanalysis of the nonlinear response of soil nitrous oxide (N 2 O) emissions to fertilizer nitrogen[END_REF] . The relatively low cost of mineral N fertilizers in developed countries compared to the price of agricultural products incentivizes farmers to apply more N than recommended by good practices, as an 'insurance' against unforeseen N losses due to climate variability. In some regions of the world, but not all, there is considerable potential to lower agricultural N 2 O fluxes in intensive farming by reducing the N surplus without affecting farmers' incomes [START_REF] Hoben | Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in on-farm corn crops of the US Midwest[END_REF] . Therefore, the use of mineral N to increase crop productivity may induce an increase of C input into the soil but a complex balance must be found to avoid excessive N 2 O emissions and N leaching.

Reduced tillage/non tillage

The effect of reduced tillage has attracted attention as a practice leading to increased SOC storage.. However, recent meta-analyses demonstrate only a small positive effect of no-tillage on SOC stocks in the topsoil (0-30 cm layer) compared to conventional tillage, while it may vary widely across pedo-climatic situations [START_REF] Luo | Can no-tillage stimulate carbon sequestration in agricultural soils? A meta-analysis of paired experiments[END_REF][START_REF] Virto | Carbon input differences as the main factor explaining the variability in soil organic C storage in no-tilled compared to inversion tilled agrosystems[END_REF][64][START_REF] Angers | Full-inversion tillage and organic carbon distribution in soil profiles: A meta-analysis[END_REF][START_REF] Powlson | Does conservation agriculture deliver climate change mitigation through soil carbon sequestration in tropical agro-ecosystems?[END_REF] . Moreover, it must be recognised that the largest impact of reducing tillage is a redistribution of SOC toward the soil surface [START_REF] Ogle | Climate and Soil Characteristics Determine Where No-Till Management Can Store Carbon in Soils and Mitigate Greenhouse Gas Emissions[END_REF][64][START_REF] Angers | Full-inversion tillage and organic carbon distribution in soil profiles: A meta-analysis[END_REF][START_REF] Powlson | Does conservation agriculture deliver climate change mitigation through soil carbon sequestration in tropical agro-ecosystems?[END_REF][START_REF] Baker | Tillage and soil carbon sequestration-What do we really know?[END_REF] . As a consequence, data from field trials must be carefully examined to distinguish between a genuine increase in SOC stocks in the surface soil layers from a simple change in the vertical distribution of SOC concentration.

There has been considerable discussion as to whether the increased SOC in soil under zero tillage, especially near the surface, might increase N 2 O emissions, because : (i) increased organic matter content can increase N 2 O release [START_REF] Mei | Stimulation of N2O emission by conservation tillage management in agricultural lands: A meta-analysis[END_REF] , either because of increased energy supply to denitrifying organisms or because increased biological activity utilises oxygen in soil, thus possibly leading to anoxic conditions at some microsites and ii) reducing tillage can be associated in the short term with a less porous soil structure, conducive of anoxy [START_REF] Linn | Effect of Water-Filled Pore Space on Carbon Dioxide and Nitrous Oxide Production in Tilled and Nontilled Soils[END_REF] (Table 1). The different meta-analysis we compiled here (Fig. 2) suggest that N 2 O emissions may offset the C storage in no-till system when both fluxes are compared in CO 2 equivalents. However, there is conflicting evidence on whether or not this risk is actually realised [START_REF] Mei | Stimulation of N2O emission by conservation tillage management in agricultural lands: A meta-analysis[END_REF][START_REF] Van Kessel | Climate, duration, and N placement determine N2O emissions in reduced tillage systems: A meta-analysis[END_REF] . Recent meta-analyses suggest that, in the majority of situations, N 2 O emissions are either unchanged or slightly decreased under zero or reduced tillage; the result will certainly be influenced by soil type and local climate and weather conditions so it may not be possible to draw a conclusion that is universally valid [START_REF] Van Kessel | Climate, duration, and N placement determine N2O emissions in reduced tillage systems: A meta-analysis[END_REF][START_REF] Mangalassery | To what extent can zero tillage lead to a reduction in greenhouse gas emissions from temperate soils?[END_REF] . Furthermore, in some studies, N 2 O emissions were expressed on both an area basis and a yield-scaled basis [START_REF] Van Kessel | Climate, duration, and N placement determine N2O emissions in reduced tillage systems: A meta-analysis[END_REF] ; because crop yields were slightly decreased under reduced tillage in some environments, N 2 O emissions per unit of grain (or other product) were sometimes increased compared to conventional tillage.

Erosion control -terracing

Erosion control practices are able to maintain or increase SOC content at the plot scale [START_REF] Moraru | Soil tillage conservation and its effect on soil organic matter, water management and carbon sequestration[END_REF] , although on a larger scale whether erosion is net C sink or a net source is still debated [START_REF] Van Oost | The impact of agricultural soil erosion on the global carbon cycle[END_REF][START_REF] Lugato | Soil erosion is unlikely to drive a significant carbon sink in the future[END_REF][START_REF] Berhe | Role of Soil Erosion in Biogeochemical Cycling of Essential Elements: Carbon, Nitrogen, and Phosphorus[END_REF] . Erosion control encompasses a wide range of practices such as protecting the soil surface with cover crops or unharvested biomass (pruned fronds and other plant residues), agroforestry, crop rotations, conservation tillage, or terracing on steep slopes. Some of these practices are already addressed in other sections of this paper (3.2, 3.4 and 3.5), and the following focuses on terracing.

Terracing is an ancient form of erosion control and a soil conservation method performed for thousands of years in steep landscape regions [START_REF] Dotterweich | Geomorphology The history of human-induced soil erosion : Geomorphic legacies , early descriptions and research , and the development of soil conservation -A global synopsis[END_REF][START_REF] Tarolli | Terraced landscapes: From an old best practice to a potential hazard for soil degradation due to land abandonment[END_REF] . Despite its importance, studies focusing on quantifying soil erosion rates and the resulting C fluxes and SOC stocks in terraced areas are limited, especially at regional scales. Generally, terracing reduces soil erosion by reducing the slope gradient and length, and can decrease soil erosion rates by up to 95% [START_REF] Fu | Soil erosion and its control in the loess plateau of China[END_REF][START_REF] Upadhyay | A review of carbon sequestration dynamics in the Himalayan region as a function of land-use change and forest/soil degradation with special reference to Nepal[END_REF] . It accordingly preserves SOC and nutrients. A meta-analysis on the ecosystem benefits of terracing shows that, compared to unterraced slopes, soil in terraced slopes contains 28.1% and 41.7% more N and C, respectively [START_REF] Wei | Global synthesis of the classifications, distributions, benefits and issues of terracing[END_REF] . However, the overall net effect of terracing on erosion depends on the terrace structure and maintenance, crop type, soil conditions, crop management practices or agricultural machinery. To maximize its positive effects, terracing needs to be combined with other soil conservation measures such as cover crops, agroforestry, organic amendments or no-till [START_REF] Chen | Effects of terracing practices on water erosion control in China: A metaanalysis[END_REF][START_REF] Mclauchlan | The nature and longevity of agricultural impacts on soil carbon and nutrients: A review[END_REF] . Furthermore, terraces need to be sustained, otherwise abandoned terraces can become sources of substantial land degradation due to gully formation. This is the case in the Mediterranean region where over 50% of the terraces have been abandoned [START_REF] Tarolli | Terraced landscapes: From an old best practice to a potential hazard for soil degradation due to land abandonment[END_REF][START_REF] Wei | Global synthesis of the classifications, distributions, benefits and issues of terracing[END_REF] .

The N 2 O emissions associated with terracing are still poorly known. Terracing decreases the aggregate breakdown and transport of soil by erosion, which would lead to reduced N 2 O emissions. However, as stated previously, N 2 O emissions may increase with increased SOC. In addition, terracing tends to change the soil C:N ratio [START_REF] Wei | Global synthesis of the classifications, distributions, benefits and issues of terracing[END_REF] and this may change the N availability for nitrifying/denitrifying bacteria and thus affect N 2 O emissions.

Finally, to fully estimate the effect of erosion control on N 2 O budgets, it is important to measure emissions at the catchment scale not only at the field scale. Since erosion control aims to avoid lateral losses of soil material (containing various forms of N) ending up in rivers or in floodplains, it is necessary to combine measurements in the terraced or unterraced fields with measurements and modelling on the fate of eroded N in floodplains and rivers.

Cover crops

Planting cover crops is an effective management practice to increase SOC content. According to a recent meta-analysis, it leads to SOC accumulation rates in the order of 1.18 t CO 2 -eq. ha -1 yr -1 over 50 years [START_REF] Poeplau | Carbon sequestration in agricultural soils via cultivation of cover crops -A metaanalysis[END_REF] in the topsoil, with a positive effect independent of tillage method, climatic zone or plant type (leguminous vs nonleguminous). The impact of cover crops on SOC will depend on their duration and the frequency with which they are included in a crop rotation, and this information is sometimes unclear in published reports of field trials [START_REF] Poeplau | Carbon sequestration in agricultural soils via cultivation of cover crops -A metaanalysis[END_REF] . The main driver of SOC storage seems to be the extra C input, as suggested by the high correlation between rates of SOC stock change and the amounts of C returned to the soil by cover crop biomass [START_REF] Soane | No-till in northern, western and south-western Europe: A review of problems and opportunities for crop production and the environment[END_REF] . However, there are limitations to the use of cover crops depending on cropping systems and climate conditions. For example, in temperate regions they can be readily utilised during the winter period prior to sowing a spring-sown crop, when the soil would otherwise be bare. But if only autumn-sown crops are grown, there is very limited time between harvesting and the sowing of the subsequent crop.

The effects of cover crops on N 2 O emissions are more variable and contrasted than those on SOC changes (Table 1). Many factors influence the magnitude of N 2 O emissions, such as the C:N ratio of cover crop residues, their rate of decomposition, the extra inputs of fertilizer N sometimes applied to cover crops, whether the residues are ploughed or left to decay on the soil surface. Current evidence points to a negative relationship between N 2 O emissions and the C:N ratio of residues [START_REF] Chen | Soil nitrous oxide emissions following crop residue addition: a metaanalysis[END_REF] . A low C:N ratio will increase the availability of soil N for microbial transformations (e.g. nitrification and denitrification) whereas larger ratios will result in N immobilization and deplete the soil inorganic N pool [START_REF] Gentile | Interactive effects from combining fertilizer and organic residue inputs on nitrogen transformations[END_REF] . Additional C inputs from cover crops may stimulate the activity of denitrifier bacteria, which use these organic compounds as a source of energy [START_REF] Mitchell | Cover crop effects on nitrous oxide emissions: Role of mineralizable carbon[END_REF] . A meta-analysis [START_REF] Basche | Do cover crops increase or decrease nitrous oxide emissions? a meta-analysis[END_REF] reported a significant increase in N 2 O emission when leguminous cover crops were introduced. However, another review [START_REF] Han | N2O emissions from grain cropping systems: a meta-analysis of the impacts of fertilizer-based and ecologically-based nutrient management strategies[END_REF] found out that the incorporation of either legume and non-legume cover crops tended to increase N 2 O emissions but the magnitude of the effect was not significant due to the high variability of data.

The effect of cover crops on N 2 O emissions is therefore not yet fully understood and may well be highly sitespecific.

One of the key points controlling cover crop effect on N 2 O emissions is how often leguminous crops are integrated within the crop rotation. Leguminous cover crops generally have a lower C:N ratio than nonleguminous crops, and can fix substantial amounts of atmospheric N, reaching up to 0.1-0.2 t N ha -1 yr -189 . These rates may lead to a N surplus if all the leguminous cover crop biomass is incorporated. A recent study, using a biogeochemistry model framework at European scale, estimated that systematic planting of N-fixing cover crops may lead to a N surplus of about 0.04 t N ha -1 yr -1 , compared to the use of non-legumes as cover crop [START_REF] Lugato | Mitigation potential of soil carbon management overestimated by neglecting N2O emissions[END_REF] . In this scenario, the cumulative climate change mitigation effect of SOC sequestration was, on average, totally offset after 50 years since the adoption of cover crops, due to enhanced N 2 O emissions. While cover crops may induce higher N 2 O emissions, in particular if leguminous crop are extensively used, they can also reduce nitrate leaching, by about 56% on average [START_REF] Thapa | Biomass production and nitrogen accumulation by hairy vetch-cereal rye mixtures: A meta-analysis[END_REF] . This is beneficial for water quality and would be expected to lead to decreased indirect N 2 O emission through denitrification of nitrate entering surface water. Finally, another indirect effect of leguminous cover crops on N 2 O emissions will strongly depend on whether or not mineral N fertilisation rates are reduced to take account of N provided by biological fixation. The meta-analysis we compiled here indicate that additional N 2 O emissions decrease the SOC storage benefit of cover crops, but do not fully offset it (Fig. 2).

Agroforestry

Agroforestry systems include a diversity of practices ranging from complex associations found in homegardens, multistrata systems or agroforests to simpler systems such as alley crops, silvopastoral systems, riparian plantings, shelterbelts, windbreaks or hedgerows [START_REF] Nair | Classification of agroforestry systems[END_REF] . Despite this broad diversity, recent reviews and meta-analyses consistently suggest that the conversion of arable land to agroforestry systems increases SOC stocks [START_REF] Lorenz | Soil organic carbon sequestration in agroforestry systems. A review[END_REF][START_REF] Kim | Carbon sequestration and net emissions of CH4 and N2O under agroforestry: Synthesizing available data and suggestions for future studies[END_REF][START_REF] Feliciano | Which agroforestry options give the greatest soil and above ground carbon benefits in different world regions?[END_REF] . In temperate regions, SOC accumulation rates are usually around 0.92 t CO 2 -eq. ha -1 yr -1 in the topsoil (0-30 cm) [START_REF] Poeplau | Carbon sequestration in agricultural soils via cultivation of cover crops -A metaanalysis[END_REF] . They are highly dependent on local pedoclimatic conditions and on the type and design of agroforestry systems (tree density, tree species, pruning management, etc), but rarely exceed 3.67 t CO 2 -eq. ha -1 yr -1 22,26 . However, the spatial distribution of SOC stocks in agroforestry systems is usually very heterogeneous, with higher stocks under the tree canopy or along tree rows [START_REF] Cardinael | Impact of alley cropping agroforestry on stocks, forms and spatial distribution of soil organic carbon -A case study in a Mediterranean context[END_REF][START_REF] Bambrick | Spatial heterogeneity of soil organic carbon in tree-based intercropping systems in Quebec and Ontario, Canada[END_REF] . Several mechanisms contribute to explain SOC sequestration in agroforestry systems. The main one is probably being linked to higher organic inputs to the soil compared to treeless agricultural land [START_REF] Cardinael | High organic inputs explain shallow and deep SOC storage in a long-term agroforestry system -Combining experimental and modeling approaches[END_REF] , including litterfall, pruning residues, and root inputs [START_REF] Germon | Unexpected phenology and lifespan of shallow and deep fine roots of walnut trees grown in a silvoarable Mediterranean agroforestry system[END_REF] .

A recent synthesis of N 2 O emissions under agroforestry compared to adjacent agricultural lands only found minor differences in net emissions, with no clear overall direction of change [START_REF] Kim | Carbon sequestration and net emissions of CH4 and N2O under agroforestry: Synthesizing available data and suggestions for future studies[END_REF] . However, several authors found increased N 2 O emissions in agroforestry, related to a greater N supply through N 2 -fixing trees [100][101][102][103] or to the incorporation of tree residues 104,105 . By contrast, N 2 O emissions are often reduced in silvo-arable systems and in riparian buffers [START_REF] Kim | Carbon sequestration and net emissions of CH4 and N2O under agroforestry: Synthesizing available data and suggestions for future studies[END_REF] . Some authors suggest that concerns over N 2 O emissions from N 2 -fixing trees are unwarranted since fluxes from soils planted with N 2 -fixing trees are similar to those fertilized with mineral N 106 . Furthermore, the yield of crops in tropical agroforestry systems may be boosted as a result of higher N-inputs from trees. In temperate regions where agroforestry systems are generally planted with non-legume trees, N 2 O emissions are often reduced [START_REF] Kim | Carbon sequestration and net emissions of CH4 and N2O under agroforestry: Synthesizing available data and suggestions for future studies[END_REF] , with several processes contributing to the trend. Increased nitrogen utilization at the plot scale may be due to the presence of deep-rooted trees 107 , which are capable of taking up nitrate-N that has leached below crop rooting depth [108][109][110] . This process can potentially reduce the amount of N available for nitrification and denitrification, and thus reduce indirect N 2 O emissions. Soil water content is often lower in agroforestry than in treeless plots 111 , due to a higher daily water consumption by trees and crops 112 . A drier soil profile in agroforestry systems could therefore lower N 2 O emissions. In temperate silvoarable systems, tree rows are usually uncropped and unfertilized. This reduction in the fertilized cropping area indirectly leads to lower N 2 O emissions per hectare. An obvious consequence of agroforestry, especially as tends to be practiced in temperate regions, is that a smaller area of land is devoted to the agricultural crop being grown. So the impact of decreased N 2 O emissions may be different if expressed on an area basis compared to per unit of production.

Non-pyrogenic organic amendments

A literature review 113 reported increases in SOC (sometimes expressed as stocks and sometimes as concentration) after prolonged large applications of organic amendments under several different agro-climatic conditions. These increases ranged from 20 to 90% of the initial total SOC after few years (3-60 years), compared to unfertilized controls or treatments receiving only synthetic mineral N fertilisers, with most being in the range 20-45%. A meta-analysis 114 based on 130 observations worldwide quantified the response of SOC stocks to manure application over periods ranging from 3 to 82 years. The mean manure-C retention coefficient defined as the average proportion of manure-C remaining in the soil was estimated at 12% for an average study duration of 18 years. The authors finally estimated a relative SOC stock change factor of 26% which was also related to cumulative manure inputs. Concerning Mediterranean cropping systems, and shorter durations, a metaanalysis 115 reported that the application of organic amendments increased SOC stocks by 23.5% with an average SOC storage rate of 4.81 t CO 2 -eq. ha -1 yr -1 calculated for an average duration of 7.9 years. From these metaanalyses, it seems that there is a consensus that organic amendments lead, on average, to a relative increase of SOC stocks in the top soils (roughly 20-30cm) of about 25% on a 20-year time frame (or 3 times the '4 per mil' target). In one example [START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF] where manure was applied annually at a high rate compared to what is usual in agrosystems, the annual rate of SOC accumulation averaged 18‰ per year in the first 20 years, then declined to 6‰ per year after 40-60 years, and to only 2‰ per year after 80-100 years. However, from the perspective of mitigating climate change, it is arguable whether any increase in SOC stocks resulting from applications of manure or similar materials can be considered as C mitigation in the sense of either a transfer of C from atmosphere to land or an avoided emission. Manure is generated in agricultural systems and is almost always used in some way by application to soils, though often quite inefficiently. Thus, an increase in SOC stocks at a given location mainly represents a transfer of C from one site to another as opposed to a net removal of atmospheric carbon [START_REF] Poulton | Major limitations to achieving '4 per 1000' increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom[END_REF] . Local additional SOC storage may not represent a CO 2 sink, i.e. a net transfer of carbon from the atmosphere to the soil at the landscape scale.

Because organic amendments such as manures contain readily-decomposable N-rich compounds, there is a significant risk that they may enhance N 2 O emissions [116][117][118] . Conversely, their use permits decreased use of mineral N fertilizers, thereby saving N 2 O emission from this source and fossil energy and the associated GHG emissions from fertilizer manufacture. A further complicating factor in assessing the overall impact of manure use is that indirect emissions due to storage or management are not negligible 119 . There are few reports in the literature of long-term monitoring of N 2 O emissions compared to data on SOC stock changes, primarily because the former are much more difficult to measure. However, the effects of multiple types of organic amendments on SOC storage and N 2 O emissions have been evaluated in short-term experiments for various soil types, climates, soil incorporation practices, and amendments types including crop residues, manure, composts of various origin and maturation stages, and sewage sludge. A meta-analysis 120 concluded that the N 2 O emission factors (EFs) related to N inputs were mainly controlled by the C:N ratios of the added material, but that many other factors influenced emission, such as soil properties (texture, drainage, SOC and N content), and climatic factors. For instance, the authors observed that the EFs were on average 2.8 times greater in fine-textured soils compared to coarse-textured, consistent with a previous meta-analysis [START_REF] Chen | Soil nitrous oxide emissions following crop residue addition: a metaanalysis[END_REF] . However, we should mention that the value of metaanalyses is often limited due numerous controlling factors that are not always correctly reported in the papers reviewed, and the general paucity of organic amendments characterisation in the literature. For instance, the two meta-analyses mentioned in this paragraph only involved 28 to 38 individual journal articles. [START_REF] Chen | Soil nitrous oxide emissions following crop residue addition: a metaanalysis[END_REF]120 . Another approach is to compare organically managed soils with those managed without organic amendments 121 . Results from such a comparison seem to indicate reduced N 2 O emissions compared to situations relying totally on mineral fertilizers, as show in Fig. 2. It should be noted that there is limited data from long-term studies on N 2 O emissions associated with additions of organic amendments; the data covers only a limited diversity of pedoclimatic conditions, and especially the range of soil water filled pore space values explored.

Biochar amendments

Biochar (pyrolyzed organic matter amended to the soil) technology is considered by some authors to be one of the methods with the highest potential to sequester carbon in soils compared to natural C cycle without biochar production step [START_REF] Paustian | Climate-smart soils[END_REF] . The aim of biochar production from biomass pyrolysis is to produce recalcitrant organic matter (i.e., charcoal and biomass-derived black C) which is then added to the soil. For this reason, biochar can be considered as a negative emission technology different from other soil C sequestration methods [START_REF] Smith | Biophysical and economic limits to negative CO2emissions[END_REF] .

Biochar properties and effects on SOC stabilization strongly depend on the feedstock material and pyrolysis conditions (e.g., maximum temperature, heating rates) [122][123][124][125][126] , as well as biochar ageing and soil properties 127,128 .The efficiency of biochar for C sequestration is two-fold as compared to simply relying on soil stabilization processes. First, slow pyrolysis for biochar production results in a much higher proportion of the feedstock C bound in persistent molecular structures than through in situ stabilization by addition of unprocessed organic matter to soil 129 . With a slow pyrolysis at about 500°C, approximately 50% of the carbon contained in a feedstock of Miscanthus or maize cobs ended up within the biochar and can therefore be assumed to be more stable than carbon in the raw biomass 130 . This compares with only 8-12% of straw residue returned to the field being transformed into longer-lived SOM forms [START_REF] Fujisaki | Soil carbon stock changes in tropical croplands are mainly driven by carbon inputs: A synthesis[END_REF]131 .Thus, pyrolysis is about four times more efficient than SOM-formation processes to produce persistent C in soils. Second, field studies show that biochar has a longer mean residence time in soils than SOM, i.e. >100 years 132 vs. about 50 years for the latter 133 . Combining effects of the higher persistent-C yield with that of the longer mean residence time, biochar appears at least 8 times more efficient at storing SOC than the return of non-pyrolysed residues. In meta-analyses, biochar amendment tends to increase the soil organic carbon stocks by 40% but the studies used were generally short term (no more than 4 years) 134 . Nevertheless, this result must be considered with due care since it is not straightforward to measure the effect of biochar, which is mainly C, on native SOC but one published study suggests that biochar amendment increase total SOC including non-biochar C 135 . In addition to the direct inputs of pyrolyzed biomass to the soil, recent studies showed that biochar amendments could increase (positive priming), decrease (negative priming) or have no effect on the mineralisation of native SOM. The biochar effect on the magnitude and direction of priming is influenced by the incubation period and pyrolysis temperature 122,123,136 . Positive priming, which would cause destabilization of SOM, thus offseting part of the increased SOC storage, could result from the biochar affecting microbial biomass activity and enzyme production 137 through changes in availability of organic substrates and nutrients, and modification of microorganism habitat associated with the great porosity and large specific surface area of charcoal particles 137 . Conversely, some studies showed that biochar-induced negative priming, leading to further SOC storage in addition to direct biochar-C inputs, resulted from the enhancement of organo-mineral interactions and soil aggregation with biochar 138,139 , and a greater adsorption of dissolved organic carbon onto biochar particles 140 . In addition, biochar amendments have been shown to increase soil water holding capacity, the availability of some nutrients (Ca 2+ , Mg 2+ in particular) and to increase soil pH.

All of these mechanisms could further enhance crop productivity and biomass inputs into soil [141][142][143][144] , with clearer effects on crop yields in highly weathered tropical soils 145 . Nevertheless, to process biomass into biochar, transport it and incorporate it to the soil, some energy is needed (possibly produced by pyrolysis) and the related GHG emissions associated with this process must be accounted for to calculate a full GHG balance for biochar.

Life cycle assessments (LCA) have shown that a positive balance can be obtained, illustrated by GHG reductions up to 2.74 t CO 2 equivalent per ton of biochar amended on volcanic soils from Southern Chile 146 . The balance can also significantly increase when plant biomass production is accompanied by an efficient use of the bioenergy produced during the pyrolysis process in order to maximize climate benefits from biochar production followed by addition to soil, as shown by an LCA performed in Spain 147 ; implementation costs were also decreased. One simulation study suggests that the maximum sustainable technical potential of biochar to mitigate climate change, involving the widespread use of biochars, without threatening food security and landscapes, could be a mitigation of 12% of current anthropogenic CO 2 emissions (1.8 Pg CO 2 -C equivalent per year) 148 . The C:N ratio of SOM approximates 14 149 while that of biochar is generally higher than that of its feedstock, i.e. generally > 50 for straw biochar and > 100 for wood biochar . It takes therefore at least five times less N to stabilize organic C in the form of biochar than in the form of SOM. Beyond this critical observation, biochar has other important interactions with the N cycle, notably: 1) volatilization and immobilization of N during the pyrolysis process 150 , 2) reduction of N 2 O emissions after application to arable fields 151 , 3) reduction of NO 3 and NH 4 leaching 152 . Emissions of N 2 O from soils are in most cases substantially reduced by biochar addition: a recent meta-analysis reported an average decrease of 38% across studies 151 . However, most measurements are faily short-term, the majority in this meta-analysis being <30 days . This effect appears consistent when biochar is produced at over ~450C, so that the product is both high pH and high surface area while containing very little labile C 125 . The contribution of N 2 O emissions attenuation with biochar was shown to be negligible in the LCA performed in Southern Chile, compared to the climate change mitigation associated to C storage 146 . Furthermore, reductions in N 2 O emission with biochar appears only significant for the first year after application, which suggests that frequent applications are necessary to maintain such an effect. In view of the large quantities of biochar usually applied in such studies, this may greatly limit the practical and/or economical potential for using biochar as a method for decreasing N 2 O emissions.

A way to capitalize on the positive interactions between biochar properties and the N cycle is through the development of biochar-based fertilizers made by mixing biochar with mineral or organic sources of nutrients 153 . This method aims at reducing nitrate losses and N 2 O emissions, and at increasing N use efficiency.

Moreover, biochar tends to adsorb mineral N and the mixing with a nutrient-rich material prevents potential N deficiency created by field application of large amounts of raw biochar. Some biochar structures have been successfully loaded with nitrate ions through co-composting, which could greatly increases the fertilisation value of the product 154 . Producing biochar fertilizers requires the development of appropriate technologies. For example, mixing raw biochar -a high pH product-with manure and slurries can result in large amounts of NH 3 being volatilized. Therefore, biochar acidification is generally required when making biochar fertilizers from organic feedstocks. However, biochar is also a strong sorbent for NH 3 155 , which may be captured from the atmosphere during the pyrolysis process and made available to plants later. This is a promising technology to abate anthropogenic emissions of NH 3 155 as well as directly reduce NH 3 volatilization from soils 150 . In conclusion, pending proper technology, biochar may be intimately mixed with N sources and applied each year as a fertilizer to maximize reductions in both N 2 O emissions and nitrate leaching, while sequestering C in a structure requiring little N. However, further studies are needed to validate the scant results currently available. . The data on Fig. 2 were taken from several metaanalyses and review papers. Here, we did not re-analyse the data gathered by such meta-analysis, but rather presented the mean effect size from each study converted in CO 2 equivalents. When results were given for the whole experiment duration, we divided by the duration of the experiment to obtain the mean annual SOC storage/ N 2 O emissions (see supplementary information for detailed methods). The data in Fig. 2 is based on over 700 measurements of SOC change and 200 measurements of N 2 O. Even allowing for some papers being cited in more than one meta-analysis, this is a large body of data and, to our knowledge, has not previously been assembled in this way. At first sight it appears that SOC increases produced by the four sets of treatments varied widely from -0.52 ±0.46 to -6.74±1.21 t CO 2 -eq. ha -1 yr -1 , the negative sign representing accumulation of SOC, i.e. transfer of C from atmosphere to soil. However, this wide range is somewhat deceptive as the two largest values are from very specific situations. The value of -6.74±1.21 t CO 2 -eq. ha -1 yr -1 for agroforestry (Kim et al, 2016) is from 34 sets of data for systems with a particularly high tree density (see legend to Fig. 2); the other two meta-analyses for agroforestry, based on >200 datasets, give values of less than half this at around -3 t CO 2 -eq. ha -1 yr -1 . For cover crops the majority of data, based on 186 datasets, lead to mean rates of C accumulation in the range of -1.2 to -2.0 t CO 2 -eq. ha -1 yr -1 . The highest value observed for cover crop was 3.67 t CO 2 -eq. ha -1 yr -1 but it specifically refers to cover crops included between the wide rows of Mediterranean woody crops (olive, almond and vineyards) where the soil would otherwise be bare 157 . This is clearly an important management change within this environment but represents a much greater input of plant material than, say, cover crops grown during winter within temperate arable cropping systems. For both agroforestry and cover crops, and even ignoring the two sets of data for SOC increases that are especially large, it appears from the summarized data in Fig. 2 that SOC increases resulting from these two changes in management considerably outweigh increased N 2 O emissions when both are expressed on a CO 2 -eq basis and these management changes can be expected to beneficial for climate change mitigation. For no-tillage the situation is different: the relatively small rates of SOC accumulation are approximately equal to the increases in N 2 O emission when both are expressed on a CO 2 -eq basis, so there appears to be no overall climate change benefit. For organic amendments the results indicate that N 2 O emissions are decreased and thus reinforce the SOC benefit, though this is based on a very limited amount of data and also, as discussed earlier, it is questionable whether SOC increases from addition of organic amendments can be fully regarded as climate change mitigation. It should be noted that most of the studies are performed over a few years and assessment of GHG balance in the long term, especially for N 2 O, are still missing [START_REF] Lugato | Mitigation potential of soil carbon management overestimated by neglecting N2O emissions[END_REF] . Some practices were too little documented or with not enough information to be compared with the others (e.g. , biochar or erosion control).

Overview of the current evidence

DISCUSSION AND OUTLOOK

Overall, the meta-analysis we gathered here (Fig. 2) suggest that, with the exception of reduced tillage practices, increased N 2 O emissions are not sufficient to invalidate the GHG abatement potential achieved by SOC sequestration strategies. Some sequestration strategies (e.g, biochar or non-pyrogenic organic amendment application) may even generate win-win situations through a decrease in N 2 O emissions, although the experimental evidences are still scant. In addition, the economic impacts and large-scale effects of the options examined here warrant further assessment. Some practices may affect crop yields or farmers' income, depending on pedo-climatic conditions and the details and practicalities of the cropping systems. For instance, conservation practices, and especially no-till may slightly decrease crop yields under temperate climates but be beneficial in drier conditions 158 . Similarly, the yield of arable crops is usually reduced in agroforestry systems in temperate regions 159 but in more arid climates, crops perform better 160 . Nevertheless, for agroforestry, trees produce timber, firewood, honey, fruits, etc. that are also a source of incomes for the farmers and may lead to greater overall sustainability. Beyond yield impacts, some socio-cultural or economic factors come into play that may hinder the adoption of carbon sequestration practices. For example, in the United States, the cost of carbon capture through Natural Resources Conservation Service programs is estimated at US $ 32-442 per tonne of CO 2 , with an average of US $ 183 161 . A carbon price much higher than the present value (around US $ 10 as a global average 162 ) would be necessary to promote carbon sequestration practices, as well as a regulation to direct the financial flow of industrial and energy emitters to the agricultural sector.

To be deployed at large scale and to enter emission trading systems, the GHG fluxes of each change in agricultural practices should be estimated precisely. Various models may be used to account for scale or leakage effects such as indirect land-use changes 163 . The methods currently available include data-driven approaches based on worldwide measurement networks 164 , statistical or empirical flux-upscaling models [START_REF] Wang | Data-driven estimates of global nitrous oxide emissions from croplands[END_REF]164 , process-based models and, lastly, integrated assessment models (IAM) 165 . This carbon mitigation option, if achievable in practice, would reduce the burden of climate stabilization for all sectors of the economy, including agriculture. In addition, SOC increases are often correlated with higher crop yields and contribute to a range of other environmental benefits and increased sustainability of agricultural systems. Practices designed to increase SOC can offer a win-win solution vis a vis food security, by mitigating food calorie losses resulting from the application of emission reduction targets (e.g. through decreased applications of mineral fertilizers) and reducing undernourishment.

Finally, many of the practices reviewed (Table 1) here may be combined on a given field: for example, no-till can be combined with cover crops, organic amendments, or agroforestry. Such combinations have been little tested in practice and in particular synergetic effects between them have not been evaluated in depth 170,171 .

Conversely, they may come with trade-off, antagonistic or synergistic effects regarding SOC storage rates, as well as N 2 O emissions or other impacts and these needs to be identified and quantified. Furthermore, proper assessment of carbon sequestration measures raises classical GHG accounting issues, such as double counting, improper setting of system boundaries and counterfactual scenarios 169 . Although further research is still needed to quantify the potential of SOC sequestration options on a local to regional basis, it appears that their potential to mitigate climate change, even when factoring in N 2 O emissions is still significant and that they deserve further consideration in climate stabilization scenarios. Including the state-of-the-art knowledge reviewed here on the effectiveness of such measures in land system or integrated assessment models could be a prime target to assess their impacts at global scale. In each of these processes, nitrous oxide (N 2 O) can be generated 175 . Net N 2 O emissions from the soil will usually be lower when the amount of soil mineral N is low and when soil pH is alkaline [START_REF] Hénault | Nitrous Oxide Emission by Agricultural Soils: A Review of Spatial and Temporal Variability for Mitigation[END_REF]175 but also when C and oxygen availability are reduced [START_REF] Butterbach-Bahl | Nitrous oxide emissions from soils : how well do we understand the processes and their controls ?[END_REF]175 . N can also be lost as

Fig. 2

 2 Fig. 2 summarises published data on rates of change in SOC and rates of emission of N 2 O resulting from four prominent sets of practices designed to increase SOC, namely agroforestry, cover crops, no-tillage and organic amendment. All fluxes are expressed in CO 2 equivalents, using a global warming potential value integrated over 100 years and including global warming potential (GWP) of 298 for N 2 O as recommend by last IPCC report 156 . GWP is the time-integrated radiative forcing induced by a pulse emission of a given component (here N 2 O), relative to a pulse emission of an equal mass of CO 2 156

  . Process-based models include a representation of N cycling processes, which are an essential tool in assessing and predicting the terrestrial N cycle and N 2 O fluxes in response to multi-factor global changes. Such models have been used to estimate N 2 O emissions from natural and agricultural soils at various scales, from field to global level via the integration of a prognostic N cycle into different land surface models 166 . As an example, Fig. 3 shows the results of simulations by various models at global scale. Most of N 2 O emissions from cropland are due to the use of mineral fertilizers (Fig. 3a) and are mainly located in USA, Europe, India and China. They may be used to quantify carbon sequestration in soil minus the N 2 O emission trade-off at global scale, based on ensemble runs as was initiated in the global N 2 O Model Inter-Comparison Project 167 .IAMs focus on the interactions between the economic activities and earth system responses and are vital for estimating what socioeconomic changes would be needed to reduce GHG emissions across sectors and increase biospheric C sinks 168 . Until recently, most IAMs did not explicitly take into account SOC restoration practices169 . A recent study that did include them found that soils could be a sink of 3.5 GtCO 2 -eq/yr by 2050 under a carbon price of 190 USD/tCO 2 13

ACKNOWLEDGEMENTS:

  This paper stemmed from a workshop "Emerging challenges in large scale soil carbon sequestration" held in Paris on 8-10 October 2018. The workshop was financially supported by the French government under the ANR "Investissements d'avenir" program with the reference CLAND ANR-16-CONV-0003. F.Z. acknowledges support from the National Natural Science Foundation of China (grant no. 41671464). D.A. is coordinator of the research consortium GLADSOILMAP supported by LE STUDIUM Loire Valley Institute for Advanced Research Studies. The authors acknowledge John Bazire for all the help to organize the workshop and Chris Van Kessel for data sharing. AUTHOR CONTRIBUTIONS: BGu, BGa and CC organized the workshop. All the authors participated to the workshop and discussed the structure of the manuscript during the workshop. All the authors wrote a section of the manuscript based on their main research field. BGu and BGa gathered all the written contributions and harmonized the text. All the authors reviewed and edited the manuscript before submission. COMPETING INTERESTS: The authors declare no competing interest

Figure 1 :

 1 Figure 1: A schematic representation of C-N interactions in the terrestrial ecosystem. Note that biological nitrogen fixation and denitrification are process performed by microorganisms that also need C as substrate.

Figure 2 :

 2 Figure 2: Estimation of the SOC storage and N 2 O emissions of land-based mitigation options expressed in CO 2 equivalents. Negative values indicate a net reduction in GHG emissions in terms of CO 2 equivalents, while positive values show a net increase of CO 2 equivalent emissions. All values refer to the difference between the land-based mitigation option in question and a "control" land (e.g. no-tillage vs conventional tillage). For agroforestry, the control land is cropland and different types of agroforestry systems were considered. NB: In Kim et al. (2016) the majority of soil C storage data comes from intercropping, improved fallows and rotational woodlots, which are systems with high tree density. This could partially explain the very high estimation of soil C storage found in Kim et al. (2016) compared to other papers. Organic amendments do not include biochar. The control used for comparison with organic amendments is an experiment managed with inorganic fertilizers. For cover crops meta-analysis, Vicente-Vicente et al. (2016) only consider Mediterranean woody crops (olive, almond and vineyards), which could also explain the large soil C rates estimated. Uncertainty is given as standard error (SE) for every paper. If it was provided as a confidence interval (CI) or standard deviation (SD) it has been adequately transformed to unify the units. (*Reviews; ** For these meta-analysis the values reported in the graph have been recalculated as the weighted mean across all experiments, from the database provided by the authors, because the values coming from the papers could not be used as they were reported as a percentage only).121,157,172,173

Figure 3 :[Box 1 :

 31 Figure 3: Spatial and latitudinal patterns of contributions of fertilizer (a) and manure (b) on cropland soil N 2 O emissions obtained during the global N 2 O Model Intercomparison Project 167 . Average over the 2006-2015 period.

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