Christophe Rizet
email: christophe.rizet@ifsttar.fr

Antoine Montenon

Reducing Freight Transport Pollution by using electric vehicles

Keywords: (5 words) LEZ, freight, electric, pollution, companies

To mitigate the pollution of road transport, several cities in Europe have implemented 'Low Emission Zones' (LEZ). In order to plan the future of such questions for the Paris area, and to assess their impacts on traffic and air pollution, LEZ scenarios have been defined for freight vehicles: a 'Business as Usual (BAU) scenario, a LEZ scenario, in which only the most recent vehicles are authorized, and two 'electric scenarios, in which only electric vehicles can enter the zone limited by the motorway A86, an area of 385 km² for about 4.8 million inhabitants. The difference between the two electric scenarios is related to the hypothesis on the evolution of the technologies: In the first scenario, Diesel trucks can be replaced by equivalent electric powered trucks of the same payload. In the second scenario, only the vehicles with a payload below 5 tons could be replaced by the electric equivalent. So, every vehicle with a load over 5 tons has to be replaced by several smaller vehicles in the LEZ area.

This communication follows the RETMIF project (Reduction of Emissions from Freight Transport in the Greater Paris Region). Its main objective is to understand the potential impacts of a LEZ in the

Paris metropolitan area, in particular for goods vehicles. In conjunction with this development of LEZ, there has been a huge evolution in the motor technologies for commercial diesel vehicles. The industrial companies have also began to develop new hybrid and electric technologies, in anticipation of the scarcity of fossil oil and the future potential regulations only enabling non-polluting vehicles.

Introduction

Air quality is a very important topic for cities. Proportion of population exposed to air quality problems is high: in 2012, 36% of European urban population is concerned by a day limit on air concentration of PM10 (EEA, 2013). The city of Paris follows this assessment. In 2013, more than 9 on 10 inhabitants of the city were concerned by an overexposure to NO2 and PM10 in the air. Peaks of pollution levels in 2014 and 2015 reopened the debate on that topic into media and politics.

Above the punctual regulations, the city of Paris actually wants to lower the pollutant emissions, and become a model to follow in Europe. Some initiatives have been taken: reduction of parking places, peak-hour restrictions, working groups on "100% non-Diesel deliveries", prefiguration of a Low Emission Zone, etc.

Low Emission Zones (LEZ) is one of the solutions proposed by European Commission to lower the air pollution. Some have already been implemented in Europe, but their effect on the city economy, on workers, and on the environment is poorly known. The objective of the RETMIF project, funded by ADEME (French Agency on Energy and Environment) is to better know the effects of this regulation on companies and environment. Phase 1 of the project aimed to specify the different characteristics of Low Emission Zones in Europe, interviews with professionals throughout Europe, especially in London, Berlin, Göteborg and Paris, have been lead during the phase 2. This paper follows the phase 3 of RETMIF. The objective is to know what effect a Low Emission Zone would have on goods transport in the Paris Region. It is structured as follows. Section 1 explains the impacts of European, national and local environmental programs, especially Low Emission Zones, with a focus on the reactions of companies concerning environment, as well as the difficulties that could be created by these regulations. Section 2 introduces the methodology of the quantitative part of the study, by defining 4 scenarios of transport regulations for Paris region from now to 2025: 1 "Business as Usual scenario", 1 Low Emission Zone scenario, and 2 scenarios 100% electric: one with goods vehicles replaced by electric vehicles with the same capacity, one with a maximum payload of 5 tones for goods vehicles (gross weight of 12 tones).

Politics for Environment and Transport

In 2002, European Commission implemented the sixth Environmental Action Program which set out to achieve a high level protection of human health. Different environmental goals have been defined to be achieved in 2020 or 2050. This program has been reinforced by the seventh Environmental Action Program which aims to define new higher standards for air quality. Current European goals in terms of pollution follow the Directive 2008/50/EC. This directive is based on Euro standards for vehicles. These standards, defined by a number (roman for HGV, Arabic for other vehicles), correspond to a limit of pollution mass per energy unit developed depending on the type of vehicle, the type of engine, the year of commercialization, and the type of pollutant. This limit is calculated through a driving-cycle standard (NEDC).

Member countries of the European Union and the European Environment Agency have to apply these programs by implementing national action programs in order to satisfy the pollution criteria. In France, two roadmaps for Environment have been set out during the past ten years : "the Grenelle Environnement (1 et 2)" and the "Loi pour la transition énergétique". These two roadmaps consider the Environment as a global initiative, which has to be interpreted in the different faces of the national economy : energy, housing, transport, etc.

The goals of the Directive 2008/50/EC are not being reached in Europe, especially in urban areas. The European Commission launched in 2014 legal proceedings against countries in order to enhance the respect of the Directive.

Road transport is one of the main sources of pollutants, especially Nitrogen Oxides (NOx) and Particulate Matters (PM). In 2011, road transport produced 47% of all NOx and 14% of all PM10 (EEA, 2013). Diesel engines particularly are concerned by these pollutions, by being 4 to 100 times more polluting for NOx than gasoline enginesequivalent power and weight. Urban goods transport is a great contributor of pollution and greenhouse gases. In London, in 2010, freight transport was responsible for 24% of the CO2 emitted by road transport, 38% for the NOx and 37% for the PM10 according to [START_REF] Allen | London Freight Data report[END_REF] and In French cities, this type of transport represent 35% of the NOx, and about 50% of the PM10 emitted by road transport, according to [START_REF] Gonzales-Feliu | End consumer goods movement generation in french medium urban areas[END_REF].

In parallel, the most recent white paper on transport for European Union stresses the need to develop new sustainable transport systems, which consumes less energy, especially in urban areas. In France, the "Loi pour la Transition Energétique" has the ambition to reach a relative energetic independence in the country. That means to use less petrol and more biogas and electrical power (France has the capacity of producing more electricity than needed). It therefore enhances the use of electrical power within all the faces of economy, especially transport. At present, two types of electrical engines are promoted by the Ministry of Sustainable Development: direct electric engines using batteries and fuel cell engines powered by hydrogen, which creates the needs of new production and value chains in the energy economy, but which is much less restricting for vehicles by reducing the size and the weight of batteries.

One of the reasons of the growth of pollution in urban areas is the growth of mobility, and the use of older vehicles for doing short routes. This assessment has been verified for big cities like Paris through interviews during the RETMIF project.

A tool which is enhanced by the European Commission is Low Emission Zones (LEZ). These are "geographically limited zones that cover more than a very local area where there are problems from the point of view of atmospheric pollution, noise, the quality of urban life, traffic congestion and/or safety and in which specific restrictions are applied with regard to the volume and/or the nature of traffic in the zone" [START_REF] Ue | Report from the Working Group on Environmental Zones-Exploring the Issue of Environmentally-Related Road Traffic Restrictions[END_REF].

The types of vehicles and associated Euro standards concerned by the LEZ are defined by the national, regional and local authorities. In 2015, there are about 210 LEZ in Europe, in 11 countries. Half of them are combined with a toll. The areas covered by these zones vary between 0.6 km² in Lisbon and 5700 km² in Styria (Austria). 99% of them concern HGVs, and about 75% concern personal cars and LCVs.

The method of enforcement widely influences the efficiency of the LEZ. In Europe, the two main methods of enforcement are the automatic enforcement by cameras reading the license plate on the vehicles, and the manual enforcement by municipal or national agents. In Amsterdam, the compliance rate rose from 66% in 2008 to 97% in 2010, while cameras have been implemented in 2009 (Broaddus et al., 2012).

Low Emission Zones are environmental policies, which has to be implemented and applied by the transport departments in the municipalities. That creates some troubles around the measure.

In fact, goods transport is often "forgotten" by the authorities [START_REF] Dablanc | Une grande oubliée, la logistique[END_REF], and is seen as the biggest pollution (air and noise) problem. But, reducing pollution by banishing different types of HGVs and LCVs creates number of evolutions in this economic sector, especially when speaking about investments for alternative fuel engines.

Low Emission Zones and Road Freight Transport.

This part is directly following two previous publications [START_REF] Rizet | The Constraints of Vehicle Range and Congestion for the Use of Electric Vehicles for Urban Freight in France, 9th Conference on City Logistics[END_REF][START_REF] Dablanc | Impacts of Environmental Access Restrictions on Freight Delivery Activities -Example of Low Emissions Zones in Europe[END_REF] and the qualitative part of the RETMIF project.

The part 2 of the RETMIF project aims at interviewing goods transport professionals (companies, federations, associations, researchers, public authorities) in four cities : Paris, which hadn't any LEZ at this time, London, Berlin and Göteborg (+ Stockholm), which have implemented their LEZ few years before. These interviews were separated in several parts: presentation of the company/institution, what they know and think about the environmental policies in their city/country, how they implement the notion of sustainable development in their institution, how they welcome the LEZ, what has changed several years after.

Most of goods transport companies and companies which use own-account transport can be divided in three types, which we won't define precisely because of the fuzziness of the limits between them:

-Larger companies. They have enough funds for renewing their fleet quickly, by modifying their business models or not. In these companies, HGVs are renewed every 5 or 6 years, LCVs between every 6 months and every 3 years. Very few LEZ concern vehicles which are less than 6 years. Therefore, the measure doesn't concern them at all, except for special vehicles, like waste collection vehicles. These companies have to ask an exemption pass, of redefine the routes of the vehicles for the time just after the implementation of the LEZ. The older vehicles drive outside the LEZ and the newer ones inside the LEZ.

-SMEs which depend on larger ones. Most of the time, the larger companies ask their subcontractor to have new vehicles in order to get a greener image. So, the LEZ doesn't have any influence on them. We have the same thing for SMEs for which environment is a great part of their business model and competitiveness. But, LEZ can be a little loss of their competitiveness in comparison of other SMEs.

-The smaller companies have big troubles for complying with the LEZ. Some decide to merge with others or to use vehicle location for long time, but a lot have to shut up because the investment can be too high, or the subsidies provided by public authorities are too low. This case is very common in bigger cities because the vehicles cover less kilometers and therefore can be used longer.

Speaking about the link between environment and transport, especially goods transport, can have a different meaning depending on the location and the policies. When German, Swedish or Italian transport companies speak about environment, they speak more about ISO 14001 or gas and hybrid vehicles. In France and in London, most of the companies say: "we actually try to be aware of the new electric technologies for vehicles, but actually we don't have a right business model for using it". From a first point of view, big cities like Paris of London are particularly adapted for using electric vehicles: smaller trucks, less need of range, lots of stops in a small area, low speed, etc.

Environment concerns everybody, adapting it in a business way can be hard. Therefore, almost everybody feel reticent for using electric vehicles for a lot of reasons:

-A lack of supply for vehicles. Vehicle manufacturers have the financial capacity for conceiving and market electrical trucks, but because of the price and of the small market, public authorities have to be very clear about what the future program on transport for cities and the regulations that will be defined. The regulations would help and constraint users for using electric vehicles. The business model for manufacturers is already too tight for evolving that way, in comparison to hybrid or gas vehicles.

-The supply of energy and fuel. There are quite few gas and electricity supply stations in France. And, refueling trucks during the night would be complicated; today, a lot of drivers take their trucks at home, in order to avoid the travel time between their home and the warehouse in the morning before doing their deliveries.

-The very low payload, compared to the GVW. The battery needed for bigger urban trucks is much too heavy, so there are no such trucks in the market. There are no such hybrid vehicles for the same reason. A change of regulations would force companies to use a number of smaller vehicles to replace one bigger. The price of the vehicles would not be a problem (with public funds, the price could be cheaper), but every vehicle would need a driver, which costs much more that the vehicles.

-But, thanks to the news developments works on electric vehicles, especially new fuel cells systems which are tanked by hydrogen composites, the battery and size problems about electric vehicles would be partially cleared.

Methodology

The scenarios -The Business As Usual (BAU) scenario -The Low Emission Zone (LEZ) scenario -The 2 'electric' scenarios These two scenarios "100% electric" scenarios aim at quantifying what could be the evolution of emissions in a hypothetical situation where all freight vehicles would be electric. These two scenarios are the following:

In the first electrical scenario, we assume that any freight vehicle -regardless of its total maximum weight -is replaced by an electric vehicle of the same capacity. In this scenario, we assume that in 2025, the commercial vehicle manufacturers will manufacture electric vehicles with a payload up to 25 tons. Thus, the utility park used is identical to that found in the BAU scenario for capacity, but all vehicles run on electricity.

In the second electrical scenario, the maximum payload of an electric vehicle is assumed to be 5 tons, with a maximum gross weight of 12 tones. All vehicles with a payload higher than 5 tones are replaced by smaller electric vehicles, while maintaining constant total payload to keep a constant volume of potentially cargo. For simplicity, we consider two types of electric trucks: GVW vehicles included in 7.5 and 12 tones (maximum payload of 5 tons) and vehicle gross weight less than 7.5 tons (maximum payload of 2 tones). Eg a 11 tons of payload diesel truck, will be replaced in the second scenario by two electric L 5 tons of payload trucks and a two tones of payload truck. In contrast, a truck 14 tons of payload will be replaced in the second electric scenario by three trucks with a payload of 5 tons to minimize the increase in traffic : More electric commercial vehicles than today will be needed to transport the same load. We will estimate the increase in traffic

In each scenario, we will focus on CO2 and PM emissions, the only pollutants generated by electric vehicles. We study the same road sections and therefore the same traffic in each scenario.

In order to calculate road transport pollutant emissions in Paris region, and their evolution according to the different scenarios, the software CopCETE v4 has been used. CopCETE v4 has been developed by the CEREMA, and uses a Copert v4 basis with French data and adapted for a use with a Microsoft Excel interface.

CopCETE, as Copert, uses the following characteristics of vehicles:

-Personal cars, Light Commercial Vehicles, Rigid Trucks, Articulated Trucks and Buses -Different types of fuel: gasoline, Diesel, electric, hybrid, LPG -Different sizes of engines -Euro standards from Euro 0 to Euro 6. 30 types of emissions can be calculated in CopCETE but we will calculate only 3 of them:

-Nitrogen oxides (NOx). In 2011, about 61% of the NOx emissions in France come from road transport (CITEPA, 2014) -Particulate matter (PM10). In 2011, about 15% of the PM10 emissions in France come from road transport -Carbone dioxide (CO2). In 2011, 33% of the CO2 emissions in France come from road transport.

CopCETE also distinguish hot engine and cold engine tail pipe emissions, and non-tail pipe emissions (PM, HAP, some metals) for brake and tires use.

Fleet evolution

To simulate the fleet evolution, we computed from statistical data on fleet, for each type of vehicles and for each year, the rate of vehicles still working for a given generation. On the other hand, we estimated the number of new vehicles entering the fleet per vehicle type, from what has been observed in other LEZ and from our scenarios. From the computed rate of survival, we estimate a theoretical law, in order to smooth the irregularities and to simulate the future evolution of the fleet, using the parameters of the statistical law. As suggested by Koli (2012), we used a Weibull statistical law, in which the rate of survival is : S(t)= e^(-〖(t/β)〗^α ) α determines how 'incurved' the function is and β is a parameter of temporal scale, which influences the vehicle median duration of life. Given the available statistics, we assumed α to be independent of the type of vehicle and we used the α estimated by [START_REF] Bourdeau | Evolution du parc automobile français entre 1970 et 2020[END_REF], which means that the speed of wear per type of vehicle does not varies along the years. The matching between the data observed and the Weibull law is then done by optimizing the parameter β for two types of vehicles: 'road tractors' and 'trucks and vans'.

We are therefore interested in the categories of commercial vehicles, for which data of registrations are available far enough in the past. We then compare the results of optimal beta in the case of tractors and the trucks and vans, and then trace the evolution of optimal beta coefficient obtained between 1993 and 2014. In the case of vans (Figure 1), beta varies in steps: around 8 between 1993 and 1998 and between 10 and 12 until 2010 when it stabilizes at 14.9. These different levels can have various explanations, including buyer behavior can be encouraged by policy measures favoring renewal. But the landing between 2009 and 2010 was most likely a change in the way of counting vehicles: before 2009 were accounted for all vehicles less than twenty years after that date then only vehicles less than fifteen years old are counted.

Figure 2 : Determination of beta for trucks

The variations of beta are much more pronounced in the case of heavy goods vehicles (Figure 2). Beta increases or decreases over long periods. As in the previous chart, we see the peak in 2009 probably due to the change in the way of counting. The beta value is between 15 and 17. Finally, to define the forecast changes, we retain the alpha values of Bourdeau, and we hold the average value of beta from 2009 as shown in the table below. Implementation for fleets of the studied areas:

We evaluated the fleets circulating in four distinct areas: inner Paris, the first ring road (boulevard périphérique), the intra-A86 area and the A86 (second ring road). For each area, the categories of vehicles have been adapted to the structure of CopCETE software. Not having enough accurate data to estimate the parameters for each vehicle types, we assume that all types of vehicles of the same category (i.e. personal cars, vans, rigid trucks or trailer) follow the same law and, within the class, the types of vehicles are only differentiated by the data of the initial year.

In addition, as observed in the cases of Berlin and London during the RETMIF project, we assumed that the total traffic remains constant per vehicle subcategory, that is to say that any abandoned vehicle is replaced by a newer model of the same vehicle. This assumption allows analyzing the impact of fleet renewal, regardless of the trends between different subcategories.

Moreover, given that the share of electric vehicles is extremely low, and no regulatory or strong political will exists today on their development, we keep constant the percentage in both scenarios "over water" and " Reduced Emissions Zone. » The "electric" scenario will assess the possible impact of a change of clean vehicles.

The fleets in the 'Business as Usual' scenario Our approach is therefore to estimate the fleet in each of the different area, implementing our statistical laws of survival defined above. Specifically, for a given vehicle type (eg diesel PL standard Euro 4), we assume that it is as likely to have been made in each year the standard was the latest in the market (Tables 2 and3), and then we apply the survival coefficient corresponding to the years ahead.

For example, PL for Euro 4 diesel in 2014, about a third was registered in 2006 and the same for 2007 and 2008. For Euro 6, the number of vehicles also split in the different years, according to the previous assumption of replacement of discarded vehicles. This enabled to estimate the fleets in the "Business As Usual" scenario on each area for the years following the counting. These BAU fleets, compared to the "LEZ" scenario fleets, show the impact of a LEZ on the fleet. To determine the LEZ fleets scenario, we rely on rates of compliance of the LEZ observed in Berlin and London, taking into account the specificities of each of the capitals (eg Parisian fleet is initially oldest) and the type of control implemented. This gives the following table of compliance: These rates have been obtained by modifying the beta coefficient of the Weibull distribution from the year of announcement of the new regulation, which corresponds to the acceleration of the fleet renewal observed in all low emission zones. Beta determines the median age of survival and its decrease leads to a lower lifetime for old vehicles. Figure 3 shows an example of a van registered in 2008). So we get ultimately estimates of the structure of the fleet (percentage of each traffic) for each year, for each zone and for scenarios BAU and LEZ.

Without a LEZ, 'natural' renewal of the old fleet with new vehicles in the latest Euro standard is gradual; the announcement and the implementation of a LEZ accelerates this renewal. Table 6 take the example of the evolution of trucks in Paris and allows us to further analyze the impact of the LEZ. La part des poids lourds les plus anciens (<= Euro II) est faible en 2021 dans le scénario au fil de l'eau et devient nulle ou négligeable dans notre scénario ZER ; l'impact de la mise en place d'une ZER sur ce parc ancien est donc faible (2,5%). En revanche, ces vieux véhicules ayant des émissions très importantes par kilomètre, l'impact sur les émissions est beaucoup plus important.

Pour les poids lourds plus récents (Euro III à V) qui représente 76 % du parc en 2014, l'impact de la ZER est beaucoup plus sensible sur l'évolution du parc : dans Paris la part de ces PL passe de 76% en 2014 à 7% dans le scénario ZER 2021 (-69%) contre 52% en 2021 dans le scénario fil de l'eau (-24%). La tendance est la même pour les VUL dans Paris.

Pour les zones géographiques hors Paris, le fait que les véhicules Euro 5 sont en service depuis 2009 pour les PL et 2011 pour les VUL implique que le scénario ZER 2021 n'aura que peu d'influence sur le renouvellement effectif du parc. Le taux de renouvellement naturel (au fil de l'eau) des PL et VUL est suffisamment rapide pour que la ZER ne provoque pas de bouleversement important. Dans Paris, l'impact est plus fort car la ZER implique de l'Euro 6 donc des véhicules 4 à 5 ans plus récents (Erreur ! Source du renvoi introuvable. et Erreur ! Source du renvoi introuvable.).

La ZER va augmenter la proportion (et donc le nombre) de véhicules utilitaires 'propres' de façon significative. Cette modernisation accélérée du parc a été déjà observée dans les autres pays (Londres et Berlin).

The share of the oldest trucks (<= Euro II) is low in 2021 in the BAU scenario r and becomes nil or negligible in our LEZ scenario; the impact of the implementation of the LEZ on this old stock is low (2.5%). However, for these older vehicles with very high emissions per kilometer, the impact on emissions is much more important than the impact on fleet.

For newer trucks (Euro III to V) representing 76% of the park in 2014, the impact of the LEZ is much more sensitive on the evolution of the park: in Paris the share of these trucks from 76% in 2014, decreases to 7% in the 2021 LEZ scenario (minus 69%) against 52% in 2021 in the BAU scenario (-24%). The trend is similar for vans in Paris.

For locations outside Paris, the fact that the Euro 5 trucks are in operation since 2009 (2011 for vans) means that the LEZ 2021 scenario will have little effect on the actual fleet renewal. The natural turnover (BAU) for trucks is fast enough for the LEZ does not cause major upheaval. In Paris, the impact is stronger because the LEZ involves Euro 6 vehicles, therefore 4-5 years more recent.

LEZ will increase significantly the proportion and therefore the number of clean freight vehicles. This accelerated modernization of the park following the implementation of a LEZ has already been observed in London and Berlin.

Computing emissions for electric vehicles

Electric vehicles do not emit CO2 directly during a journey, but we take into account the CO2 emissions upstream from the production of electricity used to operate the vehicle. For passenger cars (PC), the estimated average consumption (ADEME, 2014) per 100 km is 20 kWh. We will assume that all cars have the same consumption. Knowing that the French electricity emission factor is 53 gCO2 / kWh (MEDDE, 2012), a private car therefore emits around 10.6 gCO2 / km. For goods vehicles, emissions are calculated according to the power consumption, estimated by the following formula (Rizet et al, 2014b):

Electricity consumption per km (kWh / km) = 0.43 + 0.033 x GVW (tones) This formula was found from consumption data of different types of electric trucks literature on the assumption that the relationship between power consumption and the GVW is linear. For light commercial vehicles (vans), the average gross weight is taken equal to 3 tons. In the same way as cars, we will use the emission factor of 53 g CO2 / kWh for the CO2 emissions for each type of commercial vehicles (Heavy Commercial Vehicles).

From the number of trucks and vans and composition of fleet in the four zones in 2025, we were finally able to calculate, for each zone, the CO2 emissions per linear kilometer of road. In the different scenarios -Calculation method of PM emissions Electric vehicles do not emit directly particulate matters out of their tailpipe like other ICE vehicles, but they produce emissions of particles, by resuspension as well as abrasion (tires, brakes and road surface).

Vehicle type Trucks Vans Cars

Non tail pipe PM emission (g/km) 0,5732 0,05 0,0444

The values shown in Table 7 are the unit emissions per vehicle class data CopCETE. By summing the products of unit emissions by the number of vehicles, we finally get the PM emissions generated by the traffic of electric vehicles.

Results: vehicles and emissions

Local pollution of freight vehicles: the situation in 2011

Within the RETMIF project, the city of Paris sent us the data of the survey 'number plates' realized in 2011 to know various vehicle types circulating to Paris. These data were used with the software COPCETE V4 which calculates emissions by vehicle type, using the equations of COPERT 4. We present here the results of this analysis, focusing on the share of freight vehicles in the traffic and in its emissions.

In this plates survey, vehicle registration numbers are raised at various points, allowing, with the file of French vehicles registrations, to know their type, their energy, their year of first registration and pollution standard (Euro norm). The survey was conducted in week, 22 to 23 and June 28, 2011 from 8am to 10am and from 14h to 16h. The recorded sample of registrations recovered with a comprehensive traffic count.

We used these data for three types of roads: Motorways arriving in Paris, the ring road around central Paris (périphérique) and inside Paris. In this latter category, the survey points are mostly located on relatively large boulevards. The result of a road type is obtained by the weighted average traffic results of each point of inquiry of this type of road. For example Table 8 for motorway is the average of results from 3 survey points, one on the motorway A1, one on the motorway A4 and the third on the motorway A13, on their arrival in the capital. Emissions and traffic tables below are for 1 km of road, for 4 hours x 3 days of the week. Unfortunately we had not sufficient information to make an estimate of the overall emissions over the city for a year or even over a year at a given point, or even over 24 hours in a given point.

Importance of freight vehicles in the road traffic emissions in 2011: results by type of road:

In Table 1, freight vehicles account in 2011 for 23% of the number of registered vehicles (vans 17% HGV 6%), 53% of CO2 emissions (23 and 30%), 68% of NOx (21 and 47%) and 54% of PM (15 and 39%). On the ring road (Table 2), freight vehicles account for 28% of traffic (19 and 8% respectively for vans and trucks), 63% of CO2 emissions (25 and 38), 77% NOx (19 and 58) and 64% PM (15 and 49). Compared to motorways, the volume of traffic is close, the share of commercial vehicles is somewhat higher (28% against 23%) and the share of emissions from these freight vehicles is more important: 63% against 53% for CO2, 77% against 68% for NOx and 64% against 54% for PM. Inside Paris (Table 3), the traffic is on average much lower than on motorways or on the ring road (about 6 times less) and freight vehicles account for 24% of traffic but with less truck (2%). The share of emissions from freight vehicles is also lower: 48% for CO2 (11% for PL), 61% of NOx (25%) and 46% PM (18%). For the 3 types of roads where traffic is known, freight vehicles (vans and trucks) are much more important for their emissions than for their traffic. They represent overall around a quarter of road traffic (23, 28 and 24% of vehicles km), half (53, 63 and 48%) of CO2 emissions, two thirds of NOx (68, 77 and 61%), and half of the particles (54, 64 and 46%). This share of freight vehicles in the total road emissions is higher than previous estimate for NOx (35%, according to Gonzales-Feliu 2010) and close for PM. So specific regulations on emissions of commercial vehicles could have a significant impact on the total emissions in the capital. We also noted that commercial vehicles have a greater share in traffic on motorways and ring road than within Paris. For CO2 emissions there is not much difference between BAU and LEZ scenarios: The LEZ has very little influence on CO2 emissions since CO2 is not included in the emission Euro norms.

In electrical scenarios 1 and 2, the CO2 emissions have been considerably reduced in 2025 regardless of the geographical area. The reduction is less for electric scenario 2 because the number of vehicles increases. Indeed, the maximum threshold of 5 tones payload causes an increase in the number of vehicles and thus traffic. Compared with the BAU scenario, in the city of Paris CO2 emissions declines of 95% for the electric scenario 1 and 90% for the electricity scenario 2. These decreases are very important because the French electricity emission factor is low (53 g CO2 / kWh, ADEME, 2014), thanks to the high proportion of nuclear generation. These scenarios "100% electric" are extremely effective in terms of reducing CO2 emissions, for the three areas concerned, from the current French context. The energy situation may however change in the coming years. On PM emissions (Figure 5), the results are surprising at first. Indeed, we observe a very small decrease in emissions for the electricity scenario 1 compared to BAU and a real increase in emissions for electric Scenario 2. These phenomena are explained by the PM emission processes. The emissions in electric scenario 1, compared with the BAU scenario, decreased by 14.2%, respectively, 7.6% and 7.0% for Paris, the ring road and intra A86. These decreases are low because non tailpipe emissions (abrasion and re suspension) predominate for PM. For example, for a Euro 5 truck of less than 7.5 tones gross weight, non tailpipe PM emissions represent 96.6% of total PM. The phenomena of abrasion and resuspension also explain the significant increase in PM emissions for the electric scenario 2. Indeed, the traffic of trucks and vans increased by 293% 363% and 278% respectively, 2% for Paris, the ring-road and the intra A86. Thus the increase in emissions of the most important PM, 148.9% was observed for the ring-road, which is experiencing the greatest increase in traffic.

These results allow us to step back on the benefits of an electric scenario. But we must remember that the PM and indirect (upstream) emissions of CO2 are almost the only emissions of electric vehicles. For example, electric trucks don't emit NOx nor PAH.

Urban trip distances and electric vehicles range

The electric trucks range is an important constraint for the electrification of urban freight1 : we analyzed the distribution of freight traffic according to the distance of the legs in the electrical scenarios. The graph below (Fig. 6) shows the distribution of the number of urban legs according to their distance and thus the proportion of these legs which could be electrified in the electric scenario, according to the maximal distance considered for electric trucks. Nearly 90% of the legs are less than 50 km long. The figure below also shows the distribution of the urban CO2 emissions according to this distance. The percentage of trip legs can be read on the left axis and the corresponding CO2 emission level on the right axis: 87.6% of trip legs are less than 50 km and they emit 86% of the whole CO2 of the urban freight in the data base. At the opposite, legs beyond 100 km only represent 0.45% of the legs. The range currently announced for electric vans (around 100 km) would thus allow covering approximately 90% of the return electric routes of a French urban scenario. For the remaining 10%, it would be advisable either to improve electric trucks range (up to approximately 140 km), or to find a way to recharge battery before ending the round trip, or still to optimize the location of platforms by agreeing to reduce the zones of limitation of the Diesel traffic of freight below urban areas.

Electric vehicle capacity and the congestion in electric scenario 2

In electric scenario 2, a diesel truck with a payload of 11 tones will for example be replaced by two trucks of 5 tons payload and one 2 tones payload. In contrast, a 14 tons payload truck will be replaced in this second electric scenario by three 5 tons payload trucks, to minimize the increase in traffic.

More electric freight vehicles will be needed to transport the same quantity of goods than today with large diesel trucks. The increase in traffic in electric scenario 2 compared with BAU, expressed in truck miles, is presented by region in

Conclusion

The implementation of a LEZ will force road transport professionals to adapt their behavior to the new regulations. Impacts will depend on the size of actors: bigger ones will not be much concerned by the measure, thanks to rental services and adaptation of their business models, smaller ones will have more trouble, they have less long-term visions and their evolution will depend on the time of preparation and the helps (services or funding) provided by authorities.

But, in Paris, despite the great funding provided by the city of Paris and the national funding for buying an electric vehicle, the cost of such vehicles seems to be too high for a punctual use. Another difficulty will be that most of these companies have no time for doing complicated administrative tasks. The companies which count more than 10 employees won't have any funding from the municipality, so time for the evolution of the restriction has to be long enough for enabling these companies to comply.

Quantitatively speaking, the Low Emission Zone speeds up the modernization of the fleet. The impact the use of oldest vehicles will be low, but the resulting emissions will decrease much, due to the high emission factors. For newer vehicles, the LEZ would not have a great influence on their renewal, because the natural renewal of these utility vehicles is fast enough.

The impact on the fleet is higher in Paris than around the city, and reducing the flow of the utility vehicles is a priority because of the resultant polluting emissions. The highest impact would be for the NOx The most realistic electric scenario shows an increase of the congestion and of the nonexhaust emissions.

With these results, the industrialization of new fuel cell engines tanked by alternative fuels like hydrogen seems to be a good complement of the regulations, by reducing the cost of the vehicles (mostly due to the price of the battery). Therefore, such "electric policies" have to go with most globalized industrialization plans, developing technological and logistics solutions.

Figure 1 :

 1 Figure 1: Determination of beta for vans (<3.5 t.)

Figure 3 :

 3 Figure 3: changes in the law of survival: for a van registered in 2008

Freight

  an average emission value, weighted by the amount of traffic, is calculated from emissions on the different sections considered in the plate survey of the City of Paris in 2011. It is the same for the value of the emissions on the ring road (périphérique), where three sites are considered. For non-electric scenarios, BAU and LEZ, we used the CopCETE tool for computing CO2 emissions and PM. So we can make a comparison of emissions based on different scenarios. Figures 4 (for CO2) and Figure5(for PM) here under show the evolution of computed emissions for different scenarios in 2025 for the three areas: Paris, ring-road (périphérique) and intra A86 motorway.

Figure 4 :

 4 Figure 4 : CO2 emissions (tones / km of road) by scenario and zone in 2025

Figure 5 :

 5 Figure 5: Estimation of PM emissions (kg/km of road) by zone in 2025 for the different scenarios

Figure 6 :

 6 Figure 6: Trip legs number and CO2 distribution according to the trip leg distance.

Table 1 : The coefficients alpha and beta we used

 1

		Vans	Rigide trucks	Tractors & trailors
	Alpha	2,1	3,3	3,9
	Beta	14,9	15,17	15

Table 2 : Years of registration of each Euro standard for HDV

 2

	Conventionnel	1988 1989 1990 1991 1992
	Euro 1	1993 1994 1995
	Euro 2	1996 1997 1998 1999 2000
	Euro 3	2001 2002 2003 2004 2005
	Euro 4	2006 2007 2008
	Euro 5	2009 2010 2011 2012 2013
	Euro 6	2014 2015 …

Table 3 : Years of registration of each Euro standard for vans

 3

	Conventionnel	1990 1991 1992 1993 1994
	Euro 1	1995 1996 1997 1998
	Euro 2	1999 2000 2001
	Euro 3	2002 2003 2004 2005 2006
	Euro 4	2007 2008 2009 2010
	Euro 5	2011 2012 2013 2014
	Euro 6	2015 …

Table 4 : compliance rate of the LEZ per area and per year

 4

		2018 (manual control)	2021 (RFID control)
	Inside Paris	Limitation : E5	Limitation : E6
		VUL 70%, PL 65%	VUL 96%, PL 93%
	Ring road	Limitation : -	Limitation : E5
	(Périphérique)	E5: VUL 65%, PL 60%	E6 : VUL 90%, PL 85%
			E5 : VUL 95%, PL 90%
	Intra A86	Limitation : -	Limitation : E5
		E5: VUL 65%, PL 60%	E6 : VUL 90%, PL 82%
			E5 : VUL 95%, PL 90%
	A 86	Limitation : -	Limitation : -
		E5: VUL 65%, PL 60%	E6 : VUL 85%, PL 80%
			E5 : VUL 90%, PL 85%

Table 5 : The beta values used for the previous graph

 5

	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017
	14,9	14,9	14,9	14,9	14,9	14,9	14,9	14,9	13,6	12,3
	2018	2019	2020	2021	2022	2023	2024	2025	2026	2027
	11	9,7	8,4	7,1	7,1	7,1	7,1	7,1	7,1	7,1
	2028	2029	2030
	7,1	7,1	7,1

Table 6 : Evolution of the split of trucks in Paris according to their Euro norm

 6

	Euro standard	2014	BAU 2018	2021	2025	2014	LEZ 2018	2021	2025
	Conventional	1,5%	0,2%	0,04% 0,02%	1,5%	0,03%	0,00%	0,00%
	Euro I	1,2%	0,4%	0,1% 0,05%	1,2%	0,1%	0,00%	0,00%
	Euro II	11, 6%	5,5%	2,3%	1,1% 11,6%	3,0%	0,00%	0,00%
	Euro III	23,5% 16,6%	10,6%	5,1% 23,5%	13,2%	0,00%	0,00%
	Euro IV	23,4% 20,3%	16,1% 10,1% 23,4%	18,7%	0,2%	0,1%
	Euro V	29,5% 28,3%	25,7% 21,7% 29,5%	27,7%	6,8%	3,3%
	Euro VI or more	9,4% 28,7%	45,2% 61,9%	9,4%	37,3%	93,0%	96,6%

Table 7 : Non tail pipe PM emissions per vehicle type, in g/km.

 7

Table 8 : Motorways arriving in Paris -% of freight vehicles in the traffic and in the emissions

 8

	Motorways	Vehicles km (x1000)	tones CO2	Kg NOx	Kg PM
	Vans	5,08	0,95	3,85	0,42
	%	17%	23%	21%	15%
	Trucks	1,72	1,24	8,77	1,13
	%	6%	30%	47%	39%
	∑ freight vehicles	6,80	2,19	12,6	1,55
	%	23%	53%	68%	54%
	All vehicles	30,1	4,16	18,6	2,88

Table 9 : Ring road (périphérique) -% of freight vehicles in the traffic and in the emissions

 9

	Ring road	Vehicles km (x1000)	tones CO2	Kg NOx	kg PM
	Vans	6,09	1,43	5,65	0,55
	%	19%	25%	19%	15%
	Trucks	2,61	2,19	17,4	1,81
	%	8%	38%	58%	49%
	∑ freight vehicles	8,70	3,62	23,0	2,35
	%	28%	63%	77%	64%
	All vehicles	31,3	5,78	29,9	3,68

Table 10 : Inside Paris -% of freight vehicles in the traffic and in the emissions

 10

	In Paris	Vehicles km (x1000)	tones CO2	kg NOx	kg PM
	Vans	1,12	0,33	1,28	0,12
	%	22%	36%	36%	28%
	Trucks	0,10	0,10	0,88	0,076
	%	2%	11%	25%	18%
	∑ freight vehicles	1,22	0,44	2,15	0,19
	%	24%	48%	61%	46%
	All vehicles	5,12	0,92	3,51	0,43

Table 11 Tableau 11: Induced traffic in the electric Scenario 2 (% increase of freight vehicles) by Zone

 11

	Area	Paris	Ring-road	Intra A86
			(Périphérique)
	Increase in the number of	293 %	363 %	278 %
	freight vehicle

Cf. Rizet et al.

(City Logistics)


Manon C M Hess
email: hess@tourduvalat.org

Mélissa De Wilde

Nicole Yavercovski

Loic Willm

François Mesléard

Elise Buisson

Microwave soil heating reduces seedling emergence of a wide range of species including invasives

Keywords: eradication, germination, restoration prerequisite, seed bank reduction Implications

A microwave-based process to control invasive species (seeds and adult stages) in the field prior to plant community restoration is currently under development. We investigated the effect of four microwave treatment pairings of Power × Duration (2kW4min, 4kW2min, 2kW8min, and 4kW4min) on seedling emergence of (1) an abandoned field seed bank used as a model for various plant species and

(2) seeds of three species invasive in Europe: Datura stramonium (Jimson weed), Reynoutria × bohemica (Bohemian knotweed), and Solidago gigantea (Giant goldenrod). In addition, we assessed whether nontreated seeds of these three invasive species could germinate on microwave-treated soils.

As microwave soil heating usually leads to nonhomogeneous temperatures within a sample, we also assessed whether the position of seeds in the soil during microwave treatment (center vs. edges) impacted seedling emergence. Results show that the most intensive microwave treatments (2kW8min and 4kW4min), enabling the soil to reach 85∘C, are highly effective in inhibiting seedling emergence of invasive species and the seed bank. The 2kW8min treatment, combining lowest power and longest exposure, is the most effective, reducing seed bank seedling emergence by 98% compared to control.

The results also reveal a species-specific response to heat, and lower microwave effectiveness on seeds located at the soil surface, which may explain the incomplete effectiveness of intensive treatments. Our results also suggest that prior microwave soil treatment does not prevent a considerable proportion of newly arrived invasive species seeds from germinating, suggesting that this method should be followed by restoration of native plant cover.

Introduction

Several control methods are commonly used to reduce the dominance of invasive plant species, such as physical plant removal, herbicides use, or introduction of biological agents. However, large-scale interventions often complex and expensive [START_REF] Rejmânek | When is eradication of exotic pest plants a realistic goal? Pages[END_REF] and are rarely successful [START_REF] Kettenring | Lessons learned from invasive plant control experiments: a systematic review and meta-analysis[END_REF]. Eradications of invasions covering more than 1,000 ha are generally not feasible because of the high cost of the initial treatment coupled with the multiple repetitions [START_REF] Rejmânek | When is eradication of exotic pest plants a realistic goal? Pages[END_REF]. Eradication is only feasible if undertaken soon after the introduction of the invasive species [START_REF] Rejmânek | When is eradication of exotic pest plants a realistic goal? Pages[END_REF] and germination over time of invasive plants from a long-lasting seed bank remains a main factor explaining the failure of many restoration programs [START_REF] Berger | Ecological restoration and nonindigenous plant species: a review[END_REF][START_REF] Richardson | Seed banks of invasive Australian Acacia species in South Africa: role in invasiveness and options for management[END_REF]. Indeed, invasion strategies are often based on a combination of vegetative (i.e. rhizomes) and sexual (i.e. seeds) reproductive capacities [START_REF] Gioria | Soil seed banks in plant invasions: promoting species invasiveness and long-term impact on plant community dynamics[END_REF], leading to the establishment of a highly persistent seed bank [START_REF] Holm | The world's worst weeds[END_REF]. Thus, ensuring the elimination of propagules and the loss of seed viability is crucial to the successful eradication of many invasive species and to the effective restoration of native communities [START_REF] Myers | Eradication and pest management[END_REF][START_REF] Regan | Optimal eradication: when to stop looking for an invasive plant[END_REF]).

Biological control using natural enemies is an effective method for reducing seed production and accumulation in the soil, through damage to or destruction of buds, flowers, and young pods [START_REF] Morris | Plant pathogens and biological control of weeds in South Africa: a review of projects and progress during the last decade[END_REF][START_REF] Zimmermann | Biological control in the management of invasive alien plants in South Africa, and the role of the working for water programme: working for water[END_REF][START_REF] Adair | The biology of Dasineura dielsi Rübsaamen (Diptera: Cecidomyüdae) in relation to the biological control of Acacia cyclops (Mimosaceae) in South Africa[END_REF]). In the absence of natural enemies, alternative methods enabling the reduction of the soil seed bank of nondesirable species have been considered, mainly in agrosystems [START_REF] Kremer | Management of weed seed banks with microorganisms[END_REF][START_REF] Mirsky | Reducing the germinable weed seedbank with soil disturbance and cover crops[END_REF]. Hand-pulling before establishment of the seeds has been shown to be effective in depleting the seed bank of small patches of invasives along roadsides or hiking trails [START_REF] Concilio | Effectiveness and cost of downy brome (Bromus tectorum) control at high elevation[END_REF] or in isolated nature reserve [START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF], but requires annual treatments for at least 3 years and becomes impractical over large areas [START_REF] Concilio | Effectiveness and cost of downy brome (Bromus tectorum) control at high elevation[END_REF]. Soil solarization, which consists in heating the soil by mulching it with plastic sheeting to inhibit seed viability [START_REF] Cohen | Reducing persistent seedbanks of invasive plants by soil solarizationthe case of Acacia saligna[END_REF], has certain limitations, such as length of treatment (i.e. several months), sunlight dependency, and variable effectiveness [START_REF] Cohen | Soil solarization and weed management[END_REF]. Fire is another relatively cost-effective option for reducing seeds in leaf litter and upper seed banks in various ecosystems [START_REF] Holmes | Effects of different clearing treatments on the seed-bank dynamics of an invasive Australian shrub, Acacia cyclops, in the southwestern Cape, South Africa[END_REF][START_REF] Richardson | Seed banks of invasive Australian Acacia species in South Africa: role in invasiveness and options for management[END_REF].

Carefully designed prescribed burning combined with adequate restoration measures (e.g. grazing, seed sowing) have shown promising results for invasion control and other conservation goals (Valkô et al. 2014). Applying fire as a control method is, in some cases, less easily feasible in biomes such as dune systems or riparian zones [START_REF] Richardson | Seed banks of invasive Australian Acacia species in South Africa: role in invasiveness and options for management[END_REF]. Furthermore, using fire is controversial because intense uncontrolled fires can sterilize soil for several years [START_REF] Campbell | Development of integrated control strategies for wattle. II. Control of unwanted wattle[END_REF]).

Microwave treatments have been proposed as an alternative eradication method for invasive and pest species in soil, such as insects, nematodes, soil pathogens, fungi, and weed seeds [START_REF] Nelson | A review and assessment of microwave energy for soil treatment to control pests[END_REF].

Microwave radiation induces the dielectric heating of moist materials, making it possible to reach the soil temperatures required for seed viability loss (i.e. 60-90∘C; [START_REF] Barker | Inhibition of weed seed germination by microwaves[END_REF][START_REF] Thompson | The effect of temperature on viability of imbibed weed seeds[END_REF][START_REF] Sahin | Effects of microwaves on the germination of weed seeds[END_REF]. The use of such thermal methods is attracting interest mainly due to increased awareness of the environmental risks associated with chemical-based herbicides (i.e. growing list of herbicide-resistant weed biotypes; Heap 1997) and the need for a rapid, effective procedure free of nondesirable residues [START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF][START_REF] Sahin | Effects of microwaves on the germination of weed seeds[END_REF]. However, some studies question the effectiveness of soil microwave treatments, in particular due to the rapid attenuation of microwave radiation in moist soils [START_REF] Nelson | A review and assessment of microwave energy for soil treatment to control pests[END_REF][START_REF] Sartorato | Observations on the potential of microwaves for weed control[END_REF][START_REF] Ascard | Thermal weed control[END_REF]. Control of weed seed germination is seen as the most promising soil application of microwave technology [START_REF] Nelson | A review and assessment of microwave energy for soil treatment to control pests[END_REF].

The study reported herein was realized in the framework of a project aiming to develop a microwave-based method for use directly in the field. It is thus part of a set of studies undertaken to help determine the technical characteristics of a continuous conveying tunnel with microwave equipment, designed for the treatment of infested soil collected in situ. The field process involves extracting infested soil, passing it through a microwave oven on a conveyor belt, and putting it back in place. This process is designed to treat large areas infested with a wide range of invasive plant species (vegetative and reproductive parts, including seeds). Several studies have tested the effect of microwave radiation on germination. However, most of them were conducted more than 20 years ago with microwave ovens whose characteristics (e.g. frequency and power, volume treated) may well have compromised the effectiveness of the treatments (e.g. [START_REF] Diprose | The effect of externally applied electrostatic fields, microwave radiation and electric currents on plants and other organisms, with special reference to weed control[END_REF][START_REF] Nelson | A review and assessment of microwave energy for soil treatment to control pests[END_REF]. Furthermore, the studies subjected only a few species to the impact of microwave treatments on germination [START_REF] Bebawi | Effect of microwave radiation on seed mortality of rubber vine (Cryptostegia grandiflora R. Br.), parthenium (Parthenium hysterophorous L.) and bellyache bush (Jatrophagos sypiifolia L.)[END_REF][START_REF] Brodie | Microwave soil heating for controlling ryegrass seed germination[END_REF][START_REF] Sahin | Effects of microwaves on the germination of weed seeds[END_REF], rather than testing a diversity of species.

Among our set of studies, the first one was conducted to define the most effective microwave treatment pairing of Power x Duration to inhibit seedling emergence of three invasive species in Europe (Datura stramonium, Reynoutria × bohemica, and Solidago gigantea) depending on soil moisture (10, 13, 20%) and seed burial depth (2, 12 cm) (De [START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF]. Treatments requiring relatively high power and duration were the most effective (e.g. the 2kW8min and 4kW4min treatments), and their effectiveness decreased with increasing soil moisture. Emergence of seeds located at the 12 cm depth was occasionally more strongly affected. This experiment, focusing only on three species, was carried out in conditions far from the future field process (i.e. seeds not directly mixed with soil, seedling emergence in Petri dishes placed in a growth chamber).

As seed survival and germination responses to heat vary greatly between species or genotypes within species [START_REF] Melander | Soil steaming to reduce intrarow weed seedling emergence[END_REF][START_REF] Wahid | Heat tolerance in plants: an overview[END_REF][START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF], we aimed to evaluate the response of a higher diversity of species. Thus, we first assessed the capacity of four different microwave treatments previously tested by De [START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF] to destroy a seed bank of an abandoned organic cultivation field, chosen not for its invasion status (non-invaded) but because it contained a great diversity of species (Table S1, Supporting Information). Additionally, we introduced into the soil samples a specific number of seeds from three species invasive in Europe (D. stramonium, R. × bohemica, and S. gigantea), also used in a previous study (De [START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF]). This seed introduction was intended (1) to test the effect of microwaves directly on three important invasive plant species and (2) to evaluate the percentage of seedlings able to emerge after each treatment, when directly mixed with soil. The three species, noxious in some European countries [START_REF] Weber | Invasive plant species of the world: a reference guide to environmental weeds[END_REF]Weber & Jakobs 2004;[START_REF] Van Kleunen | Reproductive assurance through self-fertilization does not vary with population size in the alien invasive plant Datura stramonium[END_REF]), were chosen not only because they reproduce by seeds [START_REF] Saayman | Influence of intraspecific competition on seed production of Datura stramonium L[END_REF]Weber & Jakobs 2004;[START_REF] Gillies | Knotweed (Fallopia spp.) invasion of North America utilizes hybridization, epigenetics, seed dispersal (unexpectedly), and an arsenal of physiological tactics[END_REF] but also mostly because their seeds differ in shape and size.

Indeed, these characteristics are known as major determinants of seed sensitivity to microwave heating [START_REF] Bebawi | Effect of microwave radiation on seed mortality of rubber vine (Cryptostegia grandiflora R. Br.), parthenium (Parthenium hysterophorous L.) and bellyache bush (Jatrophagos sypiifolia L.)[END_REF][START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF].

Microwave treatments expose soils to high temperatures, with a predictable effect on soil-living organisms (especially microorganisms) and soil properties. This could disturb plant germination [START_REF] Certini | Effects of fire on properties of forest soils: a review[END_REF][START_REF] Brodie | Assessing the impact of microwave treatment on soil microbial populations[END_REF] and the recovery of fundamental ecosystem functionalities after treatment [START_REF] Herrick | Monitoring ecological processes for restoration projects[END_REF]. Through soil disturbance, soil microwave treatment could therefore prevent or limit invasive species reinvasion. To determine whether the response to microwave treatment in terms of seedling emergence is (1) only due to a direct effect of heat on seeds or (2) the result of this direct effect on seeds combined with soil heating disturbances to soil properties and biota, we further assessed the capacity of seeds of D. stramonium, R. × bohemica, and S. gigantea to germinate on soil previously subjected to microwave treatments. [START_REF] Brodie | Microwave heating in moist materials[END_REF] showed that microwave treatment commonly leads to the nonhomogeneous heating of exposed materials: temperatures at the edges of the exposed object are several degrees lower than in the center (output power 750W, after 150 seconds of heating). De [START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF] found seeds located at 12 cm depth more affected by microwave treatment than those at 2 cm depth. However, the question remains of whether seeds located at the surface (0-0.5 cm from the edges) of treated samples might be subjected to lower, nonlethal temperatures. The projected design for field use includes a conveyor belt carrying the extracted soil through a microwave oven. A relatively thin layer of soil containing seeds would thus be subjected to microwave radiations potentially resulting in heterogeneous temperatures (and potentially lower temperature close to the surface) that could seriously threaten complete eradication of invasive species in situ. We therefore also investigated whether the position (center vs. edge) of seeds in the soil sample during microwave treatment modified seedling emergence responses to microwave treatments.

Methods

Biological Materials

The experiments were performed on a seed bank contained in soil extracted in situ and also on seeds of three important plant species invasive in Europe (Datura stramonium, Reynoutria × bohemica, and Solidago gigantea), which are abundant seed-producers [START_REF] Weaver | The biology of Canadian weeds: 64. Datura stramonium L[END_REF]Weber & Jakobs 2004;[START_REF] Gillies | Knotweed (Fallopia spp.) invasion of North America utilizes hybridization, epigenetics, seed dispersal (unexpectedly), and an arsenal of physiological tactics[END_REF]). Information about D. stramonium, R. × bohemica, and S. gigantea ecology and invasion history is available in Appendix S1.

Soil Sampling

Topsoil, characterized as a silty loam (60% silt, 15% clay), was collected from uncultivated species-rich grassland that had developed on an abandoned organic hemp field (cultivation stopped in 2013) located near two rivers, La Cèze and La Tave (Laudun-L'Ardoise, France). The 25 fresh soil samples (135 cm± 15 length x 50 cm± 10 width x 10 cm depth) were mixed to ensure the homogeneity of soil and seed bank. Blended soil cleared of stones and organic residues was distributed in 1-kg batches just prior to microwave treatments. This soil was used for all the experiments.

Seed Sampling of the Three Invasive Species

Seeds of D. stramonium were gathered in October 2014 from stands growing along the Rhône River, Drôme (France).Achenes of R. × bohemica and S. gigantea were collected in March 2015 for the first experiment and in March 2016 for the second experiment along the Isère River, Savoie (France). The achenes and seeds were collected randomly from 10 ramets of the same population and then pooled. Then achenes and seeds were stored under dry conditions at room temperature until used for experiments. We assume that these storage conditions do not affect the achene and seed viability and germination capacity of these species [START_REF] Benvenuti | Light environment, phytochrome and germination of Datura stramonium L. seeds[END_REF][START_REF] Engler | Seed characteristics and germination limitations in the highly invasive Fallopia japonica sl (Polygonaceae)[END_REF][START_REF] Bochenek | Do the seeds of Solidago gigantea Aiton have physiological determinants of invasiveness?[END_REF]). The term "seed" will be used for both seeds and achenes.

Microwave System

An AMW200 batch microwave system (SAIREM SAS, Neyron, France) was used in this study [START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF]. The 304L microwave stainless steel chamber can contain a block of 600 x 400 x 250 mm maximum size and 30 kg maximum weight. The microwave system has a polyethylene sliding table with a usable surface of 840 x 620 mm and a pneumatically driven sliding door, both making block loading/unloading easier. The microwave system operates at 915 MHz, which allows greater penetration and treatment of thicker materials (up to 25 cm) than the frequently used 2,450 MHz microwave frequency (approximately a 3-fold increase). Coupling microwave heating from above and below and rotating the turntable enhance heating homogeneity. The output power of 10 kW maximum is produced by 2 x 5 kW generators (magnetron), adjustable from 1 to 10 kW. The system is watercooled.

Experimental Design

Effect of Microwave Treatments on Seedling Emergence From Soil Seed Bank and Sown Invasive

Species. We applied four microwave treatments using the following Power x Duration pairings: 2kW4min, 2kW8min, 4kW2min, and 4kW4min. Microwave treatments were performed on the premises of SAIREM (Neyron, France), each treatment being applied on 1-kg soil replicates placed in 16 cm high x 7.5 cm diameter plastic cylinders. For each treatment, we used eight replicates of soil containing (1) only the natural seed bank, (2) 10 seeds of each tested invasive species sown before treatment, or (3) 10 seeds of each tested invasive species sown after treatment. Controls without treatment were soil containing (1) only the natural seed bank and (2) 10 seeds of each added invasive species. Soil temperature was measured immediately after microwave treatment in each replicate, using mercury thermometers. Before treatment, soil temperature was 18°C, and soil moisture content was 7.6%±0.5 (mean±SD). After treatment, samples were dried for 8 days at 30°C.

Seedling emergence was monitored in an outdoor, pollen-free greenhouse, reducing the risk of granivory and contamination by seeds from the outside (Institute Tour du Valat, Arles, France). After microwave treatment and drying, each sample was spread in a 1-1.5 cm thick layer over 19 cm diameter perforated plastic saucers already containing, from bottom to top: (1) a 21 cm diameter Diatex filter (500-60m mesh size to prevent loss of coarse sand), (2) 1.5 cm of sterilized and desalted sand, and (3) a 23 cm diameter Diatex filter (50-7 m mesh size to avoid the loss of silt and seeds). The plastic saucers were randomly placed in containers where a 1 cm water layer was constantly adjusted so as to maintain soil moisture at a level conducive to germination and prevent submersion. To detect possible contamination, three control saucers containing only sterilized and desalted sand were added to the experimental design. Seedling emergences were recorded weekly for 8 weeks from 28 September to 6 November 2015. Identified individuals were regularly removed to reduce competition.

Effect on Seedling Emergence of Seed Position in Soil During

Microwave Treatment. This experiment was performed using seeds of R. × bohemica and S. gigantea. Three seed positions in the soil during microwave treatments were tested: (1) at the soil surface (0-0.5 cm depth, in the center), (2) 7 cm below the surface at the periphery (0-0.5 cm from the edge of the cylinder), and (3) 7 cm below the surface in the center. These three positions will be hereafter called (1) "at the surface," (2) "at periphery," and (3) "in the center," respectively. We applied three microwave treatments using the following Power x Duration pairings: 2kW4min, 2kW8min, and 4kW4min. Before treatment, soil temperature was 17°C and soil moisture content was 10.7%±0.5 (mean±SD). One replicate consisted of three paper baking bags containing 10 seeds of each of the two selected invasive species (R. × bohemica and S. gigantea), one bag placed at each of the three test positions, in a 15 cm high x 8 cm diameter plastic cylinder containing the same soil as in the previous experiment. We chose this cylinder height because the thickness of the soil layer used in the final design will be close. There were eight replicates per treatment. As a control treatment, eight paper bags were not subjected to microwave radiation. After microwave treatment, soil temperature was immediately measured in each replicate, using mercury thermometers inserted in the center and at the periphery of the cylinder.

After exposure to treatments, the seeds from both bags (20 seeds) were stuck to double-sided plastic films, themselves stuck to tissues (Diatex filters permeable to 0.2-20mm particles), and placed on a Petri dish (6 cm diameter) on cotton soaked in distilled water. The Petri dishes were placed in a growth chamber (Hotcold-GL: 12 K lux) with a photoperiod of 14h day/10 h night and temperature controlled at 25°C day/ 15°C night. Seedling emergence was monitored every 3 days until no seedling emergence was observed (from 5 April 2016 to 5 May 2016). Seedlings were removed as they germinated, to avoid their pre-empting of resources and thereby potentially preventing the germination of other individuals.

Statistical Analysis

Effect of Microwave Treatments on Seedling Emergence of Soil Seed Bank and Sown Invasive Species. We analyzed variation in numbers of seedling emergences for each species category (total seed bank, monocotyledons, dicotyledons, Asteraceae, Fabaceae, Poaceae) depending on the explanatory variable "microwave treatment" (control: absence of microwave treatment and four Power x Duration microwave treatment pairings: 2kW4min, 4kW2min, 2kW8min, and 4kW4min) using generalized linear models (GLMs) with negative binomial error distribution to account for overdispersion [START_REF] Crawley | Generalized linear models[END_REF]. Tests were performed in R with function "glm.nb" in the package "MASS" (R Core Team 2015).

We analyzed variation in percentages of seedling emergences of each species (D. stramonium, R.

× bohemica, and S. gigantea) depending on the explanatory variable "microwave treatment" using GLMs with binomial error distribution [START_REF] Crawley | Generalized linear models[END_REF]. Tests were performed with function "glm" in R (R Core Team 2015).

When the effect of microwave treatment was significant, we conducted pairwise comparisons with Tukey's pairwise comparison tests using function "glht" in the package "multcomp" in R (R Core Team 2015). For all tests, p-values lower than 0.05 were considered as statistically significant.

Effect on Seedling Emergence of Seed Position in Soil During Microwave Treatment. We analyzed variation in percentages of seedling emergence for each species (R. × bohemica and S. gigantea) and each Power x Duration microwave treatment pairing (2kW4min, 2kW8min, and 4kW4min) depending on the explanatory variable "seed position" ("center," "periphery," and "surface") using GLMs with binomial error distribution [START_REF] Crawley | Generalized linear models[END_REF]. Tests were performed with function "glm" in R (R Core Team 2015).

Thep-values lower than 0.05 were considered as statistically significant.

Results

Effect of Microwave Treatments on Seedling Emergence of Soil Seed Bank and Sown Invasive

Species

Soil Temperature. After treatment, we recorded mean soil temperatures close to 85°C for the 2kW8min and 4kW4min treatments, and temperatures around 65°C for the 2kW4min and 4kW2min treatments. Temperatures exceeding 100°C were not recorded whatever the treatment.

Total Seed Bank. Over the course of the experiment, 797 individuals were recorded (all treatments combined), distributed among 28 genera, including 31 identified species (Table S1). Four families of monocotyledons accounted for 11 genera, 12 species, and 489 individuals. Ten families of dicotyledons accounted for 17 genera, 21 species, and 308 individuals. The plant community was dominated by Poaceae, with 60.3% of the total number of individuals. Two Poaceae species, Rostraria cristata and Lolium rigidum, accounted for 20.7 and 36.3% of the total number of individuals, respectively. Dominant dicotyledon families were Asteraceae and Fabaceae, with 14.6 and 10.4% of the total number of individuals, respectively.

Soil exposure to microwave treatment significantly impacted the number of seedling emergences of the total seed bank (degrees of freedom [df] = 35, Wald x2 = 37.5, p < 0.001). The four microwave treatments significantly reduced the number of seedling emergences compared with the control (p < 0.001; Fig. 1A). The number of seedling emergences was signifi-cantly higher in the 2kW4min and 4kW2min treatments than those in the 2kW8min and 4kW4min treatments (p<0.01; Fig. 1A). The other comparisons did not reveal any significant difference.

Dicotyledons. Soil exposure to microwave treatment significantly impacted the number of seedling emergences of the dicotyledons (df = 35, Wald x 2 =38.6, p<0.001). The four microwave treatments significantly reduced the number of seedling emergences compared with the control (p<0.05; Fig. 1B).

The number of seedling emergences was significantly higher in the 2kW4min treatment than those in the 2kW8min treatment (p <0.01; Fig. 1B). The number of seedling emergences was also significantly higher in the 4kW2min treatment than those in the 2kW8min and 4kW4min treatments (p <0.05; Fig. 1B). The other comparisons did not reveal any significant difference.

Monocotyledons. Soil exposure to microwave treatment sig-nificantly impacted the number of seedling emergences of the monocotyledons (df = 35, Wald x 2 = 33.9, p < 0.001). The 2kW4min, 4kW2min, and 4kW4min treatments significantly reduced the number of seedling emergences compared with the control (p <0.001; Fig. 1C). No seedling was found in the 2kW8min treatment; thus, statistical tests were not performed for this treatment. The number of seedling emergences was significantly higher in the 2kW4min and 4kW2min treatments than in the 4kW4min treatment (p <0.01; Fig. 1C). The other comparisons did not reveal any significant difference.

Asteraceae. Soil exposure to microwave treatment significantly impacted the number of seedling emergences of the Asteraceae (df =35, Wald x 2 =34.4, p<0.001). The 2kW4min, 4kW2min, and 4kW4min treatments significantly reduced the number of seedling emergences compared with the control (p <0.05; Fig. 1D). No seedling was found in the 2kW8min treatment; thus, statistical tests were not performed for this treatment. The number of seedling emergences was significantly higher in the 4kW2min treatment than in the 4kW4min treatment (p < 0.05; Fig. 1D). The other comparisons did not reveal any significant difference. Values are expressed as mean ± SE of the eight replicates. Letters refer to the results of post hoc Tukey's test. Treatments with no letter in common are significantly different (p < 0.05). n.s. means "nonsignificant," when the general effect of microwave treatment is not statistically significant (p < 0.05).

Invasive Species Sown Before Microwave Treatment. Soil exposure to microwave treatment significantly impacted the percentage of seedling emergences of Datura stramonium and Reynoutria × bohemica (df = 35, Wald x 2 =31.3 and 32.6 respectively, p<0.001). As no seedling of Solidago gigantea was found after microwave treatment, no statistical test was performed for this species. Both the 2kW4min and 4kW2min treatments significantly reduced the seedling emergence percentage of D. stramonium and R. × bohemica compared with the control (p < 0.001; Fig. 2A). For D. stramonium, the 4kW4min treatment significantly reduced the seedling emergence percentage compared with the control (p <0.001; Fig. 2A), while for R. × bohemica, no seedling was found. For both species, no seedling was found after the 2kW8min treatment; thus, statistical tests were not performed for this treatment. The other comparisons did not reveal any significant difference.

Invasive Species Sown After Microwave Treatment. While prior exposure of soil to microwave treatment did not significantly impact the seedling emergence percentage of R. × bohemica (df = 35, Wald x 2 = 65.5, p = 0.487, respectively; Fig. 2B), the 2kW4min and 4kW4min treatments significantly reduced the seedling emergence percentage of D. stramonium and S. gigantea compared with the control (df= 35, Wald x2 = 91.1 and 78.8, respectively, p < 0.001; Fig. 2B). The percentage of seedling emergence of D. stramonium after the 2kW4min treatment was significantly lower than after the 4kW2min treatment (p<0.01, Fig. 2B). The percentage of seedling emergence of S. gigantea was significantly lower after the 2kW4min treatment than after the 2kW8min and 4kW2min treatments (p < 0.05, Fig. 2B). The other comparisons did not reveal any significant difference.

Effect of Seed Position in Soil During Microwave Treatment on Seedling Emergence

Soil Temperature. We recorded higher mean temperatures in the center than at the periphery after exposure to all the microwave treatments (eight replicates). In the 2kW4min treatment, soil temperature was around 55°C in average with a difference of 8°C between the center and the periphery. Soil reached mean temperatures close to 85°C in both the 2kW8min and 4kW4min treatments, with a difference of 6°C in average between the center and the periphery. Temperatures exceeding 100°C were not recorded whatever the treatment.

Reynoutria × bohemica. With the 2kW8min and 4kW4min treatments, seedlings were only recorded for seeds placed at the surface during microwave treatment (Fig. 3A). Thus, statistical tests were not performed for these treatments. The 2kW4min treatment showed no significant difference between the three seed positions (df = 21, Wald x 2 = 0.1, p = 0.046 but post hoc tests nonsignificant; Fig. 3A).

Solidago gigantea.

With the 4kW4min treatment, seedlings were only recorded for seeds placed at the surface during microwave treatment (Fig. 3B). Thus, statistical tests were not performed for this treatment.

Seed position in the soil during microwave treatment significantly impacted the seedling emergence percentage for the 2kW4min treatment (df =21, Wald x2 = 42.4, p < 0.001) and for the 2kW8min treatment (df =12, Wald x 2 = 85.2, p < 0.001). With the 2kW4min treatment, the seedling emergence percentage was significantly higher at the periphery and at the surface than in the center (p<0.01; Fig. 3B). For the 2kW8min treatment, the seedling emergence percentage was significantly higher at the surface than at the periphery and in the center (p < 0.001; Fig. 3B). The other comparisons did not reveal any significant difference.

Figure 3. Effect of position of seeds in the soil on seedling emergence percentages of (A) Reynoutria × bohemica and (B) Solidago gigantea without microwave treatment (control) and after microwave treatment (2kW4min, 2kW8min, and 4kW4min Power x Duration). "Center" indicates seeds placed 7 cm below the surface in the center of the cylinder, "periphery" indicates seeds placed 7 cm below the surface at the periphery of the cylinder, and "surface" indicates seeds placed close to the soil surface at the center of the cylinder. Values are expressed as mean ± SE of the eight replicates. Letters refer to the results of post hoc Tukey's test. Uppercase and lowercase letters refer to different microwave treatments. Seed positions with no letter in common are significantly different (p < 0.05). n.s. means "nonsignificant," when the general effect of seed position is not statistically significant (p < 0.05).

Discussion

Seed susceptibility to microwave treatment is mainly attributed to the direct effect of heat transfer from the surrounding soil [START_REF] Nelson | A review and assessment of microwave energy for soil treatment to control pests[END_REF] and is entirely temperature-dependent [START_REF] Barker | Inhibition of weed seed germination by microwaves[END_REF]. Studies have shown that soil microwave treatment can kill seeds of several species in the soil [START_REF] Davis | Ultrahigh-frequency electromagnetic fields forweed control: phytotoxicity and selectivity[END_REF][START_REF] Barker | Inhibition of weed seed germination by microwaves[END_REF][START_REF] Brodie | Microwave soil heating for controlling ryegrass seed germination[END_REF][START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF]). In our experiment, the drastic reduction of seedling emergences from a seed bank exposed to certain treatments confirms the effectiveness of soil microwave treatment in inhibiting the seedling emergence of a wide diversity of species (here, at least 31 species). We found the 2kW8min treatment, the lowest power, and the longest exposure, to be the most effective: enabling soil temperature to reach 85°C, it reduced total seedling emergence by 98% compared to the control. Possible reasons for the extreme effectiveness of this treatment are that (1) a lethal temperature is maintained for a longer time than with the shorter treatment (i.e. 4kW4min), and (2) temperature homogeneity within the sample is greater due to longer thermal diffusion from the zones where the microwave energy is being absorbed to the rest of the material.

As the effectiveness of microwave treatments in reducing seedling emergence is dependent on soil conditions such as soil moisture and texture [START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF][START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF], treatment power and/or duration would probably need to be adapted to each site's specific soil characteristics. Naturally, economic viability calls for the shortest and least energy-consuming treatment. While the 2kW4min and 4kW2min treatments were not very effective in our experiment, not permitting the soil to reach mean temperatures above 65°C, they may work better under other soil conditions [START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF] and should be explored as an option. None of the four treatments tested completely inhibited seedling emergence: we recorded a few seedling emergences even with the two most intensive treatments (i.e. 4kW4min and 2kW8min). A species-specific response to soil microwave heating seems a strong hypothesis to explain this result. Studies have already shown that seed survival and seedling emergence responses to soil heating vary greatly between species or between genotypes within species [START_REF] Melander | Soil steaming to reduce intrarow weed seedling emergence[END_REF][START_REF] Wahid | Heat tolerance in plants: an overview[END_REF][START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF]. S. gigantea appeared to be very sensitive to soil microwave heating compared to D. stramonium and R. × bohemica. This high sensitivity may be related to its small seed size (large size and mass are correlated with high tolerance to heat; [START_REF] Davis | Ultrahigh-frequency electromagnetic fields forweed control: phytotoxicity and selectivity[END_REF][START_REF] Diprose | The effect of externally applied electrostatic fields, microwave radiation and electric currents on plants and other organisms, with special reference to weed control[END_REF][START_REF] Gashaw | Soil seed bank dynamics and above-ground cover of a dominant grass, Hyparrhenia confinis, in regularly burned savanna types in Gambella, western Ethiopia[END_REF]. It must, however, be noticed that, for each invasive species, response to soil microwave heating was tested on seeds originating from a single population: the response could thus be population-specific [START_REF] Wahid | Heat tolerance in plants: an overview[END_REF] and not reflect the average response of the species. To generalize our results, experiments examining the response of several populations should be implemented.

Regarding the dominant plant families in the seed bank, Asteraceae and Poaceae exhibited very different seedling emergence responses compared to Fabaceae. While Asteraceae and Poaceae showed a drastic decrease in the number of seedling emergences after microwave treatment compared with the control (e.g. absence of seedling emergence in the 2kW8min treatment), Fabaceae showed no significant response to soil microwave heating. Most Fabaceae species produce hard seeds [START_REF] Baskin | Seeds: ecology, biogeography, and evolution of dormancy and germination[END_REF], characterized by a thick seed coat impermeable to water and/or gases and which imposes physical dormancy [START_REF] Herranz | Influence of heat on seed germination of seven Mediterranean Leguminosae species[END_REF]. Hard seeds are highly resistant to extreme temperatures [START_REF] Bradbeer | Seed dormancy and germination[END_REF] and can be softened by short exposure to high temperatures (i.e. 60-100°C for 5 minutes; [START_REF] Lodge | The effect of temperature on the hardseed content of some annual legumes grown on the northern slopes of New South Wales[END_REF][START_REF] Baskin | Seeds: ecology, biogeography, and evolution of dormancy and germination[END_REF][START_REF] Herranz | Influence of heat on seed germination of seven Mediterranean Leguminosae species[END_REF]. The absence of significant response of Fabaceae to soil microwave heating suggests that the treatments do not overcome seed dormancy, nor inhibit seedling emergence. Some Fabaceae seeds, resistant to high temperatures, may remain dormant in soil.

To assess the proportion of dormant seeds, complementary experiments should test the sensitivity of a controlled number of Fabaceae seeds to microwave treatment, followed by a viability test. Adaptation of the treatment (e.g. longer treatment, higher power) may be required to eradicate Fabaceae seed banks, as well as seeds of other species having a high resistance to heat (e.g. adapted to fire; [START_REF] Overbeck | No heat-stimulated germination found in herbaceous species from burned subtropical grassland[END_REF]. It should also be noticed that the number of Asteraceae and Fabaceae seedlings recorded during the experiment is small: additional experiments using a larger number of seeds should be conducted to reinforce our conclusions. Further experiments to determine how seed traits can influence the effectiveness of microwave treatment (i.e. size and/or mass, coat characteristics, type of dormancy, composition, etc.) are needed to explain differences in seedling emergence.

In addition to a species-specific response to heat, the few seedlings observed after the most intensive treatments (i.e. 4kW4min and 2kW8min) could also be the result of nonuniform microwave heating of soil samples. [START_REF] Brodie | Microwave heating in moist materials[END_REF] found the hottest place in the heating pattern of a soil cylinder a few centimeters below the surface along the center line, and temperatures at the edges of the exposed object several degrees lower than in the center (output power 750 W, after 150 seconds of heating). In our experiment, we found seedling emergence percentages of seeds placed at the surface (0-0.5 cm depth) higher than seedling emergence percentages of seeds placed 7 cm below the surface at the periphery and in the center. As seeds' susceptibility to soil microwave treatment is due more to heat transfer from the soil than to direct interactions of seeds with microwave radiation [START_REF] Nelson | A review and assessment of microwave energy for soil treatment to control pests[END_REF], the reduced treatment effectiveness on seeds placed at the surface could be due to the reduction in contact between seed surface and soil. Thus, the incomplete effectiveness of even the most intensive treatments could also be explained by seed position: some seeds could have survived at the surface. As the projected design involves passing a given thickness of extracted soil through the microwave oven, such ineffectiveness at the surface would compromise the overall effectiveness of the method in the field. Solutions (i.e. technical improvements to the machine or increasing overall soil temperature during treatment) need to be found to ensure satisfactory impact on seeds located at the surface. In this experiment, the lower effectiveness of the 2kW8min treatment compared with the 4kW4min treatment can be explained by temperature variations between replicates, most likely caused by an experimental problem. We found seedlings of S. gigantea in the center and at the periphery in the three replicates where center temperatures were lower than 85°C, highlighting the importance of reaching a lethal temperature to inhibit seedling emergence. Between the first and the second experiments, we noticed differences in the invasive seedling emergence percentages after exposure to the 2kW4min treatment, independently of seed position in soil (close to 10 vs. 90%, respectively). The use of different germination methods could explain this variability. In the first experiment, soil samples containing seeds germinated in an outdoor greenhouse, whereas in the second one, seeds were placed in a growth chamber with optimal germination conditions.

Raising soil temperature by microwave treatment also potentially affects soil-living organisms and soil abiotic properties [START_REF] Barker | Killing and preserving nematodes in soil samples with chemicals and microwave energy[END_REF][START_REF] Vela | Mechanism of lethal action of 2,450-MHz radiation on microorganisms[END_REF][START_REF] Brodie | Assessing the impact of microwave treatment on soil microbial populations[END_REF]. Such disturbances can indirectly impact seedling emergence [START_REF] Dawson | Influence of steam/air mixtures, when used for heating soil, on biological and chemical properties that affect seedling growth[END_REF]. Soil microwave treatment could therefore decrease invasive species seedling emergence through a direct effect of heat on seeds, and in a second step, through modifications of soil properties and functioning. In the study of De [START_REF] Wilde | Using microwave soil heating to inhibit invasive species seed germination[END_REF], heating effects on soil properties and biota could not be assessed because seedling emergence was not performed on treated soil. In the present study, the significant decrease in D. stramonium and S. gigantea percentages of seedling emergence after soil exposure to the 2kW4min and 4kW4min treatments is surprising, as the longest and highly effective treatment, 2kW8min, had no impact compared with the control. Given the high variability in the percentages of seedling emergence, an experimental artifact seems the most likely explanation. The seedling emergence percentage of R. × bohemica sown after treatment was not impacted. These results show that a substantial proportion of invasive species seedlings is able to emerge, when sown on previously treated soil. This suggests that microwave heating does not prevent newly arrived invasive species seeds from germinating.

Consequently, this method could be less effective in the long term to control invasive species having high dispersal abilities, such as S. gigantea. Microwave heating induces a rapid dehydration of plant tissues, causing plant death within a few seconds of treatment [START_REF] Brodie | Microwave technologies as part of an integrated weed management strategy: a review[END_REF]. Rhizomes would therefore be easily destroyed by microwave treatments used to control seedling emergence. However, it will be necessary to test the effectiveness of the method for each targeted clonal invasive species. In cases where rhizomes are buried too deep in soil to be extracted, this method would be less suitable.

Because this method would lead to bare soil cleared of viable seeds favorable to plant recolonization, restoration measures such as rapid revegetation would be required, to recover ecosystem functions, and prevent recolonization by invasive species. It is therefore essential now to assess seedling emergence response of native species sown on previously treated soil. In addition, it would be vital to investigate the effect of soil microwave heating on ecosystem components playing essential functional roles, such as some soil abiotic parameters (e.g. pH, nutrient availability, organic matter content) and soilliving organisms (e.g. microorganisms, nematodes, insects; [START_REF] Neary | Fire effects on belowground sustainability: a review and synthesis[END_REF][START_REF] Van Der Heijden | Arbuscular mycorrhizal fungi as a determinant of plant diversity: in search of underlying mechanisms and general principles[END_REF][START_REF] Herrick | Monitoring ecological processes for restoration projects[END_REF][START_REF] Meysman | Bioturbation: a fresh look at Darwin's last idea[END_REF]) before applying the method in the field. The absence of selectivity of this thermal method should also be borne in mind: only after careful consideration of the impact on the whole seed bank (native and invasive species) should it be implemented. In cases where plants with high ecological values are present, efforts to ensure their survival (e.g. seed or plant individual collection) must be undertaken. By removing the upper soil layer, this method may also not be feasible on rocky and heavy (clay reach) soils, because it may cause a severe disturbance to soil structure.

In conclusion, the findings of this study lend support to developing the in situ process.

Consistent with other studies, our results show that Power × Duration pairings enabling soil to reach 85∘C are effective in reducing seedling emergence of a wide variety of species. Our study also raises two major issues to be considered when selecting the power and duration of the treatment: the speciesspecific response to soil microwave heating and the treatment's limited effectiveness on seeds located at the soil surface. Determining more specifically which seed traits influence the effectiveness of microwave treatment is the next step in refining the applicability of the method. Our results indicate that prior exposure of soil to microwave treatment does not prevent seedling emergence of a considerable proportion of invasive species sown after treatment, suggesting that invasive species would be able to reinvade previously treated soils. This control method should therefore be coupled with restoration measures (e.g. rapid revegetation) to occupy niches that will not be available to invasive species colonization. This result also suggests that this method could be less suitable to control invasive species having high dispersal abilities because of their rapid recolonization potential. The suitability of the method to control invasive plants reproducing by rhizomes must also be assessed. Finally, it will be essential to test the effectiveness of this process in different soil types, especially in rocky and heterogeneous ones.

Fabaceae.

  Soil exposure to microwave treatment did not significantly impact the number of seedling emergences of the Fabaceae (df = 35, Wald x2 = 35.1, p = 0.747; Fig.1E).Poaceae. Soil exposure to microwave treatment significantly impacted the number of seedling emergences of the Poaceae (df = 35, Wald x2 = 30.9, p < 0.001). The 2kW4min and 4kW2min treatments significantly reduced the number of seedling compared with the control (p<0.001; Fig.1F). No seedling was found in the 2kW8min and 4kW4min treatments; thus, statistical tests were not performed for these treatments. The other comparisons did not reveal any significant difference.

Figure 1 .

 1 Figure 1. Effect of microwave soil heating on the number of seedling emergences of seed bank plants by category (A) the whole seed bank, (B) dicotyledons, (C) monocotyledons, (D) Asteraceae, (E) Fabaceae, and (F) Poaceae that emerged without soil microwave treatment (control) and after different soil microwave treatments (2kW4min, 2kW8min, 4kW2min, and 4kW4min Power x Duration).Values are expressed as mean ± SE of the eight replicates. Letters refer to the results of post hoc Tukey's test. Treatments with no letter in common are significantly different (p < 0.05). n.s. means "nonsignificant," when the general effect of microwave treatment is not statistically significant (p < 0.05).

Figure 2 .

 2 Figure 2. Effect of microwave soil heating on the percentages of seedling emergence of Datura stramonium, Reynoutria × bohemica, and Solidago gigantea sown (A) before and (B) after soil microwave treatment (2kW4min, 2kW8min, 4kW2min, and 4kW4min Power x Duration) or without soil microwave treatment (control). Values are expressed as mean ± SE of the eight replicates. Letters refer to the results of post hoc Tukey's test. Uppercase and lowercase letters refer to different species. Treatments with no letter in common are significantly different (p < 0.05).

Acknowledgments

We thank three anonymous reviewers and coordinating editor O. Valkó for improving the manuscript, as well as M. Sweetko for reviewing the English. This work is part of an FUI (French Inter-Ministerial R&D Fund) research and development project, cofinanced by BPI France, Provence Alpes Côte d'Azur (PACA) and Occitanie/Pyrénées -Méditerranée Regions and the Ain local authority. In addition to the authors, the participants in this project also include scientists from the companies GECO Ingénierie et Travaux, EDF, NGE-Guintoli, SAIREM and from the laboratory ST&M. No conflicts of interest have been declared.

Supporting Information

The following information may be found in the online version of this article: Appendix S1. Characteristics of the invasive species tested. Table S1. Seed bank composition classified by group (dicotyledon or monocotyledon), family, and species.

Guest Coordinating Editor: Orsolya Valkó Received: 11 May, 2017;First decision: 26 June, 2017;Revised: 5 December,2017;Accepted: 5 December, 2017