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With a lifetime of more than 100 years in the atmosphere, N2O is an important greenhouse gas with a global warming potential (GWP) of ~ 300 for a 100-year timescale that also takes part in the destruction of the ozone layer (Ravishankara et al., 2009). While N2O emissions are projected to increase in the coming years (Aneja et al., 2019), the promotion of the N2O reduction to N2 in soils could appear as a possible solution for mitigating soil N2O emission (Hénault et al., 2019). The N2O reduction path appears to be mainly driven by the soils’ pH with a progressive inhibition when pH is lower than 6.8. The aim of the proposed study was to test during laboratory experiments the time and the dose required for different liming products (CaCO3 > 99 % reagent, and tow R&D products coded as RD1 and RD2), to concomitantly increase soil pH and soil capacity to reduce N2O.During a first experiment, an acidic soil (pH=5.6) developed on the granitic substrate from the Morvan Region in France and cultivated with maize was sampled in July 2019, sieved and organized in 4 sets, three of them receiving the equivalent of 1.4 t ha-1 of neutralizing value (NV) respectively as CaCO3, RD1 and RD2 while the last set of soil did not receive any liming product (control). All were placed at a soil moisture of 20 % w/w and at 15°C. Soil pH (ISO 10390: 2005) and soil capacity to reduce N2O (ISO / TS20131-2: 2018) were then monthly determined on each of these 4 sets of soil. During a second experiment, the same acidic soil received different doses of RD1 from 0 to equiv. 1.4 t of NV. Soil samples were incubated at 15°C and both soil pH and soil capacity to reduce N2O were monthly determined.Both changes in soil pH and soil capacity to reduce N2O were observed after one month of incubation and continue to be observed over all the experiment. Changes in soil pH and in soil capacity to reduce N2O were especially rapid and important for CaCO3 and RD1 and consistent with the relation between soil pH and soil capacity to reduce N2O reported by Hénault et al., 2019. Significant changes could be obtained at dose lower than the theorical 1.4 t VN ha-1. We will now continue to define conditions of application of liming products for mitigating soil GHG emission (N2O and CO2) during experiments conducted from the laboratory to the field scales.

National audience; Introduction. Shifts in the frequency and magnitude of rain events (precipitation regime) associated with climate change may affect plant morphological and physiological strategies as well as soil microbial activity.Objectives. The objective of this study was to determine i) how precipitation history shapes the response dynamics of soil bacterial and fungal communities to rewetting, as well as plant-microbial competition for N, and ii) how the N status of the system may modulate the effect of precipitation regime.Materials & methods. The legacy effects of 12 weeks of contrasting precipitation (frequent or infrequent watering, equal total water input) and N inputs was assessed in soil mesocosms on wheat plants, soil microbial communities and N cycling. The legacy effects on the response to rewetting of potentially active (rRNA-based) bacterial and fungal communities was then documented over 29h, by sequencing phylogenetic marker genes, and following the dynamics of plant-microbial coupling and competition for N using 13C-CO2 and 15N-NH4 labeling.Results. Precipitation and N input history had contrasting effects on plant physiology, fungal:bacterial ratio, microbial community composition and plant-microbial coupling (reduced C transfer to microbes), but showed no effect on microbial response to rewetting. After an initial short-term response, potentially active soil microbial communities changed little, regardless of precipitation or N input history. Upon rewetting, microbes outcompeted plants for N but plant competitiveness increased over time, in particular after a history of more favorable conditions. Soil CO2 efflux upon rewetting was higher in systems with a history of frequent precipitation inputs and was not modulated by N availability.Conclusion. We suggest that by altering C and N dynamics between plants and soil microbes and reducing soil C sequestration potential, more extreme fluctuations in soil moisture may affect future ecosystem functioning.

International audience; Cover cropping plays a key role in the maintenance of arable soil health and the enhancement of agroecosystem services. However, our understanding of how cover crop management impacts soil microbial communities and how these interactions might affect soil nutrient cycling is still limited. Here, we studied the impact of four cover crop mixtures varying in species richness and functional diversity, three cover crop termination strategies (i.e., frost, rolling, and glyphosate) and two levels of irrigation at the cover crop sowing on soil nitrogen and carbon dynamics, soil microbial diversity, and structure as well as the abundance of total bacteria, archaea, and N-cycling microbial guilds. We found that total nitrogen and soil organic carbon were higher when cover crops were killed by frost compared to rolling and glyphosate termination treatments, while cover crop biomass was positively correlated to soil carbon and C:N ratio. Modifications of soil properties due to cover crop management rather than the composition of cover crop mixtures were related to changes in the abundance of ammonia oxidizers and denitrifiers, while there was no effect on the total bacterial abundance. Unraveling the underlying processes by which cover crop management shapes soil physico-chemical properties and bacterial communities is of importance to help selecting optimized agricultural practices for sustainable farming systems.

International audience; Introduction. Shifts in the frequency and magnitude of rain events (precipitation regime) associated with climate change may affect plant morphological and physiological strategies as well as soil microbial activity. Objectives. The objective of this study was to determine i) how precipitation history shapes the response dynamics of soil bacterial and fungal communities to rewetting, as well as plant-microbial competition for N, and ii) how the N status of the system may modulate the effect of precipitation regime. Materials & methods. The legacy effects of 12 weeks of contrasting precipitation (frequent or infrequent watering, equal total water input) and N inputs was assessed in soil mesocosms on wheat plants, soil microbial communities and N cycling. The legacy effects on the response to rewetting of potentially active (rRNA-based) bacterial and fungal communities was then documented over 29h, by sequencing phylogenetic marker genes, and following the dynamics of plant-microbial coupling and competition for N using 13C-CO2 and 15N-NH4 labeling. Results. Precipitation and N input history had contrasting effects on plant physiology, fungal:bacterial ratio, microbial community composition and plant-microbial coupling (reduced C transfer to microbes), but showed no effect on microbial response to rewetting. After an initial short-term response, potentially active soil microbial communities changed little, regardless of precipitation or N input history. Upon rewetting, microbes outcompeted plants for N but plant competitiveness increased over time, in particular after a history of more favorable conditions. Soil CO2 efflux upon rewetting was higher in systems with a history of frequent precipitation inputs and was not modulated by N availability. Conclusion. We suggest that by altering C and N dynamics between plants and soil microbes and reducing soil C sequestration potential, more extreme fluctuations in soil moisture may affect future ecosystem functioning.

Les légumineuses ont un rôle majeur à jouer dans la transition vers une agriculture plus durable, en réduisant notamment le recours aux intrants azotés du fait de leurs propriétés spécifiques vis-à-vis du cycle de l’azote. Pourtant peu de références sont disponibles pour une diversité d’espèces quant à leurs performances agronomiques et aux flux d’azote qu’elles induisent. Une étude comparative sur dix espèces de légumineuses à graines a été menée afin i) de quantifier la fixation symbiotique, ii) d’évaluer sa réponse à la présence d’azote minéral du sol, et iii) de quantifier l’effet des légumineuses sur le rendement d’un blé suivant (non fertilisé) en lien avec la minéralisation de leurs résidus de culture. Pour neuf espèces sur dix la fixation symbiotique a permis d’assurer une croissance et une nutrition azotée des plantes aussi bonnes qu’avec une nutrition basée sur le prélèvement d’azote minéral. Des différences d’inhibition de la fixation symbiotique par l’azote minéral ont été mises en évidence et ont été partiellement expliquées par des différences d’efficience de prélèvement de l’azote minéral entre les espèces. Ces différences ont été corrélées à des différences de vitesse d’expansion latérale des racines entre les espèces. La grande majorité des légumineuses a engendré des rendements de blé supérieurs à ceux de blés cultivés après des céréales. La variabilité des rendements du blé a en partie pu être reliée à la minéralisation de l’azote des résidus, variable entre espèces selon leur rapport carbone / azote. Cependant l’azote fourni au sol par les légumineuses peut être perdu pendant la période d’interculture, avant que cet azote ne soit prélevé par la culture suivante. Ce risque augmente pour les espèces de légumineuses récoltées en été contrairement au début automne, du fait d’une interculture plus longue. Abstract: Symbiotic nitrogen fixation and pre-crop effect : are all grain legumes the same? Legumes should play a key role in the transition towards a more sustainable agriculture by allowing a reduction of nitrogen inputs due to their specific properties relative to the nitrogen cycle. However, there is a lack of references for a wide diversity of species regarding their agronomic performances and the nitrogen fluxes they induce. A comparative study on ten grain legumes was conducted to i) quantity symbiotic nitrogen fixation, ii) evaluate its response to inorganic soil nitrogen, and iii) quantify the effect of legumes on the yield of the subsequent wheat (unfertilised) as related to the N mineralisation of their crop residues. For nine species out of ten, symbiotic fixation was able to ensure plant growth and nitrogen acquisition at levels similar to those achieved by nutrition based on inorganic nitrogen. Differences in symbiotic fixation inhibition were observed and were partially explained by differences in inorganic nitrogen uptake efficiency among species. Those differences were related to differences in root lateral expansion rate among species. The great majority of legumes led to higher wheat yields compared to those of wheat cultivated after cereals. Wheat yield variability was partially related to differences in crop residue nitrogen mineralisation among species according to their carbon /nitrogen, Legumes should play a key role in the transition towards a more sustainable agriculture by allowing a reduction of nitrogen inputs due to their specific properties relative to the nitrogen cycle. However, there is a lack of references for a wide diversity of species regarding their agronomic performances and the nitrogen fluxes they induce. A comparative study on ten grain legumes was conducted to i) quantity symbiotic nitrogen fixation, ii) evaluate its response to inorganic soil nitrogen, and iii) quantify the effect of legumes on the yield of the subsequent wheat (unfertilised) as related to the N mineralisation of their crop residues. For nine species out of ten, symbiotic fixation was able to ensure plant growth and nitrogen acquisition at levels similar to those achieved by nutrition based on inorganic nitrogen. Differences in symbiotic fixation inhibition were observed and were partially explained by differences in inorganic nitrogen uptake efficiency among species. Those differences were related to differences in root lateral expansion rate among species. The great majority of legumes led to higher wheat yields compared to those of wheat cultivated after cereals. Wheat yield variability was partially related to differences in crop residue nitrogen mineralisation among species according to their carbon /nitrogen ratio. However, nitrogen supplied to the soil by legumes can be lost during the fallow period before nitrogen is retrieved by the subsequent crop. This risk increases with legumes harvested in summer vs. beginning of autumn, because of a longer fallow period.

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National audience; Agriculture is the main source of terrestrial N2O emissions. This gas is the main depleting substance of the ozone layer and contributes to about 6% of total global warming. The unique known biological process able to convert N2O is its reduction to N2 by organisms possessing the nosZ gene, which encodes the nitrous oxide reductase. A recent publication (Jones et al., Nature Climate Change, in press) showed that the abundance and the diversity of a recently discovered clade of nosZ_carrying microorganisms are important players of soil N2O sink capacities. Therefore, enhancing the comprehension of the role of the abundance and the diversity of microbial populations in N2O emissions and looking for agricultural practices that could favour microbial populations able to reduce N2O into N2 is key in determining N2O emissions mitigation strategies. In this study, two experimental sites comprising nine management strategies that differ in crop rotation, tillage depth, fertilization, straw incorporation and cover crop were chosen. To characterize the activity of soil microbial communities, potential denitrification rates and N2O/N2O+N2 emission ratios were measured. The abundance of the different microbial guilds involved in N-cycling was quantified by real-time PCR, and the diversity of the nosZ gene was determined by 454 pyrosequencing. Our results suggest that the agricultural practices we tested were not sufficient to modify deeply the abundance and the diversity of denitrifying microbial communities related to the N2O emissions. However, the diversity and the abundance of these guilds were strongly dependent on the experimental site and responded to soil physico-chemical parameters (e.g.: pH, loam). We also confirmed the link between the diversity of nosZ microorganisms and the soil N2O sink capacity (Fig.1), which emphasize the importance of microbial diversity for ecosystem functioning.

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National audience; Microbial communities have a central role in ecosystem processes by driving the Earth’s biogeochemical cycles. However, the importance of microbial diversity for ecosystem functioning is still challenged, notably because of the “functional redundancy”. We recently showed that a reduction in denitrifier diversity resulted in a significant decrease in potential denitrification activity. However, our comprehension of the rules governing the assembly of bacterial populations and determining their structure is limited. In this study, we aimed at understanding how communities of different levels of diversity assemble when colonizing a sterile soil, how invasions with exogenous populations impact their structure and composition, and if these changes in population structure and diversity are causing changes in N-cycle activities. Therefore, we experimentally manipulated the soil microbial community structure and the history of communities’ assembly. We separately inoculated sterile soil microcosms with two different microbial communities extracted from native soils (Epoisses in France and Ulleråker in Sweden) at four different dilution levels in triplicate. After 44 days of colonization, flasks were reinoculated with one or the other community at two different dilution levels in triplicate mimicking an invasion process, while control microcosms for each soil and at each dilution were either sampled after 44 days or kept unchanged until the end of the experiment. The invaded and the control microcosms were finally sampled after 105 days. For each microcosm, the abundance of different N-cycle microbial guilds was measured by qPCR and total bacterial diversity was determined by 16S rRNA gene amplicons sequencing. NH4 +, NO3 - pools, and total mineral N content were quantified as a proxy for global N-cycle activities. Our results revealed that the dilution/colonization experiment impacted drastically the structure and the activity of the soil microbial communities with either nitrification or denitrification favoured depending on the initial dilution. Invasions of established populations with a 2nd one had various effects on microbial community structure and activities depending on the origin of the established populations and the level of diversity of the invading ones. Altogether, our work suggests that changes in microbial community diversity resulting from erosions of diversity and/or invasions of exogenous populations can have dramatic consequences on N-cycle functioning in soils.

International audience

International audience; Cultivated soils have been widely highlighted as a major source of nitrous oxide (N2O) emissions. This suggests that greenhouse gas emissions should be taken in account when evaluating the impact of new cropping systems. The development of integrated weed management in cropping systems introduces new agricultural practices (combinations of crop rotation, soil management, fertilization, and mechanical and chemical weed control, etc.), which may affect the microbial processes responsible for N2O production in soils. However, the effect of those practices remains to be assessed. Thus, the main objectives of our study is to provide (i) an accurate estimation of the intensity of N2O emissions from an integrated weed management system and (ii) a monitoring of soil chemical, physical, and biological parameters likely to affect N2O emissions over one year.

A multi-partners strategy is engaged at level of Burgundy-France territory, to evaluate potentials and identify levers for the development of a chain from production to uses of grain legumes. The analysis of genotype x environment interaction and climatic factors is supported by a second year of field trials of grain legumes varieties (pea and faba bean of spring or winter sowing types. The statistical analysis of climatic data over the 1961-2009 period, has allowed to build Burgundy maps of freezing risk and late spring heat stress risk and we are able to suggest pea or faba bean genotypes, better adapted to the region. The sustainability of cropping systems involving or not grain legumes was evaluated according several criteria (agronomy, energy and other input, work organisation). It has confirmed the positive effect of inserting grain legumes, on the reduction (i) of N fertiliser, energy and pesticide uses, (ii) for easier weed control (iii) on the economic bilan assessed over whole cropping system. On 2 sites, « in situ » most measurement of N2Oemissions by soils containing grain legume crop residues were higher than those containing cereal residues. Offering to farmers varieties with good level of stress resistance and good productivity will be a key point for the development of grain legume crops. The economic analysis has identified the potential value of developing contracts between produces and collectors which will secure prices and market. The technical development of a high quality “chain” and its promotion may be valuable provided efforts on traceabilty and on labelling.; Une démarche multi-partenaires est engagée sur le territoire de la région Bourgogne, pour évaluer les potentiels et identifier des leviers au développement d’une filière protéagineuse. L’analyse des interactions génotype x environnement et des facteurs climatiques est appuyée par une deuxième année d’expérimentation variétale de protéagineux (pois et féveroles d’hiver et de printemps). Une étude fréquentielle sur la période 1961-2009 a permis de cartographier les risques de gel hivernal et de fortes chaleurs en fin de cycle cultural en Bourgogne et nous pouvons proposer des variétés de pois et de féveroles mieux adaptées à la région. Le volet d’évaluation multicritères de la durabilité des systèmes de culture bourguignons, comparant des systèmes avec et sans protéagineux, a permis de confirmer et de quantifier l’intérêt de l’insertion de protéagineux, qui passe notamment par une diminution de l’IFT moyen et des doses d’azote, une augmentation de la robustesse économique (économie d’engrais azotés et énergie fossile sur l’ensemble de la rotation), la diversification des rotations en offrant de nouvelles possibilités de gestion des adventices. Sur 2 sites, le volet « environnemental » a montré en conditions « in situ », que les émissions de N2O par les sols contenant des résidus de pois ne sont en général pas supérieures à celles des sols contenant des résidus de céréales. La proposition de variétés tolérantes aux principaux stress en culture et de bonne productivité sera une clef essentielle du développement des cultures protéagineuses. L’analyse économique a identifié un intérêt du développement de la contractualisation dans la filière et de la mise en place et de la promotion d’une filière de qualité, en accentuant les exigences en matière de traçabilité et d’étiquetage.

International audience; Microbial carbon mineralization in soils leads to the production of different gaseous or dissolved components that have environmental impacts. Our study deals with the influence of soil temperature on the production of gaseous and dissolved carbon components during carbon mineralization in forest soils in France. After an incubation of soil samples for 42 days at 4 different temperatures, we determined both size and 13C isotopic signature of dissolved organic carbon and CO2 pools. We also characterised the soil microbial community structure (PLFA profiles). While temperature clearly increases the CO2 production, a low decrease of the dissolved organic carbon pool was observed whatever the temperature of incubation. The isotopic fractionation of the dissolved organic pool was not affected by temperature while a correlation was observed between temperature and isotopic fractionation of the produced CO2. The soil microbial structure was observed to change during incubation. Changes were affected by the soil temperature, specially the relative proportion of fungi decreased with temperature incubation. Correlations between the relative proportion of fungi and isotopic fractionation of the CO2 pool were observed. This result suggests that soil microbial structure can control fractionation during C mineralization.

Domaine SCIENCES DE L’ALIMENT ET AGRO-ENVIRONNEMENT

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