Etude de la réponse des communautés bactériennes du sol aux changements des modes de gestion dans les agrosystèmes

Diplôme : Dr. d'Universit

Soil microbial diversity in grasslands and its importance for grassland functioning and services

International audienceThis chapter contributes to understanding the role and impact of soil microbe populations and communities, in the light of the new molecular technologies that allow analysis of the diversity of these microbes and their functional role in C and N cycles. It also discusses the importance of soil biodiversity for grassland functioning (species assemblage and species removal) and representation of grassland soil biodiversity in models of grassland functioning and service delivery

Etude de la réponse des communautés bactériennes du sol aux changements des modes de gestion dans les agrosystèmes

POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Grassland management history affects the response of the nematode community to changes in above-ground grazing regime

International audienceChanges in grassland management induce disturbances that influence both soil functioning and soil fauna. This study aimed at determining the extent to which the composition of a grassland soil nematode community could be altered by a shift of grazing regime and the potential feedback that these alterations could provoke on grassland functioning. Therefore, we monitored the composition of the soil nematode community (i.e., plant-, bacterial- and fungal-feeders, omnivores and carnivores) of mesocosms that were sampled from two contrasted long-term field trials (high vs low grazing treatments) and subsequently subjected to high or no grazing for 2 years. The soil nematofauna responded faster and more strongly to the application of an intensive grazing regime on a previously extensively exploited system than the other way round. The application of an intensive grazing regime induced a significant increase in numbers of bacterial feeders and a decrease of the relative abundance of fungal-feeding nematodes. The nematofaunal community structure was determined by both the past and current grazing regimes throughout the 2 years of monitoring. Observed effects on soil microbivores seemed to reflect the 'immediate' above-ground primary production potential and to follow micro-organism dynamics. On the other hand, observed effects on root-feeding nematodes seemed to reflect the integral effect of past and current grazing regimes on plant community root biomass and quality

Impacts de changements d’occupation et de gestion des sols sur la dynamique des matières organiques, les communautés microbiennes et les flux de carbone et d’azote

National audienceLes modes d’occupation des sols et de gestion des terres ont des effets importants sur la nature des restitutions organiques et, par les techniques culturales qui en découlent, sur un certain nombre de facteurs qui affectent les cycles biogéochimiques et les communautés microbiennes du sol. Dans ce contexte, le projet COSMOS-flux (programme GESSOL2), avait pour objectif d’étudier deux premiers cas d’évolution de la gestion des sols : la conversion labour-non labour (étudiée sur le site Arvalis de Boigneville, Essonne, sur des parcelles en semis direct depuis 14 ans) et la conversion prairie-rotation de cultures annuelles étudiée sur le site INRA de l’ORE de Lusignan (Vienne), dans le cadre de la problématique d’introduction de prairies temporaires dans les rotations. Les évolutions constatées pendant 18 à 36 mois après l’application des changements ont été suivies à la fois sur les compartiments organiques des couches superficielles du sol (0-20 et 0-30 cm pour Boigneville et Lusignan, respectivement), sur les biotransformations de l’azote (minéralisation, organisation, nitrification) et sur les communautés microbiennes impliquées dans la nitrification (nitratants) et la dénitrification en termes d’activité, d’effectif et de diversité. Les résultats obtenus ont tout d’abord confirmé des situations initiales bien contrastées du point de vue des compartiments organiques et des activités microbiennes et flux d’azote entre sol sous prairie âgée de 5ans et sol en rotation de cultures annuelles d’une part, sol en semis direct et sol labouré d’autre part. Ces contrastes sont dus à l’accumulation significative de matière organique en surface. Les seconds cas concernent la conversion réciproque, pour lesquels nous avons observé que le travail du sol appliqué sur les parcelles en semis direct (Boigneville) ou pour détruire la prairie (Lusignan) constitue une perturbation très forte ; celleci conduit à l’évolution très rapide des caractéristiques des compartiments organiques et activités microbiennes vers celles observées dans les sols annuellement labourés ou en rotation de cultures annuelles. Par contre, les changements de pratique inverses (passage du labour au non labour ou implantation d’une prairie) ne conduisent pas ou peu à l’évolution des caractéristiques du sol à l’échelle de 2 à 3 ans. Les résultats obtenus permettent aussi de discuter la hiérarchie des facteurs expliquant la réponse de la nitrification et de la dénitrification. La diversité des communautés microbiennes semble mal expliquer les niveaux d’activité. Les effectifs expliquent bien les changements de niveaux de nitrification, mais pas ceux de dénitrification. Enfin, le carbone organique est la variable environnementale qui joue le rôle de facteur-clé pour les réponses observées sur la dénitrification

Cloudborne microbial populations at the puy de Dôme (France) and their putative participation to atmospheric physico-chemical processes

National audienceRecent observations have raised the hypothesis that biochemical pathways could be involved in cloud chemistry and physics. Indeed, cloud water carries about ~104 microorganisms mL-1, and a metabolic activity has been detected in cloud water samples. Since 2003, more than 34 cloud events have been sampled from the puy de Dôme summit (1465 m a.s.l., France) using droplet samplers. These have been characterized for their chemical and microbiological contents, and viable bacterial strains have been isolated by culture. In laboratory, we are currently focusing on the capacity of isolates in (i) initiating the formation of ice crystals and act as ice nuclei (IN), (ii) producing biosurfactants and initiate the formation of cloud droplets (cloud condensation nuclei (CCN)) and (iii) transforming the organic compounds present in the atmosphere (formate, acetate, lactate, succinate, oxalate, formaldehyde and methanol). Experimental setups (microcosms) consist of bacterial suspensions incubated in liquid solutions of which the chemical composition reflects that of our cloud water samples: these have been designed for mimicking the conditions encountered under either oceanic or anthropic influence. The capacity of selected isolates to catalyze the formation of ice was tested by immersion freezing and in cloud chamber (AIDA, Karlsruhe). Ice nucleation active bacteria at -5°C were found in cloud samples; in cloud water, such cells could be responsible for the freezing of droplets into crystals, which often initiates precipitations. Preliminary analysis of the data showed that the IN efficiency of the isolates was dependent on the chemical composition of cloud water. Isolates have also been screened for biosurfactant production, by emulsification, oil spreading and surface tension meniscus tests. A majority of strains were positive to at least one test, suggesting that such cells would be efficient CCN in the atmosphere and could participate to the formation of liquid clouds. In order to investigate the implication of microbes in the chemistry of cloud water, the rates of biotransformation of atmospheric organic compounds by isolated strains under microcosm conditions have been determined. They were in the range of ~10-2 pmol-1 cell-1 d-1 at 5°C and were in general decreased in the “anthropic” versus “oceanic” medium. The presence of free radicals such as OH● was also detrimental. A basic numeric model of atmospheric chemistry taking into account realistic concentrations of organic compounds, of bacteria, and of OH● and NO3● radicals in a liquid cloud at 5°C for daytime and nighttime and their respective reaction rates towards the tested organic acids was applied on the results. This confirmed that the presence of OH● radicals was largely responsible for the oxidation of organic species during the day, but it is noteworthy that during the night, bacterial activity would dominate the reactivity of formic, acetic and succinic acids by contributing to more than 90 % of their degradation. Overall, our results suggest that cloudborne microorganisms are actors of the physico-chemical processes occurring in the atmosphere. The composition of cloud water modulates these roles, and this raises the question of the negative impact that human emissions could have on the capacity of microbes to interact with atmospheric processes and disseminate by the atmosphere

Living microorganisms in clouds: their identifi cation and possible roles in cloud physicochemical processes

Poste

Ecotoxicology of silver nanoparticles and their derivatives introduced in soil with or without sewage sludge: A review of effects on microorganisms, plants and animals

International audienceSilver nanoparticles (AgNPs) are widely incorporated in many products, partly due to their antimicrobial properties. The subsequent discharge of this form of silver into wastewater leads to an accumulation of silver species (AgNPs and derivatives resulting from their chemical transformation), in sewage sludge. As a result of the land application of sewage sludge for agricultural or remediation purposes, soils are the primary receiver media of silver contamination. Research on the long-term impact of AgNPs on the environment is ongoing, and this paper is the first review that summarizes the existing state of scientific knowledge on the potential impact of silver species introduced into the soil via sewage sludge, from microorganisms to earthworms and plants. Silver species can easily enter cells through biological membranes and affect the physiology of organisms, resulting in toxic effects. In soils, exposure to AgNPs may change microbial biomass and diversity, decrease plant growth and inhibit soil invertebrate reproduction. Physiological, biochemical and molecular effects have been documented in various soil organisms and microorganisms. Negative effects on organisms of the dominant form of silver in sewage sludge, silver sulfide (Ag2S), have been observed, although these effects are attenuated compared to the effects of metallic AgNPs. However, silver toxicity is complex to evaluate and much remains unknown about the ecotoxicology of silver species in soils, especially with respect to the possibility of transfer along the trophic chain via accumulation in plant and animal tissues. Critical points related to the hazards associated with the presence of silver species in the environment are described, and important issues concerning the ecotoxicity of sewage sludge applied to soil are discussed to highlight gaps in existing scientific knowledge and essential research directions for improving risk assessment

Delayed and asymmetric responses of soil C pools and N fluxes to grassland/cropland conversions

International audienceAgricultural lands are subject to changes in use for a variety of environmental and economic reasons. Among these changes, shifts between crops and grasslands are particularly common. Understanding the effects of these changes on soil functioning is crucial to anticipating crop productivity and environmental impacts and yet has rarely been studied. The purpose of this study was to investigate the in situ temporal dynamics of soil N cycling and C pools after grassland/cropland conversions and risks of N losses from such systems. The experiment was conducted in western France at a long-term experimental site with a temperate oceanic climate. We followed soil N cycling and C pools for four experimental treatments over 36 months: (i) continuous grassland, (ii) continuous rotation of annual crops, (iii) conversion from grassland to a rotation of annual crops, and (iv) conversion from a rotation of annual crops to grassland. Gross N mineralization, potential N immobilization and potential nitrification were estimated using the 15N dilution/enrichment technique and FLUAZ model, based on samples from the 0–10 cm soil layer (0, 0.75, 3, 12, 24 and 36 months after change in land use) and 10–30 cm soil layer (after 24 and 36 months) in all 4 treatments. Nitrogen pools (NH4+, NO3−, and organic N), total carbon (C), dissolved organic C, microbial biomass C, and C mineralization were also determined on all soil sample

Biological aerosol particles in the atmosphere and their impact clouds (bioclouds project)

PosterNational audienc