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The emission of nitrous oxide (N

International audience; The priming effect describes the change in the rate of soil organic matter (SOM) mineralisation due to the addition of fresh organic matter (FOM), and is thus central to the evaluation of carbon fluxes in terrestrial ecosystems. However, little is known about how litter quality interact with soil properties and microbial communities to explain the priming effect. Here, we provide new empirical evidence showing how litter, soil and microbial parameters control FOM and SOM decomposition. We monitored the kinetics of carbon fluxes over 202 days and at 23 dates after additions of two 13C-labelled plant litters of contrasted quality (nutrient-rich and labile litter [i.e., high quality] vs nutrient-poor and recalcitrant litter [i.e., low quality]) using four soil types originating from different ecosystems (forest, grassland, cropland and plantation). We found that the priming effect varied over time with: (i) an ‘immediate’ priming effect after the addition of high quality litter, which rapidly decreases when nitrogen is available; and (ii) a ‘delayed’ priming effect after the addition of low quality litter, which increases continuously when nitrogen is depleted. The ‘immediate’ priming was mainly related to the decomposition of accessible carbon compounds, whereas the ‘delayed priming’ was mainly related to the decomposition of recalcitrant carbon compounds. These results were linked to changes in microbial resource acquisition strategies as indicated by shifts in enzyme activities from hydrolytic to oxidative and nitrogen-related enzymes, therefore suggesting that two series of mechanisms act in succession at different time scales in relation to litter quality and nitrogen availability. Furthermore, our data demonstrate that incorporating carbon in the soil via the increase in soil microbial biomass can counterbalance carbon losses via priming effect. This highlights the importance to assess the balance between the carbon outputs and inputs to determine whether terrestrial ecosystems act as a sink or a source of carbon. We conclude that shifts in litter quality, nutrient availability and microbial resource acquisition strategies need to be taken into consideration for accurate assessment of the variability of the priming effect from the short to the long term

The emission of nitrous oxide (N2O), a strong greenhouse gas, during crop residue decomposition in the soil can offset the benefits of residue recycling. The IPCC inventory considers agricultural N2O emissions proportional to the amount of nitrogen (N) added by residues to soils. However, N2O involves several emission pathways driven directly by the form of N returned and indirectly by changes in the soil induced by decomposition. We investigated the decomposition factors related to N2O emissions under controlled conditions. Residues of sugar beet (SUB), wheat (WHT), rape seed (RAS), potato (POT), pea (PEA), mustard (MUS), red clover (RC), alfalfa (ALF), and miscanthus (MIS), varying by maturity at the time of collection, were incubated in two soils (GRI and SLU) at 15 °C with a water-filled pore space of 60%. The residues contained a wide proportion range of water-soluble components, components soluble in neutral detergent (SOL-NDS), hemicellulose, cellulose, and lignin. Their composition drastically influenced the dynamics of C mineralization and soil ammonium and nitrate and was correlated with N2O flux dynamics. The net cumulative N2O emitted after 60 days originated mostly from MUS (4828 ± 892 g N-N2O ha-1), SUB (2818 ± 314 g N-N2O ha-1) and RC (2567 ± 1245 g N-N2O ha-1); the other residue treatments had much lower emissions ( 25% DM) were also non-senescent and promoted high N2O emissions (representing 1–5% of applied N), likely directly by nitrification and indirectly by denitrification in microbial hotspots. Crop residue quality appears to be valuable information for accurately predicting N2O emissions and objectively weighing their other potential benefits to agriculture and the environment.

The use of natural fibers such as hemp (Cannabis sativa L.) as substitutes for nonrenewable fibers increases the life cycle performance of composite materials. The management of retting in fields as a natural pretreatment prior to fiber extraction remains challenging due to a lack of knowledge about the relative importance of environmental and biotic factors, which continually interact under field conditions. Here, we studied the dynamics of hemp retting under controlled air temperature and humidity conditions and with simulated rain. We tracked the color and infrared spectral absorbance of the stem surface, the chemical composition and microbial enzyme activities of the bast tissues, and the stem architecture over 42 days at 15 °C. Color changes on the stem surfaces were the first indicators of retting progress, with a significant decrease in the L*, a* and b* values (CIELAB) from day 14 onward. These findings were closely correlated with the surface colonization progress as revealed by scanning electron microscopy, the changes in enzymatic activities and the decohesion of the bast tissues. Additional investigations are needed to study other environmental scenarios to provide an accurate assessment of the retting process over time.

Increasing the accumulation of organic carbon (C) in soils is a crucial challenge both for soil fertility and for climate change mitigation. Heterotrophic microbial communities are key drivers of C cycling in the soil and are influenced by cultural practices, among other factors. However, whether changes in microbial communities in turn affect their C degradation functions is not well understood. Here, we studied the effects of prior soil management on the microbial taxonomic composition and activity of soils amended with wheat litter. Prior soil management was either conventional (CONV) (i.e., full inversion ploughing) or reduced tillage (RT) during a 5 year period in the same loamy soil in northern France. Soil samples taken from the top 5 cm of field plots were incubated with C-13-labelled litter of either flowering wheat or mature wheat for 29 days at 15 degrees C. We measured the C-CO2 and C-13-CO2, microbial biomass C (MBC) and C-13, and hydrolytic enzyme activities during decomposition. The initial bacterial and fungal community diversity was studied via high-throughput sequencing of ribosomal genes. The results showed that the MBC in the RT soil was initially 1.5-fold greater than that in the CONV soil; contrasting taxonomic compositions were also recorded. The soil biotic legacy impacted the degradation functions when the soils were amended with wheat litter. Compared with that in the CONV soil, the enzymatic efficiency of microorganisms in the RT soil increased by 49% and 61% in the presence of mature and flowering wheat litter, respectively. Enzyme efficiency was positively correlated with microbial litter C use efficiency (CUE) (r = 0.92, P-Value < 0.001) but negatively associated with the priming effect (PE) (r = -0.85, P value < 0.001) across all soils and litter treatments. These findings demonstrated that the RT soil benefited both from an increase in litter C incorporated in the microbial biomass and from a reduction in soil C loss due to the PE, regardless of the quality of the decomposed litter. Our study indicated that agricultural practices such as RT, which enriches the amount of soil organic C (SOC) in the topsoil layer, can lead to positive feedback against C stabilization functions.

International audience; Increasing soil organic matter in cropped soils is a major challenge. Soil carbon in arable systems is mainly acquired through above and below-ground litter decomposition, with an efficiency depending both on litter chemical characteristics and on the composition and activity of the soil microbial communities; however, the interactions between these two factors remain poorly understood. To answer to this question, we selected fields from a same soil type which underwent different agricultural practices during 5 years. We tested whether the impacts of these practices on soil biotic characteristics will have legacy effects on litter-C microbial assimilation strategy. The study was conducted at the long-term experimental site SOERE ACBB, located in Northern France (49.873 N, 3.032 E), on a loamy soil (Orthic Luvisol, FAO classification) where six treatments varying by their tillage, N fertilization and crop residues management were set up in 2010 (Coudrain et al., 2016). We sampled in 2015 the 0-5 cm layer of the deep-tillage (CONV) and the reduced-tillage (RT) treatments. A laboratory incubation was performed during 29 days with, for each soil, three litter treatments, i.e. (i) no litter added (control), and uniformly (ii) 13C-labelled mature wheat (recalcitrant litter), and (iii) 13C-labelled flowering wheat residues (labile litter), mixed into the soil at the rate of 3 mg C g-1 dry soil. Dynamics of C-CO2 and 13C-CO2, microbial biomass C and 13C, and hydrolytic enzymatic activities were measured during the decomposition. Non amended RT soil (control) showed higher total C content, C-CO2 emitted, microbial biomass C and enzymatic activities than CONV soil. RT litter-amended treatments exhibited both higher carbon use efficiency (proportion of the litter-13C decomposed incorporated into 13C-microbial biomass) and enzymatic efficiency (amount of litter-13C mineralized per amount of enzymes produced), yet had lower or equivalent priming effects than CONV treatments. These results suggest legacy effect of agricultural practices such as reduced tillage on the microbial C assimilation strategy in soil. We observed also that differences between CONV and RT microbial activities and carbon use efficiency increased with the recalcitrance of the decomposing litter.

The role of precipitation on the decomposition of residue mulches is of primary importance for the adequate management of nutrients in no-tilled agrosystems. The objective of this work was to understand the interactions between water dynamics and crop residue quality and their effect on carbon (C) and nitrogen (N) mineralization. The decomposition of two residue mixtures (wheat + alfalfa and maize + lablab) left at the surface of repacked soil columns, was studied under controlled conditions, at 20 A degrees C over 84 days. Simulated rain pulses were either light and frequent or heavy and infrequent. A loamy soil (Luvisol) and a sandy soil (Ferralsol) were used. The maize/lablab mulch remained wetter between rain pulses which induced greater decomposition than the wheat/alfalfa mulch. Frequent/light rain pulses maintained the mulches wetter between pulses than infrequent/heavy rain pulses, and therefore these mulches decomposed faster. The loamy soil favored the moistening of the mulch layer in contact with the soil which enhanced its decomposition, compared to the sandy soil. The water dynamics (water content of the mulches and soil, evaporation, and drainage) was highly modified by residue quality, rain regime and soil type, which in turn significantly affected the mineralization of C and N.

International audience; The occurrence and diversity of biological soil crusts (BSC) a thin soil surface layer composed of phototrophic and heterotrophic microorganisms intimately associated with soil particles, are known as strong indicators of ecosystem health and sensitivity to soil surface disturbances and are key factors in the biochemical cycling of carbon and nitrogen in drylands. However, the impact of land use on C and N budgets related to BSC dynamics is poorly understood and hinders the prediction of changes in soil fertility in response to future land use scenarios. In this study, we examined the C and N exchange rates of BSC sampled along a north-south pluviometric gradient of the Sub-Sahel, which provides evidence of increasing human land pressure, leading to a gradient in fallow duration and trampling intensity. We demonstrate that the net and gross photosynthesis by BSC significantly increases with fallow duration and a reduction of trampling intensity, thus affecting BSC fine particles and relative water content. Conversely, no effect of land use was found on N fixation or mineralisation rates, which are instead regulated by the N availability within the crust. Simple statistical models were derived from the relationships between C exchange rates and BSC intrinsic characteristics related to soil surface disturbances. The proposed statistical models were tested for C gas exchange with independent data obtained from a new BSC dataset sampled in Burkina Faso and Niger. A simple equation using BSC fine particle content as a unique variable was found to explain between 60 and 70% of the gross photosynthesis. Our findings will help in mapping photosynthesis and estimating the contribution of BSC to the carbon budget at a regional scale in the dryland area of the Sahel and to further testing of land use change scenarios

National audience; Les Croûtes Biologiques de Surface (BSC), ou croûtes microbiotiques, résultent essentiellement de la colonisation de la surface du sol par des micro-organismes, en majorité des cyanobactéries. Ces croûtes représentent un état structural typique des sols nus dans les écosystèmes semi-arides, arides et désertiques. Les micro-organismes qui les composent ont la particularité de fixer l'azote et le carbone atmosphérique. Ils contribuent ainsi à l'amélioration de la qualité des sols dégradés, notamment via la formation de substrats stables vis-à-vis de l'érosion et l'apport de nutriments. Le développement et le fonctionnement de ces croûtes sont profondément affectés par les changements climatiques et les modalités d'usage des sols. Le projet BIOCRUST a pour objet l'étude des croûtes microbiotiques en zone sahélienne (Fig.1). Les sites d'études choisis permettront l'étude de la dynamique spatio-temporelle des BSC dans des contextes climatiques contrastés et sous la pression anthropique des pratiques pastorales et culturales locales.

International audience; Lyophilisation of K(2)SO(4) soil extracts has been proposed as a sample preparation technique before elemental analysis of carbon or nitrogen. However, previous measurements, based on wet oxidation or catalytic combustion, indicated that C measurements in lyophilised samples not always proved to be accurate. To determine whether the C analysis was affected by the lyophilisation process, an exploratory study was conducted to investigate the potential effects of the sample pre-treatment and of the lyophilisation process itself. This paper puts forward that the use of soil extracts, previously stored at -20 degrees C, may affect the recovery of salt in the samples and that contamination of the soluble carbon with exogenous C during lyophilisation is feasible. Therefore we recommend to use freshly prepared soil extracts for lyophilisation and always to include an internal standard among the unknown samples to account for a possible contamination.

International audience; The carbon decomposition of crop residue and agro-industrial effluent was measured under controlled conditions in a highly calcareous and a loamy soil using three different methods: the dynamic cylinder method (DCM), the static cylinder method (SCM) and the static incubation method (SIM). For the latter two static methods, the CO2 trapped by NaOH was analyzed with continuous flow colorimetry, while the accumulated CO2 accumulated in the closed chamber of the dynamic method was measured directly with an infrared gas analyzer. CO2 emission fluxes obtained from the three methods were in good agreement and significantly correlated for both soils. Nevertheless, for fluxes ranging from 10 to 130 μg C-CO2 m−2 s−1, SCM overestimated CO2 emission fluxes by about 15% in comparison to DCM in highly calcareous soil. For fluxes ranging from 0.007 to 1.3 μg C-CO2 kg−1 s−1, SIM slightly underestimated CO2 fluxes from the decomposition of organic substrates. The calculated C mineralization rates of organic materials also confirmed that SIM underestimated C degradation of organic materials compared with SCM and DCM. This result was probably due to the poorer efficiency of the alkali trap. Finally, the static method appears to be inadequate for measuring CO2 emissions in highly calcareous soils since it leads to overestimating C decomposition rates of organic materials due to equilibrium interactions between soil carbonates and bicarbonates.

International audience; This study aimed at determining food processing wastewater composition factors that regulate their carbon and nitrogen mineralization when added to soil. Twenty three different wastewaters from various food processing industries were characterized by C and N concentrations, liquid and solid physical separation and acid solubility. They were also incubated in a calcareous soil during six months at 28 C. The C and N concentrations were low but covered a wide range. Carbon and nitrogen were variously distributed in the liquid and solid fractions and much C was present in the acid-soluble fraction in which C to N ratios were low. The C and N mineralization measured during soil incubation covered a wide range of decomposition pathways. Carbon mineralization was linked significantly (p = 0.05) with the C to N ratio of the acid soluble fraction and C present in the liquid fraction. N mineralization was significantly correlated (p = 0.05) with the organic C to organic N ratio and the C to N ratio of the acid soluble fraction. Multiple factor analysis and clustering also enabled defining clusters which partially overlap the various origins of the wastewaters.

National audience

International audience; This study was focused on investigating the role of the initial residue community, i.e. microorganisms and enzymes from the epiphytic and endophytic compartments, in soil decomposition processes. Aerial and underground parts (leaves and roots) of maize (Zea mays L.) plants were gamma-irradiated, surface-sterilized with sodium hypochlorite (NaOCl)/ethanol or non-sterilized (controls), while the outer surface morphology of maize leaves and roots was examined by scanning electron microscopy (SEM). Non-sterilized and sterilized maize leaves and roots were incubated in soil to study carbon (C) mineralization kinetics and enzyme dynamics (L-leucine aminopeptidase, CBH-1, xylanase, cellulase and laccase). SEM results showed that initial microbial colonization was more pronounced on non-sterilized leaf and root surfaces than on sterilized samples. The hypochlorite treatment removed a part of the soluble components of leaves by washing and no specific effect of any type of colonizing microorganisms was observed on C mineralization. In contrast, gamma irradiation and hypochlorite treatments did not affect root chemical characteristics and the quantitative effect of initial residue-colonizing microorganisms on C mineralization was demonstrated. The variations in C mineralization and enzyme dynamics between non-sterilized and sterilized roots suggested that activities of epiphytic and endogenic microorganisms were of the same order of magnitude.; Cette étude a été réalisée pour étudier le rôle des communautés microbiennes initialement présentes sur les résidus végétaux, c’est-à-dire les micro-organismes et les enzymes provenant des compartiments épiphytes et endophytes, sur les processus de décomposition dans les sols. Les parties aériennes et souterraines (feuilles et racines) du maïs (Zea mays L.) ont été soit γ-irradiées, soit stérilisées en surface avec de l’hypochlorite de sodium (NaOCl)/éthanol ou enfin non stérilisées (témoin), alors que la morphologie des surfaces externes des feuilles et des racines de maïs a été observée par microscopie électronique à balayage (MEB). Les feuilles et les racines de maïs non stérilisées et stérilisées ont été incubées dans le sol pour étudier les cinétiques de minéralisation du carbone et les dynamiques enzymatiques (L-leucine aminopeptidase, CBH–1, xylanase, cellulase et laccase). Les observations au MEB ont montré que la colonisation microbienne initiale était plus importante sur les surfaces des feuilles et des racines non stérilisées que sur les échantillons stérilisés. Le traitement à l’hypochlorite a éliminé une partie des composés solubles des feuilles par lavage et donc aucun effet spécifique de la colonisation microbienne initiale sur la minéralisation du carbone n’a pu être constaté. En revanche, les traitements irradiation γ et hypochlorite n’ont pas affecté les caractéristiques chimiques des racines et l’effet quantitatif des micro-organismes colonisateurs des résidus initiaux sur la minéralisation du carbone a été démontré. Les variations de minéralisation du carbone et des dynamiques enzymatiques entre les racines non stérilisées et stérilisées suggèrent que les activités des micro-organismes épiphytes et endogènes sont du même ordre de grandeur.