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Carbon sequestration in soils became a major issue that governments have to face under their sustainable development objectives and the international 4p1000 program. Although, earthworms are recognized to play a key role in the structure and dynamics of organic matter (OM) in soils, their contribution to soil OM cycling is not taken into account in biogeochemical models nor well understood. In particular, the fate of OM protected in earthworm casts is unknown. In this study, we investigated the effects of ageing under field conditions on the OM dynamics contained in casts produced by the anecic earthworm Amynthas adexilis in North Vietnam. To this end we investigated (1) the microscale organisation of particulate organic matter and pores during the exposure of casts and control aggregates during 12 months and (2) compared it to the potential OM mineralisation during a laboratory incubation.Our results indicated that fresh casts contained significantly more particulate organic matter (POM) than control soil aggregates and field aged earthworm casts. Conversely, the porosity was higher in soil control aggregates than in casts and the porosity of casts tended to increase with their ageing. The analyses of micro-CT images also revealed that POM and Pores contents between casts samples presented strong variabilities even in the youngest casts category. We found, on average, higher mineralisation rates for casts than for controls and a reduction of the OM mineralisation with the ageing of casts. Our results also highlighted a strong positive correlation (r2 = 0.89) between POM contents determined by the segmentation of micro CT images and CO2 emissions from the incubation experiment. We conclude that earthworms impact the microscale organisation of POM and pores in their casts and thereby influence soil OM dynamics.

Many soils in the Boreal forest regions of the Arctic store very large amounts of carbon in the active layer above permafrost, and store significant amounts of carbon within the permafrost. Soils that are well drained, high in rock fragments, shallow to rock or rubble, or covered with ice are exceptions. No other region on Earth stores more carbon on average than the Arctic regions, especially in wetlands. However, changes in vegetation and soil are expected under warming climates. Research questions have arisen about future changes in vegetation and net carbon flux as soil and air temperatures climb, as precipitation amount and type changes, and as the growing season lengthens. A review of recent literature will be conducted to look at effects of vegetation change and annual carbon dynamics in Boreal forest and wetland soils under warming climates. Environmental variables such as soil temperature, hydrology, microbial and higher plant growth, digestibility of young and old carbon, fire, location zone, extent and type of permafrost thaw slow vs sudden collapse), and N and P nutrient balances will affect carbon stocks in addition to changing climate.

International audience; Biochar addition to compost is of growing interest as soil amendment. However, little is known about the evolution of material properties of biochar-compost mixtures and their effect on plants after exposure to physical weathering. This study aimed to investigate the physico-chemical characteristics of fresh and weathered biochar-compost mixtures, their biological stability and their effect on ryegrass growth. To this end, we used the contrasting stable isotope signatures of biochar and compost to follow their behavior in biochar-compost mixtures subjected to artificial weathering during 1-year of incubation. We assessed their impact on ryegrass growth during a 4-week greenhouse pot experiment. Weathering treatment resulted in strong leaching of labile compounds. However, biochar-compost interactions led to reduced mass loss and fixed carbon retention during weathering of mixtures. Moreover, weathering increased carbon mineralization of biochar-compost mixtures, probably due to the protection of labile compounds from compost within biochar structure, as well as leaching of labile biochar compounds inhibiting microbial activity. After soil application, weathered mixtures could have positive effects on biomass production. We conclude that biochar-compost interactions on soil microbial activity and plant growth are evolving after physical weathering depending on biochar production conditions.

Organic amendments may improve the quality of acidic tropical agricultural soils with low organic carbon contents under conventional management (mineral fertilization and irrigation) in Southeast Asia. We investigated the effect of biochar, compost and their combination on maize growth and yield, soil physical, biological and chemical properties at harvesting time at four sites in three countries: Thailand, Vietnam and Laos. Treatments consisted of 10 t·ha−1 cow manure compost and 7 t·ha−1 of Bamboo biochar and their combination. Maize biomass production and cop yields were recorded for two seasons. Elemental content, pH and nutrient availability of soils were analyzed after the first growing season. We also characterized macrofauna abundance and water infiltration. Few changes were noted for maize biomass production and maize cop yield. Soil chemical parameters showed contrasting, site-specific results. Compost and biochar amendments increased soil organic carbon, pH, total K and N, P and K availability especially for sandy soils in Thailand. The combination of both amendments could reduce nutrient availability as compared to compost only treatments. Physical and biological parameters showed no treatment response. We conclude that the addition of compost, biochar and their mixture to tropical soils have site-specific short-term effects on chemical soil parameters. Their short-term effect on plants is thus mainly related to nutrient input. The site-dependent results despite similar crops, fertilization and irrigation practices suggest that inherent soil parameters and optimization of organic amendment application to specific pedoclimatic conditions need future attention.

International audience; Earthworm cast ageing controls soil ecosystem services such as carbon storage and nutrient availability. To test the hypothesis that biogeochemical changes during casts ageing can be traced by infrared spectroscopy, we collected topsoil aggregates and earthworm casts produced by the anecic earthworm Amynthas khami belonging to five distinct degradation stages in woodland in northern Vietnam. We analysed mid-infrared (MIR) spectra of both sample types using two approaches: (1) calculation of the proportion of aliphatic compounds and the humification index of organic matter (OM) using specific wavenumbers and (2) linear discriminant analysis (LDA) of the most important peaks and shoulders of the entire spectrum. Our results showed that aliphatic compound contribution to OM was similar and the humification index of OM lower (2-fold) in casts as compared to topsoil aggregates independent of their degradation stage. In contrast, LDA showed a continuous shift from fresh casts to aged casts and topsoil aggregates in agreement with biogeochemical changes of cast properties. We thus conclude that MIR analysis of bulk samples and LDA taking into account the absorbance intensity of most important wavenumbers is a promising tool to capture biogeochemical alterations of organic and mineral earthworm cast properties during ageing.

Changes in climate, land use, and land management impact the occurrence and severity of wildland fires in many parts of the world. This is particularly evident in Europe, where ongoing changes in land use have strongly modified fire patterns over the last decades. Although satellite data by the European Forest Fire Information System provide large-scale wildland fire statistics across European countries, there is still a crucial need to collect and summarize in-depth local analysis and understanding of the wildland fire condition and associated challenges across Europe. This article aims to provide a general overview of the current wildland fire patterns and challenges as perceived by national representatives, supplemented by national fire statistics (2009–2018) across Europe. For each of the 31 countries included, we pres ent a perspective authored by scientists or practitioners from each respective country, representing a wide range of disciplines and cultural backgrounds. The authors were selected from members of the COST Action “Fire and the Earth System: Science & Society” funded by the European Commission with the aim to share knowledge and improve communication about wildland fire. Where relevant, a brief overview of key studies, particular wildland fire challenges a country is facing, and an overview of notable recent fire events are also presented. Key perceived challenges included (1) the lack of consistent and detailed records for wildland fire events, within and across countries, (2) an increase in wild land fires that pose a risk to properties and human life due to high population densities and sprawl into forested regions, and (3) the view that, irrespective of changes in management, climate change is likely to increase the frequency and impact of wildland fires in the coming decades. Addressing challenge (1) will not only be valuable in advancing national and pan-European wildland fire management strategies, but also in evaluating perceptions (2) and (3) against more robust quantitative evidence.

Interactions of earthworm species with their environment and in particular the soil they inhabit is poorly understood. In this study, we aimed to use mid infrared absorption (MIR) spectroscopy of earthworm bodies in combination with discriminant analysis of principal components (DAPC), in order to investigate if their biogeochemical composition is determined by their species belonging or the soil they inhabit. Our conceptual approach included the comparison of MIR spectra of four earthworm species (Lumbricus terrestris, Allolobophora chlorotica, Aporrectodea caliginosa and Aporrectodea icterica) sampled from four different agricultural fields in France. Moreover, we tested the influence of the habitat on the earthworm MIR spectra by exchanging individuals and soils in an incubation experiment. MIR spectra varied according to species and sampling site. The MIR spectra of earthworm bodies also changed during the incubation experiment, and for L. terrestris, A. chlorotica and A. icterica depended on the properties of the soil ingested. This suggested that the food source in different habitats impacted the biogeochemical composition of earthworm bodies. DAPC allowed for the identification of earthworm species ranging from 64 to 100% of accuracy (mean 78%). We thus conclude that MIR spectroscopy may be a suitable approach to identify earthworm species, but that their signature may also allow to investigate their ecology as determined by the soil they inhabit.

The transformation of organic matter (OM) occluded in earthworm casts during the casts' lifetime may affect its composition and accumulation in stabilised OM pools. We collected casts produced by the anecic earthworm Amynthas khami in a tropical woodland of Northern Vietnam presenting different degradation stages. The aim of this study was to assess (1) the biogeochemical properties of OM in earthworm casts compared to control aggregates from the surrounding soil, and (2) the fate of occluded OM during cast ageing. We analysed intact casts and their density fractions at five ageing stages for organic carbon (OC) and nitrogen (N) content, OM chemical composition by analytical pyrolysis, and OM stability by Rock-Eval thermal analysis. Compared to control aggregates, fresh casts had higher OC values (5.3 vs. 2.6%) and increased OC in all density fractions. Cast OM had more lignin (1.1 vs.

Plant residues are the main precursors of soil organic matter (SOM) and soil macrofauna is an important driver of ecological processes involved in the sequestration of carbon (C) in soils. In particular, earthworms are one of the largest contributors to soil matter formation in most terrestrial ecosystems. In the short term, they may increase the rate of OM turnover by mineralization, fragmentation and stimulation of microbial activity. On the other hand they may reduce OM degradability by forming stable aggregates and organo-mineral complexes protecting C from mineralization for longer time scales. Earthworms are classified in three main ecological groups depending on their behaviors and on their morpho-functional traits. However, their intra- or inter- ecological group effect on C stabilization needs to be investigated.In this study, we explored the impact of earthworm diversity (composed of several species belonging to different ecological groups) on the physicochemical properties of casts, related to CO2 emissions. We hypothesized that C mineralization in casts would be related to the ecological category.We studied casts of 6 species (2 anecic species: Lumbricus terrestris & Aporectodea nocturna, 2 endogeic species: Allolobophora icterica & Aporrectodea caliginosa and 2 epigeic species: Lumbricus castaneus & Eisenia fetida) produced in a silty subsoil with addition of plant litter. Casts were incubated for 140 days under similar laboratory conditions. We measured CO2 mineralization, pH, elemental composition and physical cast organization by X-ray microtomography (resolution of 9.49 µm voxel) at 7, 42, and 140 days.Our results showed lower CO2 mineralization in aggregates produced without earthworms than all earthworm casts. In the beginning of the incubation casts showed similar CO2 emissions and specific physicochemical properties as OC content and pH. After 140 days, CO2 emissions were earthworm species specific with Aporectodea nocturna showing highest CO2 emissions, and Aporrectodea caliginosa the lowest values. Microtomographic analyses showed that this is due to an increase of cast porosity with increasing cast age coupled with a concurrent decrease particulate organic matter (POM) structures. Our first results seemed to suggest that earthworms belonging to the same ecological category influence similarly C mineralization through their impact on the cast organization.

International audience; Clay-sized soil minerals are known to protect organic carbon (OC) from mineralisation by formation of organo-mineral associations limiting its availability to microorganisms. The impact of soil fauna on these processes is poorly known. The aim of this study was to investigate the effect of earthworms on organic matter (OM) decomposition and association with minerals during a laboratory experiment. We used a model system consisting of fresh OM incubated with and without epigeic earthworms (Eisenia andrei and foetida) in presence of different types and amounts of phyllosilicates (kaolinite, montmorillonite) and an iron oxide (goethite) and combinations of these minerals. Our experimental setup included a high OM:mineral ratio to represent the soil-litter interphase. We monitored OC mineralisation during 196 days. Additionally, we investigated physicochemical parameters and chemical OM characteristics of decomposition products by determination of water-soluble OC (WSOC) and acquisition of solid-state 13C NMR spectra. We also analysed microscale organisation of the organo-mineral associations produced with and without earthworms by transmission electron microscopy (TEM).Earthworms enhanced OC mineralisation in all treatments. They also led to greater reductions of OC emissions in the presence of minerals as compared to the mineral-free control, depending on the type and amount of minerals added. The presence of earthworms affected microbial biomass, the concentration of WSOC and increased the contribution of aromatic compounds to OM decomposition products. Microscale analyses by TEM showed that earthworms favoured association of minerals with partly degraded OM along with completely degraded material, while in absence of earthworms only completely degraded OM was associated with minerals. We conclude that earthworms impact OM decomposition through (1) their effect on microbial biomass and the physicochemical parameters of microbial habitat and (2) the formation of OM associations by changing the OM types associated to minerals and possibly by creating closer association of partly degraded OM and iron oxides. The stability of these associations remains to be investigated.

Grassland management practices, such as grazing with varying animal density and mowing may impact the processes leading to soil organic carbon (SOC) accumulation. Although, they serve similar agricultural purposes, they differ in their effect on plant physiology and their influence on SOC remains uncertain. We hypothesised that both practices affect SOC storage differently due to an altered plant C input and changed growth and physiological response leading consequently to contrasting soil microbial activity.Based on this, our experiment included the investigation of three grassland treatments: grazing at two intensities and mowing which are located at the experimental station of SOERE ACBB (Clermont-Ferrand, France). Additionally, we included bare soil and unmanaged abandoned site considering as negative and positive controls, accordingly. The aim of the study was to estimate how grazing and mowing affect SOC chemical characteristics and its link with microbial activity.Our results show highest SOC contents under low grazing intensity, whereas SOC content under high grazing intensity was lower and did not differ from abandoned grassland. SOC content under mowing was lowest among all treatments but still higher compared to bare soil. Microbial biomass C (MBC) followed a similar pattern under high grazing intensity and positive control whereas it was similar under mowing and low grazing intensity and lowest under bare soil. Absolute enzyme activities showed a similar tendency as SOC content. However, enzyme activities per MBC resulted in highest values under low grazing intensity and similarly lower values under all other treatments.These results demonstrate that microbial parameters responded to management in various ways most probably related to the differences in dung and litter inputs. We suggest that dung input under high grazing intensity increased MBC and consequently compensated for plant removal thus keeping SOC contents increasing. Consequently, grazing at both intensities allows to maintain SOC at similar levels as in absence of management. While on unmanaged land high SOC may be related to absence of harvest, on grazed land it may be related to stimulation of microbial activity due to animal activity. Mowing treatment on the other hand did not allow to increase SOC.We conclude that the presence of animals in the system is essential to improve soil heath, biogeochemical cycling, and SOC storage.

International audience; Evaluation of the temperature sensitivity of soil organic matter (SOM) decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon and, therefore, accelerate or mitigate climate warming. It is usually described, using Arrhenius kinetics, as increasing with the stability of the substrate in laboratory conditions, where substrate availability is non-limiting and where chemical recalcitrance, therefore, predominantly regulates stability. However, conditions of non-limiting subtrate availability are rare in the undisturbed soil, where physicochemical protection of substrates may control their stability. The aim of this study was to assess the temperature sensitivity of decomposition of SOM with contrasting stability in the field. Our conceptual approach was based on in situ measurements of soil CO2 efflux at a range of temperatures from root exclusion plots of increasing age (1 month and three decades) and, therefore, with SOM of increasing stability. From a set of short-term measurements in spring, using diurnal temperature variation, the relative temperature sensitivity of SOM decomposition decreased significantly (p < 0.0001) with increasing SOM stability, and was weak (Q(10) < 1.3) in long-term root exclusion plots. This result was confirmed in a similar set of short-term measurements repeated later in the year, in summer, as well as from an analysis perfomed at the seasonal timscale. We provide direct field evidence that the temperature sensitivity of SOM decomposition decreases with increasing stability, in direct contrast with Arrhenius kinetics prediction, and therefore show that stability of SOM in the field cannot be the sole result of chemical recalcitrance. We conclude that the physicochemical protection of SOM, which controls SOM stability in the field, constrains the temperature sensitivity of SOM decomposition under field conditions. (C) 2019 The Author(s).

International audience; Climate change adaptation, mitigation and food security may be addressed at the same time by enhancing soil organic carbon (SOC) sequestration through environmentally sound land management practices. This is promoted by the “4 per 1000” Initiative, a multi-stakeholder platform aiming at increasing SOC storage through sustainable practices. The scientific and technical committee of the Initiative is working to identify indicators, research priorities and region-specific practices needed for their implementation. The Initiative received its name due to the global importance of soils for climate change, which can be illustrated by a thought experiment showing that an annual growth rate of only 0.4% of the standing global SOC stocks would have the potential to counterbalance the current increase in atmospheric CO2. However, there are numerous barriers to the rise in SOC stocks and while SOC sequestration can contribute to partly offsetting greenhouse gas emissions, its main benefits are related to increased soil quality and climate change adaptation. The Initiative provides a collaborative platform for policy makers, practitioners, scientists and stakeholders to engage in finding solutions. Criticism of the Initiative has been related to the poor definition of its numerical target, which was not understood as an aspirational goal. The objective of this paper is to present the aims of the initiative, to discuss critical issues and to present challenges for its implementation. We identify barriers, risks and trade-offs and advocate for collaboration between multiple parties in order to stimulate innovation and to initiate the transition of agricultural systems toward sustainability.

International audience; The quality of organic soil amendments such as composts and vermicomposts can be improved by using additives during their production. However, little is known about the impact of these materials on carbon flow in the plant-soil system. We investigated the impact of organic amendments produced through composting and vermicomposting in the presence of clay minerals (montmorillonite) and/or earthworms on plants (perennial ryegrass, Lolium perenne and common bean, Phaseolus vulgaris) and carbon flow in soil. We carried out a growth chamber experiment with continuous 13C labelling and analysed above-/below-ground biomass production, as well as biogeochemical parameters of plant and soil (rhizospheric and non-rhizospheric) compartments. Quantitative biogeochemical analyses were further merged with microscale elemental and isotopic information (NanoSIMS) to investigate carbon transfer in the rhizopheric soil. Our results showed lowest biomass production for treatments amended with organic amendments produced without minerals probably related to the release of harmful substances (excess of dissolved organic carbon, salts and inorganic nitrogen) due to the immature nature of these materials. The use of montmorillonite as additive during the preparation of the amendments alleviated these adverse effects and resulted in a significant increase of above-/below-ground plant biomass production. When the organic amendments were produced in the presence of eathworms and montmorillonite, the higher plant biomass promoted the release of root-derived carbon within the rhizospheric soil (compared with amendments without clay minerals), which was used as an energy-rich substrate by microorganisms. We conclude that the use of mineral additives during composting or vermicomposting favours plant biomass production and carbon transfer to the soil and its microorganisms, which could enhance soil carbon storage in the longer term.

The objective of the study was to characterise co-composting dynamics and maturity of the end products of sewage sludge/ green waste mixtures. We analysed all initial substrates and co-composts, sampled at different composting times for studying (1) physicochemical parameters and (2) organic matter biodegradation and biotransformation using pyrolysis coupled to gas chromatography and mass spectrometry. Physicochemical analysis indicated compost maturity of the three co-composting mixtures after 200 days. Pyrolysis-GCMS showed that organic matter in sewage sludge generated a large number of different pyrolysis products, most of which had unspecific origin. The pyrolysis products of the three co-compost were classified according to their chemical composition (benzene derivatives, N-containing compounds, phenol derivatives, furan derivates) and according to their chemical structure (polyaromatics, aromatics compounds and aliphatic compounds). These compound classes evolved differently during composting. We suggest two new specific ratios of peak areas from pyrolysis products to describe the biodegradation and to evaluate the degree of maturity during co-composting sludge-green waste: (1) the ratio benzene, methyl (methylethenyl) /benzene, which decrease during composting, and (2) the ratio of Indene/2-methoxy-4 vinyl-phenol which increase during composting. The good correlation between these ratios and physico-chemical parameters confirms their usefulness to evaluate the co-compost maturity.

Sustainable soil carbon sequestration practices need to be rapidly scaled up and implemented to contribute to climate change mitigation. We highlight that the major potential for carbon sequestration is in cropland soils, especially those with large yield gaps and/or large historic soil organic carbon losses. The implementation of soil carbon sequestration measures requires a diverse set of options, each adapted to local soil conditions and management opportunities, and accounting for site-specific trade-offs. We propose the establishment of a soil information system containing localised information on soil group, degradation status, crop yield gap, and the associated carbon-sequestration potentials, as well as the provision of incentives and policies to translate management options into region- and soil-specific practices., Reducing soil degradation and improving soil management could make an important contribute to climate change mitigation. Here the authors discuss opportunities and challenges towards implementing a global climate mitigation strategy focused on carbon sequestration in agricultural soils, and propose a framework for guiding region- and soil-specific management options.

Response to “The “4p1000” initiative: A new name should be adopted” by Baveye and White (2019)

Alkaline industrial wastes such as red mud and fly ash are produced in large quantities. They may be recycled as bulking agent during composting and vermicomposting, converting organic waste into soil amendments or plant growth media. The aim of this study was to assess the microbial parameters, greenhouse gas emissions and nutrient availability during composting and vermicomposting of household waste with red mud and fly ash 15% (dry weight). CO2, CH4 and N2O emissions were monitored during 6 months in controlled laboratory conditions and microbial biomass and phospholipid acids, N and P availability were analysed in the end-products. Higher CO2 emissions were observed during vermicomposting compared to composting. These emissions were decreased by red mud addition, while fly ash had no effect. Nitrate (NO3-N) content of the end-products were more affected by worms than by alkaline materials, while higher ammonium (NH4-N) contents were recorded for composts than vermicomposts. Red mud vermicompost showed higher soluble P proportion than red mud compost, suggesting that worm presence can counterbalance P adsorption to the inorganic matrix. Final composts produced with red mud showed no harmful heavy metal concentrations. Adding worms during composting thus improved the product nutrient availability and did not increase metal toxicity. From a practical point of view, this study suggests that for carbon stabilisation and end-product quality, the addition of red mud during composting should be accompanied by worm addition to counterbalance negative effects on nutrient availability.

Differentiation of biochar and native soil organic matter (SOM) is required to assess the effect of biochar amendment on in-situ changes of SOM. Therefore, we used C-4 biochar produced at high temperature (1200 degrees C) by gasification (BCGS) and measured the C-13 abundance of density and particle size fractions. We quantified the BCGS effects on distinct native C-3-SOM pools of a grassland topsoil one year after BCGS amendment. The chemical composition was analyzed with solid-state C-13 CPMAS NMR, whereas information on the nanostructure of BCGS were obtained by Raman microspectroscopy measurements. Our aim was to assess BCGS induced chemical changes of SOM and physical fractions and to validate the accuracy of BCGS detection by C-13 NMR spectroscopy. Quantification by isotopic measurements and C-13 NMR spectroscopy for aromatic C yielded similar estimates of BC in soils. Of the total BCGS, 52% were recovered as free particulate organic matter (POM) and 33% were located in aggregated soil structures isolated as occluded POM particles. Around 4% of the total BCGS was detected in the clay fraction. After one year of field exposure, the surface of the BCGS particles decreased in unordered graphitic-like structures. The higher ordered BC residue is supposed to be more recalcitrant. The native SOC stock increase (p = 0.06, n = 4) in the clay fraction indicated increased sequestration of organic matter as mineral-bound SOM due to BCGS amendment. With respect to soil functionality, the BCGS amendment induced a tremendous shift from a soil system dominated by organo-mineral associations to POM-dominated OC storage, resulting in increased soil air capacity. (C) 2017 Elsevier Ltd. All rights reserved.

International audience; Multivalent cation interaction and mineral association play a vital role in organic carbon (OC) stability in tropical soils. However, the retention mechanisms of OC in mineral-cation-OC systems remain largely unclear at the molecular level. Of all OC forms, low molecular weight organic acids (LMWOAs) are readily biodegradable. Given the general acidic conditions and dominance of kaolinite in tropical soils, we investigated the retention mechanisms of malic acid (MA) in kaolinite-Fe(III)-MA systems with various Fe/MA molar ratios at pH similar to 3.5 using Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Our results indicated the formed ferrihydrite mainly contributed to the enhanced MA retention through adsorption and/or coprecipitation at high Fe/MA molar ratios in kaolinite-Fe(III)-MA systems relative to kaolinite-MA systems. With the decrease of Fe/MA molar ratios, ternary complexation of MA with kaolinite via a tetrahedral Fe bridge occurred in kaolinite-Fe(III)-MA systems. This study highlighted the significant impact of Fe speciation on MA retention mechanisms in kaolinite-Fe(III)-MA systems under acidic conditions, and clearly showed the impact of ferrihydrite-induced adsorption/coprecipitation and Fe-bridged ternary complexation on MA retention in ternary kaolinite-Fe(III)MA systems, which will enhance our understanding of the dynamics of MA and other LMWOAs in tropical soils.

Take these eight steps to make soils more resilient to drought, produce more food and store emissions, urge Cornelia Rumpel and colleagues. Take these eight steps to make soils more resilient to drought, produce more food and store emissions, urge Cornelia Rumpel and colleagues.

To maintain grassland productivity and limit resource depletion, scarce mineral P (phosphorus) fertilizers must be replaced by alternative P sources. The effect of these amendments on plant growth may depend on physicochemical soil parameters, in particular pH. The objective of this study was to investigate the effect of soil pH on biomass production, P use efficiency, and soil P forms after P amendment application (100 mg kg&minus, 1 P) using poultry manure compost (PM), rock phosphate (RP), and their combination (PMRP). We performed a growth chamber experiment with ryegrass plants (Lolium perenne) grown on two soil types with contrasting pH under controlled conditions for 7 weeks. Chemical P fractions, biomass production, and P concentrations were measured to calculate plant uptake and P use efficiency. We found a strong synergistic effect on the available soil P, while antagonistic effects were observed for ryegrass production and P uptake. We conclude that although the combination of PM and RP has positive effects in terms of soil P availability, the combined effects of the mixture must be taken into account and further evaluated for different soil types and grassland plants to maximize synergistic effects and to minimize antagonistic ones.

International audience; The aim of this study was to determine if three cereal crops differed in their behavior to take up soil and fertilizer P, with emphasis on the relationship between phosphatase activity and P fractionation. We used a vertical rhizobox experiment with wheat, oat, and barley sown on Chilean Andisol (Barros Arana Series) with low P availability under greenhouse conditions. Plants were fertilized with the equivalent of 100 kg P ha−1 of triple superphosphate (TSP) or rock phosphate (RP). Plant biomass was determined for each of the three cereal plant species. Additionally, phosphatase (P-ase) activity in roots, soil in presence of roots (soil+R), and soil in absence of roots (soil−R) after 60-day growth were evaluated, and soil P fractionation was determined using the Hedley procedure. Fertilizer increased both P uptake and biomass production, particularly in shoots. The P uptake efficiency (∆P uptake between fertilized and unfertilized treatment/P input) was low (4.6%) and similar for both fertilizers for oat, but RP was more efficient for wheat (> 30%) and even more so for barley (nearly threefold), due the higher shoot P concentration of RP fertilized plants, which could be attributable to a major P-ase activity in plants fertilized with RP. Despite, fertilizer P was most clearly identified in labile inorganic soil fractions with Olsen P being greater after TSP addition than RP. In particular, plants showed contrasting soil+R P-ase activity inducing differences in soil+R P speciation, increasing labile NaHCO3-Pi and NaOH-Pi fractions with TSP. Strong relationships were found between the sum of labile Pi fractions and P uptake. We conclude that slower release of RP has a positive impact on P-ase activity and leads to better fertilizer efficiency than TSP, especially for barley.

In terrestrial ecosystems, plants are the transducers that provide the energy for microbial metabolism through root exudation, cell sloughing, and the input of leaf and root litter. They have profound impacts on biogeochemical cycles and are pivotal control points in the soil for the regulation of ecosystem biogeochemistry. Plant biomass is composed of C-, N-, and P-containing molecules, which are synthesized during plant growth after assimilation of atmospheric CO2 and mineral nutrients, thus leading to coupling of elemental cycles. Plant-derived litter compounds will undergo different fates depending upon their properties, their localization, and availability to the soil microbial biomass. Microbial degradation leads to decoupling of C, N, and P cycles, and it results in CO2 emission and nutrient release. Soluble N and P forms are susceptible to be lost from the system if not taken up by plants or microorganisms. On the other hand, microbial activity stimulated by plant-derived organic matter input may also reuse these mineral N and P and recouple them with C. All three processes may be influenced by plant activity. Plants are able to control microbial processes by exudation of signalling molecules and to closely interact with rhizosphere microorganisms. In addition, CNP coupling and decoupling may be controlled by plants through their symbiosis with mycorrhizal fungi. The aim of this chapter is to shed light on plants' impact on the processes involved in the coupling and decoupling processes, which control stoichiometric relationships in different ecosystems, and to show how they control carbon sequestration and other ecosystem services. By understanding the plants' control on CNP cycles, important advances for the understanding of biogeochemical feedbacks, which may ultimately constrain long-term ecosystem responses to global change, can be achieved.

Decomposition is a major flux of the carbon cycle in forest soils and understanding the involved processes is a key for budgeting carbon turnover. Decomposition is constrained by the presence of biological agents such as microorganisms and the underlying environmental conditions such as water availability. A metabarcoding approach of ribosomal markers was chosen to study the succession of bacterial and fungal decomposers on root litter. Litterbags containing pine roots were buried in a pine forest for two years and sequentially sampled. Decomposition and the associated communities were surveyed under ambient dry and long-term irrigation conditions. Early decomposition stages were characterized by the presence of fast-cycling microorganisms such as Bacteroidetes and Helotiales, which were then replaced by more specialized bacteria and litter-associated or parasitic groups such as Acidobacteria, white rots, and Pleosporales. This succession was likely driven by a decrease of easily degradable carbohydrates and a relative increase in persistent compounds such as lignin. We hypothesize that functional redundancy among the resident microbial taxa caused similar root decomposition rates in control and irrigated forest soils. These findings have important implications for drought-prone Alpine forests as frequent drought events reduce litter fall, but not litter decomposition, potentially resulting in lower carbon stocks.

10 pages; International audience; Stable isotopes probing is among the most used methods applied in the studies of paleoenvironment. Isotope signatures of sediments are influenced by different environmental conditions such as climate, salinity, and plant coverage at the time those sediments were deposited, representing an inestimable value for paleoenvironmental reconstruction and interpretation. In this study the carbon stable isotopic composition of organic matter (δ13CORG) and lignin monophenols (δ13CVSC) extracted from this organic matter of submerged vascular plant Stuckenia pectinata (L.) Böerner 1912 syn. Potamogeton pectinatus L 1753 was investigated. Samples were collected from five different environments along a gradient of salinity from brackish Baltic Sea and freshwater lake in Estonia. The salinity influenced δ13CDIC of ambient water, in the freshwater 13C depletion was observed compare to brackish sites. S. pectinata have an intraspecific variation of the carbon stable isotope signature of organic matter depending on the environmental conditions, samples were 13C enriched in the more brackish sites. Strong positive correlation between δ13CORG and C/N ratio for both leaves and stems were found. Lignin monophenols were 13C depleted if compared with bulk organic matter, reflecting a common biochemical pattern of C fixation during lignin synthesis as for terrestrial plants. To conclude, organic matter isotopic signature and lignin composition of aquatic plants responded to environmental salinity. The innovative combination of these analyses provides a more refined interpretation of δ13CORG recorded in sediment deposited in the zones colonized by those plants.

Increasing soil carbon stocks in agricultural grasslands has a strong potential to mitigate climate change. However, large uncertainties around the drivers of soil respiration hinder our ability to identify management practices that enhance soil carbon sequestration. In a context where more intense and prolonged droughts are predicted in many regions, it is critical to understand how different management practices will temper drought-induced carbon losses through soil respiration. In this study, we compared the impact of changing soil volumetric water content during a drought on soil respiration in permanent grasslands managed either as grazed by dairy cows or as a mowing regime. Across treatments, root biomass explained 43% of the variability in soil respiration (p &lt, 0.0001). Moreover, analysis of the isotopic composition of CO2 emitted from the soil, roots, and root-free soil suggested that the autotrophic component largely dominated soil respiration. Soil respiration was positively correlated with soil water content (p = 0.03) only for the grazed treatment. Our results suggest that the effect of soil water content on soil respiration was attributable mainly to an effect on root and rhizosphere activity in the grazed treatment. We conclude that farm management practices can alter the relationship between soil respiration and soil water content.

International audience; Soil is considered as a non-renewable resource, which may be lost in sloping land more rapidly than it is formed thus leading to loss of fertility and ecosystem deterioration. We hypothesized that earthworms could counteract this process due to their cast forming activity. To test this hypothesis, we quantified the production of casts in small plots of 0.25 m2 established in three vegetation units (woodland, shrubland and meadow) in a catchment of 46 ha for 2.5 years in relation to their (micro-) pedoclimatic conditions. We also assessed the impact of cast formation on water runoff and soil detachment. Moreover, we quantified the mass of casts deposited in the entire catchment on a regular grid of 50 m and we measured soil erosion at the outlet of the catchment. Our results showed a high and variable production of casts (from 16 to 219 t ha−1 year−1) depending on vegetation, season and year. The mass of casts found in the entire catchment represented on average 7.4 t ha−1 with an annual production rate of 35 t ha−1. Since the annual erosion rate measured for the entire catchment (3 t ha−1 year−1) was much lower than the cast production rate, our results indicated that most of the soil bioturbated by earthworms remained in the catchment. Indeed, water runoff and soil detachment measured in small plots showed that casts were not transported in the water runoff but degraded by raindrop impacts with the material remaining at the place of deposition. This process led to the generating of a new soil horizon at the culmination point of the catchment of up to 6.5 cm after 2.5 years. We conclude that the surface activity of anecic earthworms could influence soil generation at scale and conteract the effects of soil erosion.

International audience

Atténuation des émissions des gaz à effet de serre dans les systèmes prairiaux

In this study we evaluated CO2 emissions during composting of green wastes with clay and/or biochar in the presence and absence of worms (species of the genus Eisenia), as well as the effect of those amendments on carbon mineralization after application to soil. We added two different doses of clay, biochar or their mixture to pre-composted green wastes and monitored carbon mineralization over 21 days in the absence or presence of worms. The resulting co-composts and vermicomposts were then added to a loamy Cambisol and the CO2 emissions were monitored over 30 days in a laboratory incubation. Our results indicated that the addition of clay or clay/biochar mixture reduced carbon mineralization during co-composting without worms by up to 44 %. In the presence of worms, CO2 emissions during composting increased for all treatments except for the low clay dose. The effect of the amendments on carbon mineralization after addition to soil was small in the short term. Overall, composts increased OM mineralization, whereas vermicomposts had no effect. The presence of biochar reduced OM mineralization in soil with respect to compost and vermicompost without additives, whereas clay reduced mineralization only in the composts. Our study indicates a significant role of the conditions of composting on mineralization in soil. Therefore, the production of a low CO2 emission amendment requires optimization of feedstocks, co-composting agents and worm species.

We identified and quantified specific biomarkers of shoots and roots (cutin and suberin-derived compounds, respectively) of three grassland species (Dactylis glomerata L., Festuca arundinacea Schreb. and Lolium perenne L.) in soil under different land use (grass, crop and bare soil) of the SOERE-ACBB experimental site in Lusignan (France). We also investigated the fate of these markers in soil after conversion from grassland (C3 plants) to Zea mays L. (maize) (C4 plant) with natural 13C isotope abundances. Our results indicated that 9-hydroxy hexadecanedioic acid and 8(9)(10),16-dihydroxy hexadecanoic acid may be used as biomarkers for above-ground biomass, whereas 1,22-docosandioic acid, 22-hydroxy docosanoic acid and 24-hydroxy tetracosanoic acid might be the best below-ground biomarkers for the plants investigated under the experimental conditions studied. The presence, concentration and shoot–root allocation pattern of these markers were different from those described for other species, which demonstrates the importance of verifying biomarker specificity for each species. Concentrations of cutin and suberin were largest in soil under maize and smallest under bare soil; this corresponded to the biomass added to the two soils. Suberin decreased by 40–64% and cutin by 24–40% during a 6-year bare fallow, which indicates that root markers were more sensitive than shoot markers to degradation. Changes in 13C isotopic signatures of specific biomarkers after 6 years of maize showed a faster turnover of root than shoot biomarkers, in spite of the much smaller root inputs from maize than from grasses. The sequestration of suberin in soil was more rapid but less durable than that of cutin.

9 páginas.-- 2 figuras.-- 2 tablas.-- 35 referencias.-- Supplementary information related to this article can be found at http://dx.doi.org/10.1016/j.chemosphere.2015.09.055., Carbonaceous materials like biochars are increasingly recognized as effective sorbent materials for sequestering organic pollutants. Here, we study sorption behavior of two common hydrophobic organic contaminants 2,2′,5,5′-tetrachlorobiphenyl (CB52) and phenanthrene (PHE), on biochars and other carbonaceous materials (CM) produced at a wide range of conditions and temperatures from various feedstocks. The primary aim was to establish structure–reactivity relationships responsible for the observed variation in CM and biochar sorption characteristics. CM were characterized for their elemental composition, surface area, pore size distribution, aromaticity and thermal stability. Freundlich sorption coefficients for CB52 and PHE (i.e. LogKF,CB52 and KF,PHE, respectively) to CM showed a variation of two to three orders of magnitude, with LogKF,CB52 ranging from 5.12 ± 0.38 to 8.01 ± 0.18 and LogKF,PHE from 5.18 ± 0.09 to 7.42 ± 1.09. The highest LogKF values were observed for the activated CM, however, non-activated biochars produced at high temperatures (>700 °C) sorbed almost as strongly (within 0.2–0.5 Log units) as the activated ones. Sorption coefficients significantly increased with pyrolysis temperature, CM surface area and pore volume, aromaticity, and thermal stability, and decreased with H/C, O/C, (O + N)/C content. The results of our study contribute to the understanding of processes underlying HOC sorption to CM and explore the potential of CM as engineered sorbents for environmental applications, We would like to thanks Irène Criscuoli and Christophe Naisse for contribution to elemental analysis of the biochar samples. Johannes Lehmann and Kelly Hanley (Cornell University) are thanked for providing PMW, DDM and FW biochars. Funding was provided by the Norwegian Research Council project 217918 “Biochar”, a FriPro stipend to GC, the Eurochar project FP7-ENV-478 2010 ID-265179, as well as by NGIs R&D Basic Funding Scheme.

Introduction of temporary grasslands into cropping cycles could be a sustainable management practice leading to increased soil organic carbon (SOC) to contribute to climate change adaption and mitigation. To investigate the impact of temporary grassland management practices on SOC storage of croplands, we used a spatially resolved sampling approach combined with geostatistical analyses across an agricultural experiment. The experiment included blocks (0.4- to 3-ha blocks) of continuous grassland, continuous cropping and temporary grasslands with different durations and N-fertilizations on a 23-ha site in western France. We measured changes in SOC storage over this 9-year experiment on loamy soil and investigated physicochemical soil parameters. In the soil profiles (0&ndash, 90 cm), SOC stocks ranged from 82.7 to 98.5 t ha&minus, 1 in 2005 and from 81.3 to 103.9 t ha&minus, 1 in 2014. On 0.4-ha blocks, the continuous grassland increased SOC in the soil profile with highest gains in the first 30 cm, while losses were recorded under continuous cropping. Where temporary grasslands were introduced into cropping cycles, SOC stocks were maintained. These observations were only partly confirmed when changing the scale of observation to 3-ha blocks. At the 3-ha scale, most grassland treatments exhibited both gains and losses of SOC, which could be partly related to soil physicochemical properties. Overall, our data suggest that both management practices and soil characteristics determine if carbon will accumulate in SOC pools. For detailed understanding of SOC changes, a combination of measurements at different scales is necessary.

Lagooning sludge (LS), which is used as soil amendment in Morocco, may contain microplastics (MPs). The aim of this study was to examine the effect of dewatering and co-composting of LS with green waste (GW) on the MPs’ evolution. In this context the present study proposes fast-preliminary steps to detect plastics in lagooning sewage sludge before the extraction and identification process. We used pyrolysis GC/MS spectrometry to investigate the presence of chemical compounds possibly derived from plastics, and fluorescence staining by Nile Red to detect fluorescent particles suspected as plastics. Thereafter, we quantified the MPs particles after density fractionation and investigated their nature by Raman spectroscopy. Results indicated the presence of an average of 40.5 ± 11.9 × 103 MPs particles/kg (dry matter) and 36 ± 9.7 × 103 MPs particles/kg (dry matter) in fresh sludge and dewatered sludge respectively. Sludge dewatering in drying beds resulted a loss of small MPs (

International audience; Silicon (Si) exerts beneficial effects in mitigating aluminum (Al) toxicity in different plant species. These include attenuating oxidative damage and improving structural strengthening as a result of the increased production of secondary metabolites such as phenols. The aim of this research was to evaluate the effect of Si on phenol production and composition in two barley cultivars under Al stress. Our conceptual approach included a hydroponic experiment with an Al-tolerant (Sebastian) and an Al-sensitive (Scarlett) barley cultivar treated with two Al doses (0 or 0.2 mM of Al) and two Si doses (0 or 2 mM) for 21 days. Chemical, biochemical and growth parameters were assayed after harvest. Our results indicated that the Al and Si concentration decreased in both cultivars when Al and Si were added in combination. Silicon increased the antioxidant activity and soluble phenol concentration, but reduced lipid peroxidation irrespective of the Al dose. Both barley cultivars showed changes in culm creep rate, flavonoids and flavones concentration, lignin accumulation and altered lignin composition in Si and Al treatments. We concluded that Si fertilization could increase the resistance of barley to Al toxicity by regulating the metabolism of phenolic compounds with antioxidant and structural functions.

Feeding behavior of large herbivores determines the composition of their dung and together with environmental factors the intensity of decomposition processes leading to the recycling of nutrients in tropical forests. Large herbivore dung and its decomposition has so far been characterized by stoichiometric analyses of elements such as C and N. The objective of our study was to examine the suitability of biomarker analyses and analytical pyrolysis to infer large herbivore feeding behavior and the decomposition of their dung in different environments. Our conceptual approach included exposure of fresh dung of a grazing ruminant (gaur, Bos gaurus) and a non-ruminant mixed-feeder (the Asian elephant, Elephas maximus) in two tropical forest types (dry and moist) and analysis of dung biochemical composition in two seasons (dry and wet). To this end we characterized the dungs’ lignin and carbohydrate (sugar) signatures and pyrolysis products before and after 28 days of exposure. Our results showed that stoichiometric as well as biomarker analyses were able to differentiate gaur and elephant dung independent of season and forest type, while analytical pyrolysis products did not differ between dung types. The lignin signature of fresh dung additionally indicated the forage preference of animals in different forest types and seasons. During decomposition, C and N contents decreased and the chemical composition of both dung types converged. The lignin signature of dung at the end of the experiment showed higher lignin decomposition in moist forest and wet season than dry forest and dry season. We conclude that detailed biochemical analyses can provide deeper insights into the main controls of large herbivore dung and its decomposition in tropical forests than stoichiometric analysis. In particular lignin may be a suitable indicator to investigate large herbivore feeding behavior and the environmental conditions of their habitat.

International audience; Stabilization of organic matter (OM) against decomposition and its de-stabilization leading to mineralization, are important processes in the control of greenhouse gas emissions and carbon sequestration in soils. Soil organisms, particularly earthworms, may influence these processes greatly through their potential impacts on the interaction of OM with clay minerals, which is a mechanism which may protect OM for medium (years) to long (decades, centuries) time scales. The aim of this study was to use earthworms and pure minerals in order to investigate enhancement of carbon sequestration through the formation of aggregates containing particulate OM and organo-mineral associations, during a composting experiment. To this end we compared OM transformations in composting treatments with and without (i) clay minerals (montmorillonite) and (ii) epigeic earthworms (Eisenia andrei and Eisenia foetida). We used density fractionation of the end-products to quantify the amount of aggregates, including organomineral associations, formed during 196 days. Chemical composition and stability were examined by elemental analyses, Rock-Eval 6 thermal analysis and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The results indicated that the addition of minerals led to the formation of heavy fractions, most probably containing aggregates and organo-mineral associations, in treatments with and without earthworms. While OM showed higher oxidative transformation in all treatments, addition of earthworms changed organic carbon (OC) and nitrogen (N) concentrations in specific density fractions and increased the thermal stability of OM in heavy density fractions. These fractions contained higher proportions of aromatic and proteinaceous materials. We conclude that, earthworms under composting conditions, earthworms may be able to effectively to stabilize OM through the formation of aggregates and/or organo-mineral associations containing greater proportions of microbially-derived material.

‘4 per 1,000’ initiative will boost soil carbon for climate and food security

Soil organic carbon is essential to improve soil fertility and ecosystem functioning. Soil microorganisms contribute significantly to the carbon transformation and immobilisation processes. However, microorganisms are sensitive to environmental stresses such as heavy metals. Applying amendments, such as biochar, to contaminated soils can alleviate the metal toxicity and add carbon inputs. In this study, Cd and Pb spiked soils treated with macadamia nutshell biochar (5% w/w) were monitored during a 49 days incubation period. Microbial phospholipid fatty acids (PLFAs) were extracted and analysed as biomarkers in order to identify the microbial community composition. Soil properties, metal bioavailability, microbial respiration, and microbial biomass carbon were measured after the incubation period. Microbial carbon use efficiency (CUE) was calculated from the ratio of carbon incorporated into microbial biomass to the carbon mineralised. Total PLFA concentration decreased to a greater extent in metal contaminated soils than uncontaminated soils. Microbial CUE also decreased due to metal toxicity. However, biochar addition alleviated the metal toxicity, and increased total PLFA concentration. Both microbial respiration and biomass carbon increased due to biochar application, and CUE was significantly (p < 0.01) higher in biochar treated soils than untreated soils. Heavy metals reduced the microbial carbon sequestration in contaminated soils by negatively influencing the CUE. The improvement of CUE through biochar addition in the contaminated soils could be attributed to the decrease in metal bioavailability, thereby mitigating the biotoxicity to soil microorganisms. Refereed/Peer-reviewed

International audience; Individual molecular components of soil organic matter (SOM) have been studied since the early 19th century. Their characterization is essential, because knowledge of the molecular structures constituting SOM allows for the detailed understanding of its origin and the processes implicated in soil C sequestration. This provides the basis for target-oriented development of management practices to optimize ecosystem services provided by soil. The aim of this review is to give an overview about the major analytical developments and the information gain that could be achieved by application of molecular methods in SOM research. Up to the 1980s, studies concentrated on the characterization of the chemical nature of SOM. Analyses were mainly based on soluble OM compounds, which were fractionated chemically into humic and fulvic acids. During the 1990s, the focus changed and scientists were more interested in biological processes controlling SOM formation and its dynamics. Introduction of physical fractionation and combination of molecular and isotopic techniques allowed assessment of composition, origin, and turnover of SOM within specific localizations in the mineral soil matrix. Analyses of the dynamics of single molecules led to a massive change of paradigms. Long residence times of SOM are no longer explained by chemical recalcitrance but by microbial products being stabilized by the interaction with soil minerals and microbial inaccessibility. In recent years, techniques yielding results with high molecular and spatial resolution were introduced, which will allow the acquisition of much more detailed information, moving a step further toward elucidating the nature and properties of SOM.

International audience; AbstractComposting and vermicomposting are sustainable strategies to transform organic wastes into organic amendments, valuable as potting media or soil conditioner. However, the negative aspects of these processes are emissions of greenhouse gases and odorous molecules and final product potentially containing toxic compounds. These negative aspects can be limited through the addition of organic, inorganic or biological additives to the composted or vermicomposted mixture. The aims of this review are (1) to present the main characteristics of composting and vermicomposting processes with and without additives, (2) to show the influence of additives on greenhouse gas emissions during waste degradation and (3) to report the effects of additives on the properties of the final products (heavy metal and nutrient contents), in view of their use as a soil conditioner or potting media. Finally, the feasibility and potential environmental benefits of co-composting and co-vermicomposting are discussed. Our results show that additives affect composting parameters such as temperature, pH and moisture and thus have an impact on the composting process. They may be used to reduce gas emissions and mobility of mineral ions. The various additives have contrasting effects on the quality of the final product and its impact on soil quality. The use of worms and additives seems to increase plant available nutrient contents, while decreasing N leaching, heavy metal mobility and composting time. Co-composting and co-vermicomposting strategies need to be locally optimised, involving the generated amendments in a circular economy to improve sustainability of agricultural systems.

Temporary (ley) grassland introduced into cropping cycles has been advocated as being beneficial for the delivery of ecosystem services by agricultural soils. The management of these temporary grasslands has unknown effects on soil organic matter (SOM) concentrations and biogeochemical properties of the cropland soils following the grassland phase. Here, we investigated the legacy effect of differently managed temporary grasslands, i.e. change of soil properties lasting beyond three years of crop. We assessed soil organic carbon (SOC) quantity and SOM biogeochemical signature (composition of soil neutral carbohydrates and lignin), as well as microbial activities (potential C and N mineralization and denitrification). We used a long-term field experiment on Cambisol with temperate climate in western France, where temporary grassland management practices differed in terms of duration (3 or 6 years) and presence or absence of N fertilisation. Topsoil (10 cm) samples were collected after a 3-yr crop rotation (maize, wheat, barley). Our results showed that N fertilisation during the grassland phase was necessary to maintain soil C and N concentrations beyond three years of crop. Temporary grassland management may affect microbial activities as indicated by contrasting polysaccharide and lignin composition. It had however, no effect on potential CO2 and N2O emissions during laboratory incubations. The biogeochemical signature of SOM was close to continuous grassland only in treatments with 6 yrs of fertilized temporary grassland. We thus, conclude that the legacy effects of a grassland phase on SOC quantity and properties of SOM depend on its management.

International audience; Evidence indicates that silicon (Si) alleviates diverse stresses by improving the antioxidant capacity and phenolics metabolism of plants. We assessed the effect of sodium silicate (Na2SiO3) and calcium silicate (CaSiO3) on Si and aluminium (Al) uptake, antioxidant performance and phenolics (with antioxidant or structural function) of ryegrass cultivated on an acid Andisol under greenhouse conditions. Ryegrass was treated with either sodium silicate or calcium silicate at gradually increasing doses (250, 500 and 1000 mg Si kg(-1) soil). Yield and concentrations of Si and Al were measured in roots and two shoot cuts. At the first cut, phenols, antioxidant enzymes, antioxidant capacity, lipid peroxidation and lignin production and composition were also determined. Ryegrass supplied with sodium silicate exhibited the highest Si content. Root Si was closely correlated with Al or Si : Al ratio. Shoot Si uptake increased total phenols and activities of antioxidant enzymes (CAT, APX and POD), but reduced lipid peroxidation. Silicon also changed the lignin production and composition in shoots at the highest sodium silicate dose. Silicon uptake reduced the deleterious effect of soil acidity in ryegrass. Sodium silicate had the greatest influence on the antioxidant system through enhancement of phenols production and antioxidant enzyme activation. Peroxidase activity appears to be associated with increased lignin biosynthesis in plants supplied with sodium silicate.