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Earthworm species are grouped into three ecological categories, endogeic, epigeic and anecic from their nutritional behaviors, their strategies of morpho-functional adaptations and localization in the soil profile. These earthworms play a major role in the biogeochemical cycling of carbon and nitrogen through their bioturbation activity and cast production that have specific physical, chemical and biological characteristics. They affect the sequestration of soil C through their influence on soil organic matter (SOM) protection processes. However, CO2 and N2O emissions from cast depend on earthworm species with different physical, chemical and microbiological properties even when these species belong to the same ecological category. Hence the importance of investigating their functional traits to see how they affect greenhouse gas emissions. To do this, we measured the emissions of N2O and CO2 in earthworm cast, then we characterized the physical-chemical properties from elemental and spectroscopic analyses. Finally, these results were related to different morphological (pigmentation, color, size), anatomical (gizzard, pharynx, morren's gland size) and physiological traits. Our hypothesis is that the casts of pigmented and colored earthworms, with a large pharynx and gizzard, morren's gland, secreting a lot of mucus and water produce more CO2, N2O.

Key points• Carbon occurs in different forms in soils: in living organisms, dead organic matter with particulate organic matter, organicmolecules, and as inorganic carbon• Soil carbon stock and soil carbon storage are different concepts: a carbon stock is quantified at a given date while its storageis quantified over a period of time• The carbon stock distribution across the planet is heterogeneous, and complex biotic and abiotic processes underlie itsdynamics• For a given soil and climate, the largest stocks are found in forests and grasslands. In comparison, cultivated soils are carbondepleted• Land-use changes and agricultural practices have a major impact on soil carbon stocks: they may have negative, positive ordiverse effects on carbon storage, depending on climate and soil conditions• Soil carbon storage provides socioeconomic benefits as targeted by five Sustainable Development Goals

Salt marshes are carbon rich ecosystems, but they are threatened by sea-level rise. Root litter dynamics have been little studied, despite contributing three times more to carbon burial than leaves litters and being important for salt marsh resilience to sea level rise. Root litter is defined as one single pool in salt marshes, yet this approach might fail to accurately assess root litter decay, as shown in terrestrial ecosystems where a root functional framework proved to be more accurate. We investigated if root functional types (fine absorptive roots, fine transportive roots and rhizomes) and inundation duration affected OM decay rate in a temperate salt marsh. We used a marsh organ and litterbags.We found that the decay rate strongly varied between root functional types, with fine absorptive roots decaying 30% slower than fine transportive roots and 14% slower than rhizomes.The decay rate was poorly explained by C:N ratio, N and C contents, while better correlated with root lignin composition. The duration of inundation had little effect on the belowground OM decay and did not interact with the decay of root functional types. Therefore, a shift toward experimental methods separating roots by function seems necessary to accurately assess the contribution of root litter to carbon burial in salt marshes. Overall, our results suggest that the current global carbon decay of salt marshes may have been overestimated and that fine absorptive roots could be a larger source for carbon burial than previously thought in salt marshes.

There is currently an intense debate about the potential for additional organic carbon storage in soil, the strategies by which it may be accomplished and what the actual benefits might be for agriculture and the climate. Controversy forms an essential part of the scientific process, but on the topic of soil carbon storage, it may confuse the agricultural community and the general public and may delay actions to fight climate change. In an attempt to shed light on this topic, the originality of this article lies in its intention to provide a balanced description of contradictory scientific opinions on soil carbon storage and to examine how the scientific community can support decision-making despite the controversy. In the first part, we review and attempt to reconcile conflicting views on the mechanisms controlling organic carbon dynamics in soil. We discuss the divergent opinions about chemical recalcitrance, the microbial or plant origin of persistent soil organic matter, the contribution of particulate organic matter to additional organic carbon storage in soil, and the spatial and energetic inaccessibility of soil organic matter to decomposers. In the second part, we examine the advantages and limitations of big data management and modeling, which are essential tools to link the latest scientific theories with the actions taken by stakeholders. Finally, we show how the analysis and discussion of controversies can guide scientists in supporting stakeholders for the design of (i) appropriate trade-offs for biomass use in agriculture and forestry and (ii) climate-smart management practices, keeping in mind their still unresolved effects on soil carbon storage.

The role of earthworms on biogeochemical carbon cycling is a major knowledge gap resulting from the difficulty of isolating and exploring the effects provided by the diversity of organisms. In this study, we investigated the effect of six earthworm species belonging to three ecological categories on soil organic carbon (SOC) mineralisation. To this end, we produced casts with the six species using subsoil material with low SOC content and miscanthus litter. Cast were subjected to laboratory ageing for 140 days. During this process, we monitored physicochemical parameters, CO2 emissions and determined the micro-scale organisation of the casts’ particulate organic matter and pores using X-ray tomography.Our results showed contrasting properties of fresh casts from the 3 main ecological categories, in accordance with the earthworm species’ morphological or behavioral strategies, indicating that those were maintained in artificial environments. However, species-specific changes in cast properties throughout ageing increased intragroup variability among ecological categories. As a result we observed earthworm species-specific evolution of CO2 mineralisation rates during casts ageing. We found that at least half of the variability in CO2 emissions was explained by cast microstructural changes, related to the spatial arrangement between particulate organic matter, porosity, and mineral particles. We conclude that earthworm species-specific traits may play a role in organic carbon protection through their impact on microstructural cast properties.

International audience

Microplastic (MP) input into agroecosystems is of particular concern as their sources are diverse (mulching films, biosolid application, wastewater irrigation, flooding, atmospheric input, road runoff). Compost application, which is needed to sustain soil ecosystem services in the context of a circular economy, may be a source of microplastics. The aim of this study was to evaluate how different composts derived from urban wastes impact the nature and quantity of coarse (2-5 mm) microplastics (CMP) in soils, using a long-term field experiment in France. Composts resulting from different levels of urban waste sorting were investigated. Our approach included the isolation of microplastics from composts and amended soils followed by their characterization using pyrolysis GC/MS spectrometry. We found that coarse microplastic concentrations varied from 26.9 to 417 kg per hectare depending on the compost type, after 22 years of bi-annual application. These values may be higher than for conventional agricultural practices, as application rate was twice as high as for normal practices. Composts made from municipal solid waste were by far the organic amendments leading to the highest quantity of plastic particles in soils, emphasizing the urgent need for limiting plastic use in packaging and for improving household biowaste sorting. Our results strongly suggest that standards regulating organic matter amendment application should take microplastics into account in order to prevent contamination of (agricultural) soils. Moreover, although no impacts on the soil bio-physico-chemical parameters has been noted so far. However, given the huge microplastic inputs, there is an urgent need to better evaluate their effect on soil functioning.

Being at the interface of the geosphere, the biosphere, and the atmosphere makes the soil a particularly challenging object. Nevertheless, its importance in ecological and environmental domain...

International audience; The soil, at the interface between geosphere, biosphere, and atmosphere, is a particularly challenging object to study. The ecological and environmental impacts of soil functioning encourage soil scientists to exploit new technologies to generate new data or develop new research protocols. Among these technologies, 3D printing is a promising technology for improving the understanding of soils. The general principle of 3D printing is to build parts by adding materials layer by layer following a three-dimensional solid model. The 3D printing technology makes it possible to produce almost any geometrically complex shape or feature in a wide range of materials. Its interest lies in its ability to produce customized objects easily and rapidly in an endlessly reproducible 3Darrangement. A large number of techniques and materials are available, which differ in their principles, advantages, and shortcomings. In this article, we present the main challenges and opportunities of using 3D printing in soil science. To this end, we present a review of the literature from 2000 to 2019 to identify and understand recent applications of 3D printing in this field. We first describe applications such as the manufacturing of agricultural equipment and laboratory devices in soil science, the development of new construction materials, or the geotechnical characterization of soil as a construction base. However, none of these applications requires replication of the soil functional properties as opposed to applications that would be dedicated to improving the understanding of soil functioning. We detail here the challenges and opportunities of building soil models that reproduce its physical, chemical, biological properties, and its dynamics in contact with living organisms. Despite the remarkable and rapid progress made in the development of 3D printing in recent years, this technology is still underused in the field of soil science. In particular, very few applications focus on the functioning of the soil itself as an ecological compartment. Indeed, several technical limitations have still to be overcome. 3D printed objects must be biocompatible, chemically and mechanically stable, and must be spatially resolved on the microscale. Many efforts are being made by the 3D printing community to push these boundaries. This paves the way for the wider use of 3D printing in soil science. In the near future, the availability of additive manufactured soil models, with strict and controlled composition and structure, will provide researchers with an irreplaceable opportunity to conduct reproducible experiments and better understand soil functioning factors.

International audience

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.

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.

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.

Increased mineralization of the organic matter (OM) stored in permafrost is expected to constitute the largest additional global warming potential from terrestrial ecosystems exposed to a warmer climate. Chemical composition of permafrost OM is thought to be a key factor controlling the sensitivity of decomposition to warming. Our objective was to characterise OM from permafrost soils of the European Arctic: two mineral soils—Adventdalen, Svalbard, Norway and Vorkuta, northwest Russia— and a ‘‘palsa’’ (ice-cored peat mound patterning in heterogeneous permafrost landscapes) soil in Neiden, northern Norway, in terms of molecular composition and state of decomposition. At all sites, the OM stored in the permafrost was at an advanced stage of decomposition, although somewhat less so in the palsa peat. By comparing permafrost and active layers, we found no consistent effect of depth or permafrost on soil organic matter (SOM) chemistry across sites. The permafrost-affected palsa peat displayed better preservation of plant material in the deeper layer, as indicated by increasing contribution of lignin carbon to total carbon with depth, associated to decreasing acid (Ac) to aldehyde (Al) ratio of the syringyl (S) and vanillyl (V) units, and increasing S/V and contribution of plant-derived sugars. By contrast, in Adventdalen, the Ac/Al ratio of lignin and the Alkyl C to O-alkyl C ratio in the NMR spectra increased with depth, which suggests less oxidized SOM in the active layer compared to the permafrost layer. In Vorkuta, SOM characteristics in the permafrost profile did not change substantially with depth, probably due to mixing of soil layers by cryoturbation. The composition and state of decomposition of SOM appeared to be site-specific, in particular bound to the prevailing organic or mineral nature of soil when attempting to predict the SOM proneness to degradation. The occurrence of processes such as palsa formation in organic soils and cryoturbation should be considered when up-scaling and predicting the responses of OM to climate change in arctic soils.

4 pages; International audience; Applying composts is useful for increasing soil carbon (C) stocks and improving agricultural productivity. In order to understand the effects of composts on soil organic matter (SOM) formation and mineralisation, 13C-labelled plant residues, previously composted or not, were incubated in an arable soil. The amount of 13C was quantified in the CO2 evolved, the dissolved organic carbon (DOC) and in the microbial biomass (MB). Composting decreased (from 29 to 9%) the proportion of the plant residue labile pool and increased the residence time of both labile and more stable pools from 21 to 34 days and 1.5–5.5 years, respectively. At the beginning of the incubation, the amounts of 13C in the DOC and MB were significantly higher when adding fresh residues than composted ones to the soil. A priming effect on SOM mineralisation (+21% over 3 years) was only observed for non-composted residues. These differences were attributed to changes in the chemical composition of plant materials during composting (less sugars and lipids, more lignins). In terms of C budget, the total loss of CO2 (including the composting process and the SOM priming by fresh residues) was comparable for both treatments after 600 days of incubation.

In order to understand if and how composting modifies the dynamics of incorporation of organic carbon and nitrogen to soil aggregates during residue decomposition, we studied samples of a soil amended with fresh or composted plant residues in the laboratory. Samples from the surface horizon of a Luvisol (representative of large agricultural regions in northern Europe) were amended with fresh or composted 13C–15N-labelled wheat, corn and rapeseed residues and incubated in the laboratory during three years. Aggregates of samples taken at the initial time, after 45 days and after three years of incubation were fractionated by size; OC and N concentrations plus isotopic signatures of C and N were analysed in each aggregate fraction. Both fresh and composted residues addition increased aggregate stability at 45 days, but the effect almost disappeared after three years. The addition of the residues increased OC and N contents with respect to the control soils after three years. The additional OC and N coming from residues were initially incorporated preferentially into 0.05–0.2-mm aggregates, as indicated by isotopic signature analyses, and later redistributed homogeneously among all size fractions. After three years of incubation, 11% of plant residue OC remained in soil when these were added fresh, and 22% if they were previously composted, whereas similar N percentages (26–27%) remained for fresh or composted residues. Overall, a higher amount of inputs from plant residues remained in the soil in the long term when they were previously composted, with respect to non-composted residues.

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.

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; 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.

International audience; Urban compost application in agroecosystems enhances soil fertility but can also be a source of (micro)plastics, which are not completely removed during the composting process. Knowledge of the fate of these plastics in regularly-amended soils is thus an issue for the environmental management of these soils. The aims of this study were 1) to develop a method combining soil fractionation, microscopic observation and chemical characterization to follow the fate of plastics in soils and 2) to apply this method on a long-term experimental field, where municipal solid waste composts were applied every other year during 10 years. The presence of plastics was investigated within compost and soil fractions using morphological and analytical characterization by transmission electronic microscopy (TEM-EDX) and pyrolysis coupled to gas chromatography and mass spectrometry (Py/GC/MS). Specific features of plastics allowed us to distinguish these polymers from soil organic matter even in the

A novel method coupling pyrolysis with mid-infrared spectroscopy (Py-MIRS) was developed to characterize soil organic matter (SOM) chemistry in soils. The pyrolyzer was interfaced to the MIR spectrometer by means of a Brill cell™ (CDS Analytica). The set-up generates pyrolysis fingerprint spectra from which individual pyrolysis products can be related to SOM bulk chemistry. Py-MIRS development involved the testing of experimental conditions like pyrolysis temperature (550, 700, 1000 °C), heating rate (20 °C s−1 and 20 °C ms−1) and time (15, 30 and 60 s) using reference standard compounds ranging from carbohydrates to phenols varying in chemical and structural composition like levoglusogan, gluten, tannin, syringol, pectin and leucine falling within different compound categories (carbohydrates, amino acids, proteins, phenols, etc) as well as soil samples. Pyrolysis yields of prominent specific functional groups, like aliphatics (C-H stretching at 2930 cm−1) and Cdouble bond; length as m-dashC aromatics (1510 cm−1), varied with pyrolysis temperature, heating rate and time. The preferred settings for high pyrolysis yield and minimized secondary reactions were obtained at a pyrolysis temperature of 700 °C, heating rate of 20 °C ms−1 and heating time of 30 s. The suitability of Py-MIRS to detect changes in SOM composition was evaluated by comparing Py-MIRS results to Diffuse Reflectance Fourier Transform mid-Infrared Spectroscopy (DRIFTS) results. Soil samples taken from the Static Fertilization Experiment, Bad Lauchstädt, Germany (Chernozem) revealed a major SOM contribution of the peak at 1750 cm−1 (Cdouble bond; length as m-dashO), followed by peaks at 950 (C-H), 1510 (Cdouble bond; length as m-dashC), 1176 (C-H, O-H) cm−1, with smaller contributions from the 2930 (C-H) and 3015 (CH4) cm−1 peaks, apart from a dominant CO2 peak. Using the preferred pyrolysis settings, Py-MIRS as well as DRIFTS results further indicated that soils receiving organic (e.g. farmyard manure) inputs were highly enriched in aliphatic groups, while their absence favored the accumulation of carboxyl and aromatic groups as well as polysaccharides. Py-MIRS allowed via semi-quantification of pyrolysis products a rapid monitoring of SOM bulk chemistry with a high degree of reproducibility. It was concluded that Py-MIRS represents a fast, effective and reproducible technique to characterize changes in the SOM bulk chemistry as a result of management practices. It also allows to minimize acknowledged constraints of other analytical techniques used to characterize SOM bulk chemistry such as mineral interferences and associated secondary reactions.

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.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; The international 4 per 1000 initiative aims at supporting states and non-governmental stakeholders in their efforts towards a better management of soil carbon (C) stocks. These stocks depend on soil C inputs and outputs. They are the result of fine spatial scale interconnected mechanisms, which stabilise/destabilise organic matter-borne C. Since 2016, the CarboSMS consortium federates French researchers working on these mechanisms and their effects on C stocks in a local and global change setting (land use, agricultural practices, climatic and soil conditions, etc.). This article is a synthesis of this consortium’s first seminar. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms comprising biotic and abiotic processes, which occur concomitantly and interact. Soil organic C stocks are altered by biotic activities of plants (the main source of C through litter and root systems), microorganisms (fungi and bacteria) and ‘ecosystem engineers’ (earthworms, termites, ants). In the meantime, abiotic processes related to the soil-physical structure, porosity and mineral fraction also modify these stocks. In the second part, we show how agricultural practices affect soil C stocks. By acting on both biotic and abiotic mechanisms, land use and management practices (choice of plant species and density, plant residue exports, amendments, fertilisation, tillage, etc.) drive soil spatiotemporal organic inputs and organic matter sensitivity to mineralisation. Interaction between the different mechanisms and their effects on C stocks are revealed by meta-analyses and long-term field studies. The third part addresses upscaling issues. This is a cause for major concern since soil organic C stabilisation mechanisms are most often studied at fine spatial scales (mm–μm) under controlled conditions, while agricultural practices are implemented at the plot scale. We discuss some proxies and models describing specific mechanisms and their action in different soil and climatic contexts and show how they should be taken into account in large scale models, to improve change predictions in soil C stocks. Finally, this literature review highlights some future research prospects geared towards preserving or even increasing C stocks, our focus being put on the mechanisms, the effects of agricultural practices on them and C stock prediction models.

Cost-effective and environment-friendly pretreatments, such as fungal pretreatments, are required for anaerobic digestion of lignocellulosic biomass particularly because the amount of methane produced is often limited by the lignin content. Anaerobic digestibility is estimated using a Biochemical Methane Potential (BMP) test, which lasts several weeks. Since the Py-GC–MS technique is considered to be a rapid method for obtaining information on various organic components, its suitability for the study of fungal pretreatment efficiency in anaerobic digestion was investigated here, to our knowledge for the first time. In this goal, mycelium of the white-rot fungi Polyporus brumalis BRFM 985, untreated wheat straw and straw pretreated with different fungal strains and under different conditions were analysed with Py-GC–MS. In the mycelium pyrolysate, diverse compounds, often considered as unspecific, probably derive from proteins (toluene, etc.). A strong presence of tyrosine and phenylalanine among the fungus amino acids was also suspected. As for pretreated straw samples, a correlation was observed between the amount of fungal biomass determined by qPCR (used as a reference method) and the sum of relative areas of toluene, styrene and ethylbenzene in the pyrograms, showing that it is feasible to estimate the fungal biomass amount on pretreated straws using Py-GC–MS. In addition, the H/L-Py (Holocelluloses/Lignin) ratio, determined by dividing the sums of areas of pyrolysis compounds that have a polysaccharide (PS) and lignin (LIG) origin, was correlated to the BMP values of pretreated straws, thus showing that the pretreatment efficiency can be rapidly estimated with Py-GC–MS in the tested conditions.

Implementation of ley grassland into crop rotation could have positive influence in soil ecosystem services such as C storage. The periodical changes of land-use plus the in situ labelling given by the introduction of maize crops under ley grassland induce differences in soil organic matter (SOM) that could be traced either by stable isotopes or by the characterization of plant biomarkers such as lignin derived phenols. Evaluation of SOM dynamics is often limited by the complexity of soil matrix. To override these limitations, a hierarchical approach to decompose the soil mosaic into aggregates has been proposed in this study. Soil and plant samples were collected from a long-term experimental area in Lusignan (western France). Soils from four different treatments (bare fallow, permanent maize, permanent grassland, and ley grassland based on 6 years of grassland followed by 3 years of maize) were sampled, fractionated into water stable aggregates, and finally analysed for carbon, nitrogen, and lignin contents, as well as for 13C isotopic signature. Soils under ley and permanent grassland stored higher amount of SOM in larger aggregates and preserved more efficiently the lignin stocks than the corresponding samples under permanent maize. Contemporary, finer fraction of ley grassland showed higher mean residence time of organic carbon, probably due to a legacy effect of the previous years under grassland. Even if maize derived SOM was identified, the grassland footprint was still dominating the ley grassland soils, as described by the principal component analysis. Strong correlation between these results and the quality and stoichiometry of the vegetal litter returned to soil were found, evidencing the needs for a comprehensive evaluation at a molecular level of all the parameters modified by land-use changes, including tillage, to understand the potential for carbon storage of different agroecosystems.

RATIONALE The efficiency of extraction procedures for the determination of organic compounds in soil may be affected by the presence of the mineral phase. Our aim was to analyse the magnitude of such an effect on both total polysaccharide content and 13C-isotopic signature of the polysaccharides. METHODS After acid hydrolysis of 13C-labelled wheat, soil and a mixture of these, sugars were quantified and analysed isotopically. Measured values were compared with theoretical contents. RESULTS No matrix effect was apparent for total sugar-C content of the mixture. However, a matrix effect was observed for the contribution of 13C-labelled wheat sugars. For the soil+plant mixture 13C-labelled wheat sugar contribution was overestimated. Soil-derived sugar-C contribution to the mixture was underestimated. CONCLUSIONS Studies using stable isotopes to follow the fate of added plant-derived compounds in soil need to take into account matrix effects. Further studies have to elaborate on correction procedures and/or the development of extraction procedures to overcome the influence of matrix effects and/or acid hydrolysis extraction on sugar-C contents. Copyright © 2014 John Wiley & Sons, Ltd.

In this study we evaluated CO2 emissions during co-composting and co-vermicomposting of green wastes with clay and/or biochar. The stability of the final products as well as their effect on C mineralization in soil have been evaluated. The aim of the study was to test the following hypothesis: (1) interactions between clay and biochar and organic wastes would lead to reduced CO2 emissions during the composting process, (2) these interactions would be enhanced in the presence of worms, and (3) more carbon would be sequestered in soil after the use of the resulting compost/vermicompost as amendments. We added two different doses of clay, biochar and their mixture to pre-composted green wastes and monitored C mineralisation during 21 days in presence or absence of worms (Eisenia species). The organic materials were then added to a loamy Cambisol and the CO2 emissions were monitored during 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 %. However, in the presence of worms, CO2 emissions increased for all treatments except for the low clay dose. The production conditions had more influence on C mineralization in soil for composts than for vermicomposts except for the low clay treatment, which showed a more reduced CO2 emissions compared to a regular compost. In summary, the addition of worms during co-composting with clay and biochar may be a promising technology for reducing CO2 emissions and increasing soil carbon storage. We suggest that the production of a low CO2 emission amendment requires optimisation of OM source, co-composting agents and worm species. The effect of the resulting material on soil fertility has to be evaluated.

The aim of the international "4 per 1 000" initiative is to support States and non-governmental actors in their efforts towards a better management of carbon (C) stocks in soils. These stocks depend on the inputs and outputs of C in soils. Stocks are the result of intercon¬nected mechanisms acting at fine spatial scales, leading to stabilisation/destabilisation of the C present in organic matter (OM). The need to federate research on these mechanisms and their effects on C stocks in a context of local and global changes (land uses, agricultural practices, climatic and soil conditions, etc.) led to the creation in 2015 of a group of french researchers, CarboSMS. This is the synthesis of the seminar dedicated to this issue. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms. Biotic mechanisms correspond to the effects of plants (main source of C through litter and root systems), microorganisms (fungi and bacteria) and ecosystem engineers (earthworms, termites, ants) on organic C stocks in soils. Abiotic processes are related to the physical structure of soils, to the organization of their porosity and to their mineral fraction : inclusion of the OM in aggregates, association with minerals, etc. These biotic and abiotic mechanisms take place concomitantly and interact. In the second part, we discuss the effects of agricultural practices on soil C stocks. By acting on biotic as well as abiotic mechanisms, land use and management specificities (choice of plant species and density, plant residue exports, amendments, fertilization, tillage, etc.) drive not only the spatial and temporal organic inputs into soils, but also the OM sensitivity to mineralisation. The complexity of the interactions between the different mechanisms and their effects on C stocks are evidenced by meta-analyses and long-term field studies. Organic C stabilisation mechanisms in soils are studied at fine spatial scales (mm-μm) under controlled conditions. The third part of this article addresses the issues of upscaling (up to the plot and to global scale) and validating the action of specific mechanisms in different soil and climatic contexts, before taking them into account in large scale models to improve predictions of the changes in soil C stocks. Finally, this literature review highlights some perspectives for future research aiming at preserving or even increasing C stocks. These perspectives focus on the mechanisms, the effects of agricultural practices on these mechanisms, and on predictive models of C stocks.; La iniciativa internacional "4 por 1 000" tiene como ambición sostener Estados y actores no gubernamentales en sus esfuerzos hacia una mejor gestión de los stocks de carbono (C) de los suelos. Estos stocks dependen de las entradas y salidas de C en los suelos. Los stocks son la consecuencia de mecanismos interconectados, que actúan a una escala espacial fina, que conducen al almacenaje o al desalmacenaje del C presente en las materias orgánicas (MO). Aparece necesario federar los investigadores sobre estos mecanismos y su acción sobre los stocks de C en un contexto de cambio local o global (uso de los suelos, prácticas agrícolas, condiciones climáticas o edáficas, etc.), lo que lleva a la creación del colectivo de investigadores CarboSMS. Este artículo es la síntesis del seminario dedicado a esta cuestión. La primera parte presenta los avances recientes sobre la compren¬sión de los mecanismos de estabilización. Los mecanismos bióticos corresponden a los efectos de las plantas (principal fuente de C, en particular a través de sus sistemas radiculares), de los microorganismos (hongos y bacterias) y de los ingenieros del ecosistema (lombrices, termitas, hormigas) sobre los stocks de C orgánico del suelo. Los procesos abióticos son ligados a la estructura física del suelo, a la organización de su porosidad y a su fracción mineral : inclusión de la MO en los agregados, asociación a los minerales, etc. Estos mecanismos bióticos y abióticos se desarrollan de manera concomitante y en interacción. La segunda parte discute de los efectos de las prácticas agrícolas sobre los stocks de C de los suelos. La elección de las especies vegetales y de su densidad de plantación, la intensidad de la extracción de los vegetales, la enmienda, la fertilización y el trabajo del suelo, etc. condicionan no solamente los aportes de MO al suelo en el tiempo y en el espacio, pero también la sensibilidad de estas MO a la mineralización, actuando sobre los mecanismos bióticos y abióticos. La complexidad de las interacciones entre los mecanismos y sus efectos en el tiempo sobre los stocks de C se constata por los meta-análisis y los estudios de terreno de larga duración. Se estudian los mecanismos de estabilización del C orgánico en los suelos a escala espacial fina (mm-μm), en condiciones especificas. La tercera parte de este artículo discuta proposiciones concretas para abordar las cuestiones del cambio de escala (hasta la parcela y al nivel global) y de la validación de la acción de un mecanismo en diferentes contextos pedo-climáticos antes de tomarlo en cuenta en los modelos a larga escala para mejorar las previsiones de evolución de los stocks de C de los suelos. En fin, en una óptica de preservar mejor los stocks de C, incluso de aumentarlos, esta revisión bibliográfica nos permite hacer proposi¬ciones para los ejes de investigaciones futuras sobre los mecanismos, los efectos de las prácticas agrícolas sobre estos mecanismos, y los modelos de predicción de los stocks de C.; L’initiative internationale "4 pour 1 000" a pour ambition de soutenir États et acteurs non gouvernementaux dans leurs efforts vers une meilleure gestion des stocks de carbone (C) des sols. Ces stocks dépendent des entrées et sorties de C dans les sols. Ils sont la consé¬quence de mécanismes interconnectés, agissant à une échelle spatiale fine, qui conduisent au stockage ou déstockage du C présent dans les matières organiques (MO). Il apparaît nécessaire de fédérer les recherches sur ces mécanismes et leur action sur les stocks de C dans un contexte de changement local ou global (usage des sols, pratiques agricoles, conditions climatiques ou édaphiques, etc.), ce qui a conduit à la création du collectif de chercheurs CarboSMS. Cet article est la synthèse d'un séminaire dédié à cet enjeu. La première partie présente les avancées récentes sur la compréhension des mécanismes de stabilisation. Les mécanismes biotiques correspondent aux effets des plantes (principale source de C, notamment à travers leurs systèmes racinaires), des micro-organismes (champignons et bactéries) et des ingénieurs de l’écosystème (vers de terre, termites, fourmis) sur les stocks de C organique du sol. Les processus abiotiques sont liés à la structure physique du sol, à l’organisation de sa porosité et à sa fraction minérale : inclusion de la MO dans les agrégats, association aux minéraux, etc. Ces mécanismes biotiques et abiotiques se déroulent de façon concomitante et en interaction. La deuxième partie discute des effets des pratiques agricoles sur les stocks de C des sols. Le choix des espèces végétales et de leur densité de plantation, l’intensité du prélèvement des végétaux, l’amendement, la fertilisation et le travail du sol, etc. conditionnent non seulement les apports de MO au sol dans le temps et l’espace, mais aussi la sensibilité de ces MO à la minéralisation, en agissant sur les mécanismes, biotiques comme abiotiques. La complexité des interactions entre les mécanismes et leurs effets dans le temps sur les stocks de C sont mis en évidence par les méta-analyses et les études de terrain de longue durée. Les mécanismes de stabilisation du C organique dans les sols sont étudiés à fine échelle spatiale (mm-μm), dans des conditions spéci¬fiques. La troisième partie de cet article discute de propositions concrètes pour aborder les questions du changement d’échelle (jusqu’à la parcelle et au niveau global) et de la validation de l’action d’un mécanisme dans différents contextes pédo-climatiques avant de le prendre en compte dans des modèles à large échelle pour améliorer les prévisions d’évolution des stocks de C des sols. Enfin, dans une optique de mieux préserver les stocks de C, voire de les augmenter, cette revue bibliographique nous permet de faire des propositions pour les axes de recherches futures sur les mécanismes, les effets des pratiques agricoles sur ces mécanismes, et les modèles de prédiction des stocks de C.

RATIONALE The objective of this investigation was to test gas-chromatographic compound-specific analysis for studies on the isotopic composition of 13C-enriched sugar molecules. The effects of 13C enrichment and type of sugar (C5, C6) will provide valuable information on isotopic correction for future studies employing 13C-enriched sugars. METHODS Five sugar solutions of xylose, mannose and glucose with 13C enrichments ranging between 1.1 and 1.5 atom-% were prepared. The 13C enrichments of the initial sugars were measured by elemental analyser/isotope ratio mass spectrometry (EA/IRMS); 13C enrichments for derivatised sugars were obtained by gas chromatography/combustion/IRMS (GC/C/IRMS). RESULTS The linear relationships between the 13C enrichments of the initial sugars and the values for the derivatised sugars were sugar-type dependent. Corrections for GC/C/IRMS values took into account the kinetic isotope effect (KIE) of the derivatising agent associated with the coefficient (Kd) and a newly introduced second coefficient (Kc) associated with the KIE of the sugar. While Kd was constant, Kc varied with sugar type. During derivatisation acetate groups with 12C and sugars with more 13C reacted faster. CONCLUSIONS Coefficients for the specific ranges of 13C enrichments under study have to be assessed and the reactions of different sugar types have to be taken into account to avoid underestimation of 13C enrichment of up to 9% (C5) or overestimation of up to 4% (C6). Copyright © 2012 John Wiley & Sons, Ltd.

A Research Network focusing on organic matter(Re´seau MO) was established in 2004 in France. Itsoverall goal was to encourage discussions betweenFrench speaking scientists originating from a widerange of disciplines (soil science, hydrology, ocean-ography, sedimentology, cosmochemistry, wastemanagement, etc.) studying the same subject: organicmatter (OM). The various scientific communitiesworking on OM often ignore each other. However,they focus on the same issues regarding sources,transformations and interactions of OM with othercompounds. They use similar techniques and meth-odologies to address these issues, and face similarlimitations regarding the understanding of this com-plex matrix. In order to improve scientific synergybetween the various OM approaches, the ‘‘Re´seauMO’’ intends to stimulate interactions between scien-tists from all communities working on OM. Work-shops, summer schools and conferences are organisedregularly to facilitate the sharing of recent analyticaltools, to present emerging methods, to relate the latestscientific findings, to identify crosscutting and unitingthemes, and to stimulate creativity.This special issue of Biogeochemistry presents aselection of the results from the Second Conferenceorganised by this network, which took place on25–28 January, 2009 at Sainte Maxime, in the southof France. The conference was attended by 140delegates and focused on the impact of humanactivities on OM dynamics. Five sessions were heldduring the Conference, four of them dedicated to aresearch theme and the last one specifically focusingon methodological advances for OM study. Presen-tations and debates addressed the storage of carbon insoils and wetlands, the impact of anthropogenicactivities on this storage, the sources of OM inaquatic systems, and the affinity of dissolved organicmatter for hazardous compounds. Outcomes of thesetopics are summarised below.Storage of carbon in soils and wetlands:contribution of plant versus microbial OMTerrestrial ecosystems, if properly managed, areexpected to sequester a part of the increased

The addition of composted buffalo manure may lead to qualitative and quantitative improvement of the organic matter content of degraded tropical agricultural soils in Northern Vietnam. The objectives of this study were to follow the biochemical changes occurring during composting of buffalo manure with and without earthworms during 3 months and to study the effect of the end products (compost and vermicompost) on soil biochemical parameters and plant growth after two months of incubation under controlled conditions in an open pot experiment. Our conceptual approach included characterisation of organic matter of the two composts before and after addition to soil by elemental, isotopic analysis and analytical pyrolysis and comparison with conventional fertilisation. We also analysed for lignin content and composition. Our results showed that composting in the presence of earthworms led to stronger transformation of buffalo manure than regular composting. Vermicompost was enriched in N-containing compounds and depleted in polysaccharides. It further contained stronger modified lignin compared to regular compost. In the bulk soil, the amendment of compost and vermicompost led to significant modification of the soil organic matter after 2 months of exposure to natural weather conditions. The lignin component of SOM was unaffected whatever the origin of the organic amendment. Compost and vermicompost amendments both enhanced aggregation and increased the amount of organic matter in water stable aggregates. However, vermicompost is preferable to compost due to its beneficial effect on plant growth, while having similar positive effects on quantity and quality of SOM.

Cutins of plant shoots and suberins, mostly present in roots could contribute to significant portions of stable soil organic matter. Their biomarker potential, residing in their unique compositions in different plant types, has been used previously to infer sources of organic matter in sediments. These aliphatic plant biopolyesters contain specific biomarkers, which may be used for tracing their fate in soils and sediments, when combined with stable 13C isotope labelling. In order to evaluate the potential use of cutin and suberin biomarkers as indicators of shoot and root contributions from C3 and C4 plant origins, the objectives of this study were to 1) identify their constitutive monomers, which are specific for shoots and roots of maize (C4) and wheat (C3); 2) evaluate the 13C differences between maize and wheat biomarkers. Mid-chain hydroxy carboxylic acids were mainly found in the aboveground biomass, while α,ω-alkanedioic acids were only present in the roots. The differences in the isotopic composition of the specific monomers between wheat and maize plants (17–18‰ for shoot markers, 19‰ for root markers) were larger than those observed for bulk plant tissues and close to those observed for lignin-derived phenols in other studies. These differences should make it possible to differentiate and quantify the different types and sources of organic matter in sediments and soils. In particular, the molecular and isotopic signatures of cutins and suberins can be used to evaluate the specific dynamics of root vs shoot tissues in soils using C3/C4 chronosequences.

CuO oxidation is traditionally used for soil lignin study, although, like other degradation methods it might give access only to a part of the lignin polymers. For structural characterization, lignins are conventionally isolated from plant material by combining ball-milling, cellulolysis and solvent extraction to recover a milled wall enzymatic lignin (MWEL) fraction. This method might isolate condensed lignins, which are not accounted for CuO oxidation. MWELs are still associated with polysaccharides. This study aimed to evaluate if the MWEL method can be used complementary to CuO oxidation for soil lignin studies. We assessed the purity of isolated lignins by pyrolytic characterization of MWEL as well as the efficiency of the isolation method by characterization of the MWEL-free residues. MWELs isolated from maize stems and leaves and soil were primarily composed of lignin units, with few associated polysaccharides. MWELs isolated from soils were more degraded and contained less polysaccharides than MWEL from maize tissues. Fewer lignin-derived pyrolysis products were detected in the soil residue compared to the residues obtained from plant tissues. The MWEL method appeared very efficient in isolating relatively pure lignins from soil, most probably because the lignin–polysaccharide complex is less intact than in plants.

Up to now stabilization of organic matter (OM) in soils due to mineral interactions has been assessed mainly by correlations between carbon and iron and/or aluminum oxides evidencing that metal oxides may be principal stabilization agents. The nature and morphology of stabilized OM are poorly known. Taking advantage of a field experiment, the aim of our study was to analyze the fate of 13C and 15N labeled root material at 30 and 90 cm depths after three years of incubation and to characterize the nature of OM stabilized by interactions with metal oxides. Our methodological approach included isolation of metal oxides by physical fractionation and visualization of their interaction with OM using NanoSIMS. We concentrated metal oxides in a fraction corresponding to our objectives: the heavy fraction (>3 g cm−3) of fine silt. NanoSIMS analyses of this fraction allowed us to locate unlabeled OM and OM either double labeled or carrying one single label in association with metal oxides. Our results suggest that decoupling of C and N may have happened during OM stabilization within the timeframe of the 3 year field experiment. Scanning electron microscopy (SEM) after NanoSIMS analyzes, indicated that 15N labeled OM at 90 cm were well-defined ovoid OM particles resembling to microbial cells in interaction with Fe, Al and Ti oxides. At 30 cm depth, OM associated with metal oxides was 13C and 15N labeled unstructured material, possibly derived from plant debris. We suggest that at the two soil depths under investigation different processes might be at work, leading to association of OM with mineral compounds of the isolated fraction: in upper soil layers, decomposed plant material may directly interact with metal oxides, whereas in deep mineral soil, OM could mainly interact with metal oxides after microbial turnover. Both types of interactions may be fairly stable as they persisted after ultrasonication and salt extraction.

The analysis of sugars in soil at a molecular level may help to understand their origin and function. Gas chromatographic determination of individual sugars was applied to the litter layers and all mineral soil horizons of an acid forest soil under broad-leaved trees. Hydrolysis was carried out using trifluoroacetic acid (TFA). For GC analysis, the monosaccharide units were derivatised by reduction to alditols and subsequent acetylation. The results indicate a strong contribution of microbial-derived carbohydrates in the mineral soil (Ah to Cv horizons) compared to the litter layers (L, Of and Oh horizons).

Summary Lignin has long been suspected to be a major source of stable carbon in soils, notably because of the recalcitrant nature of its polyphenolic structure relative to other families of plant molecules. However, lignin turnover studies have produced conflicting results, most of them suggesting that large proportions of plant-residue lignin decompose within a year of incorporation into soils. Here, we propose a two-reservoir model where lignin in undecomposed plant residue (Lp) can either reach soil fractions where it is somewhat protected from further decomposition (Ls) or is transformed to non-lignin products. Model calibration data were obtained through compound-specific 13C isotopic analyses conducted in a zero- to 9-year chronosequence of maize monoculture after wheat in a temperate loam soil of the Paris basin. Lignin was quantified by CuO oxidation as VSC-lignin, i.e. the sum of vanillil- (V), syringyl- (S) and coumaryl-type (C) phenols. Model calibrations indicate that Lp has a turnover rate faster than 1 year and that 92% is mineralized as CO2 or transformed into other non-lignin products, while only 8% reaches the Ls fraction. Estimated turnover rate of the Ls fraction was 0.05 years−1. The model also suggested that about half of Lp was not measured because it had been excluded from the samples in the process of sieving at 5 mm. In conclusion, the model indicates that chemical recalcitrance alone is not sufficient to explain VSC-lignin turnover in soils, and that, functionally, the most relevant mechanism appears to be the transfer of VSC-lignin molecules and fragments from decomposing plant tissues to soil-protected fractions.

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce (Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH4NO3-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day−1 g C−1 and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.

Understanding the origin of the carbon (C) stabilised in soils is crucial in order to device management practices that will foster Caccumulation in soils. The relative contributions to soilC pools of roots vs. shoots is one aspect that has been mostly overlooked, although it appears a key factor that drives the fate of plant tissueC either as mineralized CO2 or as stabilized soil organic matter (SOM). Available studies on the subject consistently indicate that rootC has a longer residence time in soil than shootC. From the few studies with complete datasets, we estimated that the mean residence time in soils of root-derived C is 2.4times that of shoot-derived C. Our analyses indicate that this value is biased neither by an underestimation of root contributions, as exudation was considered in the analysis, nor by a priming effect of shoot litter on SOM. Here, we discuss the main SOM stabilisation mechanisms with respect to their ability to specifically protect root-derived SOM. Comparing in situ and incubation experiments suggests that the higher chemical recalcitrance of root tissues as compared to that of shoots is responsible for only a small portion, i.e. about one fourth, of the difference in mean residence time in soils of root-derived vs. shoot-derivedC. This suggests that SOM protection mechanisms other than chemical recalcitrance are also enhanced by root activities: (1)physico-chemical protection, especially in deeper horizons, (2)micrometer-scale physical protection through myccorhiza and root-hair activities, and (3)chemical interactions with metal ions. The impact of environmental conditions within deeper soil layers on rootC stabilisation appear difficult to assess, but is likely, if anything, to further increase the ratio between the mean residence time of root vs. shootC in soils. Future advances are expected from isotopic studies conducted at the molecular level, which will help unravel the fate of individual shoot and root compounds, such as cutins and suberins, throughout soil profiles.

A recently developed demineralisation treatment, using 2% hydrofluoric acid, was applied to the

International audience; Lignin is an aromatic plant compound that decomposes more slowly than other organic matter compounds; however, it was recently shown that lignin could decompose as fast as litter bulk carbon in minerals soils. In alpine Histosols, where organic matter dynamics is largely unaffected by mineral constituents, lignin may be an important part of soil organic matter (SOM). These soils are expected to experience alterations in temperature and/or physicochemical parameters as a result of global climate change. The effect of these changes on lignin dynamics remains to be examined and the importance of lignin as SOM compound in these soils evaluated. Here, we investigated the decomposition of individual lignin phenols of maize litter incubated for 2 years in-situ in Histosols on an Alpine elevation gradient (900, 1300, and 1900 m above sea level); to this end, we used the cupric oxide oxidation method and determined the phenols' C-13 signature. Maize lignin decomposed faster than bulk maize carbon in the first year (86 vs. 78% decomposed); however, after the second year, lignin and bulk C decomposition did not differ significantly. Lignin mass loss did not correlate with soil temperature after the first year, and even correlated negatively at the end of the second year. Lignin mass loss also correlated negatively with the remaining maize N at the end of the second year, and we interpreted this result as a possible negative influence of nitrogen on lignin degradation, although other factors (notably the depletion of easily degradable carbon sources) may also have played a role at this stage of decomposition. Microbial community composition did not correlate with lignin mass loss, but it did so with the lignin degradation indicators (Ac/Al)s and S/V after 2 years of decomposition. Progressing substrate decomposition toward the final stages thus appears to be linked with microbial community differentiation.

Despite its importance, relatively little is known about the composition and fate of wastewater organic matter (OM) in treatment plants. Monitoring the chemical changes in OM during activated sludge treatment can improve our knowledge of the processes involved in the biological elimination of OM. Direct chemical analyses of treated water OM typically account for about 20% of the OM, and structural information was obtained in this study using Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR) spectroscopic techniques. Distinct changes in the OM during wastewater biological treatment were underlined. 13C and 1H NMR showed that aromatic carbons were minor constituents of the samples. Alkyl chains exhibited a more highly branched character in treated water, as compared to long chain aliphatic carbons present in wastewater. Carboxyl signals in the 13C NMR spectrum of wastewater could be due to peptide bonds in proteins, whereas in the treated water spectrum, this signal could be related to the presence of non-proteinaceous nitrogen. Besides the non-degraded compounds, treated water OM could contain recondensation products of simple molecules. Their refractory character probably derives from their complex structures rather than from particular chemical functions, as suggested by the lack of fundamental differences in the chemical structures of wastewater and treated water OM.

Activated sludge treatment is used to remove wastewater organic matter (OM) and residual OM is found at low levels in treated water. The molecular composition of OM of waters and sludge was determined in order to understand the fate of the organic substances during biological treatment and the nature of residual OM. Proteins, sugars, lipids and polyphenolic compounds were quantified in a municipal wastewater treatment plant via chromatographic analysis after chemical hydrolysis. The concentration of OM was decreased by more than 90% by biological treatment. However, if 63% of the OM were characterized in activated sludge, molecular analysis did not allow us to characterize all the OM in the water samples. Relatively higher amounts of total organic carbon (TOC) were identified in wastewater (50%) than in treated water (20%). Likewise, for total nitrogen, 33% were identified as amino acid nitrogen in wastewater, and only 10% in treated water. Analytical pyrolysis suggested that some of the difficulties in identifying OM of treated water with common analytical methods are due to the presence of complex structures, refractory to hydrolysis. These structures may also be refractory to microbial degradation since they are concentrated during the biological treatment of wastewater. Suggestions on the possible origin and structure of resistant OM were formulated according to the analytical results.