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Global environmental changes are threatening the productivity of agroecosystems. Floods or droughts, together with long-term decline in soil organic matter stocks are pointing to the necessity of finding solutions for sustainable performance of agroecosystems.Deep soil horizons store significant amounts of water, soil organic carbon and nutrients, and thus subsoil management is being increasingly considered as an option to sustain crop productivity under unfavorable conditions.We used samples from several field experiments in Germany designed to investigate the potential benefits of deep ploughing together with deep placement of organic fertilizers on soil organic matter stocks. We recorded hyperspectral images of 1-metre soil cores in the Vis-NIR range and modelled the C distribution at a very high spatial resolution (53×53 μm²). Using approaches for GIS analyses, we quantified the changes in C and N stocks and we will present their spatial distribution resulting from the incorporation of different types of organic fertilizer (compost vs green manure) in subsoils. The organic matter stocks and C:N stoichiometry are both impacted by the agricultural management and the imaging technique allows us to distinguish between increased amount of organic matter in hotspots or in soil mineral matrice and to discuss the mechanisms controlling the observed changes.

Almost half of Earth’s terrestrial surface is covered by drylands, where limitation of water restricts vascular plant growth. In these ecosystems, a substantial part of primary production instead takes place directly at the soil surface, within complex microbial communities forming biological soil crusts (biocrusts) that include intricately bound soil particles. These communities, composed mainly of cyanobacteria, algae, fungi, and bryophytes, are fundamental actors for dryland biogeochemical cycles, as they fix atmospheric CO2 and N2 and constitute one of the primary, if not the only, sources of soil C and N. Due to the vast spatial extent of biocrusts, accounting for up to 70% of the living land cover in drylands, their importance for C and N cycling extend to the global scale, i.e. accounting for 7% of global net primary production of total terrestrial vegetation. However, warming temperatures and increasing soil dryness following climate change are estimated to have critical implications on these systems; recent studies show i.e. warming-induced reduction of biocrust cover and, thus, reduced CO2 uptake.Our aim is to contribute to this relatively new research field by providing insights into biocrust-soil-microorganism interactions under elevated temperatures and drought at the process-scale. This was realized in a phytotron incubation experiment of soil-biocrust mesocosms with experimental warming and drought, during which CO2 uptake and heterotrophic respiration was monitored. Dual labelling pulses (13CO2 and 15N2) were applied to follow the fate of recently fixed 13C and 15N into both particulate and mineral-associated SOM pools via physical fractionation and into microbial biomass via PLFA. Further, hyperspectral VIS-NIR images of the surface were recorded to quantify and determine crust cover and composition.The results revealed clear drought effects—not only in a distinct reduction in CO2 fixation by the biocrusts, but also in the elemental distribution of soil C underneath; the effect extended down into underlying soil layers, where biocrust-derived C contents were reduced by half due to drought. The change in the translocation of biocrust-derived C into the underlying soil was reflected in the 13C-PLFA profiles, showing how mainly fungi transform recently fixed C from the biocrust into their biomass in the biocrust layer, extending down into the underlying soil via fungal hyphae expansion. While drought clearly restricted the microbial abundance, warming further induced a microbial community shift, where a greater relative fungal dominance was determined under experimental warming—a shift, however, that was not reflected under dry conditions. A further combined effect was determined in N fixation, where we confirm a decrease in biocrust-derived N under drought under warming.Our results showcase the implications of elevated temperature and drought on C and N fixation and cycling—the two most fundamental ecosystem functions in biocrust-soil systems. The results support the growing body of evidence of major implications for biogeochemical cycles in drylands in a warming world.

Soil organisms such as arbuscular mycorrhizal fungi (AMF) and the roots they inhabit are key actors for shaping soil structure, which fosters a multitude of functions such as carbon storage and water availability. The expansion of AMF external hyphae, by being in direct contact with soil particles, can promote soil structure formation and thus induce a positive feedback on plant growth under unfavorable conditions such as drought.Here, we aim at disentangling the complexity of the root-AMF-soil interface by partitioning the respective effects of AMF, of root, and of their interaction on soil structure formation and organic matter cycling, under both drought and well-watered conditions. To discriminate the effects of plant and AMF, we used the wild-type and two mutants of the plant species Lotus japonicus that cannot be properly colonized by AMF (ccamk and ram2-2). The mutant ccamk impairs root entry by the fungus and ram2-2 causes impaired arbuscule development. To exclude confounding factors, we used an artificial soil mixture (quartz, illite, goethite; loamy texture) that was free of microorganisms and native organic matter. The wild type and the mutants were grown in this substrate during a 60-day incubation in a climate chamber. Half of the mesocosms were inoculated with spores of the AMF Rhizophagus irregularis. We stopped the watering two weeks before the end of the experiment in half of the cylinders to create drought conditions. At the end, roots and shoots were sampled and the rhizosphere soil was separated from the non-rhizosphere soil. We analyzed root architecture, AMF traits (intraradical colonization, hyphae length), as well as aggregate distribution and their organic carbon and nitrogen contents in the rhizosphere soil.Our results highlight the major role of AMF in promoting plant growth, with an increase of above-ground biomass, total root length and root surface area in the soil colonized with AMF, regardless of the water conditions. While plant root vigor (biomass, length, surface area) is reduced under drought conditions, the AMF are resistant to drought, with unchanged mycorrhization intensity and hyphae length in the soil that received less water. Under well-watered conditions, we quantified a higher share of macroaggregates. While AMF did not significantly affect soil structure formation, the presence of fungal hyphae resulted in an increase of carbon and nitrogen contribution of microaggregates in the rhizosphere soil. We are thus able to demonstrate that irrespective of soil water availability, AMF foster the vigor of the host plant. Furthermore, the expansion of AMF into soil, leading to higher carbon and nitrogen storage in rhizosphere soil microaggregates, is not dependent of soil moisture conditions.

This study investigated the effect of land-use and management change (LUMC) on carbon (C) and nitrogen (N) dynamics after 15–40 years. LUMC constituted change from rice paddies to chestnut orchard, wetland, and buckwheat upland fields in Shonai region, Yamagata Prefecture, Northeast Japan. Soil samples were collected from the top – (0–15 cm) and sub-layers (15–30 cm) for analysis of soil organic C (SOC) and its δ13C value, total N (TN), and their stocks. C decomposition (Dec-C) and net N mineralization (Net Min-N) were determined according to the production of CO2 and NH4+ + NO3– by aerobic incubation and CO2 + CH4 and NH4+ by anaerobic incubation, respectively. The results reveal that 40 years after change from rice paddy to orchard and wetland, the SOC and TN contents in the top-layer were not significantly altered. However, in buckwheat upland fields and in the parking area where vegetation was absent, the SOC content decreased significantly. Conversion of rice paddies to amur silver grass wetland altered the soil δ13C the most, leading to an increase of 5.1‰ and 2.9‰ for the top – and sub-layers, respectively. In general, the incubation experiment results revealed that a change to orchard and wetland did not significantly decrease the Dec-C and Net Min-N. Whereas, the change from rice paddies to parking area significantly decreased the Dec-C and Net Min-N in both aerobic and anaerobic conditions. We conclude that LUMC over decades had various effects on the SOC and TN contents and stocks as well as their mineralization potentials.

Soil organic matter is composed of fractions with different functions and reactivity. Among these, particulate organic matter (POM) is the main educt of new inputs of organic matter in soils and its chemical fate corresponds to the first stages of the SOM decomposition cascade ultimately leading to the association of organic and mineral phases. We aimed at investigating the POM molecular changes during decomposition at a sub-millimetre scale by combining direct measurements of POM elemental and molecular composition with laboratory imaging VNIR spectroscopy. For this, we set up an incubation experiment to compare the molecular composition of straw and composted green manure, materials greatly differing in their C/N ratio, during their decomposition in reconstituted topsoil or subsoil of a Luvisol, and recorded hyperspectral images at high spatial and spectral resolutions of complete soil cores at the start and end of the incubation. Hyperspectral imaging was successfully combined with machine learning ensembles to produce a precise mapping of POM alkyl/O-N alkyl ratio and C/N, revealing the spatial heterogeneity in the composition of both straw and green manure. We found that both types of organic amendment were more degraded in the reconstituted topsoil than in subsoil after the incubation. We also measured consistent trends in molecular changes undergone by straw, with the alkyl/O-N alkyl ratio slightly increasing from 0.06 to 0.07, and C/N dropping by about 40 units. The green manure material was very heterogeneous, with no clear molecular changes detected as a result of incubation. The visualisation approach presented here enables high-resolution mapping of the spatial distribution of the molecular characteristics of organic particles in soil cores, and offers opportunities to disentangle the roles of POM chemistry and morphology during the first steps of the decomposition cascade of organic matter in soils.

Soil organic matter is composed of fractions with different functions and reactivity. Among these, particulate organic matter (POM) is the main educt of new inputs of organic matter in soils and its chemical fate corresponds to the first stages of the SOM decomposition cascade ultimately leading to the association of organic and mineral phases. We aimed at investigating the POM molecular changes during decomposition at a sub-millimetre scale by combining direct measurements of POM elemental and molecular composition with laboratory imaging VNIR spectroscopy. For this, we set up an incubation experiment to compare the molecular composition of straw and composted green manure, materials greatly differing in their C/N ratio, during their decomposition in reconstituted topsoil or subsoil of a Luvisol, and recorded hyperspectral images at high spatial and spectral resolutions of complete soil cores at the start and end of the incubation. Hyperspectral imaging was successfully combined with machine learning ensembles to produce a precise mapping of POM alkyl/O-N alkyl ratio and C/N, revealing the spatial heterogeneity in the composition of both straw and green manure. We found that both types of organic amendment were more degraded in the reconstituted topsoil than in subsoil after the incubation. We also measured consistent trends in molecular changes undergone by straw, with the alkyl/O-N alkyl ratio slightly increasing from 0.06 to 0.07, and C/N dropping by about 40 units. The green manure material was very heterogeneous, with no clear molecular changes detected as a result of incubation. The visualisation approach presented here enables high-resolution mapping of the spatial distribution of the molecular characteristics of organic particles in soil cores, and offers opportunities to disentangle the roles of POM chemistry and morphology during the first steps of the decomposition cascade of organic matter in soils.

Sustainable agricultural practices aim to ensure the rebuilt of soil organic carbon (SOC) stocks and to sustain soil fertility. One of the levers is the use of carbon and nutrient inputs in the form of organic amendments, such as farmyard manure, slurry and biogas digestate. These organic fertilizers represent a promising alternative to the mineral fertilizers, which are mainly made from non-renewable resources. The use mineral fertilizers is indeed associated with an excessive use of natural resources and a loss of biodiversity. The effect of organic amendments compared with traditional mineral fertilizers on SOC stocks and soil fertility are uncertain in the longer-term. We aimed at investigating the effects of mineral and organic fertilizers (i.e., manure, pig slurry and biogas digestate) on topsoil and subsoil biogeochemistry, after eight years of application. For this purpose, we sampled soil cores down to a depth of one meter in a randomized field experiment in Germany, running since 2011. A full-profile assessment of the carbon and nitrogen distribution, stability and bioavailability was achieved using a combination of classical bulk physico-chemical analyses (e.g., SOC and nitrogen contents, texture, pH, bulk density) and state-of-the-art imaging techniques. Selected samples were analysed for aggregate size distribution, as well as organic carbon and nitrogen contents and allocation within these aggregates. Further, undisturbed core-samples were scanned using a hyperspectral camera in the Vis-NIR range to reveal hotspots of carbon storage at the soil profile scale. Soil carbon distribution was predicted as a function of spectral response, using a variety of machine learning approaches. The application of organic fertilizers (whatever their nature) resulted in higher SOC contents in the first 10 cm, as compared to the control and the mineral fertilizer treatments. The SOC stocks were + 21-33 % higher in the soil treated with organic fertilizers as compared to the control treatment. The application of mineral fertilizer or digestate, as compared to the control, resulted in higher relative amount of microaggregates (versus macroaggregates) (+ 19-40 %) in the soil down to 80 cm. These results will provide essential information to develop management strategies that increase nutrient recycling as well as SOC stocks.

Organic matter added to agricultural soil determines the C balance and the nutrient cycling in these ecosystems. Organic fertilisation can result in the accumulation of C in soil but can also stimulate the decomposition of the existing soil C pool, as the incorporation of an easily accessible energy-rich substrate often trigger the growth and activity of decomposer. We monitored the fate of two types of organic material (wheat straw and green manure) during the first stages of their decomposition into the soil. For this, we incubated 1-m soil columns amended with the two organic fertilisers either into the topsoil or into the subsoil. We measured changes in C and N contents, and used 13C-NMR to resolve the structural group composition of the added organic material. We also scanned the incubated samples with a hyperspectral camera and developed predictive models for C to N and for alkyl to O-alkyl ratios at a very fine spatial resolution (53 x 53 µm2 per pixel) for organic particles in the whole soil cores.The approach based on hyperspectral imaging was successful to follow the decomposition dynamics of POM during the incubation, and the associated decreases in C to N and increases in alkyl to O-alkyl ratios at a very fine spatial resolution, showing how different parts of the organic particles underwent distinct decomposition. We also observed contrasting decomposition dynamics between the wheat straw and the green manure. This method can bring new information about the first steps of fresh organic matter decomposition in soils and develop our general understanding of the soil organic matter decomposition continuum.

Background and aims: Phosphorus (P) is an essential element for crop growth. However, while links of P turnover in soils to carbon (C) and nitrogen (N) availability have been described, it remains to be clarified how combinations of fertilizer C and N additions affect stocks and cycling of distinct P fractions at different soil depths. The objectives of our study were (1) to assess how soil total P stocks are affected by organic amendments and N fertilization, (2) to evaluate how different soil P fractions respond to N fertilization, and (3) to verify whether N fertilization increases soil biological P cycling. Methods: We collected soil samples from a long‐term field experiment established in 1984 in Rauischholzhausen, Germany. The soil is a Haplic Luvisol and received either no organic fertilizer (NOF), farmyard manure (FYM) or a combination of organic and mineral N fertilizer (OMF). Each treatment additionally received three levels of mineral N: 0 kg ha−1 y−1 (N0), 100 kg ha−1 y−1 (N100), and 200 kg ha−1 y−1 (N200). The organic fertilizers were applied by a manure spreader and the N fertilizer (calcium ammonium nitrate) was applied in spring as top dressing by a plot fertilizer machine. We estimated stocks of P in fractions isolated by sequential P fractionation, and assessed the oxygen isotopic composition of 1 M HCl‐extractable phosphate (δ18OP). Results: We found that increased organic matter (OM) addition and mineral N inputs caused significant decreases in the stocks of resin‐ and NaHCO3‐extractable P in the topsoil (0–30 cm). Mineral N fertilization alone resulted in significant increases in stocks of resin‐, NaHCO3‐, and NaOH‐extractable P in the upper subsoil (30–50 cm). These changes occurred for both inorganic and organic P. The subsoil δ18OP values were closer to expected equilibrium values in soil fertilized with mineral N, indicative of more intensive biological P cycling than in the treatments without mineral N inputs. Conclusions: These findings suggest that long‐term OM and mineral N fertilization promotes topsoil P losses from labile fractions by crop uptake with an enrichment of these P forms in the subsoil, and an overall increase in biological P cycling in both top‐ and subsoil horizons upon N fertilization., Journal of Plant Nutrition and Soil Science, 184 (2), ISSN:1436-8730, ISSN:0044-3263, ISSN:1522-2624

Studies on soil organic carbon (SOC) stocks mostly focus on topsoils (< 30 cm). However, 30 to 63% of the SOC are stored in the subsoils (30 to 100 cm), and the factors controlling SOC storage in subsoils may be substantially different than in topsoils. The low mean SOC content in subsoils makes its quantification and characterization challenging. Thus, new approaches are required to depict the SOC stocks distribution in full soil profile. Hyperspectral imaging of soil core samples can provide high spatial resolution of the vertical distribution of SOC in a soil profile. The main objective of the ongoing study, within the Horizon 2020 European Project Circular Agronomics, is to apply laboratory hyperspectral imaging with a variety of machine learning approaches for the mapping of OC distribution in undisturbed soil cores. Soil cores were collected down to a depth of one meter in grasslands of 15 organic farms located in the Lungau Valley, in Austria. Some samples were divided into five depths in the field for classical bulk soil measurements (total carbon and nitrogen, texture, pH, EC and bulk density) on disturbed samples. Undisturbed soil cores were sliced vertically for laboratory hyperspectral imaging in the range of Vis-NIR (400-1000 nm). We were able to reveal the hotspots of OC and map the OC distribution in soil profile by applying a variety of machine learning approaches (i.e. partial least square and random forest regression) as a function of spectral responses. A digital elevation model was further exploited to investigate the effects of topographical factors such as elevation, aspect and slope on SOC profile distribution. Landsat 8 data were also used to depict the spatial variability of land insensitive cover/vegetation in study area.

While the demographic pressure for food demand is continuously rising, global environmental changes are threatening the productivity of agroecosystems. Climatic events like floods or droughts, and long-term decrease in soil organic matter stocks due to intensive agriculture are examples pointing to the necessity to find solutions for sustainable performance of agroecosystems.Significant amounts of water and nutrients are stored in deep soil horizons, and thus subsoil management is being considered as an alternative to sustain high demand in crop productivity. We used samples from an ongoing field experiment in Germany where the agricultural management was adapted to investigate the potential benefits of deep ploughing with OM incorporation. We recorded hyperspectral images of soil cores (depth = 1 m) using Vis-NIR reflectance spectroscopy and the C distribution within the soil was modeled at a very high spatial resolution (53×53 μm). The SOC mapping revealed an increase in SOC stocks resulting from deep ploughing, and the high resolution images generated allows the observation of OM distribution in the subsoil and the response in SOM stocks to different types of organic matter incorporation (compost vs green manure). The same imaging technique was also combined with solid-state 13C NMR measurements to track the molecular composition of the organic amendment during decomposition.Hyperspectral imaging of soil cores allows the quantification of OM stocks and changes at the pedon scale, and fine scale resolution of heterogeneity in the spatial distribution of soil organic matter is helping to understand and quantify the processes related to changes in soil C stocks in subsoils.

13 pages; International audience; Soils can act as a carbon sink, and the chemical and biological transformation of vegetal litter into soil organic matter (SOM) is widely influenced by land-use and other biogeochemical parameters. However, the increase of new carbon inputs to soil has the potential to trigger the mineralization of stabilized SOM, a process called priming effect.The objective of this manuscript is to investigate, at a landscape scale (5 km²), the factors influencing the susceptibility of SOM to priming effect. To achieve this objective, physical, chemical, and metagenomics analyses were conducted on 88 soil samples and successively combined with agronomical data and soil incubation for the quantification of carbon fluxes.Variance partitioning models highlighted that priming effect is controlled by complex interactions of biotic and abiotic factors, which include soil chemistry, quality of SOM, shape and abundance of microbial communities. Fluorescence properties of the dissolved organic matter has been found as a strong descriptor for priming effect. Depending on the time of crop rotation devoted to grassland, two different components leading to priming effect were identified. The introduction of grassland for 40–60% of the time of rotation achieved the lowest susceptibility to priming effect, and higher indexes of microbial diversity, whereas higher or lower proportions of time of the rotation devoted to grassland resulted in an increase of priming effect and a decrease of bacterial evenness.

The interface between decaying plant residues and soil minerals represents an essential soil microenvironment at which soil organic matter forms. The high amount of microbial products and residues within this hot spot of microbial activity fosters the formation of mineral-associated organic matter. Besides classical quantitative analyses, our understanding of processes controlling soil organic matter formation greatly benefits from microscopic observations and measurements, which provide spatially resolved information at a meaningful scale for microbial processes and for the association between organic and mineral particles. We studied carbon and nitrogen transfer from fresh-plant residues to the mineral soil, through a litter decomposition experiment in an artificial soil mixture. Needles of Norway spruce (Picea abies L.) were placed in microbatch containers filled with an artificial soil mixture free of soil organic matter. Containers were buried in fresh organic layer material from a Norway spruce stand and incubated for 14 and 42 days. We applied nanoscale secondary ion mass spectroscopy (NanoSIMS) to investigate the spatial distribution of mineral and organic compounds at the needle vicinity and into the mineral soil (0–550 μm from the needle). After 14 days, we depicted the formation of mineral-associated organic matter in the surrounding of the decaying needles. After 42 days, we observed substantial colonization of the needles and the detritusphere by saprotrophic fungi. The fungal hyphae extended into the mineral matrix of the artificial soil acting as vectors for the transfer of litter-derived carbon and nitrogen into the bulk soil. This resulted in an increase of the area covered by organic matter in the detritusphere, with up to 10% of the total investigated area classified as organic matter closely associated with mineral surfaces. Our results provide evidence that the carbon and nitrogen derived from litter decomposition transformed by microorganisms is transferred as mineral-associated organic matter, heterogeneously distributed from the litter source, and still detected 550 μm away from the latter. The close association of newly formed soil organic matter and fine sized minerals suggests that the formation of mineral-associated OM and likely also microaggregates is directly driven by microbial activity in the vicinity of hot spots for plant carbon input (e.g. the detritusphere).

It is estimated that only 20% of fertilizers applied annually in the European agricultural systems are converted to finished products for human consumption. These low efficiencies result in large loss of nutrients into the environment with severe negative influences on soils, water and air, and constitute unacceptable health and environmental costs. In addition, around 45% of soils in the European countries have less than 2% organic carbon (OC). Low soil OC storage is linked with negative environmental impacts including soil and water quality, climate change and biodiversity. A relevant strategy to enhance soil OC is the transformation of waste products into organic amendments for application on soils. The aim of the H2020 European project Circular Agronomics is to provide a comprehensive synthesis of practical solutions to improve the current C, N and P cycling in European agro-ecosystems. This project explores the medium and long-term effects of new and classical organic fertilizers in six countries including Germany, Spain, Italy, Netherlands, Czech Republic and Austria. The study sites will be sampled before and after applying the new organic amendments using a hydraulic corer. A full profile assessment of the C, N and P distribution, stability and bioavailability will be released up to one meter depth using a combination of classical bulk chemical analyses and state-of-the-art imaging techniques. Undisturbed soil cores will be scanned using a hyperspectral camera to reveal hotspots of C, N and P storage in the soil profile, at the micro-scale. Soil C, N and P will be modelled as a function of spectral response using a variety of machine learning approaches. These results will provide essential information to develop management strategies that reduce nutrient surplus and increase C stocks.

Understanding the effects of temperature and moisture on soil organic carbon (SOC) dynamics is crucial to predict the cycling of C in terrestrial ecosystems under a changing climate. For single rice cropping system, there are two contrasting phases of SOC decomposition in rice paddy soils: mineralization under aerobic conditions during the off-rice season and fermentation under anaerobic conditions during the growth season. This study aimed to investigate the effects of soil temperature and moisture on SOC decomposition under the aerobic and subsequently anaerobic conditions. Two Japanese paddy soils (Andisol and Inceptisol) were firstly incubated under four temperatures (±5, 5, 15, and 25°C) and two moisture levels (60 and 100% water-filled pore space (WFPS)) under aerobic conditions for 24 weeks. Then, these samples were incubated for 4 weeks at 30°C and under anaerobic conditions. Carbon dioxide (CO2) and methane (CH4) productions were measured during the two incubation stages to monitor the SOC decomposition dynamics. The temperature sensitivity of SOC was estimated by calculation of the Q10 parameter. The total CO2 production after the 24-week aerobic incubation was significantly higher in both soils for increasing soil temperature and moisture (P

To understand the effects of soil temperature and moisture on rice straw decomposition during the off-rice season and subsequent CH4 production in rice growth season, we firstly incubated a paddy soil with 13C-labeled straw addition under four temperature (±5, 5, 15, and 25 °C) and two moisture levels (60 and 100 % water-filled pore space (WFPS)) under aerobic conditions for 24 weeks and subsequently incubated those samples anaerobically at 30 °C and under submerged conditions for 4 weeks. Temperature change at ±5 °C was used to simulate the diurnal variation of soil temperature between night (−5 °C, 12 h) and day (5 °C, 12 h) during the corresponding freeze-thaw cycles. Our results showed that both increased soil temperature and moisture significantly promoted straw aerobic decomposition as observed by carbon dioxide (CO2) production, soil organic carbon (SOC) content, and its δ13C value in this incubation experiment. During the anaerobic incubation, straw-amended soil samples remarkably promoted CH4 production and CH4/total decomposed C. Both CH4 and CO2 productions in straw-amended soil samples were lowest at 25 °C and 100 % WFPS and highest at ±5 °C and 100 % WFPS, which could be attributed to a depletion of labile organic C derived from straw during the 24-week aerobic incubation. Total rates of straw C decomposition calculated from decomposed C production, SOC content, and δ13C value in the whole incubation (aerobic + anaerobic) ranged from 29.8 to 48.1, 45.0 to 64.2, and 25.7 to 35.7 %, respectively. These results imply that increasing soil temperature during the off-rice season promotes the rice straw decomposition and leads to a decrease in CH4 production during the subsequent rice growth season under conditions of northeastern Japan.

In soil, the link between microbial diversity and carbon transformations is challenged by the concept of functional redundancy. Here, we hypothesized that functional redundancy may decrease with increasing carbon source recalcitrance and that coupling of diversity with C cycling may change accordingly. We manipulated microbial diversity to examine how diversity decrease affects the decomposition of easily degradable (i.e., allochthonous plant residues) versus recalcitrant (i.e., autochthonous organic matter) C sources. We found that a decrease in microbial diversity (i) affected the decomposition of both autochthonous and allochthonous carbon sources, thereby reducing global CO 2 emission by up to 40%, and (ii) shaped the source of CO 2 emission toward preferential decomposition of most degradable C sources. Our results also revealed that the significance of the diversity effect increases with nutrient availability. Altogether, these findings show that C cycling in soil may be more vulnerable to microbial diversity changes than expected from previous studies, particularly in ecosystems exposed to nutrient inputs. Thus, concern about the preservation of microbial diversity may be highly relevant in the current global-change context assumed to impact soil biodiversity and the pulse inputs of plant residues and rhizodeposits into the soil. IMPORTANCE With hundreds of thousands of taxa per gram of soil, microbial diversity dominates soil biodiversity. While numerous studies have established that microbial communities respond rapidly to environmental changes, the relationship between microbial diversity and soil functioning remains controversial. Using a well-controlled laboratory approach, we provide empirical evidence that microbial diversity may be of high significance for organic matter decomposition, a major process on which rely many of the ecosystem services provided by the soil ecosystem. These new findings should be taken into account in future studies aimed at understanding and predicting the functional consequences of changes in microbial diversity on soil ecosystem services and carbon storage in soil.

10 pages; International audience; A better understanding of the links between dissolved organic matter and biogeochemical processes in soil could help in evaluating global soil dynamics. To assess the effects of land cover and parental material on soil biogeochemistry, we studied 120 soil samples collected from various ecosystems in Burgundy, France. The potential solubility and aromaticity of dissolved organic matter was characterised by pressurised hot-water extraction of organic carbon (PH-WEOC). Soil physico-chemical characteristics (pH, texture, soil carbon and nitrogen) were measured, as was the δ13C signature both in soils and in PH-WEOC. We also determined bacterial and fungal abundance and the genetic structure of bacterial communities. Our results show that the potential solubility of soil organic carbon is correlated to carbon and clay content in the soil. The aromaticity of PH-WEOC and its δ13C signature reflect differences in the decomposition pathways of soil organic matter and in the production of water-extractable organic compounds, in relation to land cover. The genetic structure of bacterial communities is related to soil texture and pH, and to PH-WEOC, revealing that water-extractable organic matter is closely related to the dynamics of bacterial communities. This comprehensive study, at the regional scale, thus provides better definition of the relationships between water-extractable organic matter and soil biogeochemical properties.

The characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air-dried soils to compare three extraction methods for water-extractable organic matter (WEOM) as follows: (i) pressurised hot-water-extractable organic carbon (PH-WEOC), a percolation at high pressure and temperature; (ii) water-extractable organic carbon (WEOC), a 1-hour end-over shaking; and (iii) leaching-extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48-day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH-WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic-like fluorophores and a tryptophan-like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH-WEOC and LEOC while WEOC extracts were depleted in humic-like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method-induced differences in WEOM quality vary with soil characteristics.

20 pages; International audience; Dissolved organic matter in soil is a highly reactive pool of organic matter and is of great importance for biogeochemical cycles in soil. A better understanding of its dynamics relies on its molecular characterisation. Here, the molecular composition of water-extracted organic matter at elevated pressure and temperature (PH-WEOM) obtained from 120 Burgundy soils was investigated using high-field Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR MS). Unsupervised multivariate statistical analysis (UMSA) was used to retrieve classes of samples with specific molecular characteristics. Accordingly, van Krevelen diagram, Kendrick mass defect (KMD), as well as aromaticity index (AI) and aromaticity equivalent (Xc), were applied to present a pool of ubiquitous molecular formulas and to evaluate the PH-WEOM molecular heterogeneity in the sample set. Based on UMSA, the PH-WEOM from forest soils revealed a clearly distinct molecular composition, with major contributions from lignin- and tannin-like compounds, and with its aromaticity related to soil characteristics, especially the soil pH. No clear evidence of land-cover influence on the PH-WEOM molecular composition was found for cropland and grassland soils, but the role of pH was also identified for these samples, and agrees with molecular patterns attributed to microbial activity, with the presence of compounds with high H/C ratio. A group of samples from cropland soils developed on residual formations is characterised by a very specific molecular composition, rich in aliphatic organosulfur-like compounds, highlighting the importance of specific soil processes in the molecular composition of PH-WEOM. This work demonstrates the potential of FT-ICR MS to resolve the high chemical complexity of PH-WEOM in soils and the intricate influences of both biotic and abiotic environmental factors on the molecular composition of PH-WEOM in soils.

9 pages; International audience; This study characterises the sediment dredged from a lagooning system composed of a settling pond and three lagoons that receive leachates from a municipal solid waste (MSW) landfill in France. Organic carbon, carbonate, iron oxyhydroxides, copper (Cu) and zinc (Zn) concentrations were measured in the sediment collected from upstream to downstream in the lagooning system. In order to complete our investigation of sedimentation mechanisms, leachates were sampled in both dry (spring) and wet (winter) seasonal conditions. Precipitation of calcite and amorphous Fe-oxyhydroxides and sedimentation of organic matter occurred in the settling pond. Since different distributions of Zn and Cu concentrations are measured in sediment samples collected downstream in the lagooning system, it is suggested that these elements were not distributed in a similar way in the leachate fractions during the first stage of treatment in the settling pond, so that their sedimentation dynamics in the lagooning system differ. In the lagoons, it was found that organic carbon plays a major role in Cu and Zn mobility and trapping. The presence of macrophytes along the edges provided an input of organic matter that enhanced Cu and Zn scavenging. This edge effect resulted in a two-fold increase in Cu and Zn concentrations in the sediment deposited near the banks of the lagoons, thus confirming the importance of vegetation for the retention of Cu and Zn in lagooning systems.

Studies on soil organic carbon (SOC) stocks mostly focus on topsoils (< 30 cm). However, 30 to 63% of the SOC are stored in the subsoils (30 to 100 cm), and the factors controlling SOC storage in subsoils may be substantially different than in topsoils. The low mean SOC content in subsoils makes its quantification and characterization challenging. Thus, new approaches are required to depict the SOC stocks distribution in full soil profile. Hyperspectral imaging of soil core samples can provide high spatial resolution of the vertical distribution of SOC in a soil profile. The main objective of the ongoing study, within the Horizon 2020 European Project Circular Agronomics, is to apply laboratory hyperspectral imaging with a variety of machine learning approaches for the mapping of OC distribution in undisturbed soil cores. Soil cores were collected down to a depth of one meter in grasslands of 15 organic farms located in the Lungau Valley, in Austria. Some samples were divided into five depths in the field for classical bulk soil measurements (total carbon and nitrogen, texture, pH, EC and bulk density) on disturbed samples. Undisturbed soil cores were sliced vertically for laboratory hyperspectral imaging in the range of Vis-NIR (400-1000 nm). We were able to reveal the hotspots of OC and map the OC distribution in soil profile by applying a variety of machine learning approaches (i.e. partial least square and random forest regression) as a function of spectral responses. A digital elevation model was further exploited to investigate the effects of topographical factors such as elevation, aspect and slope on SOC profile distribution. Landsat 8 data were also used to depict the spatial variability of land insensitive cover/vegetation in study area.

It is estimated that only 20% of fertilizers applied annually in the European agricultural systems are converted to finished products for human consumption. These low efficiencies result in large loss of nutrients into the environment with severe negative influences on soils, water and air, and constitute unacceptable health and environmental costs. In addition, around 45% of soils in the European countries have less than 2% organic carbon (OC). Low soil OC storage is linked with negative environmental impacts including soil and water quality, climate change and biodiversity. A relevant strategy to enhance soil OC is the transformation of waste products into organic amendments for application on soils. The aim of the H2020 European project Circular Agronomics is to provide a comprehensive synthesis of practical solutions to improve the current C, N and P cycling in European agro-ecosystems. This project explores the medium and long-term effects of new and classical organic fertilizers in six countries including Germany, Spain, Italy, Netherlands, Czech Republic and Austria. The study sites will be sampled before and after applying the new organic amendments using a hydraulic corer. A full profile assessment of the C, N and P distribution, stability and bioavailability will be released up to one meter depth using a combination of classical bulk chemical analyses and state-of-the-art imaging techniques. Undisturbed soil cores will be scanned using a hyperspectral camera to reveal hotspots of C, N and P storage in the soil profile, at the micro-scale. Soil C, N and P will be modelled as a function of spectral response using a variety of machine learning approaches. These results will provide essential information to develop management strategies that reduce nutrient surplus and increase C stocks.

Poster presentation at at the 7th International Symposium on Soil Organic Matter (SOM2019), in Adelaide, South Australia, from the 6th to the 11th of October.

Sustainable agricultural practices aim to ensure the rebuilt of soil organic carbon (SOC) stocks and to sustain soil fertility. One of the levers is the use of carbon and nutrient inputs in the form of organic amendments, such as farmyard manure, slurry and biogas digestate. These organic fertilizers represent a promising alternative to the mineral fertilizers, which are mainly made from non-renewable resources. The use mineral fertilizers is indeed associated with an excessive use of natural resources and a loss of biodiversity. The effect of organic amendments compared with traditional mineral fertilizers on SOC stocks and soil fertility are uncertain in the longer-term. We aimed at investigating the effects of mineral and organic fertilizers (i.e., manure, pig slurry and biogas digestate) on topsoil and subsoil biogeochemistry, after eight years of application. For this purpose, we sampled soil cores down to a depth of one meter in a randomized field experiment in Germany, running since 2011. A full-profile assessment of the carbon and nitrogen distribution, stability and bioavailability was achieved using a combination of classical bulk physico-chemical analyses (e.g., SOC and nitrogen contents, texture, pH, bulk density) and state-of-the-art imaging techniques. Selected samples were analysed for aggregate size distribution, as well as organic carbon and nitrogen contents and allocation within these aggregates. Further, undisturbed core-samples were scanned using a hyperspectral camera in the Vis-NIR range to reveal hotspots of carbon storage at the soil profile scale. Soil carbon distribution was predicted as a function of spectral response, using a variety of machine learning approaches. The application of organic fertilizers (whatever their nature) resulted in higher SOC contents in the first 10 cm, as compared to the control and the mineral fertilizer treatments. The SOC stocks were + 21-33 % higher in the soil treated with organic fertilizers as compared to the control treatment. The application of mineral fertilizer or digestate, as compared to the control, resulted in higher relative amount of microaggregates (versus macroaggregates) (+ 19-40 %) in the soil down to 80 cm. These results will provide essential information to develop management strategies that increase nutrient recycling as well as SOC stocks.

The main goals of sustainable agricultural practices are to rebuild soil organic carbon (SOC) stocks and to sustain soil fertility. The use of organic amendments such as manure, slurry and biogas digestate, as sources of carbon and nutrients, is one of the levers to achieve these goals, as an alternative to the use of mineral fertilizers. However, the effects of organic amendments compared with traditional mineral fertilizers on topsoil and subsoil SOC stocks and soil fertility are still uncertain. Hence, we aimed at investigating the effects of mineral and organic fertilizers (i.e., manure, pig slurry and biogas digestate) on topsoil and subsoil biogeochemistry, and soil structure, after seven years of application. To this end, we sampled soil cores down to 1 m depth in a randomized field experiment in North Germany, running since 2011. We quantified the SOC and nitrogen stocks, as well as some nutrient contents (e.g., nitrate, available phosphorus). Selected samples were further analysed for aggregate size distribution, as well as organic carbon and nitrogen contents within these aggregates. A hyperspectral camera in the range of Vis-NIR was used to scan undisturbed core-samples in order to reveal hotspots of carbon storage along the soil profile. Soil carbon distribution was predicted as a function of spectral response coupled with a machine learning ensemble. Overall, the mean SOC stocks were low (53 t ha-1), reflecting the sandy loam texture of the Northeast German soils under permanent cropping. The application of organic fertilizers (whatever their nature) resulted in higher SOC contents in the first 10 cm (+26 %) and from 20-40 cm (+30%), as compared to the mineral fertilizer treatments. The application of mineral fertilizer or digestate, as compared to the control, resulted in higher relative amount of microaggregates (versus macroaggregates) (+ 19-40 %) in the soil down to 80 cm. These results will provide essential information to develop management strategies that could increase nutrient recycling as well as SOC stocks.