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The structure of soil aggregates plays an important role for the turnover of particulate organic matter (POM) and vice versa. Analytical approaches usually do not disentangle the continuous re-organization of soil aggregates, caught between disintegration and assemblage. This led to a lack of understanding of the mechanistic relationship between aggregation and OM sequestration in soils.In this study, we take advantage of a process-based mechanistic model that describes the interaction between the dynamic (re-)arrangement of soil aggregates, based on realistic shapes obtained by dynamic image analysis of wet-sieved aggregates, the turnover of POM, and simultaneous alteration of soil surface properties in a spatially and temporally explicit way.We used this modeling approach to investigate the impact of the following factors for aggregation: soil texture, OM input and OM decomposition rate. Our model enabled us to quantify the temporal development of the aggregate size distribution, the amount of OC in POM fractions of different ages and the surface coverage.The simulations provided important implications for the sequestration of OM in soils. Firstly, aggregation was largely determined by the POM input and mostly decoupled from the soil texture. Secondly, the OM storage in terms of POM increased with clay content, with both findings confirming experimental results. Thirdly, we were able to contribute to the understanding of a structural priming effect in which the increased input of POM stimulated the mineralization of old POM.

Microbial necromass is a central component of soil organic matter (SOM), whose management may be essential in mitigating atmospheric CO2 concentrations and climate change. Current consensus regards the magnitude of microbial necromass production to be heavily dependent on the carbon use efficiency of microorganisms, which is strongly influenced by the quality of the organic matter inputs these organisms feed on. However, recent concepts neglect agents relevant in many soils: earthworms. We argue that the activity of earthworms accelerates the formation of microbial necromass stabilized in aggregates and organo-mineral associations and reduces the relevance of the quality of pre-existing organic matter in this process. Earthworms achieve this through the creation of transient hotspots (casts) characterized by elevated contents of bioavailable substrate and the efficient build-up and quick turnover of microbial biomass, thus converting SOM not mineralized in this process into a state more resistant against external disturbances, such as climate change. Promoting the abundance of earthworms may, therefore, be considered a central component of management strategies that aim to accelerate the formation of stabilized microbial necromass in wide locations of the soil commonly not considered hotspots of microbial SOM formation.

The charge-and concentration-dependent sorption behavior of a range of per-and polyfluoroalkyl substances (PFASs) was studied for three organic soil samples with different organic matter quality, one Spodosol Oe horizon (Mor Oe) and two Sphagnum peats with different degrees of decomposition (Peat Oi and Peat Oe). Sorption to the two peat materials was, on average, four times stronger compared to that onto the Mor Oe material. In particular, longer-chained PFASs were more strongly bound by the two peats as compared to the Mor Oe sample. The combined results of batch sorption experiments and 13C NMR spectroscopy suggested sorption to be positively related to the content of carbohydrates (i.e., O-alkyl carbon). Sorption of all PFAS subclasses was inversely related to the pH value in all soils, with the largest pH effects being observed for perfluoroalkyl carboxylates (PFCAs) with C-10 and C-11 perfluorocarbon chain lengths. Experimentally determined sorption isotherms onto the poorly humified Peat Oi did not deviate significantly from linearity for most substances, while for the Mor Oe horizon, sorption nonlinearity was generally more pronounced. This work should prove useful in assessing PFAS sorption and leaching in organic soil horizons within environmental risk assessment.

Saturated acid sulfate soils with hypersulfidic material are productive wetland soils, but when they dry, they generate large amounts of sulfuric acid due to oxidation of pyrite to form sulfuric material (pH

Archived soil samples are a valuable tool for any long‐term soil research. We analysed total carbon (C) and nitrogen (N) content and soil organic matter fractions in 38 archived soil samples that were stored for up to 21 years and compared air‐dried storage to frozen storage conditions. Samples include top‐ and upper subsoils, different soil texture and land use with C contents between 4.3 and 174 mg g⁻¹. The results from this study reveal no changes in total C and N contents with storage time up to 21 years or type of storage (freezing vs. air drying). The analyses of soil physical fractions also revealed no significant differences between air‐dried stored and frozen stored samples for most samples. However, we found indications, that freezing of soil material might lead to changes in the mineral fractions for soils containing high amounts of water. Therefore, and as archiving soils in a frozen state is more expensive than storing air‐dried samples, we recommend the use of air‐dried samples for C quality analyses of archived soil samples.

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 microaggregates contain particles of different sizes, which may affect their potential to store organic carbon (OC). A variety of methods can be used to isolate microaggregates from the larger soil structures, among which wet sieving approaches are widely employed. We developed a novel dry crushing method that isolates microaggregates along failure planes due to mechanical stresses rather than hydraulic pressures and compared the mechanical stability, OC contents and microbial community composition between dry‐crushed and wet‐sieved samples with contrasting clay contents. Dry‐crushed samples exhibited a higher stability and bacterial diversity compared to wet‐sieved samples. As a result, the dry‐crushed microaggregates had different size distributions when analysed dry and after wetting. In the dry state, dry‐crushed microaggregates were larger and contained more sand‐sized primary particles within the aggregate structures. The wetting of dry‐crushed aggregates caused a disintegration of larger microaggregates and sand‐sized primary particles into smaller microaggregates that contained finer particles. In the soils with lower clay contents, the diameter of dry‐crushed microaggregates was 40 μm larger due to more sand‐sized primary particles remaining within the aggregates. Depending on how much volume in microaggregates is occupied by large primary particles, the OC concentration increased in the soil with higher clay content. Wet‐sieved size fractions also showed a similar pattern of OC distribution, whereas more primary particles were observed outside of aggregates. Wet sieving approaches disperse the soil into OC‐rich aggregates and might be preferable if OC dynamics are investigated. Differences in bacterial community composition in dependence on clay content were more pronounced in dry‐crushed microaggregates. If intact aggregate architectures are of interest for the isolation of soil structural units, the presented dry crushing method might provide an advantageous alternative that also better preserves bacterial diversity.

Agricultural sandy soils with high organic matter (OM) contents are generally unexpected under the current paradigm of organic matter formation and stabilization. These so-called black sand soils occur in North-Western Europe and have been related to historical heathland vegetation. The properties and mechanisms of the high OM sequestration in these soils are not clear as they exceed common observations of OM stored in coarse-textured soils. In this study, we analyzed a subset of samples with ‘black sand’ properties from the European soil database “Land Use/Cover Area frame statistical Survey” (LUCAS). Through particle size fractionation, we isolated the fine fraction -1 with a C:N ratio of 17.4 on average and was positively correlated with the bulk soil OC. The characterization of OM composition in the fine fractions by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy revealed that the share of alkyl C increased with OC concentrations whereas O/N-alkyl C decreased. To analyze the distribution of OM at the microscale, we analyzed five samples from the ‑1 with nanoscale secondary ion mass spectrometry (NanoSIMS) at a spatial resolution of 120 nm. These microscale measurements revealed fine mineral particle structures associated with heterogeneously distributed OM. Using image analysis, we found that the proportion of OM-dominated area (indicated by 12C2- and 26CN-) increased from 52 to 80 % on average with increasing OC concentration of the fine fractions. A majority of OM-dominated area was correlated with higher 42AlO- counts, which might suggest a preferential co-localization. In turn, the particle area which was dominated by minerals (indicated by 16O‑, 28Si‑, 42AlO‑ and 72FeO‑) contained less Al and more Si. This shows that the more alkylated and OM-rich fine fractions are related with distinct patterns of organo-mineral structures at the microscale.

Nutrient turnover in soils is strongly driven by soil properties, including clay mineral composition. One main nutrient is phosphorus (P), which is known to be easily immobilized in soil. Therefore, the specific surface characteristics of clay minerals might substantially influence P availability in soil and thus the microbial strategies for accessing P pools. We used a metagenomic approach to analyze the microbial potential to access P after 842 days of incubation in artificial soils with a clay mineral composition of either non-expandable illite (IL) or expandable montmorillonite (MT), which differ in their surface characteristics like soil surface area and surface charge. Our data indicate that microorganisms of the two soils developed different strategies to overcome P depletion, resulting in similar total P concentrations. Genes predicted to encode inorganic pyrophosphatase (ppa), exopolyphosphatase (ppx), and the pstSCAB transport system were higher in MT, suggesting effective P uptake and the use of internal poly-P stores. Genes predicted to encode enzymes involved in organic P turnover like alkaline phosphatases (phoA, phoD) and glycerophosphoryl diester phosphodiesterase were detected in both soils in comparable numbers. In addition, Po concentrations did not differ significantly. Most identified genes were assigned to microbial lineages generally abundant in agricultural fields, but some were assigned to lineages known to include oligotrophic specialists, such as Bacillaceae and Microchaetaceae.

The response of mineral-stabilized soil organic carbon (SOC) to environmental change is a source of uncertainty in the understanding of SOC cycling. Fluctuating wet-dry cycles and associated redox changes in otherwise well-drained soils may drive mineral dissolution, organic carbon (OC) mobilization, and subsequent OC mineralization. However, the extent to which rapid fluctuations between water-saturated and unsaturated conditions (i.e., flashy conditions) result in long-term changes in mineral composition and organo-mineral interactions is not well understood. In this study, the effect of variable saturation frequency on soil mineral composition, mineral-associated OC, and OC mineralizability was tested using selective dissolution, bulk spectroscopy, microscale imaging, and aerobic-anaerobic incubation experiments. Previous water table fluctuation measurements and diagnostic profile characteristics at Hubbard Brook Experimental Forest (NH) were used to identify soils with high, medium, and low saturation frequency regimes (defined by historical water table cycling frequency; i.e., water table presence and recession in the upper B horizon). We found the amount of OC released during extractions targeting non-crystalline minerals was of similar magnitude as extracted iron (Fe) in lower saturation frequency soils. However, the magnitude of extracted OC was 2.5 times greater than Fe but more similar to extractable aluminum (Al) in higher saturation frequency soils. Bulk soil Fe was spatially more strongly correlated to soil organic matter (SOM) in lower saturation frequency soils (Spearman Rank r(s) = 0.62, p < 0.005), whereas strong correlations between Al and SOM were observed in higher saturation frequency soils (r(s) = 0.88, p < 0.005) using nanoscale secondary ion mass spectrometry (NanoSIMS) imaging. Characterization of bulk soil Fe with X-ray absorption spectroscopy showed 1.2-fold greater Fe(II) and 1-fold lower contribution of Fe-organic bonding in soils with high saturation frequency. Fe(III) interactions with carboxylic and aromatic C were identified with C-13 nuclear magnetic resonance (NMR) spectroscopy Fe(III) interference experiments. Additionally, carboxylic acid enrichment in high saturation frequency soils quantified by C K-edge X-ray absorption spectroscopy point towards the role of carboxylic functional groups in Al-organic in addition to Fe-organic interactions. In our incubation experiments, a doubling in short-term CO2 evolution (per unit total soil C) was detected for high relative to low saturation frequency soils. Further, an order of magnitude increase in CO2 evolution (per unit water-extractable OC) following anaerobic incubation was only detected in high saturation frequency soils. The observed shift towards Al-dominated SOC interactions and higher OC mineralizability highlights the need to describe C stabilization in soils with flashy wet-dry cycling separately from soils with low saturation frequency or persistent saturation. NSF IGERT in Cross-Scale Biogeochemistry and Climate at Cornell University (NSF)National Science Foundation (NSF) [1069193]; Institute for Advanced Study (IAS) from the Technical University of Munich (TUM); Andrew W. Mellon Foundation; Cornell College of Agriculture and Life Sciences Alumni Foundation, USA; National Science FoundationNational Science Foundation (NSF) [DMR-1332208]; Canada Foundation for InnovationCanada Foundation for InnovationCGIAR; Natural Sciences and Engineering Research Council of CanadaNatural Sciences and Engineering Research Council of Canada (NSERC)CGIAR; University of Saskatchewan; Government of Saskatchewan; Western Economic Diversification Canada; National Research Council Canada; Canadian Institutes of Health Research, CanadaCanadian Institutes of Health Research (CIHR) Funding for this study was provided by the NSF IGERT in Cross-Scale Biogeochemistry and Climate at Cornell University (NSF Award #1069193) and the Institute for Advanced Study (IAS) from the Technical University of Munich (TUM) through the Hans-Fisher Senior Fellowship. Additional research funds were provided by the Andrew W. Mellon Foundation and the Cornell College of Agriculture and Life Sciences Alumni Foundation, USA. Hubbard Brook Experimental Forest is operated and maintained by the US Forest Service, Northern Research Station, Newtown Square, PA, USA. This work uses research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under award DMR-1332208. Research was performed at the Canadian Light Source (CLS), which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research, Canada. The authors thank Dr. Rong Huang of CHESS, Dr. Tom Regier of CLS, Kelly Hanley and Akio Enders of Cornell University for technical assistance; Dr. Carmen Hoschen, Johann Lugmeier and Dr. Carsten Mueller of the Technical University of Munich for their support with the NanoSIMS measurements; Dr. Carmen Enid Martinez' research group for providing iron XAS standard materials; Dr. Linda Pardo, Stephanie Duston, Brittany LeBeau, and Johali Sotelo for soil sample collection and field characterization; and the referees for their invaluable insights. Public domain – authored by a U.S. government employee

Floodplain soils are important reservoirs of organic carbon (OC) in the terrestrial carbon cycle. Few rivers and floodplains in the world and particularly in central Europe are in a natural state. They are regulated, stabilised from erosion behind artificial levees, drained and used for agriculture. Fluvisols store high amounts of OC from the topsoil to the subsoil, but little is known about the soil organic matter (SOM) quality and its vulnerability to decomposition. In this study, two regulated floodplains originating from different parent materials under grassland use in Southern Germany (Alpine Foreland and the Bavarian Forest) were sampled and analysed for the quantity and quality of inherent SOM in the topsoil and two subsoil levels. We characterised bulk soil (pH, texture, inorganic carbon and total nitrogen) and applied a combined density and size fractionation scheme to obtain six fractions. The chemical composition of the fractions was further determined using solid-state 13C NMR spectroscopy (SOM composition), as well as X-ray diffraction (clay mineralogy) and N2-BET (specific surface area (SSA)). Contributions of light fractions and especially of occluded particulate organic matter (oPOMfine) to the total OC were remarkably high in the subsoils. Organo-mineral associations (OMFfine) highly contributed to total OC at both sites but only in the topsoils. The highest OC concentrations of OMFfine were found in calcareous parent material, which shows that polyvalent cations promote OC storage. However, the inter-site as well as intra-site heterogeneity of oPOMfine and OMFfine OC contribution was highly variable. In the Bavarian Forest, oPOMfine was enriched in lipids in the subsoil. This was most probably due to a limited supply of oxygen in the aggregates through fluctuating groundwater levels that retarded decomposition and selectively preserved aliphatic compounds. In the Alpine Foreland, soil biota had mixed fresh SOM into the subsoil. There, oPOMfine contained all the functional C-groups, particularly carbohydrates. A changing global climate jeopardises OC reservoirs in floodplains, due to increased flooding and associated river bank erosion. Such disturbances not only cause losses of productive land but also release OC stored in light fractions that could be oxidised to CO2, depending on its chemical composition, thus adding to global warming.

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.

Despite a large body of studies investigating soil organic carbon (SOC) stocks and potential influencing factors, the impact of contrasting parent material, particularly in the subsoil, has received little attention. To reveal potential effects varying parent materials exert on SOC stocks, we investigated chemical (14C content and overall chemical composition via 13C NMR spectroscopy) and plant/microbial related parameters (root mass, amino sugars) of bulk soil and soil organic matter fractions from topsoil, subsoil, and rhizosphere soil at three European beech stands (Fagus sylvatica L.) only differing in parent material (Tertiary sand, Quaternary loess, and Tertiary basalt). The results suggest that the clay fraction, its amount being largely dependent on the respective parent material, took a central role in shaping differences in SOC stocks among the investigated sites by affecting soil organic matter stabilization via organo-mineral association and aggregation. This fraction was particularly relevant in the subsoil, where it accounted for up to 80% of the bulk soil SOC stocks that decreased with decreasing amounts of the clay fraction (basalt > loess > sand site). Determining the soil's nutrient composition, parent material likely also indirectly affected SOC stocks by changing rhizosphere traits (such as fine root density or mortality) and by attracting root growth (and thus organic matter inputs) to subsoil with higher nutrient contents, where in situ root inputs in the form of rhizodeposits were likely the prime source of plant-derived SOC. However, root inputs also contributed in large part to topsoil SOC stocks and were associated with higher abundance of microbial compounds (amino sugars), whose relative importance increased with increasing soil depth. Independent of soil depth and site, amino sugars and the amount of the clay fraction, combined with parameters related to the input of organic matter (root mass and amount of the particulate organic matter fraction) explained more than 90% of the variability in SOC stocks, indicating a key role of these measures in impacting SOC stocks. Because parent material directly or indirectly influenced these parameters, we demonstrate the necessity to consider differences in parent material when estimating and predicting SOC stocks.

We compared synchrotron-based C near-edge X-ray absorption fine structure (NEXAFS) and CPMAS 13C nuclear magnetic resonance (NMR) spectroscopy with respect to their precision and accuracy to quantify different organic carbon (OC) species in defined mixtures of soil organic matter source compounds. We also used both methods to quantify different OC species in organic surface horizons of a Histic Leptosol as well as in mineral topsoil and subsoil horizons of two soils with different parent material, stage of pedogenesis, and OC content (Cambisol: 15–30 OC mg g−1, Podzol: 0.9–7 OC mg g−1). CPMAS 13C NMR spectroscopy was more accurate and precise (mean recovery of different C functional groups 96–103%) than C NEXAFS spectroscopy (mean recovery 92–113%). For organic surface and topsoil samples, NMR spectroscopy consistently yielded larger O-alkyl C percentages and smaller alkyl C percentages than C NEXAFS spectroscopy. For the Cambisol subsoil samples both methods performed well and showed similar C speciation results. NEXAFS spectroscopy yielded excellent spectra with a high signal-to-noise ratio also for OC-poor Podzol subsoil samples, whereas this was not the case for CPMAS 13C NMR spectroscopy even after sample treatment with HF. Our results confirm the analytical power of CPMAS 13C NMR spectroscopy for a reliable quantitative OC speciation in soils with >10 mg OC g−1. Moreover, they highlight the potential of synchrotron-based C NEXAFS spectroscopy as fast, non-invasive method to semi-quantify different C functional groups in soils with low C content (0.9–10 mg g−1).

Plaggic Anthrosols are anthropogenic soils formed by former plaggen agriculture in north-western continental Europe. Here, we present an inventory of organic carbon (OC) concentrations and stocks of Plaggic Anthrosols and of reference soils in the vicinity without plaggen fertilization history from eight sites in northwest Germany (Lower Saxony, North Rhine-Westphalia) and the eastern parts of The Netherlands (North Brabant, Overijssel). To evaluate the enrichment of Plaggic Anthrosols with OC and the implications for OC stocks, soils were analysed for basic soil properties (bulk density, pH, OC − and nitrogen (N) concentrations), texture, and acid oxalate-(FeOX) and dithionite-citrate-bicarbonate-extractable iron (FeDCB) concentrations. Organic C stocks and plant-available water capacity (PAWC) were calculated. The plaggic topsoils are characterized by high OC concentrations (6.5–36.9 mg g−1) and a thickness of 53–124 cm, in contrast to the modern cultivated ∼30 cm thick topsoils of the reference soils with 10.6–24.0 mg g−1 OC concentration. The plaggen agriculture formed a soil with improved properties and conditions for plant growth. The addition of plaggen material to the topsoils induced a volume increase as well as higher OC concentrations in the Ap horizons, resulting in significantly higher OC stocks in the Plaggic Anthrosols (6.3–14.5 kg m−2) compared to the reference soils (4.9–10.5 kg m−2). The additional soil volume amounted to 30–56% to the total soil profile depth (1.0 to 1.6 m) and comprised 25–62% of the total OC stocks. Compared to the reference soils, the volume increase of the plaggic topsoil increased the PAWC in the effective rooting zone by 1.6 to 3.4 times. Plaggic Anthrosols represent a relic of historical farming with preserved high OC concentrations and improved storage capacity for plant-available water, causing higher nutrient availability and an increase in yields, which in turn result in higher in-situ OM input, additionally contributing to high OC stocks. The results of this study demonstrate the potential of soil melioration practices similar to the former plaggen management to induce high OC concentrations and increase OC stocks in sandy agricultural soils.

Long-term cultivation of steppe soils in a nonsustainable way caused severe soil degradation and reduced agricultural productivity in Eastern Europe, one of the world's most important areas for cereal production. In order to combat soil erosion and maintain yields, a widespread system of tree windbreaks was introduced in the 1950s, accompanied by improved agricultural practices in recent years. However, information on the effectiveness of such measures to rebuild soil organic carbon (SOC) is scarce. The objective of this study was to estimate the OC storage potential of the fine mineral fraction of degraded arable steppe soils in Moldova and to quantify SOC sequestration rates under (a) windbreaks, (b) cropland with improved crop rotation/manure application, and (c) cropland with cover cropping. Natural grassland relicts served as a reference to estimate the SOC saturation potential. Our results revealed a low SOC saturation of 50% under conventional agricultural use due to high SOC losses, indicating a high potential for SOC sequestration. Relatively high SOC sequestration rates were determined for topsoils (0–30 cm) under windbreaks (0.9 t ha−1 yr−1), improved crop rotation/manure application (1.3 t ha−1 yr−1), and cover cropping (1.9 t ha−1 yr−1). In this regard, sequestration rates derived from OC changes of the fine fraction may be more reliable than total SOC-based rates, particularly for windbreaks with high proportions of labile SOC. We conclude that implementation of improved agricultural management together with the maintenance of windbreaks is a promising strategy to rebuild SOC, reduce widespread soil erosion and compaction, and secure Moldova's agricultural productivity.

Global change contributes to the retreat of glaciers at unprecedented rates. The deglaciation facilitates biogeochemical processes on glacial deposits with initiating soil formation as an important driver of evolving ecosystems. The underlying mechanisms of soil formation and the association of soil organic matter (SOM) with mineral particles remain unclear, although further insights are critical to understand carbon sequestration in soils. We investigated the microspatial arrangement of SOM coatings at intact soil microaggregate structures during various stages of ecosystem development from 15 to >700 years after deglaciation in the proglacial environment of the Damma glacier (Switzerland). The functionally important clay-sized fraction ( 2.2 g/cm3 ). To quantify how SOM extends across the surface of mineral particles (coverage) and whether SOM coatings are distributed in fragmented or connected patterns (connectivity), we developed an image analysis protocol based on nanoscale secondary ion mass spectrometry (NanoSIMS). We classified SOM and mineral areas depending on the 16 O- , 12 C- , and 12 C14 N- distributions. With increasing time after glacial retreat, the microspatial coverage and connectivity of SOM increased rapidly. The rapid soil formation led to a succession of patchy distributed to more connected SOM coatings on soil microaggregates. The maximum coverage of 55% at >700 years suggests direct evidence for SOM sequestration being decoupled from the mineral surface, as it was not completely masked by SOM and retained its functionality as an ion exchange site. The chemical composition of SOM coatings showed a rapid change toward a higher CN:C ratio already at 75 years after glacial retreat, which was associated with microbial succession patterns reflecting high N assimilation. Our results demonstrate that rapid SOM sequestration drives the microspatial succession of SOM coatings in soils, a process that can stabilize SOM for the long term.

The interaction between mineral particles and soil organic matter (SOM) is an important and complex process during soil structure formation, in which the effects of soil texture and OM properties are intertwined. Within the SOM, there are residues of particulate organic matter (POM) of various sizes, as well as microbial necromass co-existing. These OM residues undergo microbial decay whose products stabilize particle connections and thus induce aggregate formation. We developed an experimental set-up to study early soil structure formation within a controlled lab environment. Artificial soil microcosms with a mineral mixture of different textures (clay loam, loam, and sandy loam) were used to perform a short-term incubation for 30 days under constant water tension. OM was added individually either as POM of two different size classes (milled hay litter, 0.63–2 mm and The dry mixing process and incubation of the mineral mixtures led to particle–particle interactions and fine particle coatings of the sand grains as shown by a reduction in the specific surface area (N2-BET). The OM residues were quickly accessed and degraded by microbes (peak in CO2-release within the first 10 days of incubation), which induced the formation of water-stable aggregates. The POM of both sizes induced predominantly the formation of macroaggregates (0.63–30 mm) with a mass contribution of 72% to 91%, irrespective of the mixtures’ texture. The bacterial necromass induced a texture-dependent formation of macro- and microaggregates (63–200 µm), with larger aggregates in sand-rich mixtures. The different aggregate sizes were related to differences in the developed microbial community, as obtained by PLFA analysis. The bacterial necromass induced a microbial community dominated by bacteria, whereas the POM fostered a high relative abundance of fungi, whose hyphae could enmesh and stabilize large aggregates in all textures. The formed aggregates are water-stable but have a very low mechanical stability. Dry crushing with a mechanical loading frame revealed that very low forces (

Acid sulfate soils contain hypersulfidic material, e.g. pyrite (FeS2). Under oxidizing conditions, it transforms to sulfuric material (pH We demonstrated that the jarosite can be differentiated into zones with either high or low jarosite concentrations at distances of

When acid sulfate soils containing hypersulfidic material (pH > 4) dry, oxidation of pyrite causes strong acidification with the formation of sulfuric material (pH

Base hydrolysis methods are widely used to extract bound lipid biomarkers such as cutin and suberin from plant material, soils, and sediments. However, non-uniform treatments among studies hamper their comparability, and information on differences between commonly used saponification methods does not exist. The aim of the present study was to compare frequently used base hydrolysis methods and evaluate their performance in extracting bound lipids from common European forest and agricultural plant species ( Fagus sylvatica L. and Zea mays L.), and corresponding soils and sediments. We compared extraction in Teflon-lined bombs, ultrasound assisted extraction, and extraction with reflux in quintuplicate for each plant, soil, and sediment material. The reflux method gave consistently higher yields of bound lipids as compared to the other two methods, while requiring the same amount of time and solvent and featuring lower standard errors. Some lipids, including common building blocks of cutin and, to a lesser extent, suberin, were more efficiently extracted by the reflux method than others, indicating that source assignment of organic matter in soils and sediments via biomarkers might be sensitive to the employed method. Although the other extraction methods may perform similarly under different conditions, such as other sample matrices, extraction conditions of the reflux method may potentially be more easily controlled across replicates and laboratories as compared to the bomb or ultrasound methods. Thus, the use of reflux methods ensures meaningful yields of hydrolysable lipids while providing high reproducibility.

Agricultural soils are widely recognized to be capable of carbon sequestration that contributes to mitigating CO2 emissions. To better understand soil organic carbon (SOC) stock dynamics and its driving and controlling factors corresponding with a period of rapid agronomic evolution from the 1980s to the 2010s in the North China Plain (NCP), we collected data from two region-wide soil sampling campaigns (in the 1980s and 2010s) and conducted an analysis of the controlling factors using the random forest model. Between the 1980s and 2010s, environmental (i.e. soil salinity/fertility) and societal (i.e. policy/techniques) factors both contributed to adoption of new management practices (i.e. chemical fertilizer application/mechanization). Results of our work indicate that SOC stocks in the NCP croplands increased significantly, which also closely related to soil total nitrogen changes. Samples collected near the surface (0-20 cm) and deeper (20-40 cm) both increased by an average of 9.4 and 5.1 Mg C ha-1 , respectively, which are equivalent to increases of 73% and 56% compared with initial SOC stocks in the 1980s. The annual carbon sequestration amount in surface soils reached 10.9 Tg C year-1 , which contributed an estimated 43% of total carbon sequestration in all of China's cropland on just 27% of its area. Successful desalinization and the subsequent increases in carbon (C) inputs, induced by agricultural projects and policies intended to support crop production (i.e. reconstruction of low yield farmland, and agricultural subsidies), combined with improved cultivation practices (i.e. fertilization and straw return) since the early 1980s were the main drivers for the SOC stock increase. This study suggests that rehabilitation of NCP soils to reduce salinity and increase crop yields have also served as a pathway for substantial soil C sequestration.

The physical, chemical, and biological processes forming the backbone of important soil functions (e.g., carbon sequestration, nutrient and contaminant storage, and water transport) take place at reactive interfaces of soil particles and pores. The accessibility of these interfaces is determined by the spatial arrangement of the solid mineral and organic soil components, and the resulting pore system. Despite the development and application of novel imaging techniques operating at the micrometer and even nanometer scale, the microstructure of soils is still considered as a random arrangement of mineral and organic components. Using nanoscale secondary ion mass spectroscopy (NanoSIMS) and a novel digital image processing routine adapted from remote sensing (consisting of image preprocessing, endmember extraction, and a supervised classification), we extensively analyzed the spatial distribution of secondary ions that are characteristic of mineral and organic soil components on the submicrometer scale in an intact soil aggregate (40 measurements, each covering an area of 30 μm × 30 μm with a lateral resolution of 100 nm × 100 nm). We were surprised that the 40 spatially independent measurements clustered in just two complementary types of micrometer-sized domains. Each domain is characterized by a microarchitecture built of a definite mineral assemblage with various organic matter forms and a specific pore system, each fulfilling different functions in soil. Our results demonstrate that these microarchitectures form due to self-organization of the manifold mineral and organic soil components to distinct mineral assemblages, which are in turn stabilized by biophysical feedback mechanisms acting through pore characteristics and microbial accessibility. These microdomains are the smallest units in soil that fulfill specific functionalities.

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

Organo-mineral interactions stabilize soil organic matter (SOM) by protecting from microbial enzymatic attack. Soil water content affects aggregation, mineral weathering, and microbial respiration, thus influencing the relative importance of SOM stabilization mechanisms. While the response of microbial respiration to momentary changes in water content is well established, it is unclear how microbial activity will impact stabilization mechanisms under different long-term moisture contents.To understand how long-term soil moisture affects SOM stabilization mechanisms we studied fallow soils from upstate New York situated on a naturally occurring water content gradient. Wetter (but not saturated) soils contained more exchangeable Ca and had more strongly stabilized SOM, resulting in SOM accumulation. But it was not clear whether Ca-driven surface interactions or occlusion in micro-aggregates was more important, and if interactions with Fe and Al played a role in the Ca-poor soils. Also, the role of biotic drivers in SOM stabilization at different water contents was unknown.We tested which mechanisms governed SOM stabilization by determining C and N contents and natural isotope abundances in particulate and mineral-associated organic matter fractions. We also extracted the C bound to Ca and to reactive Fe+Al phases. Wetter, Ca-rich soils had higher oPOM content, and in the heavy mineral fraction, higher relative concentrations of Ca-bound C, lower C:N values, and more oxidized C forms. In addition, wetter soils had greater microbial biomass. Together, these results showed that high long-term soil moisture increased microbial SOM cycling, and that processed SOM was better stabilized, in agreement with the recent notion that stable SOM consists of processed labile C. Additionally, higher soil moisture augmented the role of Ca in SOM stabilization over that of Al+Fe phases. We then manipulated the exchangeable Ca content and incubated soils with 13C15N labeled plant litter. Ca-amended soils emitted less CO2 while incubated with litter, confirming that Ca is instrumental in SOM stabilization. Tracing the labeled isotopes in the gaseous phase and soil fractions will allow us to gain a clearer understanding of how water content and soil Ca interact to stabilize SOM.

Alpine and prealpine grasslands provide various ecosystem services and are hotspots for the storage of soil organic C (SOC) in Central Europe. Yet, information about aggregate-related SOC storage and its controlling factors in alpine and prealpine grassland soils is limited. In this study, the SOC distribution according to the aggregate size classes large macroaggregates (> 2000 μm), small macroaggregates (250–2000 μm), microaggregates (63–250 μm), and silt-/clay-sized particles (< 63 μm) was studied in grassland soils along an elevation gradient in the Northern Limestone Alps of Germany. This was accompanied by an analysis of earthworm abundance and biomass according to different ecological niches. The SOC and N stocks increased with elevation and were associated with relatively high proportions of water-stable macroaggregates due to high contents of exchangeable Ca2+ and Mg2+. At lower elevations, earthworms appeared to act as catalyzers for a higher microaggregate formation. Thus, SOC stabilization by aggregate formation in the studied soils is a result of a joined interaction of organic matter and Ca2+ as binding agents for soil aggregates (higher elevations), and the earthworms that act as promoters of aggregate formation through the secretion of biogenic carbonates (low elevation). Our study highlights the importance of aggregate-related factors as potential indices to evaluate the SOC storage potential in other mountainous grassland soils.Graphical abstract

Physical separation of soil into different soil organic matter (SOM) fractions is widely used to identify organic carbon pools that are differently stabilized and have distinct chemical composition. However, the mechanisms underlying these differences in stability and chemical composition are only partly understood. To provide new insights into the stabilization of different chemical compound classes in physically-separated SOM fractions, we assessed shifts in the biomolecular composition of bulk soils and individual particle size fractions that were incubated in the laboratory for 345 days. After the incubation, also the incubated bulk soil was fractionated. The chemical composition of organic matter in bulk soils and fractions was characterized by 13C-CPMAS nuclear magnetic resonance spectroscopy and sequential chemical extraction followed by GC/MS measurements. Plant-derived lipids and lignin were abundant in particulate organic matter (POM) fractions of sand-, silt-, and clay-size and the mineral-bound, clay-sized organic matter. These results indicate that recent conceptualizations of SOM stabilization probably understate the contribution of plant-derived organic matter to stable SOM pools. Although our data indicate that inherent recalcitrance could be important in soils with limited aggregation, organo-mineral interactions and aggregation were responsible for long-term SOM stabilization. In particular, we observed consistently higher concentrations of plant-derived lipids in POM fractions that were incubated individually, where aggregates were disrupted, as compared to those incubated as bulk soil, where aggregates stayed intact. This finding emphasizes the importance of aggregation for the stabilization of less ‘recalcitrant’ biomolecules in the POM fractions. Because also the abundance of lipids and lignin in clay-sized, mineral-associated SOM was substantially influenced by aggregation, the bioavailability of mineral-associated SOM likely increases after the destruction of intact soil structures.

My 2002 SBB paper, The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter , brought together knowledge on the chemical composition of the diverse inputs to soil organic matter. Both plant and microbial residues were examined with the analysis of their composition using a combination of different techniques. From this, the limitations of conventional proximate analysis methods were identified and the great potential of recent techniques, in particular solid-state 13 C NMR spectroscopy and molecular level analysis, for the overall characterization of the input materials were discussed. The paper emphasised the importance of differentiating between organic matter from plants (above-ground litter, root litter and rhizodeposition), microbial residues and extracellular polymers and their breakdown products as well as the need for quantitative measurements of the amounts of these materials entering soils. In the last 14 years much new knowledge has been generated regarding these inputs and their alteration during decomposition, yet we still lack quantitative data for the amounts, composition and transformations of the many different forms of organic matter entering the soil. This is particularly the case regarding the inputs to the subsoil via root litter and rhizodeposition and the significance of microbial residues and extracellular polymers and their turnover.

Our understanding of the interactions between minerals, organic matter, and microorganisms at so-called biogeochemical interfaces in soil is still hampered by the inherent complexity of these systems. Artificial soil maturation experiments can help to bridge a gap in complexity between simple abiotic sorption experiments and larger-scale field experiments. By controlling other soil-forming factors, the effect of a particular variable can be identified in a simplified system. Here, we review the findings of a series of artificial soil incubation experiments with the aim of revealing general trends and conclusions. The artificial soils were designed to determine the effect of mineral composition and charcoal presence on the development of abiotic and biotic soil properties during maturation. In particular, the development of soil aggregates, organic matter (OM) composition and turnover, sorption properties, and the establishment of microbial community composition and function were considered. The main objectives of the research were to determine (1) how surface properties and sorption of chemicals modify biogeochemical interfaces ; (2) how much time is required to form aggregates from mixtures of pure minerals, OM, and a microbial inoculum ; and (3) how the presence of different mineral and charcoal surfaces affects aggregation, OM turnover, and the development of microbial community composition.

Accumulating evidence indicates that future rates of atmospheric N deposition have the potential to increase soil C storage by reducing the decay of plant litter and soil organic matter (SOM). Although the microbial mechanism underlying this response is not well understood, a decline in decay could alter the amount, as well as biochemical composition of SOM. Here, we used size-density fractionation and solid state 13C-NMR spectroscopy to explore the extent to which declines in microbial decay in a long-term (ca. 20 yrs.) N deposition experiment have altered the biochemical composition of forest floor, bulk mineral soil, as well as free- and occluded particulate organic matter. Significant amount of organic matter have accumulated in occluded particulate organic matter (~20%; oPOM); however, experimental N deposition had not altered the abundance of carboxyl, aryl, alkyl, or O/N-alkyl C in forest floor, bulk mineral soil, or any soil fraction. These observations suggest that biochemically equivalent organic matter has accumulated in oPOM at a greater rate under experimental N deposition, relative to the ambient treatment. Although we do not understand the process by which experimental N deposition has fostered the occlusion of organic matter by mineral soil particles, our result highlight the importance of interactions among the products of microbial decay and the chemical and physical properties of silt and clay particles that occlude organic matter from microbial attack. Because oPOM can reside in soils for decades to centuries, organic matter accumulating under future rates of anthropogenic N deposition could remain in soil for long periods of time. If temperate forest soils in the Northern Hemisphere respond like those in our experiment, then unabated deposition of anthropogenic N from the atmosphere has the potential to foster greater soil C storage, especially in fine-textures forest soils. This article is protected by copyright. All rights reserved.

Savannas are dry tropical biomes characterized by high above- and belowground biomass production, but at the same time there is extensive C loss by fire regimes. Soil-mediated responses are crucial in biogeochemical cycling and the role of soil minerals may be important in controlling organic carbon (OC) storage and stabilization. The objective of this study was to determine OC fractional distribution and saturation capacity in three soil groups (Vertisols, Lixisols, Luvisols) that are characterized by specific and different 1:1 and 2:1 clay mineral dominance. For each soil group, top- and subsoils (0–10 cm, 40–60 cm) were sampled in protected Soudanian savannas across Burkina Faso and fractionated into free and aggregate-occluded particulate organic matter (fPOM, oPOM) and mineral-associated organic matter (mOM). The results showed that oPOM and mOM were significantly affected by soil groups and topsoil/subsoil layers. We consider this to be due to interactions with fine particles (clay, fine and medium silt), the presence of exchangeable Ca and Mg and extractable Al (D) and Fe (D) as revealed by significant positive correlations. We found that oPOM (13–29% of total OC) and mOM

The enrichment culture KS is one of the few existing autotrophic, nitrate-reducing, Fe(II)-oxidizing cultures that can be continuously transferred without an organic carbon source. We used a combination of catalyzed amplification reporter deposition fluorescence in situ hybridization (CARD-FISH) and nanoscale secondary ion mass spectrometry (NanoSIMS) to analyze community dynamics, single-cell activities, and interactions among the two most abundant microbial community members (i.e., Gallionellaceae sp. and Bradyrhizobium spp.) under autotrophic and heterotrophic growth conditions. CARD-FISH cell counts showed the dominance of the Fe(II) oxidizer Gallionellaceae sp. under autotrophic conditions as well as of Bradyrhizobium spp. under heterotrophic conditions. We used NanoSIMS to monitor the fate of 13 C-labeled bicarbonate and acetate as well as 15 N-labeled ammonium at the single-cell level for both taxa. Under autotrophic conditions, only the Gallionellaceae sp. was actively incorporating 13 C-labeled bicarbonate and 15 N-labeled ammonium. Interestingly, both Bradyrhizobium spp. and Gallionellaceae sp. became enriched in [ 13 C]acetate and [ 15 N]ammonium under heterotrophic conditions. Our experiments demonstrated that Gallionellaceae sp. was capable of assimilating [ 13 C]acetate while Bradyrhizobium spp. were not able to fix CO 2 , although a metagenomics survey of culture KS recently revealed that Gallionellaceae sp. lacks genes for acetate uptake and that the Bradyrhizobium sp. carries the genetic potential to fix CO 2 . The study furthermore extends our understanding of the microbial reactions that interlink the nitrogen and Fe cycles in the environment. IMPORTANCE Microbial mechanisms by which Fe(II) is oxidized with nitrate as the terminal electron acceptor are generally referred to as “nitrate-dependent Fe(II) oxidation” (NDFO). NDFO has been demonstrated in laboratory cultures (such as the one studied in this work) and in a variety of marine and freshwater sediments. Recently, the importance of NDFO for the transport of sediment-derived Fe in aquatic ecosystems has been emphasized in a series of studies discussing the impact of NDFO for sedimentary nutrient cycling and redox dynamics in marine and freshwater environments. In this article, we report results from an isotope labeling study performed with the autotrophic, nitrate-reducing, Fe(II)-oxidizing enrichment culture KS, which was first described by Straub et al. (1) about 20 years ago. Our current study builds on the recently published metagenome of culture KS (2).

The carbon (C) balance of agricultural soils may be largely affected by climate change. Increasing temperatures are discussed to cause a loss of soil organic carbon (SOC) due to enhanced decomposition of soil organic matter, which has a high intrinsic temperature sensitivity. On the other hand, several modeling studies assumed that potential SOC losses would be compensated or even outperformed by an increased C input by crop residues into agricultural soils. This assumption was based on a predicted general increase of net primary productivity (NPP) as a result of the CO2 fertilization effect and prolonged growing seasons. However, it is questionable if the crop C input into agricultural soils can be derived from NPP predictions of vegetation models. The C input in European croplands is largely controlled by the agricultural management and was strongly related to the development of crop yields in the last decades. Thus, a glance at past yield development will probably be more instructive for future estimations of the C input than previous modeling approaches based on NPP predictions. An analysis of European yield statistics indicated that yields of wheat, barley and maize are stagnating in Central and Northern Europe since the 1990s. The stagnation of crop yields can probably be related to a fundamental change of the agricultural management and to climate change effects. It is assumed that the soil C input is concurrently stagnating which would necessarily lead to a decrease of agricultural SOC stocks in the long-term given a constant temperature increase. Remarkably, for almost all European countries that are faced with yield stagnation indications for agricultural SOC decreases were already found. Potentially adverse effects of yield stagnation on the C balance of croplands call for an interdisciplinary investigation of its causes and a comprehensive monitoring of SOC stocks in agricultural soils of Europe.

Microbial residues are important contributors to soil organic matter (SOM). However, the effect of mineral composition on microbial activity and thereby SOM development is not well understood. Here, we used artificial soils composed of four different mixtures containing illite, montmorillonite, ferrihydrite and charcoal to study the effect of mineral composition on SOM development. In addition, silt- and sand-sized quartz were used to provide texture, sterilised manure was added as OM source and the mixtures were inoculated with a microbial community extracted from a natural soil. The mixtures were incubated in the dark for 3–18 months and amino sugar and muramic acid content of the bulk soil and

The interaction between minerals and organic matter (OM) is a key to the turnover of OM in soils. In particular, clay minerals, iron oxides and charcoal are considered as important constituents affecting the sequestration of carbon (C) and nitrogen (N). Here, we incubated pre-produced artificial soils (842 days) and a natural soil (Ap, Luvisol) with 13C- and 15N-labelled plant litter over 63 days to follow OM turnover and the formation of organo-mineral associations regarding different compositions (montmorillonite (MT), llite (IL), montmorillonite + charcoal (MT+CH), illite + ferrihydrite (IL+FH)). The microbial biomass, salt extractable organic C, the isotopic C and N composition in the bulk soil and the soil fractions (combined density and particle size fractionation) were determined. By comparison of the artificial soils with the natural soil, we were able to show that the produced soil-like systems have OM dynamics comparable to the natu- ral soil. We found out that the decomposition of the added plant litter was affected by the type of clay mineral that formed the artificial soils, as the soil MT exhibited a slower mineralisation compared to IL, which was in line with a lower microbial biomass. Although a high specific surface area (SSA) provides a high sequestration capacity for C and N, smaller amounts were sequestered in the MT soil with a higher SSA compared to the soil IL. We suppose that a more intensive decomposition is associated with a higher microbial biomass and thus leads to higher amounts of microbial products sequestered in the clay-sized fraction. Charcoal and ferrihydrite had no additional effect in this experiment.

We examined the potential of stable-isotope Raman microspectroscopy (SIRM) for the evaluation of differently enriched 13C-labeled humic acids as model substances for soil organic matter (SOM). The SOM itself can be linked to the soil water holding capacity. Therefore, artificial humic acids (HA) with known isotopic compositions were synthesized and analyzed by means of SIRM. By performing a pregraphitization, a suitable analysis method was developed to cope with the high fluorescence background. Results were verified against isotope ratio mass spectrometry (IRMS). The limit of quantification was 2.1 × 10−1 13C/C tot for the total region and 3.2 × 10−2 13C/C tot for a linear correlation up to 0.25 13C/C tot. Complementary nanoscale secondary ion mass spectrometry (NanoSIMS) analysis indicated small-scale heterogeneity within the dry sample material, even though—owing to sample topography and occurring matrix effects—obtained values deviated in magnitude from those of IRMS and SIRM. Our study shows that SIRM is well-suited for the analysis of stable isotope-labeled HA. This method requires no specific sample preparation and can provide information with a spatial resolution in the micrometer range.

The study of interactions between minerals, organic matter (OM) and microorganisms is essential for the understanding of soil functions such as OM turnover. Here, we present an interdisciplinary approach using artificial soils to study the establishment of the microbial community and the formation of macro-aggregates as a function of the mineral composition by using artificial soils. The defined composition of a model system enables to directly relate the development of microbial communities and soil structure to the presence of specific constituents. Five different artificial soil compositions were produced with two types of clay minerals (illite, montmorillonite), metal oxides (ferrihydrite, boehmite) and charcoal incubated with sterile manure and a microbial community derived from a natural soil. We used the artificial soils to analyse the response of these model soil systems to additional sterile manure supply (after 562 days). The artificial soils were subjected to a prolonged incubation period of more than two years (842 days) in order to take temporally dynamic processes into account. In our model systems with varying mineralogy, we expected a changing microbial community composition and an effect on macro-aggregation after OM addition, as the input of fresh substrate will re-activate the artificial soils. The abundance and structure of 16S rRNA gene and internal transcribed spacer (ITS) fragments amplified from total community DNA were studied by quantitative real-time PCR (qPCR) and denaturing gradient gel electrophoresis (DGGE), respectively. The formation of macro-aggregates (>2 mm), the total organic carbon (OC) and nitrogen (N) contents, the OC and N contents in particle size fractions and the CO2 respiration were determined. The second manure input resulted in higher CO2 respiration rates, 16S rRNA gene and ITS copy numbers, indicating a stronger response of the microbial community in the matured soil-like system. The type of clay minerals was identified as the most important factor determining the composition of the bacterial communities established. The additional OM and longer incubation time led to a re-formation of macro-aggregates which was significantly higher when montmorillonite was present. Thus, the type of clay mineral was decisive for both microbial community composition as well as macro-aggregation, whereas the addition of other components had a minor effect. Even though different bacterial communities were established depending on the artificial soil composition, the amount and quality of the OM did not show significant differences supporting the concept of functional redundancy.

To better understand how carbon and nitrogen mineralization are linked in soils, we conducted a long-term incubation experiment and compared carbon and nitrogen dynamics in the bulk soil and in soil fractions. Topsoil of a Rendzic Leptosol from a beech forest site near Tuttlingen, Germany, was separated into three particle size classes: sand (2000–20 μm), silt (20–2 μm), and clay ( clay > silt. The fractions respired between 10.4% (sand fraction), 8.8% (clay fraction) and 4.4% (silt fraction) of total soil organic carbon. However, nitrogen mineralization was affected by the fractionation procedure (∑ fractions silt > sand. Fractionation increased the surface area and hence provided accessory mineral surfaces, which allowed new binding of especially nitrogen-rich compounds, in addition to ammonium fixation via cation exchange. As indicated by lower metabolic quotients, microbial carbon mineralization was more efficient in the bulk soil compared to the calculated sum of fractions. In the clay fraction, carbon mineralization rates, salt extractable organic carbon contents, and microbial biomass carbon and nitrogen contents declined strongly towards the end of the incubation. This indicates that in the clay fraction, organic carbon was not available for microbial degradation and that microorganisms were strongly carbon-limited causing a subsequent inhibition of nitrogen immobilization. Density fractionation revealed that organic matter in the sand fraction consisted mainly of particulate organic matter present as light material containing partly decomposed plant remnants. The organic matter in the clay fraction was mostly adsorbed on mineral surfaces. Organic matter in the sand and in the clay fraction was dominated by O/N-alkyl C indicating low recalcitrance, but the C/N ratio of organic matter narrowed with decreasing particle size. Our results suggest that carbon and nitrogen mineralization are decoupled in the mineral-associated fractions of the soil. The specific interactions of both carbon and nitrogen containing components with the mineral matrix strongly modulate the mineralization dynamics. Therefore, isolated considerations of C/N or alkyl C to O/N-alkyl C ratios of organic matter are insufficient as indicators for decomposition in plant residues. The combined consideration of C/N and alkyl C to O/N-alkyl C ratios provides a first impression about the degree of decomposition in plant residues. However, bioavailability in fractions where organic matter is mainly stabilized by spatial inaccessibility and by organo-mineral interactions cannot be explained by these ratios, but can be examined by an incubation approach.

We studied organic carbon (OC) accumulation in organo-mineral associations during soil development on calcareous parent material. Two chronosequences in the Zhejiang Province, PR China, were investigated; one under paddy cultivation with a maximum soil age of 2000 years, and the other under upland crops where the oldest soil was 700 years old. Bulk soils and soil fractions of the uppermost A horizons were analyzed for OC concentrations, radiocarbon ( 14 C) contents, total pedogenic iron oxide concentration and oxalate extractable proportions of iron (Fe OX ) oxides. The specific surface area of soil minerals was measured with the Brunauer–Emmett–Teller (BET-N 2 ) method on four conditions: untreated, after organic matter removal, after iron oxide removal and after removal of both. Initial soil formation on calcareous marine sediments includes soil decalcification and OC accumulation. Paddy soils are characterized by an accelerated decalcification, higher contents of OC and Fe OX oxides, and a pronounced accumulation of modern OC. The mineral constitution of the soil material indicated already a certain degree of weathering since the earliest stages of pedogenesis and remained unchanged in paddy and non-paddy soils. The study provides no evidence of formation of new clay-sized minerals during soil development, which could supply new surfaces for OC accumulation. However, the study revealed higher OC coverage on mineral surfaces in decalcified paddy soils. Therefore, we assume the specific surface area and the specific affinity of Fe OX oxides for OC storage to play an important role for OC accumulation in organo-mineral associations. In contrast, the surface area of minerals in non-paddy soils, in which decalcification and the proportion of Fe OX oxides were much lower, showed significantly lower OC coverage. Selective removal of SOM or iron oxides clearly showed that iron oxides and SOM protect each other in organo-mineral associations primarily in paddy fine clay-sized fraction. Thus, we explained the higher OC coverage on mineral surfaces by complex association between clay minerals, iron oxides and SOM in paddy soils.

Aggregation assembles different size mixtures of soil particles into a larger architecture. Such mixtures impede resolving which particles build aggregates and how these control the accumulation of soil organic matter (OM). Here we present an approach to differentiate the size distributions of soil fractions in the size range of microaggregates ( 250 μm were sonicated to isolate occluded size fractions 250 μm) and larger microaggregates in the 50–180 μm size range. The low-clay soils, on the other hand, contained more non-aggregated sand-sized particles >100 μm which probably hampered the buildup of larger aggregates. The size distribution of particles

The formation of aggregates is considered to happen by clustering and cohesion of mineral particles and organic matter. Up to now, little is known about the role of different organic matter types within this process. We developed an experimental set-up to study the influence of organic carbon (OC) derived from particulate organic matter (POM-C) and from dissolved organic matter (DOM-C) for aggregate formation under controlled laboratory conditions. We designed artificial soil microcosms with loamy texture, mimicking an arable Cambisol, and performed an incubation for 30 days under constant water tension. The microcosms received either POM-C as milled hay litter, DOM-C as solution derived from hay or no additional OC input. We characterized the developed soil matrix by analysing aggregate size distribution and composition as well as the associated pore system. After incubation, we observed the formation of mostly large, water-stable macroaggregates (630–3000 μm) in all treatments with OC addition. POM-C input led to an increase in mass contribution of the large macroaggregates from 6% to 88 ± 6%, when compared to the original particle size distribution, while DOM-C input led to an increase to 59 ± 9%. The size fractions of the small microaggregates and primary particles ( 50 μm pore size) and highly connected pores (Euler number change from −1519 to −5767) after incubation. This indicates a reorganization of the pore space towards a fine, connected pore system with improved gas, water and nutrient exchange and thus implications for the microbial microenvironment. DOM-C microcosms also had an increased pore connectivity (Euler number change from −122 to −665) after incubation, but overall values were smaller than in the POM-C microcosms, indicating that changes in pore structure depend on OM type initiating aggregation. We demonstrate that aggregate formation in a loamy textured artificial soil is possible within 30 days and can take place in the absence of physical interference like stirring or repeated wet-dry cycles. Different organic matter types induce specific formation of aggregates and associated pore system as the effect of microbial decomposition of the OC input and the interaction of OC compounds with mineral surfaces.

The stabilization of soil organic matter (SOM) is triggered by three main mechanisms: (i) low bioavailability due to aggregation, (ii) recalcitrance due to the chemical structure, and (iii) association of the SOM with mineral surfaces. In the present study we used particle size SOM fractions (sand, silt and clay), derived from the Ah soil horizon from a Norway spruce forest in Southern Germany, to study the effects of different stabilization mechanisms on the bioavailability of soil organic carbon (SOC) in a one year incubation experiment. The respired CO 2 was hourly recorded, additionally 13 CO 2 was analysed 20 times and 14 CO 2 three times during the incubation experiment. To better differentiate between particulate OM (POM) and mineral associated OM (MIN), the incubated fractions and bulk soil were separated according to density (1.8 g cm −3 ) after the incubation experiment. 13 C-CPMAS NMR spectroscopy was used to study the chemical composition of the incubated samples. We demonstrate a clear increase in SOM bioavailability due to aggregate disruption, as the calculated theoretical CO 2 evolution of the SOM fractions recombined by calculation was 43.8% higher in relation to the intact bulk soil. The incubated sand fraction, dominated by POM rich in O/N-alkyl C, showed a prolonged bioavailability of SOC moieties with mean residence times (MRT) of 78 years. Interestingly, the silt fraction, dominated by highly aliphatic, more recalcitrant POM, showed low mineralization rates and slow MRT's (192 years) close to values for the clay fraction (171 years), which contained a large amount of mineral-associated SOM. The recorded 13/12 CO 2 signatures showed a high depletion in 13 C during the initial stage of the incubation, but an enrichment of the respired 13 CO 2 of up to 3.4‰ relative to the incubated SOM was observed over longer time periods (after 3 and 4 days for bulk soil and sand, respectively, and after 14 days for silt and clay). Therefore, we found no evidence for a 13 C enrichment of SOM as driven by metabolic isotopic fractionation during microbial SOM mineralization, but an indication of a change in the isotopic composition of the C-source over time.

Prolonged summer droughts are projected to occur as a consequence of climate change in Central Europe. The resulting reduced soil water availability may lead to alterations in rates of soil processes such as nitrogen partitioning among soil organic matter fractions and stabilization within soil. To study the effect of climate change-induced drought on (1) the distribution of nitrogen among soil organic matter fractions and (2) nitrogen stabilization, we performed a space-for-time climate change experiment. We transferred intact plant–soil–microbe mesocosms of a Rendzic Leptosol with a young beech tree from a slope with northwestern exposure in southern Germany characterized by a cool-moist microclimate across a narrow valley to a slope with southwestern exposure with a warm-dry microclimate, which reflects projected future climatic conditions. A control transfer was also done on the northwest-facing slope within the same area of origin. We combined a homogenous 15N labeling approach using ammonium nitrate with a physical fractionation procedure and chemical soil extraction protocols. Our aim was to follow the partitioning of 15N in different soil organic matter fractions, i.e. light fractions, organo-mineral fractions, and extractable soil fractions including microbial biomass, ammonium, nitrate, and dissolved organic nitrogen. Within less than one growing season, we observed a modified partitioning of recently applied inorganic 15N between different soil fractions in relation to drier summer conditions, with attenuated nitrogen turnover under drought and consequently significantly higher 15N concentrations in the relatively labile light fractions. We ascribed this effect to a decelerated mineralization immobilization turnover. We conclude that prolonged summer droughts may alter the stabilization dynamics because the induced inactivity of microorganisms may reduce the transfer of nitrogen to stabilization pathways. A retarded stabilization in organo-mineral associations enhances the risk of nitrogen losses during extreme rainfall events, which are projected to increase in the 21st century predicted by future climate change scenarios for Central Europe.

Interactions between organic matter (OM), minerals and charcoal may play an important role in the development and stabilization of OM in soils. However, these interactions are difficult to characterize in natural soils, which are usually very complex systems with unknown initial conditions. We developed so-called ‘artificial soils’ with a texture and OM content similar to natural arable soils that were incubated up to 18 months. The aim was to determine the turnover and development of OM with incubation time, and to establish the effect of mineral composition and charcoal presence on organic carbon (OC) and N distribution and properties. Artificial soils were composed of quartz, manure as OM source and a microbial community extracted from a natural arable soil, with 8 different mixtures of montmorillonite, illite, ferrihydrite, boehmite and charcoal. We determined C and N particle size distribution with time and used solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and acid hydrolysis to determine the development of OM composition. The CO2 respiration rate and distribution of OC and N with particle size was similar for all artificial soil compositions. OC and N accumulated in the 200 μm) particulate OM was lost after 18 months of incubation. 13C NMR spectroscopy indicated accumulation of protein-rich OC in the

Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is considered to have high potential for global CO2 mitigation. However, the potential of soils to sequester soil organic carbon (SOC) in a stable form, which is limited by the stabilization of SOC against microbial mineralization, is largely unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the potential SOC saturation of silt and clay particles according to Hassink [Plant and Soil 191 (1997) 77] on the basis of 516 soil profiles. The determination of the current SOC content of silt and clay fractions for major soil units and land uses allowed an estimation of the C saturation deficit corresponding to the long-term C sequestration potential. The results showed that cropland soils have a low level of C saturation of around 50% and could store considerable amounts of additional SOC. A relatively high C sequestration potential was also determined for grassland soils. In contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites with a high degree of apparent oversaturation revealed that in acidic, coarse-textured soils the relation to silt and clay is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt CO2 -equivalents could theoretically be stored in A horizons of cultivated soils - four times the annual emission of greenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved management of cultivated land could contribute significantly to CO2 mitigation. Moreover, increasing SOC stocks have additional benefits with respect to enhanced soil fertility and agricultural productivity.

Extracellular polymeric substances (EPS) are expected to be an important source for the formation of mineral-organic associations in soil. Because such formations affect the composition of mobile and immobile organic matter as well as the reactivity of minerals, we investigated the composition of EPS before and after adsorption to goethite. Raman measurements on EPS extracted from Bacillus subtilis distinguished four fractions rich in proteins, polysaccharides, lipids, or lipids and proteins. Scanning transmission X-ray microscopy identified three different EPS-fractions that varied in their composition in proteins, nonaromatic proteins, and polysaccharides. Reaction of EPS with goethite led to a preferential adsorption of lipids and proteins. The organic coverage was heterogeneous, consisting of ~100 × 200 nm large patches of either lipid-rich or protein-rich material. Nanoscale secondary ion mass spectrometry showed a strong S enrichment in aggregates of ~400 nm in the goethite adsorbed EPS. From our simplified model system, we learned that only a small portion (

Sorption of organic chemicals in soil is affected by the properties and availability of surfaces. These surfaces are composed of diverse mineral, organic and biological components, forming a soil's ‘biogeochemical interface’. Phenanthrene was used to probe the hydrophobic sorptive capacity of the interface of an arable soil. Batch sorption experiments were carried out with the bulk soil as well as the fine (0.2–6.3 µm) and coarse (6.3–63 µm) particle size fractions of two arable topsoil samples with different organic matter (OM) contents from a Eutric Cambisol. The specific surface area (SSA) of the bulk soil and particle size fractions was determined by BET-N2 and EGME sorption. OM composition was characterized by solid-state 13C NMR spectroscopy. No clear relationship was found between phenanthrene sorption and SSA. We conclude that phenanthrene probes a specific fraction of the soil interface that is not well represented by the traditional methods of SSA detection such as BET-N2 and EGME sorption. The sorption behaviour of phenanthrene may therefore provide a useful additional tool to characterize the specific affinity of the soil biogeochemical interface for hydrophobic molecules. Sorption capacity for phenanthrene increased after particle-size fractionation, indicating that the reduced availability of the interface caused by the aggregated structure is important for the sorptive capacity of a soil. This should be considered when projecting data obtained from extensively treated and fractionated samples to the actual interaction with biogeochemical interfaces as they are present in soil.

To study the influence of the clay minerals montmorillonite (M) and illite (I), the metal oxides ferrihydrite (F) and aluminum hydroxide (A), and charcoal (C) on soil bacterial communities, seven artificial soils with identical texture provided by quartz (Q) were mixed with sterilized manure as organic carbon source before adding a microbial inoculant derived from a Cambisol. Bacterial communities established in artificial soils after 90 days of incubation were compared by DGGE analysis of bacterial and taxon-specific 16S rRNA gene amplicons. The bacterial community structure of charcoal-containing soils highly differed from the other soils at all taxonomic levels studied. Effects of montmorillonite and illite were observed for Bacteria and Betaproteobacteria, but not for Actinobacteria or Alphaproteobacteria. A weak influence of metal oxides on Betaproteobacteria was found. Barcoded pyrosequencing of 16S rRNA gene amplicons done for QM, QI, QIF, and QMC revealed a high bacterial diversity in the artificial soils. The composition of the artificial soils was different from the inoculant, and the structure of the bacterial communities established in QMC soil was most different from the other soils, suggesting that charcoal provided distinct microenvironments and biogeochemical interfaces formed. Several populations with discriminative relative abundance between artificial soils were identified.

Soils represent a very complex material where mineral, organic and biological components interact together to form a large ‘biogeochemical interface’. In order to study this complex interface in a simplified system, a so-called ‘artificial soil’ incubation experiment was carried out. This experiment was used here specifically to study the effect of microbial activity and mineral interactions on the aggregation occurring during incubation. Artificial soils with different mineral composition but the same texture, were composed of well-defined model materials. The materials considered were quartz, illite, montmorillonite, ferrihydrite, boehmite and charcoal, manure as an organic matter (OM) source and a microbial inoculum extracted from a natural arable topsoil. The artificial soils were incubated for 3 to 18 months in the dark, under constant temperature and water content. The pH, organic carbon (OC) and nitrogen content, extractable Fe, Al and Si of the incubated artificial soils were measured, and the amount of macroaggregates > 2 mm was determined by sieving. Density fractionation was performed at densities of 1.8 and 2.4 g cm − 3 to separate particulate OM, organo-mineral associates and the mineral fraction. The specific surface area (SSA) of the organo-mineral associate fraction and the amount of OC associated with organo-mineral associates and minerals was determined. The SSA of the model materials and the artificial soils at the start of incubation was determined by BET-N 2 . The artificial soils developed quickly and CO 2 respiration occurred during the entire 18 month incubation time. The actual SSA of the ‘soils’ was significantly lower than the sum of the SSA of the pure model materials indicating that occlusion of mineral surfaces by interaction between OM and minerals occurred almost immediately after incubation started. Macroaggregates and organo-mineral associates were formed within 3 months of incubation. Macroaggregation decreased after 12 months of incubation probably due to decreasing biological activity. The turnover of macroaggregates and continuing formation of organo-mineral associates was consistent with the aggregate hierarchy model. Less macroaggregation was observed in the soil where no clay mineral was present, indicating that clay minerals were important for the formation of macroaggregates. Ferrihydrite and boehmite did not affect the aggregation or formation of organo-mineral associates in this experiment. The results of density fractionation indicate that OM was mainly associated with the clay minerals, probably due to the neutral pH of the artificial soils, leading to a low or negative surface charge of the oxides. The artificial soil incubation experiment showed that interface development and the building of macroaggregates and organo-mineral associates took place within a relatively short time scale. It offers a valuable model where the formation and interactions of soil properties and processes can be studied in a well-defined system.