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The biogeochemical functioning of soils (e.g., soil carbon stabilization and nutrient cycling) is determined at the interfaces of specific soil structures (e.g., aggregates, particulate organic matter (POM) and organo‐mineral associations). With the growing accessibility of spectromicroscopic techniques, there is an increase in nano‐ to microscale analyses of biogeochemical interfaces at the process scale, reaching from the distribution of elements and isotopes to the localization of microorganisms. A widely used approach to study intact soil structures is the fixation and embedding of intact soil samples in resin and the subsequent analyses of soil cross‐sections using spectromicroscopic techniques. However, it is still challenging to link such microscale approaches to larger scales at which normally bulk soil analyses are conducted. Here we report on the use of laboratory imaging Vis–NIR spectroscopy on resin embedded soil sections and a procedure for supervised image classification to determine the microscale soil structure arrangement, including the quantification of soil organic matter fractions. This approach will help to upscale from microscale spectromicroscopic techniques to the centimetre and possibly pedon scale. Thus, we demonstrate a new approach to integrate microscale soil analyses into pedon‐scale conceptual and experimental approaches. Highlights: Quantification of soil constituents using Vis‐NIR spectroscopy.New approach to use resin embedded soil core sections with intact structure.Reproducible quantification of soil constituents important for soil carbon storage.Vis‐NIR as promising tool for upscaling from microscale to pdeon scale. [ABSTRACT FROM AUTHOR]

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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 amounts 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 results 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-texture forest soils. [ABSTRACT FROM AUTHOR]

Quantity and quality of soil organic matter (SOM) affect physical, chemical, and biological soil properties, and are pivotal to productive and healthy grasslands. Thus, we analyzed the distribution of soil aggregates and assessed quality, quantity, and distribution of SOM in two unimproved and improved (two organic and two conventional) grasslands in subarctic Iceland, in Haplic and Histic Andosols. We also evaluated principal physicochemical and biological soil properties, which influence soil aggregation and SOM dynamics. Macroaggregates (>250 µm) in topsoils were most prominent in unimproved (62–77%) and organically (58–69%) managed sites, whereas 20–250 µm aggregates were the most prominent in conventionally managed sites (51–53%). Macroaggregate stability in topsoils, measured as mean weight diameter, was approximately twice as high in organically managed (12–20 mm) compared with the conventionally managed (5–8 mm) sites, possibly due to higher organic inputs (e.g., manure, compost, and cattle urine). In unimproved grasslands and one organic site, macroaggregates contributed between 40% and 70% of soil organic carbon (SOC) and nitrogen to bulk soil, whereas in high SOM concentration sites free particulate organic matter contributed up to 70% of the SOC and nitrogen to bulk soil. Aggregate hierarchy in Haplic Andosols was confirmed by different stabilizing mechanisms of micro- and macroaggregates, however, somewhat diminished by oxides (pyrophosphate-, oxalate-, and dithionite-extractable Fe, Al, and Mn) acting as binding agents for macroaggregates. In Histic Andosols, no aggregate hierarchy was observed. The higher macroaggregate stability in organic farming practice compared with conventional farming is of interest due to the importance of macroaggregates in protecting SOM and soils from erosion, which is a prerequisite for soil functions in grasslands that are envisaged for food production in the future. [ABSTRACT FROM PUBLISHER]

Recent studies with Andisols show that the carbon (C) stabilization capacity evolves with soil age relative to the evolution of the mineral phase. However, it is not clear how soil mineralogical changes during pedogenesis are related to the composition of soil organic matter (SOM) and C activity as an indicator for the mean residence time of soil organic matter (SOM). In the present study, we analyzed the contribution of allophane and metal-SOM complexes to soil C stabilization. Soil organic matter was analyzed with solid-state C nuclear magnetic resonance spectroscopy. Additionally, the soil was extracted with Na-pyrophosphate (Al, Fe) and oxalate (Al, Si, and Fe). Results supported the hypothesis that allophane plays a key role for SOM stabilization in deep and oldest soil, while SOM stabilization by metal (Al and Fe) complexation is more important in the surface horizons and in younger soils. The metal/C ratio (C extracted in Na-pyrophosphate), soil pH, and radiocarbon age seemed to be important indicators for formation of SOM-metal complexes or allophane in top- and subsoils of Andisols. Changes in main mineral stabilization agents with soil age do not influence SOM composition. We suggest that the combination of several chemical parameters (Al, Fe and C, metal/C ratio, and pH) which change through soil age controls SOM stabilization. [ABSTRACT FROM AUTHOR]

Soil microorganisms play a pivotal role in soil organic matter (SOM) turn-over and their diversity is discussed as a key to the function of soil ecosystems. However, the extent to which SOM dynamics may be linked to changes in soil microbial diversity remains largely unknown. We characterized SOM degradation along a microbial diversity gradient in a two month incubation experiment under controlled laboratory conditions. A microbial diversity gradient was created by diluting soil suspension of a silty grassland soil. Microcosms containing the same sterilized soil were re-inoculated with one of the created microbial diversities, and were amended with C labeled wheat in order to assess whether SOM decomposition is linked to soil microbial diversity or not. Structural composition of wheat was assessed by solid-state C nuclear magnetic resonance, sugar and lignin content was quantified and labeled wheat contribution was determined by C compound specific analyses. Results showed decreased wheat O-alkyl-C with increasing microbial diversity. Total non-cellulosic sugar-C derived from wheat was not significantly influenced by microbial diversity. Carbon from wheat sugars (arabinose-C and xylose-C), however, was highest when microbial diversity was low, indicating reduced wheat sugar decomposition at low microbial diversity. Xylose-C was significantly correlated with the Shannon diversity index of the bacterial community. Soil lignin-C decreased irrespective of microbial diversity. At low microbial diversity the oxidation state of vanillyl-lignin units was significantly reduced. We conclude that microbial diversity alters bulk chemical structure, the decomposition of plant litter sugars and influences the microbial oxidation of total vanillyl-lignins, thus changing SOM composition. [ABSTRACT FROM AUTHOR]

Grazed steppe ecosystems are discussed as one of the big global carbon sinks that may have the potential to sequester large amounts of atmospheric CO and mitigate the effects of global change if grazing is abandoned or management improved. But until today, little is known about sequestration potentials and stabilisation mechanisms in complete soil profiles of semiarid grasslands and how these systems react to grazing cessation. We applied a combined aggregate size, density and particle size fractionation procedure to sandy steppe soils under different grazing intensities (continuously grazed = Cg, winter grazing = Wg, ungrazed since 1999 = Ug99, ungrazed since 1979 = Ug79). Higher inputs of organic matter in ungrazed treatments led to higher amounts of OC in coarse aggregate size classes (ASC) and especially in particulate organic matter (POM) fractions across all depth. These processes started in the topsoil and took more than 5 years to reach deeper soil horizons (>10 cm). After 25 years of grazing cessation, subsoils showed clearly higher POM amounts. We found no grazing-induced changes of soil organic matter (SOM) quantity in fine ASC and particle size fractions. Current C-loading of fine particle size fractions was similar between differently grazed plots and decreased with depth, pointing towards free sequestration capacities in deeper horizons. Despite these free capacities, we found no increase in current C-loading on fine mineral soil fractions after 25 years of grazing exclusion. Silt and clay fractions appeared to be saturated. We suppose empirical estimations to overestimate sequestration potentials of particle size fractions or climatic conditions to delay the decomposition and incorporation of OM into these particle size fractions. POM quality was analysed using solid-state C NMR spectroscopy to clarify if grazing cessation changed chemical composition of POM in different ASC and soil depths via changing litter quality or changing decomposition dynamics. We found comparable POM compositions between different grazing intensities. POM is decomposed hierarchically from coarse to fine particles in all soil depths and grazing cessation has not affected the OM decomposition processes. The surplus of OM due to grazing cessation was predominately sequestered in readily decomposable POM fractions across all affected horizons. We question the long-term stabilisation of OM in these steppe soils during the first 25 years after grazing cessation and request more studies in the field of long-term OM stabilisation processes and assessment of carbon sequestration capacities to consider deeper soil horizons. [ABSTRACT FROM AUTHOR]

Land-use change affects not only the amount of soil organic matter (SOM) but also its composition. We performed cross-polarization magic angle spinning (CPMAS) 13C and 15N nuclear magnetic resonance (NMR) spectroscopy to investigate the chemical composition of bulk SOM in topsoils (0–15 cm) under different land use, namely native forest (NF), 27-year cropland with wheat/soybean rotation (CL) and 27-year rangelands with guineagrass (RG) and with bahiagrass (RB), in south-east Bolivia. The findings of this study showed only a subtle alteration of composition of bulk SOM despite the large changes in carbon (C) content. Nevertheless, NF and RB showed a slightly lower abundance of aromatic C but a higher proportion of alkyl C compared to CL and RG where the loss of organic matter was substantial. This suggests that relatively stable components dominated by aromatic structures had relatively enriched during SOM decomposition under agricultural practices. A slight disparity of SOM composition observed between RG and RB (less O-alkyl C but more aromatic C in RG than RB) suggests that grass species influenced SOM quality even under the same land use, namely the rangeland. On the other hand, organic N composition was less affected by land use or management practice than C forms. [ABSTRACT FROM AUTHOR]

• Importance of SOM and CaCO 3 for aggregate formation depends on aggregate size class. • CaCO 3 controls aggregation of microaggregates in highly calcareous soils. • In calcareous soils SOM dynamics are predominantly associated with macroaggregate formation. • CaCO 3 does not improve the stabilisation of SOM. • CaCO 3 affects water (in)stability of aggregates. In soil, carbonates are important inorganic binding agents, controlling aggregate formation and soil structural stability. Until today, a clear understanding of the aggregate forming mechanisms in calcareous soil is missing. The objective of this study was to elucidate the initial aggregate formation in calcareous loess from the point when soil organic matter content was low and soil properties were mostly controlled by the parent material. We used a space-for-time chronosequence approach on agriculturally reclaimed loess soils in an open-cast lignite mining area in Garzweiler, Germany. We selected six time points from 0 to 24 years after reclamation in order to investigate the early processes of soil aggregate formation. Samples from two sampling depths (1–5 cm and 16–20 cm) were wet sieved into four size classes: silt- and clay-sized fraction (<63 µm), large microaggregates (63–200 µm), small macroaggregates (200–630 µm), and large macroaggregates (>630 µm). Each aggregate size class was characterized for soil organic carbon, total nitrogen and CaCO 3 content. Organic matter amendment induced the formation of large macroaggregates (>630 µm), but we did not detect an accumulation of soil organic carbon neither in microaggregates nor in the silt- and clay-sized fraction. The silt- and clay-sized fraction contained large amounts of CaCO 3 throughout the whole space-for-time chronosequence. This finding shows that in undeveloped loess material, microaggregation is controlled by cementation by carbonates inherited from the loess. These carbonates were likely easily dissolved during contact with water, therefore, aggregates in the early stage of soil development were not water-stable. Soil aggregate formation on young calcareous soils consists of two main mechanisms, 1. fresh soil organic matter predominantly contributes to macroaggregate formation, and 2. intrinsic cementing of loess through carbonates affects microaggregate stability. Therefore, the early stages of soil development in loess material do not show an aggregate hierarchy as we would expect in soils with high silt and clay content, such as Cambisols and Luvisols. [ABSTRACT FROM AUTHOR]

The distribution of soil organic carbon (OC) in the landscape shows high variability between landform positions. Soils in depositional sites are characterized by a highly diverse vertical structure and can contain significant amounts of OC over the whole soil profile. However, anthropogenic modifications of the landscape such as land use and artificial drainage can significantly alter the OC balance in a catchment. Here, we analyse the spatial distribution of OC stocks along an agriculturally used catena in a catchment, tributary to the Danube River in the Bavarian Forest in Southern Germany. The combination of geomorphic aspects with highly detailed soil data based on 16 soil profiles along a catena allowed us to closely examine the relationship between soil OC and the factors of landform position, soil inherent properties and grassland vs. cropland land use. We determined bulk density, total carbon (TC) and inorganic carbon (IC) for each soil horizon in order to quantify OC stocks and picture the vertical OC distribution in the soils. Pedogenic oxides were analysed as an indicator for the redox conditions as well as the soil development. Furthermore, we analysed the clay fraction as well as poorly crystalline iron oxides (Fe OX ) as potential binding agents for OC. The persistence of OC in the landform positions could be designated by the proportion of 14 C content of the bulk soil OC. Our data revealed that the amount of OC stored in the topsoil is mostly determined by land use, but subsoil OC stocks highly depend on landform position. Most OC of this catena is stored in the floodplain featuring particularly high amounts of OC in their subsoils. Radiocarbon signatures of the floodplain OC indicated a young 14 C age compared to the footslope subsoils. We assume that these high OC stocks were due to 1.) formerly high input of OC-rich sediments, 2.) preservation of this OC from decomposition due to high water saturation and 3.) current land use (grassland). Dropping of the ground water table due to artificial drainage measures left detectable footprints within a few decades in the depth distribution of pedogenic iron oxides and indicated currently dominant oxic conditions within the uppermost 100 cm of the floodplain soils. We suspect that under these conditions the OC stocks are now vulnerable to OC losses. Even though we found no correlation between Fe OX and OC, it can be assumed that the interplay between Fe OX and OC will play a key role for the stabilization of the floodplain OC in the future. Today most floodplains in Central Europe are not in their natural state, but drained and under agricultural land use. Therefore this work points to the necessity of monitoring the OC pools in these landform positions and to consider quick responses when it comes to changes in the hydrologic regime and/or land-use system. [ABSTRACT FROM AUTHOR]

Wildfires change the chemical composition of soil organic matter (SOM). Since the effects of fires on organic matter (OM) in soil solution are largely unknown, we sought to compare the quality of dissolved organic matter (DOM) and total organic matter (TOM = DOM + particulate OM) between burned and control sites. The sites were subjected to a low-intensity surface fire in a coniferous forest in Germany dominated by spodic Cambisols derived from Triassic sandstone. Soil solutions from three different soil horizons (O, Ah, Bw), and throughfall (TF) were analyzed using solid-state 13 C NMR spectroscopy, allowing us to track the initial fire impact on OM vertically through the soil profile and 70 days after the fire. In addition, organic layer samples were analyzed by 13 C NMR spectroscopy to compare the OM quality. Under control conditions, properties of SOM influence the chemical composition of DOM and TOM in soil solutions. However, with fire, there is an initial increase in aromatic C in SOM, but not in DOM and TOM. Seventy days after the fire treatment, the aromatic C fraction in soil solutions of O and Ah layers increased, possibly due to accelerated oxidation processes, which would make the aromatic C more water-soluble. Our findings highlight the importance of short-term low-intensity fire-induced changes on forest soils that are useful to those seeking to better understand and model the temporal variability in the response of soil chemistry to fire to improve our knowledge of TOM and DOM dynamics in soils. [ABSTRACT FROM AUTHOR]

Organic carbon (OC) stored in permafrost affected soils of the higher northern latitudes is known to be highly vulnerable to ongoing climatic change. Although the ways to quantify soil OC and to study connected C dynamics from ecosystem to global scale in the Arctic has improved substantially over the last years, the basic mechanisms of OC sequestration are still not well understood. Here we demonstrate a first approach to directly study micro scale soil structures mainly responsible for soil OC (SOC) stabilization using nano scale secondary ion mass spectrometry (NanoSIMS). A cross section from a permafrost layer of a Cryosol from Northern Alaska was analysed using a cascade of imaging techniques from reflectance light microscopy (RLM) to scanning electron microscopy (SEM) to NanoSIMS. This allowed for the direct evaluation of micro scale soil structures known to be hot spots for microbial activity and SOC stabilization in temperate soils. The imaging techniques were supported by classical soil analyses. Using this unique set of techniques we are able to evidence the formation of micro-aggregate structures in the vicinity of plant residues in permafrost soils. This clearly indicates biogeochemical interfaces at plant surfaces as important spheres for the formation of more complex soil structures in permafrost soils. Organo-mineral associations from these hot spots of microbial activity were recovered from plant residues (free particulate organic matter, fPOM) as fine grained mineral fraction with a typically low C/N ratio. This nicely illustrates the link between classical bulk analysis and state of the art spectromicroscopic techniques. [ABSTRACT FROM AUTHOR]

Abstract: Interactions between organic and mineral constituents prolong the residence time of organic matter in soils. However, the structural organization and mechanisms of organic coverage on mineral surfaces as well as their development with time are still unclear. We used clay fractions from a soil chronosequence (15, 75 and 120years) in the foreland of the retreating Damma glacier (Switzerland) and from mature soils outside the proglacial area (>700 and <3000years) to elucidate the evolution of organo–mineral associations during initial soil formation. The chemical composition of the clay-bound organic matter (OM) was assessed by solid-state 13C NMR spectroscopy while the quantities of amino acids and neutral sugar monomers were determined after acid hydrolysis. The mineral phase was characterized by X-ray diffraction, oxalate extraction, specific surface area by N2 adsorption (BET approach), and cation exchange capacity at pH 7 (CECpH7). The last two methods were applied before and after H2O2 treatment. We found pronounced shifts in quantity and quality of OM during aging of the clay fractions, especially within the first one hundred years of soil formation. The strongly increasing organic carbon (OC) loading of clay-sized particles resulted in decreasing specific surface areas (SSA) of the mineral phases and increasing CECpH7. Thus, OC accumulation was faster than the supply of mineral surfaces and cation exchange capacity was mainly determined by the OC content. Clay-bound OC of the 15-year-old soils showed high proportions of carboxyl C and aromatic C. This may point to remnants of ancient OC which were inherited from the recently exposed glacial till. With increasing age (75 and 120years), the relative proportions of carboxyl and aromatic C decreased. This was associated with increasing O-alkyl C proportions, whereas accumulation of alkyl C was mainly detected in clay fractions from the mature soils. These findings from solid-state 13C NMR spectroscopy are in line with the increasing amounts of microbial-derived carbohydrates with soil age. The large accumulation of proteins, which was comparable to those of carbohydrates, and the very low C/N ratios of H2O2-resistant OM indicated strong and preferential associations between proteinaceous compounds and mineral surfaces. In the acid soils, poorly crystalline Fe oxides were the main providers of mineral surface area and important for the stabilization of OM during aging of the clay fractions. This was indicated by (I) the strong correlations between oxalate soluble Fe and both, SSA of H2O2-treated clay fractions and OC content, and (II) the low formation of expandable clays due to small extents of mineral weathering. Our chronosequence approach provided new insights into the evolution of organo–mineral interactions in acid soils. The formation of organo–mineral associations started with the sorption of proteinaceous compounds and microbial-derived carbohydrates on mineral surfaces which were mainly provided by ferrihydrite. The sequential accumulation of different organic compounds and the large OC loadings point to multiple accretion of OM in distinct zones or layers during the initial evolution of clay fractions. [Copyright &y& Elsevier]

Visible-near infrared (vis-NIR) spectroscopy is an acknowledged technique to observesimultaneously several soil parameters, such as soil organic matter and nutrient content,moisture, texture and mineralogy. Imaging spectroscopy provides a possibility to collectspectral information from intact soil samples with a high spatial resolution of 50×50μm2/pixel. In our study we identified physico-chemical soil properties using a hyperspectralvis-NIR camera (spectral resolution 196 bands between 400-1000 nm and spatial resolutionof 50×50 μm2/pixel) and combined them with information on intact soil structure obtainedusing X-ray CT (spatial resolution of 19×19×19 μm3/voxel). We used undisturbed soilcylinders (diameter and height 3 cm) from agriculturally reclaimed soils in the open-castmining area of Garzweiler near Cologne, Germany. Soil samples were scannedwith an X-ray CT, subsequently slices from the cylinder were embedded in resin(polyester) and scanned with the hyperspectral camera. For the first time imageregistration of 2D vis-NIR and 3D X-ray CT images were performed in elastix.This allowed us to correlate organic and mineral soil materials with structural dataclassified by image processing. We identified reclamation management and plant rootinfluence on soil organic matter accumulation and soil structural development. [ABSTRACT FROM AUTHOR]

• CaCO 3 as cementing agent dominates soil aggregation in loess used for reclamation. • SOC is dominantly stored in large aggregates in reclaimed soils. • N is a limiting factor for development of the microbial community. • Crop residues are not sufficient to build up SOM content. After reclamation of open-cast mining pits, soil formation starts from the deposited calcareous loess characterised by its basic physical and chemical properties whereas soil biology and structure need to develop to achieve a fully functional soil. In this study we used a chronosequence approach to elucidate soil formation on agriculturally reclaimed loess soils in an open-cast lignite mining area in Garzweiler (Germany). We selected six fields aged 0, 1, 3, 6, 12, and 24 years after the first seeding in order to observe the initial stage of development of soil properties and assess the role of management with conventional crop rotation in soil structure formation and soil organic carbon (SOC) accumulation. Loess parent material had a strong impact on aggregation, as CaCO 3 acted as a strong cementing agent. Alfalfa cultivation in the pioneering phase was of high importance in the development of microbial biomass, as it protects microbes from N limitation. Soil macroporosity and pore connectivity increased only after compost application and ploughing during agricultural crop rotation. Soil organic matter (SOM) build-up was strongly dependent on the addition of compost, as crop residues from conventional crop rotation are not sufficient to maintain high SOC contents. [ABSTRACT FROM AUTHOR]