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Abstract: The physical and chemical heterogeneities of soils are the source of a multitude of spatial domains supporting a vast functional diversity of soil properties. But many studies do not consider the spatial variability of soil types, diagnostic horizons and properties. This lateral and vertical heterogeneities of soils or soil horizons are mostly neglected due to the limitations in the available soil data and missing techniques to gather the information. We present a fast imaging technique that enables the spatially accurate classification of diagnostic horizons and the mapping of various elemental concentrations (e.g. carbon and iron) for large, undisturbed soil samples (100×300mm) with a high spatial resolution of 63×63μm per pixel. We sampled a stagnic Luvisol (siltic, greyic) under a Norway spruce monoculture in southern Germany (Freising, Bavaria) using a stainless steel box (100×100×300mm3). A laboratory-based hyperspectral camera with a spectral range of 400 to 990nm in 160 bands and a ground sampling distance of 63×63μm per pixel was used. After recording the images, we took 66 samples from characteristic Luvisol horizons and representative spots and determined the concentrations of carbon, nitrogen, aluminium, iron and manganese with standard laboratory methods. These concentrations were correlated to the measured spectra of the sampled spots and used for regression analyses to extrapolate the elemental concentrations to the complete image. The image data of the soil profile was analysed in three different ways with increasing complexity: 1) geostatistics were used to assess the spatial variability of the soil profile as a whole without previous knowledge; 2) supervised spectral angle mapper classification was used to classify diagnostic horizons following the WRB guidelines; and 3) three different chemometric regression algorithms (narrow band indices, partial least square regression and support vector machine regression each used on reflectance spectra and continuum removed spectra) were tested to extrapolate the elemental concentrations of the sampling areas to the complete image and calculate high resolution chemometric maps of the five elements. Laboratory imaging spectroscopy enables the fast evaluation of a soil''s spatial variability without previous knowledge, the mapping of diagnostic horizons, different qualities of OM, soil mottling and various elemental concentrations for large undisturbed soil samples with a high spatial resolution. It has the potential to significantly improve soil classification, assessment of elemental budgets and balances and the understanding of soil forming processes and mechanisms. [Copyright &y& Elsevier]

Abstract: Overgrazing has led to severe degradation and desertification of semi-arid grasslands in Northern China over the last decades. Despite the fact that vegetation is often heterogeneously distributed in semi-arid steppes, little attention has been drawn to the effect of grazing on the spatial distribution of soil properties. We determined the spatial pattern of soil organic carbon (SOC), total nitrogen (Ntot), total sulphur (Stot), bulk density (BD), pH, Ah thickness, and carbon isotope ratios (δ13C) at two continuously grazed (CG) and two ungrazed (UG79=fenced and excluded from grazing in 1979) sites in Leymus chinensis and Stipa grandis dominated steppe ecosystems in Inner Mongolia, Northern China. Topsoils (0–4cm) were sampled at each site using a large grid (120m×150m) with 100 sampling points and a small plot (2m×2m) with 40 points. Geostatistics were applied to elucidate the spatial distribution both at field (120m×150m grid) and plant (2m×2m plot) scale. Concentrations and stocks of SOC, Ntot, Stot were significantly lower and BD significantly higher at both CG sites. At the field scale, semivariograms of these parameters showed a heterogeneous distribution at UG79 sites and a more homogeneous distribution at CG sites, whereas nugget to sill ratios indicated a high small-scale variability. At the plant scale, semivariances of all investigated parameters were one order of magnitude higher at UG79 sites than at CG sites. The heterogeneous pattern of topsoil properties at UG79 sites can be attributed to a mosaic of vegetation patches separated by bare soil. Ranges of autocorrelation were almost congruent with spatial expansions of grass tussocks and shrubs at both steppe types. At CG sites, consumption of biomass by sheep and hoof action removed vegetation patches and led to a homogenization of chemical and physical soil properties. We propose that the spatial distribution of topsoil properties at the plant scale (<2m) could be used as an indicator for degradation in semi-arid grasslands. Our results further show that the maintenance of heterogeneous vegetation and associated topsoil structures is essential for the accumulation of SOM in semi-arid grassland ecosystems. [Copyright &y& Elsevier]

Abstract: It is not clear from the literature whether heavy grazing leads to a deterioration of physical and chemical parameters of topsoils in steppe ecosystems. We sampled five sites in northern China with different grazing intensities, ranging from ungrazed since 1979 to heavily grazed, at 540 sampling points to a depth of 0–4 cm. Each sample was analysed for bulk density, organic carbon (OC), total nitrogen (N), total sulphur (S) and pH. The dataset was analysed using general statistics and explorative analysis (ANOVA, Kruskal–Wallis). As a result of the large number of samples, we were able to detect a change in the mean value of all parameters of less than 10%, with a statistical power of 90% and a level of significance of 0.01. Bulk density increased significantly with increasing grazing intensity. Organic carbon, total N and total S concentrations decreased significantly with increasing grazing intensity. No effect on the pH or C/N ratio was detected. Significant differences in C/S and N/S ratios between differently grazed plots were found. These differences point towards a relative accumulation of sulphur in grazed compared to ungrazed areas following an increased organic matter decline or lower inputs of diluting litter. Elemental stocks of the upper 4 cm were calculated for OC, total N and total S using the measured bulk densities. The data revealed significantly lower amounts for all three elements on the heavily grazed site, but no significant differences for the other areas. In addition, elemental stocks were calculated using an equivalent mass instead of bulk density to take into account changes in bulk density following grazing. This revealed a highly significant decrease for OC, total N and total S with increasing grazing intensity. OC, total N and total S concentrations respond similarly to different grazing intensities, showing highly significant positive correlations. OC concentrations and bulk densities were significantly negatively correlated. We found effects of grazing cessation only in the long-term, as no ameliorating effects of reduced or excluded grazing could be detected five years after grazing cessation. After 25 years of exclusion, significantly different values were found for all parameters. Thus, physical and chemical parameters of steppe topsoils deteriorated significantly following heavy grazing, remained stable if grazing was reduced or excluded for five years, and recovered significantly after 25 years of grazing exclusion. [Copyright &y& Elsevier]

• 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]

• 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]

Soil organic matter (SOM) plays a vital role for soil quality, sustainable food production and climate change mitigation. It is common knowledge that SOM consists of different pools with varying qualities, quantities, and turnover times. However, it is still poorly understood how mineral and organic fertilization affects the formation and stabilization of mineral-associated organic matter (MAOM) and how long it can remain there. Here, we report on the long-term effects of different farming systems on the stability and turnover of the fine silt and clay-sized MAOM fraction (<6.3 μm) of a Haplic Luvisol (0–20 cm) in the DOK long-term trial (Switzerland). We compared three farming systems with contrasting fertilization (CONMIN = pure mineral, CONFYM = mineral + organic, BIODYN = pure organic) with an unfertilized control (NOFERT) between 1982 and 2017. We performed specific surface area (SSA) measurements on fractionated MAOM samples (<6.3 μm) from 1982 to 2017, before and after removal of OM, measured the 14C activity of all samples during the entire period and estimated the mean residence time (MRT) with a model taking into account 'bomb 14C' and radioactive decay. We found constant MAOM-C contents under organic fertilization. Results of SSA analysis indicate best conditions for MAOM-C stabilization under organic fertilization and different sorption mechanisms in MAOM between farming systems with and without organic fertilization. The modelled MRTs were significantly higher in NOFERT (238 ± 40 yrs) and CONMIN (195 ± 27 yrs), compared to CONFYM (138 ± 18 yrs) and BIODYN (140 ± 19 yrs), implying a high C turnover (i.e. more active MAOM) at high C contents under organic fertilization. Our findings show that MAOM is not the dead OM but corroborates the concept of 'dynamic stability'. Continuous OM inputs from organic fertilizers and their rapid and constant turnover are needed to stabilize the "stable" MAOM-C fraction. • SSA analysis reveals improved aggregate stability under organic fertilization. • Significantly shorter MRT of MAOM-C under organic fertilization. • Highest MAOM-C turnover (i.e. more active MAOM) under organic fertilization. • Constant OM input and turnover is vital for maintenance of MAOM-C contents. [ABSTRACT FROM AUTHOR]

• Bulk SOC increased under long-term organic fertilization. • No SOC accumulated in the MAOM fraction (<6.3 µm) over a 36-year period. • Bulk SOC increase under organic fertilization was solely attributed to the labile POM fractions. • oPOM-C contents are highly dynamic and depend on farming system. Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO 2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned. [ABSTRACT FROM AUTHOR]

NIR (near infrared) hyperspectral reflectance imaging has been carried out with the aim of identifying biochar particles and mapping biochar concentrations in a soil monolith (12 cm × 12 cm × 30 cm), which was extracted from the top 30 cm of an agricultural field amended with biochar. Hyperspectral images were obtained directly on one vertical side of the soil monolith and also on ground soil samples extracted from the same side after scanning. Median spectra from the ground samples were used to calibrate a PLS regression model of the C content in the samples and this model was applied to predict the distribution of the biochar in the soil profile. Spectral angle mapper (SAM) was also applied as a comparative technique to predict the biochar distribution, using reference spectra from pure unground biochar. The analysis of the hyperspectral measurements was validated by conducting chemical analysis of the total C and 13 C signature on the ground samples extracted from the scanned side of the monolith. The results indicate that biochar can be qualitatively identified in soil profiles using hyperspectral NIR images. Quantifying the amount of biochar, however, is challenging on the pixel level due to the rough surface of the soil profile, non-homogenous distribution of the biochar and varying moisture conditions. [ABSTRACT FROM AUTHOR]

Temporal dynamics create unique and often ephemeral conditions that can influence soil microbial biogeography at different spatial scales. This study investigated the relation between decimeter to meter spatial variability of soil microbial community structure, plant diversity, and soil properties at six dates from April through November. We also explored the robustness of these interactions over time. An historically unfertilized, unplowed grassland in southwest Germany was selected to characterize how seasonal variability in the composition of plant communities and substrate quality changed the biogeography of soil microorganisms at the plot scale (10 m × 10 m). Microbial community spatial structure was positively correlated with the local environment, i.e. physical and chemical soil properties, in spring and autumn, while the density and diversity of plants had an additional effect in the summer period. Spatial relationships among plant and microbial communities were detected only in the early summer and autumn periods when aboveground biomass increase was most rapid and its influence on soil microbial communities was greatest due to increased demand by plants for nutrients. Individual properties exhibited varying degrees of spatial structure over the season. Differential responses of Gram positive and Gram negative bacterial communities to seasonal shifts in soil nutrients were detected. We concluded that spatial distribution patterns of soil microorganisms change over a season and that chemical soil properties are more important controlling factors than plant density and diversity. Finer spatial resolution, such as the mm to cm scale, as well as taxonomic resolution of microbial groups, could help determine the importance of plant species density, composition, and growth stage in shaping microbial community composition and spatial patterns. [ABSTRACT FROM AUTHOR]

Abstract: Despite their importance for C sequestration, especially in the subsoil, little is known about decomposition and stabilisation processes affecting root litter in soil horizons at different depths. In particular the influence of specific conditions at depth on molecular alterations of degrading root litter is unknown. We took advantage of a decomposition experiment, which was carried out at different soil depths under field conditions and sampled litterbags with 13C-labelled wheat roots, incubated in subsoil horizons at 30, 60 and 90 cm depth for up to 36 months. Changes of bulk root chemistry were studied by solid-state 13C NMR spectroscopy, and lignin content and composition was assessed after CuO oxidation. Compound-specific isotope analysis allowed assessment of the role of root lignin for soil C storage at the different soil depths. Results indicated that decomposition proceeded in a similar way at all three depths, but at a different rate. The alkyl/O-alkyl C ratio was a meaningful indicator to assess the degree of root litter degradation within the mineral soil. After three years, the greatest increase of this ratio, corresponding to the most advanced degradation degree, occurred at 30 cm compared to the lower depths despite a similar carbon loss. The greater proportion of O-alkyl C persisting in deeper subsoil horizons was consistent with their higher clay content. Root derived lignin-C concentration decreased at all soil depths and soil lignin content reached a similar level after 12 months, suggesting that microbial communities in all subsoil depths had capability to degrade lignin. However, the intensity of degradation appeared to be different at different soil depths, with lignin being less transformed at 60 and 90 cm depth. We conclude that chemistry of subsoil organic matter is determined by horizon-specific conditions, which have to be fully understood in order to explain the long residence times of subsoil C. In our study physico-chemical parameters only partly explained the observations. [Copyright &y& Elsevier]

Analysis of the spatial variability of soil properties is important to interpret the site-specific ecosystems not only with respect to process investigations but also to model upscaling. This paper aims to study the effects of the grazing intensity on soil physical and mechanical properties and their interactions in a Leymus chinensis steppe of the Xilin River Basin, Inner Mongolia, China. The investigated sites were subjected to five grazing intensities (ungrazed since 1979, ungrazed since 1999, winter grazing, continuous grazing and heavy grazing). Soil water content (SWC), hydraulic conductivity (K), water drop penetration time (WDPT), shear strength (SS), soil organic carbon (SOC) concentration, bulk density (BD), and soil texture were measured at a grid with 15m sampling distance on the surface soil during the period of 2004–2005. The data were analyzed using descriptive statistics and geostatistics. The correlation and interaction between soil properties were analyzed by the methods of Pearson correlation, partial correlation and multiple regression analysis. The results showed that spatial distributions of soil properties could be well described by spherical or exponential models. The ranges of spatial dependence were the highest for WDPT and the lowest for SS. Grazing decreased SWC, SOC and WDPT but increased BD and SS. Multiple regression analysis showed significant correlations among SWC, K, WDPT, SOC and BD; as well as between SS and silt content. Soil compaction induced by sheep trampling, especially in the heavily grazed site, inclined to a homogenous spatial distribution of soil properties, which will possibly enhance soil vulnerability to water and nutrient loss, and consequently reduce the plant available water and thus grassland productivity. [Copyright &y& Elsevier]

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

Abstract: Activity and stability phases as well as geomorphic processes within the Critical Zone are well known. Erosion and deposition of sediments represent activity; soils represent geomorphic stability phases. Data are presented from a 4m deep sediment section that was dated by luminescence techniques. Upslope erosion and resulting sedimentation started in the late Pleistocene around 18ka until 12ka. Conditions at the study site then changed, which led to the formation of a well-developed soil. Radiocarbon dating of the organic matter yielded ages between 8552 and 8995cal.BP. From roughly 6.2 to 5.4ka another activity phase accompanied by according sediment deposition buried the soil and a new soil, a Cambisol, was formed at the surface. The buried soil is a strongly developed Luvisol. The black colors in the upper part of the buried soil are not the result of pedogenic accumulation of normal organic matter within an A-horizon. Nuclear magnetic resonance spectroscopy clearly documents the high amount of aromatic components (charcoal), which is responsible for the dark color. This indicates severe burning events at the site and the smaller charcoal dust (black carbon) was transported to deeper parts of the profile during the process of clay translocation. [Copyright &y& Elsevier]