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One of the most prominent changes in the rhizospheric soil structure is associated with the formation of a strongly bound soil layer in the surroundings of the root, which is named rhizosheath. In this study, we investigated how root hair elongation, a ubiquitous root morphological trait, affect the stability of rhizosheath aggregates. Using 13CO2 pulse labeling, we tracked the fate of root-derived 13C inputted into the rhizosheath of two Zea mays L. genotypes with contrasting root hair elongation: a mutant with root hair defective elongation (rth3) and a corresponding wild type (WT). In addition, we also investigated the differences between two 13CO2 labeling approaches (single vs. multiple pulse labeling) in the distribution of 13C in the rhizosheath aggregates. We were able to demonstrate that the rhizosheath aggregate stability and the resulting aggregate size distribution follows the same mechanisms irrespective of the root hair elongation. This result reinforces the assumption that other soil properties are more decisive for the soil structure formation in the rhizosheath in comparison to root hair elongation. The majority of recently deposited root-derived C (57%) was found in the macroaggregates. Increasing the number of pulses (multiple pulse labeling approach) resulted in a higher 13C enrichment of the rhizosheath aggregates fractions in comparison to the application of a single pulse. While both labeling approaches resulted in a similar distribution of 13C in the rhizosheath aggregates, the higher enrichment given by multiple pulse labeling allowed the separation of significant differences between the genotypes in plant C allocation in the rhizosheath.

The conversion of native vegetation into agricultural lands is often associated with a decrease in soil C. The soils from the Brazilian savannah (named Cerrado), with 200 million hectares, are rich in Fe and Al (hydr)oxides, which could result in more organo-mineral associations and lead to particularly high C storage. The changes in the C stocks from the conversion of native forest into degraded pasture (DP), and the adoption of proper management to recover DP and increase C stocks in such Ferralsols are not well understood. To provide insights into the drivers of C storage, this study compared the C stocks across depth in the top 1m and the distribution of C in the soil fractions 24 years after the adoption of different management systems in degraded pastures in the Brazilian Cerrado.A DP area located in São Carlos, São Paulo, Brazil was converted into different management systems: (i) RMS: rainfed pasture with moderate animal stocking rate, (ii) RHS: rainfed pasture with higher animal stocking rate, and (iii) IHS: irrigated pasture with higher stocking rate. As a control, the adjacent native vegetation (FO) was also evaluated. The adoption of management started in 1996 with RMS and in 2002 for RHS and IHS. Except for the DP, all areas were limed and N-fertilized. RMS with 200 kg N ha, RHS 400 kg N ha, and IHS with 600 kg N ha. Soil sampling was carried out in 2020 and the C stocks were evaluated up to 1 m deep. To state vegetation change from C3 (native forest) to C4 (introduced pasture) the isotopic natural abundance of 13C was analyzed. To evaluate the contribution of mineral-associated and particulate organic matter forms to C storage, we performed a physical fractionation by size and density with SPT 1.8 g cm-3, respectively.Our results showed that the conversion of FO into DP decreased soil C stocks. Otherwise, the adoption of management in DP with RMS and RHS increased C stocks achieving levels similar to FO. RMS showed the highest C stocks with the lower dosage of N-fertilizer and animal stocking rate. IHS area did not increase their C stocks compared to DP, which may be related to limited root growth after irrigation decreasing the C input. Around 50% of the C stocks in RHS and RMS systems are pasture-derived (C4 plants) according to the 13C abundance. This shows that half of C stocks from rainfed pastures is of preserved organic matter from previous FO. While in the IHS and DP systems, the organic matter composition is mainly pasture-derived. Our preliminary data showed that the RMS topsoil contained more free particulate organic matter than the FO, suggesting that the C stocks were enhanced mainly by pasture-derived biomass input. The contribution of mineral-associated organic matter still will be evaluated.Our study shows that the recovery of degraded pasture soils by management leads to increased OC stocks derived from fertilized pasture but also higher maintenance of OC from FO.

Optimized slurry management targeted to increase nitrogen use efficiency (NUE) can be fundamental for limiting fertilizer N losses from agricultural grasslands causing eutrophication of ground- and surface water as well as air pollution. However, a holistic assessment of both agronomic and environmental impacts including key soil functions is still missing. This study aims to provide such information by assessing the impacts of traditional vs. modern slurry application techniques on NUE, hydrological and gaseous N losses, productivity and fodder quality, soil organic nitrogen formation and total N balances. In a plot-scale grassland experiment 15N enriched slurry was applied after the first cut in early summer. The application treatments included: (1) traditional slurry broadcast spreading under dry weather; (2) application like (1) followed by a heavy rainfall event to increase slurry infiltration into soil; (3) broadcast spreading of slurry diluted with water; (4) injection of slurry into soil via shallow slits; and (5) injection of slurry into soil via deep slits. Variant (4) and (5) represent modern trailing shoe injections requiring extensive machinery. Fates of fertilizer N such as plant uptake, immobilization in soil and microbial biomass as well as NO3 leaching were investigated by 15N tracing approaches in order to create full N balances. For this, biomass harvest and soil sampling were conducted after a growth period of 2 months. Here, we will present first results of our work that are expected to provide highly relevant decision support for grassland management.

Local environments such as soils and waterways, but also landscape-scale environments such as agricultural areas, are often classified as oxic based on the dominant conditions in such systems. Still, anoxic conditions do occur within these “oxic” landscapes, their upland and lowland soils, sediments and creeks at a wide range of spatiotemporal scales, and can represent hot spots and hot moments for greenhouse gas emissions but also carbon storage due to their strongly deviating biogeochemical character.Within the project “Bavarian landscapes under climate change”, located in the crystalline Bavarian Forest, Germany, we characterize and quantify the role of anoxic spots for greenhouse gas emissions and carbon storage at the scale of agricultural landscapes, including managed grassland and cropland soils, riparian soils and creeks including their special features such as beaver dams. Our focus is on mesoscale anoxic events, at the 1-10 m2 scale, that are the result of temporary conditions, such as flooding or fine sediment deposition. We place these mesoscale events into the context of the whole agricultural landscape and watershed, and characterize the carbon species, gaseous emissions, carbon stocks, and organic carbon degradation dynamics in the different oxic and anoxic parts of our larger system, both aquatic and terrestrial. First results indicate that the enhanced greenhouse gas emissions, but also carbon storage, can be large enough to be relevant offsets of the larger scale (whole stream, riparian zone) carbon dynamics. Ignoring these spatially or temporally relatively small events or locations in upscaling or modeling can thus lead to important underestimations of carbon storage, as well as to offsets of the greenhouse gas balance of an area.

Percolating dissolved organic matter (DOM) from the topsoil is considered the main source of subsoil organic carbon (OC) in temperate soils. Although DOM adsorption to minerals has been extensively studied, comprehensive knowledge about its influence on subsoil OC storage and structure development is limited.We conducted a short-term incubation experiment using artificial model soils without pre-existing aggregates to study the effects of percolating DOM within varying soil textural conditions on OC turnover and initial structure development.The model soils were designed with contrasting texture (clay loam, loam, sandy loam), but identical mineral composition (quartz, illite, montmorillonite, goethite), mimicking subsoil conditions, where mineral surfaces free of OM come into contact with percolating DOM. The regular application of DOM under a constant suction head (-15 kPa) enabled the DOM to percolate freely through the soil matrix over the course of the experiment.A higher sand content caused a lower porosity, which was accompanied by a lower moisture content. In contrast, the OC retention (21% of the OC input), and the microbial abundance and activity were unaffected by the soil texture. The percolating DOM created patches of OM covers on 10% of the mineral surfaces (N2-BET) within an otherwise OC-free mineral matrix.The biochemical processing of the percolating DOM solution induced the formation of large, water-stable aggregates (wet-sieving) in all textures without requiring the presence of physical organic nuclei. Aggregate formation was pronounced in the clay-rich soils (58% mass contribution), which also exhibited a higher mechanical stability of the aggregates.The results highlight that retention and microbial mineralization of dissolved OM are decoupled from pore sizes and soil solution exchange, but are instead driven by the mineral composition and OC input.The biochemical processing of percolating DOM can induce large soil aggregates. Here, the presence of fine mineral particles enhances the formation and mechanical stability of the aggregates, irrespective of their surface charge or sorption properties.

Organic matter (OM) and soil mineral constituents interact closely at the submicron scale forming structural units and providing biogeochemical interfaces. Soil structure itself plays a key role for carbon storage, microbial activity and soil fertility and pollutant mitigation. A better understanding to which extent biogeochemical processes and interactions in the soil are driven by the spatial arrangement of OM and mineral constituents requires advanced efforts to apply novel microspectroscopy approaches.NanoSIMS, allowing unique elemental and isotopic analyses at nanometer spatial resolution, provide valuable insights into the architecture of soil organo-mineral constituents and crucial processes taking place at the microscale.The instrument is equipped with two ion sources: the Cesium source (Cs+) convenient to detect ions related to organic matter distribution and the Oxygen source (O-) favourable to provide information on mineral phases or metals in samples. With a spatial resolution similar to the Cesium source and high stability, the upgraded radio frequency (RF) plasma Oxygen source recently installed at the TUM is now best suited for novel analytical approaches to probe elemental and isotopic composition of soil organo-mineral constituents in soils at the microscale.We will show examples of how the two primary ion sources, single or correlatively applied, enable novel experimental designs in soil biogeochemistry. Novel combinations of the OM distribution (12C, 13C and 14N, 15N) detected by the Cs+ source with the distribution of e.g. Si, Al, Fe, Ca, Mg, K, and Na of minerals as revealed by the O- source are now possible.Post-processing tools for unsupervised clustering and supervised segmentation facilitate the comparison and quantitative analysis of the spatial architecture within intact soil structures. These ongoing developed tools can contribute to the extent of our understanding of biogeochemical processes taking place at organo-mineral and mineral-mineral interfaces in soil systems at the microscale.

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

Soil organic matter consists of components that differ in their specific stabilization/decomposition dynamics, and turnovers. From a simplified viewpoint, two fractions in particular can be distinguished from one another. Particulate organic matter (POM) is predominantly unbound in the soil matrix. The POM decomposition rate is defined by its inherent chemical recalcitrance and occlusion within aggregates. Mineral-associated organic matter (MAOM) is significantly smaller and is protected from decomposition by its adsorption to mineral surfaces. MAOM-C has therefore significantly longer mean residence times in soil than POM-C. Since the soil organic carbon (SOC) stocks are determined by C input/output balances, it is important to decrease C output quantities by increasing the long-term stabilization of OC within the MAOM-C stocks. However, MAOM-C cannot be enriched indefinitely. It is limited by the amount of clay and fine silt particle surfaces it can adsorb to and to the general land-use management. We investigated the validity of a POM-C/MAOM-C ratio indicator on 25 long-term field experiments in Central Europe to evaluate the sustainability of SOC management measures. We found that the POM-C/MAOM-C ratio might be used to assess the sustainability of agricultural management in before/after management change comparisons. Accordingly, a sharply increasing ratio indicates that the change in management does not adequately affect the long-term MAOM-C storage of soil. Moreover, we found a dependence between the POM-C/MAOM-C ratio and the MAOM-C sequestration deficits in soils, where arable soils with a POM-C/MAOM-C ratio indicator > 0.35 are close to MAOM-C saturation. If these observations are repeatable on further arable soils, the POM-C to MAOM-C ratio of 0.35 could be used as a management target to avoid organic over-fertilization and N loss, especially in coarse-textured soils. Thereby, the indicator might help to optimize SOC management and sequestration on arable soils and support climate change mitigation strategies.

Plants' roots promote changes in soil structure, forming a strongly-bound soil layer in the surroundings of the root, which is named as rhizosheath. Rhizosheath formation is attributed mainly to the root hairs' presence, that favors the enmeshment of the soil particles around the roots, and the release of mucilage and exudates, which acts as gluing agents of those soil particles. In the present work, we studied the rhizosheath aggregate formation of two Zea mays L. genotypes with contrasting root hair development: a mutant with root hair defective elongation (rth3) and a corresponding wild type (WT). We also tracked the fate of recently-deposited C in the rhizosheath aggregates using two 13CO2 pulse labeling approaches (single vs. multiple pulse labeling). The sampled rhizosheath aggregates were further separated using dry-sieving fractionation into three aggregate size classes: primary small particles and smaller microaggregates (250 µm). We observed that the aggregate size distribution followed the same pattern in both genotypes. This result reinforces the assumption that other soil properties are more important for rhizosheath aggregation than root hair elongation. We observed that the higher potion of the recently-deposited root-derived C (57%) was accumulated in the macroaggregates. Moreover, the multiple pulse labeling approach proportioned a higher 13C enrichment of the rhizosheath aggregates fractions than applying a single pulse. Despite both single and multiple labeling approaches have resulted in a similar distribution of 13C in the rhizosheath aggregates, multiple pulse labeling provided a higher enrichment in the rhizosheath aggregates, which allowed a better separation of significant differences between the genotypes.

The impacts of land use on organic carbon (OC) and nitrogen (N) in different ecosystems have been widely studied. However, less attention has been paid to its influence on the change of total phosphorus (Pt) and its two fractions, inorganic P (Pi) and organic P (Po), along slopes. In addition, since N and P are the main factors causing the eutrophication of aquatic ecosystems, it is essential to link the terrestrial and aquatic ecosystems.Two slope-floodplain-creek catenas were chosen in the low mountain region of the eastern Bavarian Forest. The slopes of the two catenas have been extensively managed for grassland and cropland use, respectively. The floodplain was under grassland use. They were fertilized with sludge, occasionally with farmyard manure, under the same regime. Additional carbonic magnesium lime was applied annually at the cropland. Soil profiles at the top, middle and bottom slope, as well as the floodplain area, were sampled according to their horizons and replicated with three soil cores in each catena. Liquid-nitrogen freeze-cores of the aquatic sediments were sampled to extend each catena into the below-slope creek.We found the grassland slope had higher OC and N content in the topsoil (0-10 cm) and higher Pt and Po content in both the topsoil (0-40 cm) and subsoil (40-60 cm). The P content in the topsoil (0-40cm) of the grassland slope was dominated by Po, while the cropland slope was Pi -dominated. The OC and N stocks on the grassland slope were significantly higher than at the corresponding position on the cropland slope. Except for the lowest P stock found on the top slope of grassland, the Pt stock in topsoil (0-20 cm) was not land-use affected while the Po stock was significantly higher on the grassland slope. An accumulation of OC, N and Po in topsoil (0-20 cm) from top to bottom of the slope was observed. In the floodplain, upslope cropland use decreased the OC, N, Pt content and stocks, but reversely for Po. The upslope land use effect on OC, N and P content in the creek sediment was limited.In conclusion, this study provides a field-based observation on the change of OC, N and P (Pt and Po) under different land use upslope from terrestrial to aquatic systems through two slope-floodplain-creek catenas.

Aggregate forming and stabilizing processes have been intensively studied as they are closely linked to organic carbon (OC) sequestration. However, soils are no static systems and consequently, their structure is subject to constant breakup and turnover processes.In order to study soil structure turnover with respect to the loss of OC, we designed an incubation experiment with soil microcosms, allowing OC-loss by leaching and microbial respiration, while preventing any mechanical disturbance.We incubated intact soil cores of an arable Luvisol derived from Loess-deposits in south-east Germany for 300 days at constant water-tension and 25 °C to promote microbial activity. During the incubation, CO2-release and OC leaching from the microcosms were monitored. A subset of microcosms was sampled each month to assess the effect of progressing OC depletion on the size distribution, OC content and stability of the aggregates.The incubation led to a reduction of the initial OC (11.2 mg g-1) by 2.2 mg per g soil and a more narrow C:N ratio, which corresponded to a reduced OC coverage of the mineral surfaces (1.7 m² g-1 to 0.9 m² g-1, as determined by N2-BET). Despite the OC reduction, the aggregate size distribution (as determined both by wet- and dry-sieving) did not change significantly, although there was a trend towards a reduced aggregate mean weight diameter (higher reduction after wet-sieving). The aggregates’ mechanical stability (as determined by dry-crushing), even slightly increased with a lower OC-content in the bulk soil.Those observations highlight that OC depletion, without additional mechanical influence, does not immediately lead to the decay of soil structure. This suggests the existence of OC-storage sites that are not prone to OC-loss by leaching or microbial degradation. In contrast, the sites of initial OC-loss might not contribute to the structural stability of a soil.

Wildfires are a common disturbance in Mediterranean ecosystems; nevertheless, in temperate and higher latitude ecosystems the frequency and severity of wildfires increased because of prolonged heat waves, drought periods and changes in fire regime. High wildfire frequency and increased burned surface area may affect soil organic carbon (SOC) dynamics. In our study, we assessed the effects on different SOC pools (labile, intermediate and passive) in two areas of the Cadiretes Massif in Spain 28 years after a wildfire event. The area was burned at different severities (LS: low severity; HS: high severity) and were compared with a control (unburned) adjacent area. A physical fractionation method was used to isolate the OC and associated to dissolved organic carbon (DOC), particulate organic matter (POM) the mineral associated organic matter (MAOM). In addition, we used 13C NMR spectroscopy to get information about the quality of the different OC fractions. Results displayed that an increase in wildfire severity caused a decrease of SOC and N in the bulk soil. This was mainly observed in the labile fractions (DOC; POM). No differences were observed in the OC associated to the MAOM with the increase of the burning severity indicating that the OC protected in the mineral fraction was resilient towards wildfires.

The spatial distribution of organic substrates and microscale soil heterogeneity significantly influence organic matter (OM) persistence as constraints on OM accessibility to microorganisms. However, it is unclear how changes in OM spatial heterogeneity driven by factors such as soil depth affect the relative importance of sub-strate spatial distribution on OM persistence. This work evaluated the decomposition and persistence of 13C and 15N labeled water-extractable OM inputs over 50 days as either hotspot (i.e., pelleted in 1-2 mm-size pieces) or distributed (i.e., added as OM < 0.07 mu m suspended in water) forms in topsoil (0-0.2 m) and subsoil (0.8-0.9 m) samples of an Andisol. We observed greater persistence of added C in the subsoil with distributed OM inputs relative to hotspot OM, indicated by a 17% reduction in cumulative mineralization of the added C and a 10% higher conversion to mineral-associated OM. A lower substrate availability potentially reduced mineralization due to OM dispersion throughout the soil. NanoSIMS (nanoscale secondary ion mass spectrometry) analysis identified organo-mineral associations on cross-sectioned aggregate interiors in the subsoil. On the other hand, in the topsoil, we did not observe significant differences in the persistence of OM, suggesting that the large amounts of particulate OM already present in the soil outweighed the influence of added OM spatial distribution. Here, we demonstrated under laboratory conditions that the spatial distribution of fresh OM input alone significantly affected the decomposition and persistence of OM inputs in the subsoil. On the other hand, spatial distribution seems to play a lower role in topsoils rich in particulate OM. The divergence in the influence of OM spatial distribution between the top and subsoil is likely driven by differences in soil mineralogy and OM composition. Technical University of Munich Institute for Advanced Study; NSF [DMR-1654596]; Packard Foundation; NSF MRSEC program [DMR-1719875]; Kavli Institute at Cornell; DOE EFRC BES [DE-SC0001086, NSF-MRI-1429155]; Cornell University; Weill Institute; KIC Published version The Technical University of Munich Institute for Advanced Study provided funding for this study. MJZ and LFK acknowledge support by the NSF (DMR-1654596) and Packard Foundation. This work used the Cornell Center for Materials Research Shared Facilities supported through the NSF MRSEC program (DMR-1719875). Additional support for the FIB/SEM cryo-stage and transfer system was provided by the Kavli Institute at Cornell (KIC) for Nanoscale Science and the Energy Materials Center at Cornell, DOE EFRC BES (DE-SC0001086). The FEI Titan Themis 300 was acquired through NSF-MRI-1429155, with additional support from Cornell University, the Weill Institute, and the KIC. The authors thank Katherine E. Grant and Louis A. Derry (Cornell University Earth and Atmospheric Sciences) for providing soil samples from the Polulu Flow, HI. The authors thank Akio Enders and Kelly Hanley for their technical support.

Microbial necromass is regarded as a central pool of soil organic carbon, whose management is critical in efforts to reduce atmospheric CO2 concentrations and mitigate climate change. However, recent concepts on soil organic matter formation have ignored one of the most important factors for the formation and stabilization of microbial necromass in many soils: earthworms. Based on recent evidence, we conceptualize how the ingestion and mixing of mineral particles and organic matter by earthworms temporarily convert the egested soil to a hotspot of quick and efficient microbial growth and turnover, in which increased amounts of necromass tightly bind to mineral surfaces and stabilize within aggregates. We further stress the low dependence of this process on the quality of pre-existing soil organic matter (in contrast to the assumptions of recent concepts) and its high relevance to the resilience of soil carbon to external disturbances in extensive regions of the soil remote from classical hotspots of microbial necromass formation. We finally provide suggestions on how to close remaining research gaps.

The interactions between mineral particles and soil organic matter (SOM) are an important factor for soil structure formation. Percolating dissolved organic matter (DOM) from upper soil horizons is considered an important input pathway of organic carbon (OC) into subsoils. While DOM sorption processes have been extensively studied, the effect of DOM input on soil structure formation has rarely been looked at systematically. We conducted a 30-day laboratory incubation experiment to investigate the process of DOM-induced structure formation in artificial model soils with three contrasting textures (clay loam, loam, sandy loam).The soil texture defined the pore system and the flow characteristics of the soil solution, leading to a lower liquid retention and faster soil solution turnover in the sand-rich soils. In contrast, the OC retention was unaffected by the soil texture, indicating that only the clay minerals and iron oxides, but not the texture-defining quartz grains, contributed to the OC sorption.The total microbial biomass, as well as the CO2-release were unaffected by the texture. In contrast, the microbial community composition showed a texture-dependent development with a higher proportion of fungi and gram-positive bacteria in the sand-rich mixtures. This suggests that texture-related architectural features of the pore space shape the microbial community composition.It could be shown that the biochemical processing of the percolating DOM solution was sufficient to induce the formation of large macroaggregates in all textures without requiring mechanical stress or the presence of physical OM nuclei. Very low OC concentrations (< 0.8 mg g-1) could support the water-stability of the formed aggregates, although they were not sufficient to provide any meaningful stability against mechanical loads.

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.

Organo mineral associations intermediated by Fe and Al are considered one of the most important mechanisms for soil organic carbon (SOC) stabilization. However, since Fe and Al are normally mentioned together as stabilizing agents, we still lack knowledge about their relative role. In addition, this stabilization mechanism can be profoundly affected by climate differences, but the magnitude of this influence whether as a direct effect or an indirect consequence due to changes in soil mineralogy is not yet fully understood. In this study, we evaluated a series of subsoil samples throughout a climate gradient (1800–2400 mm precipitation year-1 and 15–24º C) on Kohala Mountain, Hawaii to understand the impact of climate differences on organic matter protection. We have used a combined approach of analyses at the bulk soil and microscale using NanoSIMS. At the bulk soil scale, we have observed a concurrent decline of subsoil Fe, Al (i.e., dithionite citrate and ammonium oxalate extractions) and SOC above a precipitation level of 2000 to 2200 mm year-1. This decline co-occurred with more reduced forms of Fe s (evaluated by Fe K-edge XANES) and declines in carboxyl-C (evaluated by CP-MAS 13C NMR). We found significant positive correlations between SOC with Fe and Al in the bulk soil throughout the gradient, and we could discern the relative role of Fe and Al in promoting organo-mineral associations in contrasting climate conditions (e.g., ~1800 and ~2300 mm year-1) using NanoSIMS. While Fe contributed to approximately 40% of the microscale organo-mineral associations in the lower precipitation site (assessed by co-localizations with OM segments), this contribution at the higher rainfall regime was only 5%. In contrast, the contribution of Al was approximately the same in both rainfall levels (approximately 30%). This fact indicates that Al may be more important than Fe in stabilizing SOC especially under high precipitation levels. The normalized CN:C ratio was higher when associated with Fe and Al especially in the high precipitation level, which demonstrates the importance of Fe and Al in stabilizing N-rich organic matter. Here we demonstrate that spatial relationships between Fe and Al with SOC at the microscale display a shift towards Al-dominated SOC associations at higher precipitation that could not be ascertained from bulk measurements alone. Thus, they are of great importance to understand the impact of climatic differences on SOC sequestration in organo-mineral associations.

Microaggregates (250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (

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.

High long-term soil moisture may either stimulate or inhibit soil organic carbon (SOC) losses through changes to mineral and chemical composition, and resultant organo-mineral interactions. Yet, the trade-off between mineralization and accrual of SOC under long-term variation in unsaturated soil moisture remains an uncertainty. In this study, we tested the underexplored relationships between long-term soil moisture and organo-mineral chemical composition, and its implications for SOC persistence. The results provide new insights into SOC accrual mechanisms under different long-term moisture levels commonly observed in well-drained soils. Differences in long-term mean volumetric water content ranging from 0.4 - 0.63 (v/v) on fallow plots in an experimental field in New York, USA, were positively correlated with SOC contents (R2 = 0.228; P = 0.019, n = 20), mineral-associated organic matter (MAOM) (R2 = 0.442; P = 0.001; n = 20) and occluded particulate organic matter (oPOM) contents (R2 = 0.178; P = 0.033; n = 20). Higher long-term soil moisture decreased the relative content of sodium pyrophosphate extractable Fe (R2 = 0.33; P < 0.005; n = 20), increased that of sodium dithionite extractable Fe (R2 = 0.443; P < 0.001; n = 20), and increased the overall importance of non-crystalline Al pools (extracted with sodium pyrophosphate and hydroxylamine extractable) for SOC retention. Higher long-term soil moisture supported up to a four-fold increase in microbial biomass (per unit SOC), and lower C:N ratios in MAOM fractions of high-moisture soils (from C:N 9.5 to 9, R2 = 0.267, P = 0.011, n =20). This was reflected by a 15% and 10% greater proportion of oxidized carboxylic-C to aromatic-C and O-alkyl C, respectively, as measured with 13C-NMR, and a more pronounced FTIR signature of N-containing proteinaceous compounds in high-moisture MAOM fractions, reflective of microbial metabolites. SOC accrual increased with increasing soil moisture (P = 0.019), exchangeable Ca2+ (P = 0.013), and pyrophosphate-extractable Al content (P = 0.0001) and Al/Fe ratio (P = 0.017). Taken together, our results show that high long-term soil moisture resulted in SOC accrual by enhancing microbial conversion of plant inputs to metabolites that interact with reactive minerals.

Soil salinization is an emerging problem worldwide as a result of unsustainable land management practices and climate change. However, salt-affected soils under agricultural use could act as a C sink if these negative effects can be offset by combination of sustainable land management practices (SLM). In this study, we assessed the effect of (i) intensive tillage along with flood irrigation (IT); (ii) combination of no-tillage with pruning residues (branches and leaves) as mulch, and drip-irrigation (NT + PM); and (iii) combination of reduced tillage with the incorporation of pruning residues and drip-irrigation (RT + PI), on physico-chemical soil parameters, aggregate stability, amount and quality of organic matter fractions and soil organic carbon (SOC) sequestration in a lemon tree orchards (Citrus limon var. Verna) under semi-arid climate conditions. The RT + PI management system showed a decrease in salinity and bulk density, and increased soil porosity, soil OC and N stocks, and percentage of OC-rich macroaggregates as compared to the IT system. The aggregate-occluded particulate organic matter fraction (oPOM) played a key role in macroaggregate stability. The NT + PM treatment also showed positive effects on the investigated soil properties, but this was limited to the upmost topsoil (0−5 cm). The IT management system revealed highest values of salinity and bulk density, and considerably lower SOC stocks. Moreover, a degradation of soil structure with a low percentage of macroaggregates depleted in SOC was observed. We conclude that the incorporation of pruning residues in combination with reduced tillage and drip-irrigation is an effective management system to improve soil structure and facilitate SOC sequestration. Therefore, conventional management systems based on intensive tillage and flood irrigation should be abandoned in salt-affected soils under semi-arid climate conditions in favour of systems with higher organic matter inputs incorporated into the soil combined with measures to reduce the salt content.

Agricultural soils are highly important organic carbon (OC) reservoirs in Central Europe. Their OC stocks are crucial for a wide range of ecosystem services such as food security or climate regulation through atmospheric CO2 storage. There is increasing evidence that the controlling factors of OC stocks may differ between topsoil and subsoil, but distinct knowledge about the factors at a regional scale for temperate soils is scarce. In order to identify the controlling factors of OC storage in agricultural soils we implemented a random forest model on a data set of 692 soil profiles located in Bavaria, Southeast Germany. We differentiated between topsoil (0–30 cm profile depth) and subsoil (30–100 cm) and predicted OC stocks (kg OC m−2) using 13 predictor variables including soil type, topographical factors, climatic factors and information on land use. The explained variance of the topsoil random forest was higher than that of the subsoil (51 and 23%, respectively). The fact that major proportions of the variance in the modelling of subsoil OC remained unexplained, revealed high uncertainties that persist about subsoil OC storage. OC storage of both, topsoils and subsoils, was to a large extend determined by four factors: land use history, soil type, major landform and topographic wetness index (TWI). Climatic factors as well as clay content had surprisingly low impact on the model. Topsoil OC was mainly controlled by the legacy of the land use resulting in highest stocks under permanent grassland use. In contrast, soil type and topographical factors, i.e. major landform and TWI, largely determined subsoil OC stocks. The topography indicates the lateral transport of OC due to soil erosion and its subsequent deposition in low lying landform positions. There, high soil moisture as indicated by the TWI, hampers the decomposition of the OC. Human activities such as tillage amplify the lateral OC transport and therefore the amount of buried OC. As a result of these processes, highest subsoil stocks were found in alluvial, colluvial and groundwater affected soils. The findings demonstrate the close link between the generic soil type, topography, land use and OC stocks. We further conclude that the legacy of human activities affect both, topsoil and subsoil OC storage.

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 CaCO3 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.

Grassland management and climate change may have profound effects on soil organic carbon (SOC) storage in temperate grasslands, but to date only limited information on the long-term SOC development derived from grassland monitoring sites is available. In 1986, the Bavarian State Research Center for Agriculture established 20 long-term grassland soil monitoring sites in the state of Bavaria, Germany. We analyzed SOC changes and their main controlling factors in these monitoring sites between 1989 and 2016 in four intervals using the machine learning algorithm conditional random forest. The results showed that changes of SOC stocks in grassland monitoring sites were affected by the multifactorial and interactive nature of changing climate conditions, pedogenic-topographic factors and management practices. Seasonal climate variables explained the highest variability of SOC stock changes in long-term grassland sites, followed by organic fertilizer application. Increasing autumn precipitation led to decreased SOC stocks, whereas increasing spring and summer precipitation led to increased SOC stocks on sites at high elevations with low slopes. Furthermore, we observed that the organic fertilizer application resulted in an increase and/or preservation of SOC stocks, whereas conventional farming with a low degree of organic fertilization resulted in SOC stock decreases. Regarding the acceleration of climate change and the complex interactions between site conditions, changing climate and management, there is an urgent need to collect improved management data in future long-term studies.

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.

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.

The heterogeneity of soil organic matter (SOM) and the small changes in soil organic carbon (SOC) compared to large total SOC stocks hinder a robust estimation of SOC turnover, in particular for more stable SOC. We developed a simple fractionation protocol for agricultural topsoils and tested it extensively on a range of soils in southern Belgium, including farmed soils, soils from long-term field trials, and paired sites after recent conversion to conservation farming. Our simple fractionation involves shaking the soil, wet sieving over 20 μm and analysing the SOC concentration in the soil as well as in the fine fraction (< 20 μm). Eight biological indicators measured in an earlier study across the same monitoring network for the 0–10 cm topsoil were analysed in a conditional inference forest model in order to investigate the factors influencing the SOC fractions. Soil microbial biomass N explained the largest proportion of variation in both fractions. The fine fraction was also associated with factors explaining the regional trend in SOC distribution such as farmyard manure input, precipitation, land use and flow length. The variation in SOC content between treatments both in long-term trials and in farmers’ fields converted to conservation management was mainly attributed to changes within the coarse fraction. Thus, this fraction proves to be sensitive to management changes, although care should be taken to sample deep enough to represent the former plough layer inherited from the conventional tillage practice. Furthermore, the ratio between the coarse and the fine fraction showed a linear relationship (r²=0.66) with the relative changes in SOC concentration over the last ten years. These fractions derived from a simple analytical approach are thus useful as an indicator for changes in SOC concentration. In analogy to biological indicators such as the soil microbial biomass C, the relationship between the fractions and relative changes in SOC concentration are likely to depend on climate conditions. Our methodology provides an indicator for use in routine analysis of agricultural topsoils, which is capable of predicting the effects of management practices on SOC concentrations in the short to mid-term (5–10 years)