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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.

Wildfires occur regularly in boreal forests of Northern Canada and are increasing in frequency and intensity due to the impacts of projected global climate change. A by-product of these forest fires is pyrogenic carbon (PyC) as a residue of incomplete combustion. The short-and long-term dynamics of this important soil organic carbon (SOC) pool in permafrost-affected mineral soils, however, is largely unknown. Here we studied eleven boreal forest soils at distinct landscape positions under continuous (northern sites) and discontinuous (southern sites) permafrost. In these we assessed the short-term fate of 13C-labeled PyC and its precursor grass organic matter over two year in-situ soil core incubations. Further, we isolated PyC by hydrogen pyrolysis (PyCHyPy) for quantification and radiocarbon measurements to investigate long-term pools across the landscape.Losses of PyC after two years were dominated by decomposition with up to three times more PyC losses at northern sites (36%) compared to the southern sites (11%). The losses of the grass organic matter were substantial (69-84%) but losses were larger in southern soils. The PyC persistence depended on site and soil specific properties and not solely on its chemical resistance. Fresh PyC was increasingly decomposition in nutrient limited mineral soils under continuous permafrost, indicating that polyaromatic compounds can act as a nutrient source. Mineral interactions were important and contributed to the stabilization of ~40% of recovered PyC. Mineral-associated PyC mainly remained in particulate forms as identified on the microscale using NanoSIMS. Beside large grass organic matter losses, remaining fractions were recovered predominantly as particles in northern soils but highly dispersed on mineral surfaces in the southern soils on the microscale. Our results highlight that permafrost-affected boreal forest soils are sensitive to fresh PyC and organic matter inputs with substantial losses by decomposition even under continuous permafrost conditions with unique stabilisation mechanisms at the organo-mineral interface.On the long-term, we identified that the native PyCHyPy represents a millennial age carbon pool with significantly higher ages in continuous (5.5-7.8 cal. ka BP; F14C=0.44-0.54) than in discontinuous (1.2-2.2 cal. ka BP; F14C=0.76-0.88) permafrost soils in 0-15 cm soil depth. The PyCHyPy was markedly older than the bulk SOC (modern with F14C=0.65-1.11). With soil depths, PyCHyPy ages increased to >18 cal. ka BP (F14C

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.

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.

Organic matter management can improve soil structural properties. This is crucial for agricultural soils in tropical regions threatened by high rainfall intensities. Compared to conventional farming, organic farming is usually deemed to increase organic carbon and improve soil structural properties such as stability and permeability. However, how much, if any, buildup of organic carbon is possible or indeed occurring also depends on soil type and environmental factors. We compared the impact of seven years of organic farming (annually 13.6 t ha−1 of composted manure) with that of conventional practices (2 t ha−1 of farmyard manure with 150–170 kg N ha−1 of mineral fertilizers) on soil structural properties. The study was conducted on a Vertisol in India with a two‐year crop rotation of cotton soybean wheat. Despite large differences in organic amendment application, organic carbon was not significantly different at 9.6 mg C g−1 on average in the topsoil. However, the size distribution of water‐stable aggregates shifted toward more aggregates, Biovision Foundation for Ecological Development http://dx.doi.org/10.13039/501100015593, Coop Sustainability Fund, Swiss Agency for Development and Cooperation (SDC), Foundation fiat panis http://dx.doi.org/10.13039/501100011087, Liechtenstein Development Service http://dx.doi.org/10.13039/501100015698, https://doi.org/10.6084/m9.figshare.18665612

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 (

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.

There is an intimate relationship between microorganisms and the formation and stability of soil microaggregates, realized by the immobilization and occlusion of microorganisms. Little is known about the initial aggregate formation phase and the role of microorganisms in this process under the impact of environmental changes such as wetting and drying. We investigated this initial aggregate formation process of montmorillonite and goethite in combination with two bacterial strains, Pseudomonas protegens strain CHA0 and Gordonia alkanivorans strain MoAcy 2, in the presence or absence of stress conditions in form of wetting and drying cycles for up to eight days. Montmorillonite and goethite formed microaggregates instantaneously, the size of these aggregates being enhanced in the presence of microorganisms, resulting in up to twofold larger aggregates. This increase in aggregate size was strain-dependent. However, the aggregates that developed during the first 48 h broke into smaller structures later on. A microscopic analysis of the microaggregates revealed that notably the larger microaggregates harbored bacteria and that microaggregates had a sheltering effect on living cells, especially when exposed to desiccation stress. Additionally, aggregate formation was analyzed in the presence of a Pseudomonas protegens mutant strain (CHA211) with increased production capability of extracellular polymeric substances (EPS). About fivefold higher survival rates of culturable cells were observed after desiccation for this EPS overproducing mutant strain in comparison to the wild-type. Our results hint at an aggregate formation process characterized by a rapid occlusion of mineral compounds, and, after the addition of microorganisms, the bacterial colonization of small microaggregates, leading to an increase in aggregate size. The further development of the aggregate size distribution varied depending on the presence of microbial taxa and was modulated by environmental conditions like desiccation events.

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

The conversion of tropical forest for cassava cultivation is widely known to decrease the soil organic matter (OM) and nutrient contents of highly weathered soils in the tropics. Amazonian Dark Earth (ADE) might be more resistant to this process due to their historical anthropogenic amelioration with e.g. charcoal, ceramics and bones, leading to higher soil OM and nutrient concentrations. In this study, we analyzed the effect of land use change on the OM dynamics under tropical conditions and how this is related with P distribution at the microscale, using ADE and an adjacent Acrisol (ACR) as model systems. Soil samples were obtained south of Manaus (Brazil), from a secondary forest and an adjacently located 40-year-old cassava plantation. The land use change induced a severe decrease of organic carbon (OC) concentrations in ADE (from 35 to 15 g OC kg‑1) while OC in the adjacent ACR was less affected (18 to 16 g OC kg‑1). The analysis by 13C NMR spectroscopy showed that the conversion of secondary forest to cassava changed the chemical composition of OM to a more decomposed state (increase of alkyl:O/N-alkyl ratio) in the ADE whereas the OM in ACR changed to a less decomposed state (decrease of alkyl:O/N-alkyl ratio). According to neutral sugar and lipid extraction analyses, land use change led to a larger impact on the microbial-derived and plant-derived compounds in the ADE compared to the ACR. In order to analyze the interactions of OC and P at the microscale, we conducted an incubation experiment with 13C glucose for the analysis with Scanning X-ray Microscopy (SXM) and Nano scale Secondary Ion Mass Spectrometry (NanoSIMS). In both soil types ADE and ACR, land use change caused a reduction of the total 13C glucose respiration by approximately one third in a 7-days incubation, implying lower microbial activity. Microorganisms in both soil types appear to be more readily active in soils under forest, since we observed a distinct lag time between 13C glucose addition and respiration under cassava planation. This indicated differences in microbial community structure, which we will be assessed further by determining the 13C label uptake by the microbial biomass and the microbial community structure using 13C PLFA analysis. Preliminary results from synchrotron-based STXM demonstrate a distinct arrangement of OM at fine-sized charcoal-particle interfaces. From ongoing NanoSIMS analyses, we expect further insights on the co-localization of P and 13C-labelled spots at the microscale. Despite the high loss of OC in the ameliorated ADE through land use change, the remaining OM might foster nutrient dynamics at the microscale thanks to charcoal interactions compared to the ACR. Our results contribute to a better understanding of the C and P interactions and how these respond to land use change in highly weathered tropical soils.

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

The largest terrestrial organic carbon pool, carbon in soils, is regulated by the intricate connection between plant carbon inputs, microbial activity, and soil matrix. This is manifested by how microorganisms, the key players in transforming plant-derived carbon into soil organic carbon, are controlled by the physical arrangement of organic and inorganic soil particles. We studied the role of soil structure on the fate of litter-derived organic matter and we propose that the persistence of soil carbon pools is directly determined at plant–soil interfaces. We show that while microbial activity and fungal growth is controlled by soil structure, occlusion of organic matter into aggregates and formation of organo-mineral associations occur in concert on litter surfaces regardless of soil structure. These two mechanisms—the two most prominent processes contributing to the persistence of organic matter—occur directly at fresh litter that constitutes a key nucleus in the build-up of soil carbon persistence.

The conversion of tropical forest for cassava cultivation is widely known to decrease the soil organic matter (OM) and nutrient contents of highly weathered soils in the tropics. Amazonian Dark Earth (ADE) might be affected less due to their historical anthropogenic amelioration with e.g. charcoal, ceramics and bones, leading to higher soil OM and nutrient concentrations. In this study, we analysed the effect of land use change on the OM dynamics and its composition under tropical conditions, using ADE and an adjacent Acrisol (ACR) as model systems. Soil samples were obtained south of Manaus (Brazil), from a secondary forest and an adjacently located 40-year-old cassava plantation. The land use change induced a severe decrease of organic carbon (OC) concentrations in ADE (from 35 to 15 g OC kg‑1) while OC in the adjacent ACR was less affected (18 to 16 g OC kg‑1). Soils were analysed by 13C NMR spectroscopy to obtain information on how the conversion of secondary forest to cassava affected the chemical composition of OM. Our results show that land use change induces differences in the OM composition: The OM in ADE changes to a more decomposed state (increase of alkyl:O/N-alkyl ratio) whereas the OM in ACR changes to a less decomposed state (decrease of alkyl:O/N-alkyl ratio). According to a molecular mixing model, land use change influenced mostly the proportion of lipids, which might be related with a change of the plant input. The incubation of the soils with 13C glucose enabled resolving how soil microorganisms were affected by land use change. In both soil types ADE and ACR, land use change caused a reduction of the total 13C glucose respiration by approximately one third in a 7-days incubation, implying lower microbial activity. Microorganisms in both soil types appear to be more readily active in soils under forest, since we observed a distinct lag time between 13C glucose addition and respiration under cassava planation. This indicated differences in microbial community structure, which we will assess further by determining the 13C label uptake by the microbial biomass and the microbial community structure using 13C PLFA analysis. Preliminary results from synchrotron-based STXM demonstrate a distinct arrangement of OM at fine-sized charcoal-particle interfaces. Samples of soils receiving 13C label will be further analysed by NanoSIMS with the hypothesis that charcoal interfaces foster nutrient dynamics at the microscale. Despite the high loss of OC in the ameliorated ADE through land use change, the remaining OM might improve the nutrient availability thanks to charcoal interactions compared to the ACR. Our results contribute to a better understanding of the sensitivity of OM upon land use change and how the microbial community is responding to land use change in highly weathered tropical soils.

Soil texture and microorganisms are key drivers controlling the fate of organic matter (OM) originating from decaying plant litter, and thus the stabilization of soil organic matter (SOM). However, the understanding of the mutual interactions between microbial litter decay and soil structure formation controlled by different soil textures remains incomplete. We monitored the fate of litter-derived OM (using 13C isotopic enrichment) from decaying litter (shredded maize leaves) to microorganisms and SOM in two differently textured soils (sand and loam). The two soils were incubated with litter mixed in the top layer in microcosms for 95 days during which regular CO2 and 13CO2 measurements were conducted. After the incubation, each microcosm was divided in three to separate a top, center and bottom layer. Using a physical soil fractionation scheme, we assessed the fate of litter-derived OM to free and occluded particulate OM (POM), as well as mineral associated OM (MAOM). All SOM fractions were analysed with respect to their mass distribution, C, N, and 13C contents, and for their chemical composition using compound-specific 13C-CPMAS NMR spectroscopy. The effects of contrasting textures on the total microbial community structure were studied using phospholipid fatty acids (PLFA) and the incorporation of litter-derived C into individual PLFAs was assessed via 13C-PLFA. Lastly, scanning electron microscopy and nano scale secondary ion mass spectroscopy (NanoSIMS) analysis of free POM of both textures enabled qualitative insights directly at the biogeochemical interface of the microbial hot spot of decaying plant litter.We were able to clearly demonstrate higher contents of litter-derived OM still residing as free POM in the loamy textured soil after the 95 day-incubation, while higher contents were found in occluded and MAOM in the sandy textured soil. This indicated that the overall litter decomposition was refrained in the finer-textured soil, whereas microbial alteration and allocation of litter-derived compounds was promoted in the coarser textured soil. This was further corroborated by higher respiration and higher amounts of respired litter-derived CO2-C in the sandy soil. The PLFA analysis showed a coherent pattern between the textures, with similar community structures in all treatments and significant increases in microbial abundance in the top layers induced by litter addition. This increase was found most pronounced in fungal biomarkers, which was in line with the 13C-PLFA measurements revealing over 90% of fungal biomarkers to be of litter-origin (compared to 30-40% in the other microbial groups). The labelled PLFA profiles also confirmed the importance of fungi as a vector for litter-derived OM into deeper layers of the soil columns, with significantly higher litter-derived fungal markers also in center and bottom layers. The NanoSIMS measurements verified the high 13C enrichment in fungal hyphae and further revealed clay minerals embedded in enriched microbial-derived extracellular polymeric substances and intertwined with hyphae directly on top of the POM. Based on this comprehensive data, we highlight that regardless of the texture, plant litter in association with microbial-derived products represent a hot spot for soil structure formation by harbouring a core for aggregation and MAOM formation.

Earthworms are considered as “ecosystem engineers” impacting soil properties as well as nutrient and element cycles. As they move through soil, earthworms secrete cutaneous mucus which is metabolized by soil microorganisms and a source of plant-available nutrients. Earthworm-processed soil contains carbon enriched, earthworm-specific soil aggregates (e.g. casts and middens) in comparison to earthworm unaffected soil. The reason could be that organic polymeric substances in earthworm mucus bind to soil minerals. The objective of this study was to investigate the small-scale interactions between earthworm mucus and secondary soil minerals, e.g. goethite and illite, leading to the formation of organo-mineral associations. We characterized the chemical composition of earthworm mucus by FTIR and 13C-NMR spectroscopy and compared spectra of mucus to microbial extracellular polymeric substances (EPS), an abundant and well-known type of organic matter that binds with soil minerals. Mucus from anecic (Lumbricus terrestris L.) and endogeic (Aporrectodea caliginosa Sav.) earthworm species was dominated by proteins and carbohydrates. Between 21 and 36% of the total organic carbon in the mucus containing treatments adsorbed to illite and goethite, and most of the binding with goethite was associated with phosphorus containing mucus compounds. The surface charge of newly-formed organo-mineral associations was determined by measuring the isoelectric point (IEP). The IEP of mucus-goethite associations was 6.8, which was lower than the bare goethite IEP of 8.4. The zeta potential of mucus-illite associations was greater than bare illite. We conclude that the specific adsorption of earthworm mucus constituents to soil minerals leads to the formation of mucus-mineral associations. These associations contribute to retention of organic substances from earthworm mucus in soil (micro-)aggregates and explain the altered physicochemical properties of earthworm-formed aggregates in comparison to the earthworm unaffected “bulk” soil material.

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

Conversion of forest to agricultural land for maize cultivation is known to negatively affect soil fertility. However, limited knowledge is available of the impact on aggregate stability and interconnected soil properties. The aims of the present study were to (1) quantify soil aggregate stability, (2) assess aggregate stability changes after land use change, and (3) determine the interactions of aggregate stability with clay, soil organic carbon (SOC), exchangeable Ca 2+ and Mg 2+ , and soil erosion rates. The topsoils of three soil types in Northwest Vietnam were analyzed in chronosequences (0–18 y after land use change from forest to continuous maize) by two methods: Wet sieving and sonication. By differentiation of these aggregate stability measurements, the impact of both methods on aggregate size distribution could be quantified separately and compared. The sonication method indicated a more homogeneous disaggregation whereas the wet sieving method was more suitable to detect low aggregate stabilities. Soil aggregate stability declined simultaneous with a decrease of SOC and exchangeable Ca 2+ and Mg 2+ , which both declined with increasing time since land use change. The Alisol and Luvisol chronosequences were 1.9 times more stable under primary forest than under maize (shown by sonication) whereas the Vertisol chronosequence was 2.5 times more stable under primary forest (shown by wet sieving). Over the 18 y chronosequence the topsoils had 1.6 kg m −2 lower SOC and 3.2 g kg −1 lower Ca 2+ contents. This study highlights the destabilization of soil in interaction with a degradation of relevant chemical soil properties with differentiated aggregate stability methods.

Zinc (Zn) deficiency is a widespread problem in human nutrition and wheat grains are a major source of Zn intake in large parts of the population. It remains unclear to what extent organic and conventional farming practices, differing in organic matter management, influence Zn availability and uptake by wheat grains. Factors leading to an increased Zn uptake may also increase the Cd uptake in wheat grains, which can be harmful for humans. Here, we investigated the effects of different farming practices on Zn and Cd concentrations in wheat grains and their relationships with total and available soil Zn and Cd concentrations, and other soil properties. In northern Switzerland, 28 farms were sampled including 11 organic farms with compost use, 10 organic farms without compost use, and 7 conventional farms without compost use. Soil organic matter was a key factor for soil Zn and especially Cd concentrations across all three farming systems. Total and available soil Cd concentrations as well as soil organic carbon concentration (SOC) were significantly higher on the organic farms with compost use than on the conventional farms. However, only the compost farms with livestock showed significantly higher grain Cd concentrations in comparison to conventional and organic farms without compost use, although a nested effect of cultivar within the system also had an influence. In contrast to Cd, the soil and grain Zn concentrations showed no significant farming system effect although there was a correlation between total soil Zn and SOC when all farms were pooled. Grain Zn was decoupled from soil Zn indicating that under agricultural field conditions the farming systems are a minor factor in increasing grain Zn. Our study suggests that the Zn and Cd soil and grain concentrations were mediated by a combination of on-farm organic matter management, soil properties, cultivar, and livestock production.

As soil microaggregates (250 μm) were dispersed ultrasonically to study occluded SMA and single building units. Finally, we analyzed the size distribution of the small SMA by laser diffraction analysis. The total mass distribution of free and occluded SMA grouped soils into those with small (19, 22, and 24%) and large (32 and 34%) clay contents. The finer textured soils exhibited larger portions of occluded SMA, with a gamma size distribution of small SMA peaking at 6 μm. Yet the occluded small SMA in the finer textured soils showed an additional enrichment of colloids

Today’s food production and consumption go hand in hand with immense damages to humans and nature. Change is needed, but where to start and which direction to go? This article tries to give an interdisciplinary answer by taking recourse to a vision, that is, an ideal image of the future which is drawn upon ethical reflection and beyond the limits of actual political and economic constraints. The main purpose of this paper is to show that generating and discussing visions can be a powerful process in order to regain ability to act in the face of the complex challenges of our time and that Education for Sustainable Development (ESD) plays an important role to enable current and future generations to become actors of change. First, a students vision on agriculture and society in 2050 is presented, followed by a theoretical examination of visions, their potentials, limits and practical implications. Subsequently, the results of a field analysis of current innovative solutions to local agriculture are given. These include intercultural gardens and community supported agriculture. Claiming that a sustainable development can only be reached if people are not only able to envision a desirable future, but to develop small scale, locally adapted solutions as answers to challenges such as climate change, this paper then focuses on the competence oriented educational concept of ESD. Here, an approach of integrating ethics in the course of studies of agricultural sciences implemented by a student’s initiative serves as practice example.

Rice (Oryza sativa) is usually grown under flooded conditions, leading to anoxic periods in the soil. Rice plants transport oxygen via aerenchyma from the atmosphere to the roots. Driven by O2 release into the rhizosphere, radial gradients of ferric Fe and co-precipitated organic substances are formed. Our study aimed at elucidating the composition and spatial extension of those gradients. Air-dried soil aggregates from a paddy field were embedded in epoxy resin, cut, and polished to produce cross sections. Reflected-light microscopy was used to identify root channels. With nano-scale secondary ion mass spectrometry (NanoSIMS), we investigated transects from root channels into the soil matrix and detected 12C−, 16O−, 12C14N−, 28Si−, 27Al16O−, and 56Fe16O− to distinguish between embedding resin, organic matter, oxides, and silicates. Image analyses reveal high occurrences of 56Fe16O− within and in close proximity of oxide-encrusted root cells, followed by a thin layer with high occurrences of 27Al16O− and 12C14N−. In two of the three transects, 28Si− only occurs at distances larger than approximately 10 µm from the root surface. Thus, we can distinguish distinct zones: the inner zone is composed of oxide encrusted root cells and their fragments. A thin intermediate zone may occur around some roots and comprises (hydr)oxides and organic matter. This can be distinguished from a silicate-dominated outer zone, which reflects the transition from the rhizosphere to the bulk soil.

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

This contribution presents how the Ethical Matrix was used in teaching and evaluating a course on ‘Ethics of Food and Nutrition Security’ (EFNS) at Hohenheim University in the winter semester 2010/2011 and in the winter semester 2011/12. We will first provide some information on the development of the course and present the results of the general evaluation of the course by the department. We will then introduce the newly developed instrument in which the Ethical Matrix was embedded. The instrument aims at testing individual skills necessary for identifying ethical issues. Finally, we will draw some conclusions that refer to the further development of the instrument.