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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

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.

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 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.