Marcus Schiedung

Organo-mineral interactions underpin long-term stabilization of soil organic carbon (OC). It remains unclear to which extent soil OC stabilization is coupled to the abundance of fine mineral particles or which compositional changes underlie the formation of OC in the mineral-associated pool. Here, we identify that soil OC content plays a larger role in determining the OC composition of fine fractions than the abundance of fine mineral particles (<20 µm). The formation of soil OC occurs in microscale patches which depend on the selective anchoring of newly forming organic matter to minerals or native organic matter. Using X-ray absorption and mid-infrared spectroscopy, we observed that the fine fraction of low-OC soils (7-16 g OC kg-1 soil) contained more oxidized OC than high-OC soils (46-73 g OC kg-1 soil) regardless of the abundance of fine mineral particles. A majority (mean of 61% (17-86%)) of new 13C-enriched litter-derived OC was found associated with mineral-dominated surfaces, which indicated a greater degree of oxidative transformation (increased carboxylic compounds). In contrast, new OC associated with native organic matter, occurred in larger patches and was less transformed. These results demonstrate that whether new OC is selectively anchored on mineral surfaces or native organic matter, which plays a central role for OC stabilization and consequently it’s resistance to future environmental and climate changes.

Accurate monitoring of soil organic carbon (SOC) is essential for the development of national greenhouse gas inventories as well as for the assessment of strategies implemented to maintain and increase SOC stocks in agricultural systems. Limited access to high quality SOC data across large spatial and temporal scales remains the main barrier for scientists, policy makers and practitioners to make informed decisions about soil management. To address this knowledge gap, the Thünen Institute of Climate-Smart Agriculture conducted the German Agricultural Soil Inventory (BZE-LW), with the first campaign carried out between 2010 and 2018 and the second campaign initiated in 2022. During the first campaign, 3,104 arable, grassland, and permanent crop sites were sampled on an 8×8 km grid and analyzed for SOC contents and stocks, along with additional soil parameters, down to a depth of 1 m. In addition to soil sampling, annual farm management data were collected via questionnaires. The second campaign, which involves resampling of the same sites, enables to quantify and explain changes in SOC contents and stocks at the decadal scale. Based on the most recent results (n=587), minor changes in SOC contents were observed in cropland soils. Average SOC stocks declined significantly in 0–30 cm (−1.6%) and 0–50 cm (−2.7%) with a statistically insignificant increase of 0.9% in the top 10 cm. Grassland soils showed more pronounced SOC losses, with significant declines in both 0–30 cm (−5.9%) and 0–50 cm (−5.1%). Current hypotheses attribute SOC losses to land use history, soil type, rapid climate warming characterized by a 2.1°C rise in the mean air temperature in Germany over 50 years, and farm management activities such as declining nitrogen fertilization. Conversely, increased adoption of cover cropping and reductions in tillage intensity might have partially mitigated against SOC stock losses in croplands. Disentangling these drivers while accounting for several methodological adjustments to the first soil sampling campaign are key tasks, which will serve to inform future soil monitoring efforts in Germany and beyond. To further highlight the importance of data quality, we will emphasize the methodological challenges associated with detecting SOC changes across spatial scales ranging from individual sites to regional levels.

Soil organic carbon (SOC) is the largest terrestrial carbon reservoir and acknowledged to play an important role in climate mitigation. It has been advocated to store additional SOC in stable forms to provide efficient climate change mitigation. For this, mineral associated organic matter (MAOM) is considered to be relatively stable and has longer residence times than faster cycling particulate organic matter. However, we require a better mechanistic understanding of microscale interactions and the controlling factors of MAOM formation. Here we used arable soils across Germany representing three texture classes (sandy, loamy and clayey), two levels of SOC contents (low and high; 7-73 mg OC g-1 soil) and consequently a range of SOC loading in the MAOM fraction (2-122 mg OC g-1 fraction). The soils were incubated for two years after an addition of highly 13C labelled (8 atom%) barley-litter and fractionated by particle size to extract the MAOM (

The accrual of stabilized soil organic carbon (SOC) can mitigate the atmospheric CO2 concentration and thus climate change. Organic carbon in the fine silt and clay size fraction (OCfine) is typically mineral-associated and thus considered relatively stable compared to the coarse fraction. The SOC saturation concept suggests that this pool has a limited storage capacity, for which the fine soil particle content constrains SOC sequestration. Therefore, fine-textured soils with low OC loading of the fine fraction are thought to have a greater potential to stabilize additional OC than soil with high OC loading and coarse-texture due to their higher available storage space. Here, we assessed soils’ potential to stabilize additional OCfine using 21 temperate agricultural soils. The soils were selected from the archive of the German Agricultural Soil Inventory to analyze SOC gradients (0.7-10.2 %) in three texture classes (sandy, loamy, clayey). After a two-years incubation, we investigated the recovery of 13C labeled litter in two size-based fractions: OCcoarse (>20µm) and OCfine (

Soils store both organic (SOC) and inorganic carbon (SIC), yet biogenic processes driving SIC formation remain poorly quantified. One such process is the oxalate-carbonate pathway (OCP) where plant-derived calcium oxalate is transformed by oxalotrophic microorganisms into SIC, which can sequester atmospheric CO₂ in non-calcareous soils. Yet, the OCP has rarely been investigated in connection to trees with significant agroforestry potential. To further investigate the OCP and its connection to species with agroforestry potential, we investigated three East African fig species (Ficus glumosa, F. natalensis, and F. wakefieldii) in semi-arid Samburu County, Kenya.Across contrasting parent materials devoid of primary carbonates, soils adjacent to fig trees exhibited significantly higher pH, exchangeable Ca, SOC, and SIC content compared to control soils, indicating the trees maintained hotspots of distinct biogeochemical conditions. Fig biomass samples contained substantial calcium oxalate contents (4.9±0.5 % dry weight), predominantly as prismatic whewellite crystals (CaC2O4.H2O). Calcium carbonate coatings were observed on trunks and roots of all three species, which reacted strongly to hydrochloric acid. Synchrotron-based μ-X-ray Fluorescence coupled with μ-X-ray absorption near-edge structure spectroscopy (Ca K-edge) revealed that CaCO₃ had precipitated deeply into woody tissues, providing direct evidence for aboveground OCP. Amplicon-based sequencing showed diverse and abundant microbial communities on the aboveground biomass, litter, roots, and adjacent soils. In addition, a co-occurrence analysis of fungal and bacterial communities showed specific fungal genera and fungal oxalate-producers are tightly linked to known baterial oxalotrophs, indicating that bacterial-fungal interactions could be essential for oxalotrophy. Combined these results demonstrate an active OCP both above and belowground in connection to the food-providing fig trees (Ficus spp.) of semi-arid East Africa.Our findings identify East African fig trees as previously unrecognised drivers of biogenic SIC sequestration. Integrating specific fig species into agroforestry systems could therefore represent a novel nature-based solution that couples food production with SOC and long-term SIC storage in dryland landscapes.

Soil organic carbon (SOC) models need independent evaluation against field measurements, but those latter are rarely publicly available and harmonized. In this study, we collected and shared data from 167 agronomic treatments in 34 agronomic long-term experiments (LTEs) located in temperate croplands, allowing the evaluation of several soil organic C models such as RothC, Century, AMG, MIMICS, ICBM, Millenial, and CTOOL. The dataset includes climate data, soil properties, C inputs from crops (n = 4588 records) and organic amendments, irrigation data, monthly soil cover, as well as SOC stock measurements in the topsoil layer (n = 1328 records). Climate, soil moisture, and soil temperature data were extracted from daily climate databases. Carbon inputs from crops were calculated from observed yields and harvest index, with some harvest index values estimated, combined with crop allometric coefficients from the literature. Descriptions of LTE, agronomic treatments, methodological metadata, and a part of the code, accompanies the dataset. The dataset can be reused to evaluate single SOC models, or to evaluate an ensemble of models.

Stabilized soil organic carbon (SOC) accrual plays a crucial role in long‐term atmospheric CO ₂ sequestration. The organic carbon in the fine silt and clay size fraction (OC _fine ) is typically mineral‐associated and thus relatively stable. However, the SOC saturation concept suggests that the OC _fine has limited capacity for additional carbon (C) storage, thereby constraining further C sequestration. Low‐OC and fine‐textured soils are thought to have greater potential to stabilize additional OC than High‐OC and coarse‐textured soils due to their higher available storage space. Here, we assessed soils' potential to stabilize additional OC using 21 temperate agricultural soils, varying in SOC (0.7%–10.2%), silt + clay content (32%–92%), and OC loading of fine fraction (17–135 g C kg ⁻¹ ). We investigated the decomposition and recovery of uniform ¹³ C labeled litter after 2 years in two size‐based fractions: OC _coarse (> 20 μm, the OC associated with coarse silt and sand) and OC _fine (< 20 μm). Litter‐derived OC retention increased significantly with initial SOC content and fine fraction OC loading, primarily driven by the OC _coarse fraction, which indicated that less added C was utilized by microbes when enough C was already abundant. In contrast, litter‐derived OC _fine formation was negatively correlated with initial SOC and fine fraction OC loading. However, when normalized to the amount of actually decomposed litter, initial SOC and texture did not significantly affect the efficiency of OC _fine formation. NanoSIMS showed litter‐derived OC forming at distinct microscale patches, partly overlapping with OM‐ and mineral‐dominated sites. Both findings together revealed that initial SOC content in the studied range, OC loading of the fine fraction, or even soil texture may not be major limiting factors of new OC _fine formation. Instead, increasing initial SOC content appeared to have a positive effect on litter‐derived OC retention by retarding its mineralization.