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 (<20 µm). We applied mid-infrared spectroscopy and synchrotron-based scanning transmission X-ray microscopy coupled with near-edge X-ray absorption focusing on C K-edge spectra (STXM C NEXAFS) to investigate MAOM composition. Following the STXM C NEXAFS analysis, we conducted nano-scale secondary ion mass spectrometry imaging (NanoSIMS) to separate the labelled new from native organic matter. Organic matter composition derived from mid-infrared spectroscopy of the bulk MAOM fraction aligned well with the total STXM C NEXAFS spectra obtained for the different soils. Overall, the composition of the MAOM was controlled by SOC loading rather than texture with more processed and oxidized organic matter in soils with low organic carbon contents. On the microscale, organic matter was patchy distributed and the majority of the mineral surfaces were free of organic matter. Newly formed ¹³C-enriched patches that formed directly on mineral surfaces were more oxidatively-transformed (carboxylic groups) compared to ¹³C co-located with existing organic matter (dominant aromatic moieties). These findings provide direct evidence that soil carbon storage is governed by distinct and preferential accrual pathways shaping local ‘anchoring’ types, and thus, surface attachment mediates the composition of newly incorporated MAOM.

How to cite: Schiedung, M., Rowley, M., Colocho Hurtarte, L. C., Hu, Y., Beinik, I., Hoeschen, C., Begill, N., Poeplau, C., and Schweizer, S. A.: Soil organic matter and mineral surface interactions are governed by the anchoring pathway, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1627, https://doi.org/10.5194/egusphere-egu26-1627, 2026.

Buried soils (paleosols) represent vast but under-characterized reservoirs of long-term soil organic carbon (SOC) that can persist for millennia when isolated from surface processes (Marin-Spiotta et al., 2014; Berhe et al., 2018). Their stability depends on geomorphic protection and mineral–organic interactions that constrain microbial decomposition (Kleber et al., 2007; Kögel-Knabner et al., 2022), but this protection may be compromised when erosion, hydrologic variability, or land-use change reconnect buried carbon to the atmosphere (Doetterl et al., 2016; Berhe et al., 2012).

Using the late Pleistocene Brady paleosol in Nebraska (USA) as a model system, we combined geochemical, isotopic, and incubation approaches to examine mechanisms controlling SOC persistence and reactivation across burial and erosional settings. Radiocarbon and spectroscopic data show that millennia-old SOC is stabilized by fine-textured minerals and polyvalent cation bridging (Ca²⁺, Mg²⁺), which promote aggregation and organo-mineral bonding. Burial enhanced these stabilization mechanisms, whereas erosional exposure induced geochemical convergence toward modern surface soils and faster SOC turnover.

Incubation experiments further revealed that drying–rewetting cycles accelerate decomposition and destabilize even the slow-cycling pool, while continuously moist, deeply buried horizons retained low decomposition rates and greater mineral-associated carbon fractions. These results demonstrate that SOC persistence is jointly controlled by geomorphic position, ionic environment, and moisture regime, linking ancient pedogenesis with modern disturbance.

Because loess–paleosol sequences also occur throughout Central and Eastern Europe, these findings provide a valuable framework for assessing the vulnerability of deep-soil carbon pools to future climate and land-use change. Integrating paleosol processes into soil–climate models will improve predictions of carbon feedbacks and inform management of legacy carbon reservoirs.

How to cite: Nel, T., Dolui, M., McMurtry, A. R., Chacon, S., Phillips, L. M., Mason, J. A., Marin-Spiotta, E., de Graaff, M.-A., McFarlane, K., Tfaily, M., Moreland, K., Ghezzehei, T. A., and Berhe, A. A.: From time capsule to carbon source: Paleosol exposure as a missing component in soil-climate feedbacks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-168, https://doi.org/10.5194/egusphere-egu26-168, 2026.

Clay minerals are widely recognized as key regulators of soil organic matter (SOM) stabilization, yet their interactions with litter quality, soil fauna, and plant roots remain insufficiently understood. We investigated carbon (C) storage and partitioning among particulate (POM) and mineral-associated organic matter (MAOM) in substrates dominated by kaolinite, illite, and montmorillonite using two controlled microcosm and pot experiments. In the first experiments litter of contrasting quality (oak vs. alder) was added to clay minerals with and without earthworms, in second mineral organic matter was delivered by growing  plants  (Festuca rubra and Lotus corniculatus), in the same clay minerals.

 Contrary to expectations based expectation that  surface area (S_BET) would be major predictor of SOM storage, illite consistently supported high C storage, particularly through enhanced incorporation of POM, while montmorillonite promoted MAOM accumulation. Earthworms and easily decomposable litter increased total C storage by facilitating transfer of litter-derived POM into mineral soil. Pore size distribution emerged as a critical factor: illite contained a higher proportion of micrometer-sized pores conducive to POM occlusion, whereas montmorillonite was dominated by nanometer-scale pores favoring MAOM formation.

How to cite: Irshad, S. and Frouz, J.: Specific surface area of clays affects accumulation of mineral associated organic matter while larger pore volume is crucial for particulate organic matter accumulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7476, https://doi.org/10.5194/egusphere-egu26-7476, 2026.

Process-based soil carbon (C) models are increasingly used to project regional and global C cycle responses to climate change. However, the development and evaluation of these models has largely focused on temperate regions of North America and Europe. This geographic bias raises a critical question: Do these models capture generalizable mechanisms that can be applied to underrepresented pedological regions, or do they encode processes specific to their developmental context? Through collaboration between modelers and experimentalists, we evaluated three process-based models—Century, Millennial, and MIMICS—across 777 topsoil samples spanning the climate and pedological diversity of sub-Saharan Africa. Despite their differences in mechanistic detail, all three models performed similarly (adjusted R² = 0.09–0.18) in predicting soil organic carbon (SOC) stocks. Using random forest algorithms trained on observed and modeled SOC data, we identified divergences between drivers of SOC. All three models overemphasized net primary productivity as a SOC driver and misrepresented the role of organo-mineral interactions. Bias analyses revealed that the three process-based models inadequately capture exchangeable calcium, which is increasingly recognized as an important control on SOC. Notably, increased mechanistic complexity did not improve transferability. Our results have significant implications for regional C budgets and global climate projections. They underscore the need for tighter feedback between modelers and experimentalists to incorporate region-specific biogeochemistry—particularly organo-mineral interactions and calcium dynamics—into future soil C models in (sub-)tropical regions. We propose that targeted experimental work on these mechanisms, coupled with model re-parameterization, offers a path toward more reliable climate projections.

How to cite: von Fromm, S. F., Rocci, K. S., Anuo, C. O., Asabere, S. B., Kanyiri, J., Kengdo, S. K., Mureva, A., Nketia, K. A., Zhang, L., and Abramoff, R. Z.: Reconciliation of Process-Based Models and Observations: Collaborative Pathways to Improve Soil Carbon Predictions Across Sub-Saharan Africa, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12784, https://doi.org/10.5194/egusphere-egu26-12784, 2026.

Soil organic matter (SOM) stocks in mountain ecosystems play a crucial role in carbon and nutrient cycling, thereby mitigating climate change and providing many important ecosystem services. However, their response to rapid forest succession resulting from the combined effects of land abandonment and global warming, which is particularly pronounced in mountain areas, remains strongly context-dependent. The influence of soil-forming processes, represented by soil types, is still insufficiently understood. This study investigates how SOM stocks and their stability change along forest succession in the Gorce Mountains in southern Poland, with a specific focus on the role of soil type.

We analyzed soils across different forest succession stages, ranging from grasslands through shrubs and young successional forests to permanent old-growth forests, and across different soil types (Cambisols, Gleysols, and Podzols). We assessed SOM stocks in the O horizons and in the 0–5 cm (topsoil) and 20–30 cm (subsoil) mineral soil layers. In addition, we performed soil density fractionation of the mineral topsoil and subsoil layers into free light fraction, occluded light fraction, and heavy fraction, the relative proportions of which were used as indicator of SOM stability. Our results show that both SOM quantity and stability vary significantly along the forest succession gradient; however, these patterns are strongly modified by soil type. In some soil-contexts, the transition from grassland to forest led to increased SOM stocks but also to a greater vulnerability to SOM decomposition, whereas in other soil types, the highest SOM stocks and greatest stability occurred at an intermediate forest succession stage (tall-shrub communities).

These findings highlight that soil type is a key contextual factor controlling SOM storage in mountain ecosystems. Accounting for soil-specific responses is therefore essential for predicting SOM sequestration potential under ongoing environmental change.

This project has received funding from the Central European Leuven Strategic Alliance (grant CELSA/24/002) and has been supported by a grant from the Priority Research Area Antropocene under the Strategic Programme Excellence Initiative at Jagiellonian University.

How to cite: Musielok, Ł., Nijdam, S., Gus-Stolarczyk, M., Kramarczuk, P., Vancampenhout, K., and Muys, B.: Soil type matters: forest succession and soil organic matter stability in the Gorce Mountains (S Poland), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13706, https://doi.org/10.5194/egusphere-egu26-13706, 2026.

The large carbon storage in terrestrial soils underscores the need for mechanistic soil process representations in Earth System Models (ESMs) aimed at simulating carbon-climate feedbacks under changing climate and land use. However, ESMs participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) still exhibit large uncertainties in simulating historical and future soil carbon stocks, reflecting incomplete process understanding and discrepancies between model assumptions and emerging empirical knowledge. 

We present the recently developed NOAA/GFDL Global Integrated Microbial Interactions with Carbon in Soil (GIMICS) model, which integrates key advances in soil biogeochemistry and addresses limitations of the previous soil component, CORPSE, in GFDL ESM4.1. GIMICS has been incorporated into the CMIP7-class GFDL ESM4.5. GIMICS explicitly represents soil microbial dynamics and physicochemical stabilization mechanisms, as well as interactions among microbes, minerals, and vegetation, for both aboveground organic materials (leaf and coarse wood litter) and belowground, vertically resolved mineral soils (rhizosphere and bulk soil). GIMICS also accounts for the effects of spatially and vertically varying soil temperature and moisture on microbial processes and organic matter turnover, and represents redistribution and transport via bioturbation, diffusion, advection, and runoff to rivers. 

GIMICS has been integrated with the GFDL Land Model LM4.2, which includes dynamic vegetation and wildfire processes. The resulting LM4.2-GIMICS configuration is evaluated in a stand-alone mode against a widely used, observationally derived global soil inventory dataset (Harmonized World Soil Database, HWSD), reported global synthesis estimates, and other global model results. Results show improved simulations of global and regional soil carbon stocks relative to models that do not explicitly represent microbial processes and mineral-associated organic matter (MAOM) stabilization. Analyses of pool–specific carbon stocks and fluxes highlight the critical role of soil mineral–microbe–vegetation interactions in regulating terrestrial carbon persistence. 

How to cite: Lee, M., Shevliakova, E., and Malyshev, S.: Coupling vegetation dynamics, soil microbial processes, and physicochemical stabilization mechanisms within the new NOAA/GFDL Global Integrated Microbial Interactions with Carbon in Soil (GIMICS) model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13714, https://doi.org/10.5194/egusphere-egu26-13714, 2026.

The simulation of soil organic matter (SOM) dynamics, including SOM persistence, is a vital component of broader models representing vegetation dynamics or the impact of environmental change on the biosphere and climate. One of the biggest challenges in the application of SOM models is that their complexity is often not supported by sufficient data for parameter optimization. Inevitably, this leads to the calibration of more parameters than can be reliably optimised with available data, resulting in equifinality. This is the phenomenon that multiple parameter sets generate behavioural models: similarly well-performing models that cannot be ruled out.

This study assessed how equifinality affects the variability of predictions made by behavioural SOM models. We used a mechanistic, microbially-driven soil organic carbon and nitrogen model and evaluated it against an artificial data set. After the models were successfully calibrated and run into steady state, carbon inputs were doubled to evaluate how models with different mathematical formulations and different amounts of data used to optimize parameters reacted to this external forcing.

The key results are summarised as follows. (1) The accurate simulation of total SOM in steady state is an insufficient criterion to evaluate model performance. (2) The amount of calibration data determines how many model parameters can be jointly optimised without their values compensating for each other (i.e., identifiable parameters). And (3) the type of calibration data is equally important, as it dictates which pools can have their size and turnover rate constrained. For example, the size of particulate organic matter (POM) and mineral-associated organic matter (MAOM) can only be accurately simulated when data on these pool sizes are available. Similarly, the turnover rate of MAOM can only be reliably simulated if Δ¹⁴C data for MAOM are present. Our results emphasise the necessity of optimising only identifiable model parameters to avoid hidden uncertainty in model predictions. Adopting this approach consistently represents an important step forward to increase confidence in predictions made by SOM models.

How to cite: Van de Broek, M., Doetterl, S., and Six, J.: Equifinality and overparameterisation undermine confidence in predictions by soil organic matter models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14121, https://doi.org/10.5194/egusphere-egu26-14121, 2026.

Long-term bare fallow (LTBF) field experiments provide a unique framework to investigate soil organic carbon (SOC) persistence in the absence of fresh organic inputs. The Versailles 42-plots LTBF, established in 1928, is the oldest continuously managed bare fallow worldwide. Initially designed to assess the effects of fertilisers and amendments on loess-derived Luvisols, it offers a rare opportunity to quantify and isolate a centennially stable SOC pool. In 2008, total SOC in the reference plots was shown to equate estimated centennially stable SOC pool (Barré et al. 2010).

Here, we used repeated soil sampling to assess (i) whether the SOC content have stabilised over time and (ii) how long-term fertilisation and amendment practices affect the size of this pool. Treatments included mineral N fertilisers (ammonium, nitrate), basic amendments (lime, basic slag), mineral amendments containing P or K, an organic fertiliser (horse manure), and no-input reference plots. Topsoil (0–25 cm) was sampled in all 42 plots in 2008, 2014, 2017, 2021, and 2025 complemented by archived samples from 1929, 1949 and 1962.

In all plots except those receiving annual manure inputs, SOC contents have stabilised over recent decades, with no significant variation between 2008 and 2025. These steady values show that a centennially stable SOC pool has been reached but with different pool sizes across treatments. SOC contents were higher in plots receiving mineral N fertilisers or basic amendments than in no-input controls, whereas plots amended with monovalent cations (e.g. Na⁺, K⁺) or phosphates exhibited lower SOC levels.

These patterns suggest that long-term soil chemical and physical conditions, shaped by fertilisation and amendment regimes, influenced stabilisation processes and ultimately, SOC persistence. We suggest that low pHs (<5) resulting from with mineral N fertilisation may favour SOC stabilisation, while enhanced physical protection is promoted in lime- and carbonate-amended plots. Conversely, poor soil structure in monovalent cations amended plots may explain less SOC persistence.

These results underscore the high scientific value of long-term experiments, which need to be maintained and valorised, for understanding SOC dynamics and stabilisation.

How to cite: Delahaie, A., Plessis, C., Girardin, C., and Chenu, C.: Long-term bare fallows reveal centennially stable soil carbon pools under contrasting fertiliser and amendment regimes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16960, https://doi.org/10.5194/egusphere-egu26-16960, 2026.

Persistence of soil organic carbon (SOC) is largely controlled by interactions between organic matter and mineral surfaces, particularly iron oxides. These interactions are further influenced by the presence of competing species and various cations in soil systems. Despite their importance, the geochemical mechanisms by which competing species regulate organic matter and mineral interactions remain poorly understood, representing a critical knowledge gap in SOC stabilization processes. DNA is a ubiquitous biomolecule in soils and sediments. It is a key component of microbial necromass and extracellular polymeric substances. While DNA represents a small fraction of SOC, the mechanisms of DNA and mineral interaction can reveal broader principles applicable to other organic matter to elucidate mechanisms that contribute towards SOC formation and stabilization.

In this study, we used DNA of varying lengths representing organic matter fractions of different sizes, and phosphate as a model competing anion, to investigate the effects of phosphate on DNA adsorption to goethite. Batch adsorption experiments were complemented by enzymatic hydrolysis studies to assess the influence of phosphate and divalent cations (Ca²⁺ and Mg²⁺) on the degradation of DNA adsorbed on goethite. Our results show that DNA adsorption to goethite is strongly influenced by DNA length, phosphate concentration, and adsorption time. Phosphate significantly reduced DNA adsorption through competitive surface site occupation. Although the presence of Ca²⁺ and Mg²⁺ enhanced DNA adsorption under phosphate concentrations that were otherwise unfavorable for adsorption, this increased adsorption did not translate into protection against enzymatic hydrolysis.

These findings demonstrate that enhanced adsorption alone does not necessarily confer long-term protection of organic matter and highlight the complex roles of competing ions and cations in regulating mineral-associated SOC persistence. Our study provides mechanistic insights into how nutrient and cation availability may influence the stabilization and turnover of reactive, phosphorus-containing organic matter in soils.

How to cite: Brijit, A. J., Klier, P., Singh, V. V., Kumar, N., Kimber, R., Berthelemy, P., Rose, J., and Kraemer, S. M.: Controls on mineral-associated organic matter stability: Insights from DNA adsorption and degradation on goethite in the presence of phosphate and cations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17326, https://doi.org/10.5194/egusphere-egu26-17326, 2026.

How to cite: Kingsland-Mengi, A., de Boer, E., Kowalchuk, G., Barel, J., and Barry, K.: Linking Root Traits and Soil Organic Matter Pools in a Grassland Diversity Experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18950, https://doi.org/10.5194/egusphere-egu26-18950, 2026.

Mineral-associated organic matter (OM) exhibits a heterogeneous arrangement in soils at the microscale and nanoscale as revealed by high-resolution imaging techniques. The arrangement of OM at the microscale has broad implications for biogeochemical cycles of major elements such as C and N by compartmentalizing their dynamics into distinct micropatches and a few µm-sized hotspots. It is crucial to understand the organization of this heterogeneous microscale arrangement across diverse soil systems. Here, we present a meta-analysis of spatial patterns of OM patches based on unsupervised segmentation of nanoscale secondary ion mass spectrometry (NanoSIMS) measurements. Using a dataset of over 450 measurements of fine fractions from soils with different texture and C content, we evaluated the spatial coverage, clustering, and heterogeneity of OM micropatches across mineral surfaces. The OM coverage across mineral surfaces linearly correlated with the bulk soil C content, indicating a spatially expanding arrangement of OM whereas large parts of mineral-dominated surface remain. Higher OM coverage was related to more connected and more clustered OM patches. Within the OM patches, we found evidence of recurring µm-sized distinct C-rich and N-rich subunits based on a fractal geography approach. Within more homogeneous OM patches, subunits showed stronger differentiation in C and N composition, whereas subunits within more heterogeneous patches exhibited less differentiated C and N composition.  The distinct spatial organization of OM micropatches observed here suggests a compartmentalized framework of OM dynamics with implications for C and N cycling in soils.

How to cite: Schweizer, S. A., Hu, Y., Zollner, J. M., Inagaki, T., Höschen, C., and Werner, M.: Spatial pattern analysis of soil organic matter micropatches show distinct µm-sized C-rich and N-rich subunits, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20930, https://doi.org/10.5194/egusphere-egu26-20930, 2026.

Forest soils play a key role in global carbon (C) storage. Important for long-term C storage in soil is the formation of mineral-associated organic matter (MAOM), which is protected through chemical bonds and occlusion from decomposition. Recent research demonstrated shifts in forest C cycles depending on tree diversity and mycorrhizal type, but we still lack mechanistic knowledge about the role of tree diversity and associated mycorrhizal symbiosis in soil C stabilization.

This study aims to quantify particulate organic matter (POM) and MAOM in relation to tree diversity and mycorrhizal type, i.e., ectomycorrhiza (ECM) and arbuscular mycorrhiza (AM) and to investigate the contributions of plant and microbial residues to POM and MAOM. It was conducted in the MyDiv tree experiment (Bad Lauchstädt, Germany), established in 2015 with 10 different deciduous tree species planted across a gradient of species richness. After density separation of POM and MAOM fractions, their mass, carbon and nitrogen (N) contents as well as ¹³C and ¹⁵N isotopic composition were determined. Soil samples were also taken from the grassland next to the site as background values. In addition, we measured ¹³C and ¹⁵N natural abundances of leaf litter, fine roots, saprotrophic and ectomycorrhizal sporocarps as well as ectomycorrhizal and soil hyphae.

There was no significant difference in the amount and C content of POM and MAOM between treatments. However, there was a tendency towards more POM (and POM-C) in diverse - especially ECM - systems. Compared to the grassland, C and N contents in POM were lower, which may result from reduced litter input after planting trees (soil was covered with a weed tarp until canopy closure). In line with this, POM of forest soil samples was enriched in ¹³C and ¹⁵N compared to grassland POM, suggesting a higher share of microbial residues in forest POM due to litter exclusion. Interestingly, also C content in MAOM declined due to litter exclusion, whereas ¹³C in MAOM seemed unaffected by the transition from grassland to forest.

The higher C/N ratio of POM compared to MAOM aligned with the expected greater contribution of plant residues to POM and microbial residues to MAOM. This is in accordance with a higher ¹³C-enrichment of MAOM compared to POM, especially in diverse systems and irrespective of the mycorrhizal type.

All in all, the analysis of natural stable isotope abundances is a powerful tool to elucidate the composition of POM and MAOM in temperate forests, which may change depending on forest age, mycorrhizal type and tree diversity.

How to cite: Marburger, C., Kochniss, L., Meier, I., Mohamed, A., and Pausch, J.: How tree species diversity and mycorrhizal association type influence contributions of plant and microbial residues to soil organic matter fractions in temperate forests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21021, https://doi.org/10.5194/egusphere-egu26-21021, 2026.

Soil organic matter persistence has been recognized as an ecosystem property emerging from environmental and biological controls rather than intrinsic molecular recalcitrance. Yet a theoretical framework operationalizing this insight into predictive equations remains elusive. Here we present a coupled transport-reaction model where persistence emerges from the dynamic interplay of water, heat, and oxygen transport with kinetically-controlled mineral associations. The framework explicitly couples: (1) environmental state variables (θ, T, O₂) that modulate all reaction rates, (2) transformation kinetics from particulate to dissolved to reactive intermediates, (3) two-stage mineral association with direction-dependent sorption and desorption rates (αs > αd) following Langmuir-Freundlich kinetics, and (4) diffusive and advective transport controlling substrate accessibility. Analytical steady-state solutions in dimensionless form reveal fundamental parameter groupings—including a combined affinity parameter β that governs saturation behavior.

The model predicts distinct persistence regimes: environmental (decomposition suppressed by moisture, temperature, or oxygen limitation), kinetic (asymmetric mineral association creates hysteresis and path-dependence), and transport-limited (micro-site isolation restricts accessibility). These regimes explain why particulate organic matter can persist for centuries under certain conditions while mineral-associated carbon exhibits dynamic exchange in others. The framework also resolves apparent contradictions between saturation theory and field observations—total soil carbon can increase linearly while mineral-associated efficiency declines. Validation against long-term field experiments demonstrates predictive capability across contrasting sites and input regimes. We show that persistence is not a property to be measured but an outcome to be predicted from coupled dynamics—providing a quantitative foundation for the paradigm shift from intrinsic to emergent controls on soil carbon.

How to cite: Ghezzehei, T. and Berhe, A. A.: Kinetic and Transport Controls on Soil Organic Matter Persistence: A Coupled Framework for an Emergent Ecosystem Property, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15388, https://doi.org/10.5194/egusphere-egu26-15388, 2026.

Fen peatlands are essential ecosystems that support high biodiversity, buffer hydrological extremes such as droughts and flooding, sequester carbon (C), and contribute to human well-being. However, increasing climate anomalies and anthropogenic disturbances are accelerating peat degradation, potentially triggering abrupt shifts in peat integrity and function – with significant implications for the global C cycle. Our study investigated the rapid peat subsidence observed in Belgium’s oldest nature reserve ‘De Zegge’, which represents an unheard form of fen ecosystems deterioration, and an environmental alarm. Thus, this case study may provide insights for land managers and researchers working in similar peat systems worldwide. To determine how hydrological stress, coupled with chronic hydro-chemical pressures − may push the system beyond a critical threshold, and lead to peat collapse,  we: (1) estimated the loses in elevation and C stocks using field-based digital elevation models, (2) compared peat characteristics between collapsed, adjacent non-collapsed and distant non-collapsed areas, and (3) experimentally assessed the effects of potential collapse triggers,- hydrological alterations and hydro-chemical additions (control, ditchwater, and sulfate [SO­₄^2-])- as on peat stability using a mesocosm experiment by measuring greenhouse gases (GHGs) emissions and porewater chemistry as indicators. Our findings demonstrate that fen peatland collapse led to significant lowering of the surface level (-12.8 ± 2.3 cm) accompanied by significant carbon losses (-21.6 ± 6.1 kg−C m^-2), alongside structural and functional shifts across biological, vegetative, physiochemical, and molecular dimensions (P < 0.05). In the mesocosm experiment, hydrologically perturbed peat exhibited reduced stability compared to undisturbed monoliths, particularly in regulating GHGs fluxes. During the successional phase, SO­₄^2- emerged as a key stressor, exerting pressure on system-wide stability. SO­₄^2- intrusion indirectly increased N₂O emissions during the re-saturation, with high spatial variability. Collectively, our study provides new insights into long-term pedological shifts affecting peat integrity and function in fen ecosystems.

How to cite: Kim, K., Emsens, W.-J., Ottoy, S., Schellekens, J., Deswert, T., Desie, E., Muys, B., J. I. Briones, M., Jansen, B., and Vancampenhout, K.: Peat Collapse as a new threat to fen hydrological stability: the case of Nature reserve De Zegge (Belgium), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20252, https://doi.org/10.5194/egusphere-egu26-20252, 2026.

Understanding the persistence, stability and turnover time of soil organic carbon (SOC) is essential for predicting terrestrial carbon storage, ecosystem responses to climate change, and strategies to enhance sequestration. However, linking SOC stability to specific soil properties remains challenging, with global models often underestimating SOC residence times compared to empirical observations (e.g., Shi et al., 2020). Radiocarbon (¹⁴C) provides a powerful tool for addressing these gaps by adding a temporal dimension to SOC studies.

Conventional approaches typically use physical or chemical fractionation to create operational pools for ¹⁴C analysis (e.g., Haddix et al., 2020), such as mineral-associated organic matter (MAOM). While informative, these pools themselves represent mixtures of diverse chemical components. An emerging approach is that of thermal-based methods such as ramped oxidation (ROx). These offer an alternative by partitioning SOC according to activation energy (e.g., Hanke et al., 2023), providing valuable insights into mechanisms of SOC stabilization (Stoner et al., 2023).

We applied ROx combined with ¹⁴C analysis to samples from a study that examined how converting temperate grassland to coniferous forest influences below-ground carbon dynamics in Scotland (Joly et al., 2025). As part of this work, we also quantified pyrogenic carbon (PyC), a fire-derived SOC fraction known for its long environmental residence times, in bulk soils and sub-fractions to assess its contribution to SOC persistence. By comparing ¹⁴C signatures from conventional fractionation with those from thermal fractions, including PyC, we evaluate the added value of ROx in revealing SOC age structure and persistence. These insights advance understanding of SOC stabilization processes and inform predictions of land-use change impacts on soil carbon storage.

Acknowledgments

We acknowledge support from the UK Natural Environment Research Council (NERC) via the National Environmental Isotope Facility (NEIF) grant (NE/S011587/1) and the NERC project grant NE/P011098/1.

References

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Hanke UM, Gagnon AR, Reddy CM, Lardie Gaylord MC, Cruz AJ, Galy V, Hansman RL, Kurz MD. 2023. . Radiocarbon 65(2): 389-409. 10.1017/RDC.2023.13

Joly F, Cotrufo MF, Garnett MH, Johnson D, Lavallee JM, Mueller CW, Perks MP & Subke J. 2025. . Journal of Environmental Management, 374, Art. No.: 124149.

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How to cite: Ascough, P., Garnett, M., Subke, J.-A., Street, L., Joly, F.-X., Harman, N., and Murdoch, I.: Evaluating Soil Carbon Persistence Using Ramped Oxidation and Radiocarbon Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11600, https://doi.org/10.5194/egusphere-egu26-11600, 2026.

Mediterranean mountain regions provide essential ecosystem services and resources to surrounding urban areas. Since the mid-20th century, however, rural depopulation has led to widespread land abandonment, triggering natural revegetation and consequent shrub encroachment. These changes have been associated with increased wildfire risk, reduced agro-pastoral resources, and altered hydrological functioning. Soil, a non-renewable resource on human timescales, plays a key role in ecosystem resilience due to its influence on biogeochemical cycles and carbon accumulation, making it a potential tool in Climate Change adaptation strategies.

This study, grounded in the MANMOUNT project (PID2019-105983RB-I00) investigates post-abandonment land management strategies in marginal Mediterranean mountain areas, focusing on their effects on soil quality and soil organic carbon (SOC) storage. The Leza Valley (Iberian System, Spain) was selected as the study area due to its representativeness of the historical and ecological context of Mediterranean mountains. Three management strategies were evaluated: passive management through secondary succession; forest management through conifer afforestation and dehesa systems; and shrub clearing to establish pastures for extensive grazing. Analyses considered soil environment (acid or alkaline), soil depth (0–40 cm), time since management implementation, and the presence of active management practices (e.g., thinning, selective cutting).

A total of 453 soil composite samples were collected and analysed to assess physicochemical properties and SOC stocks. Carbon stabilization mechanisms were examined using aggregate stability tests and fractionation techniques. Future SOC dynamics were projected under different Climate Change scenarios using the CarboSOIL predictive model, while integrated soil–water balance was evaluated with the RHESSys model.

Results revealed significant effects of post-abandonment management on soil quality and SOC storage. All management strategies increased carbon stocks compared to the initial unmanaged shrubland. Forest systems accumulated higher total SOC, whereas pasture systems promoted greater mineral-associated carbon, indicating enhanced long-term stability. Soil environment was a major driver of SOC responses, with marked differences between acid and alkaline soils. Model projections highlight the importance of sustained active management to maintain soil functions in abandoned areas under future climatic conditions.

These findings provide valuable insights for land managers and policymakers, underlining the potential of post-abandonment management in marginal Mediterranean mountain landscapes as an important tool for Climate Change adaptation. The promotion of mosaic landscapes is proposed as an effective strategy to achieve both ecological resilience and socio-economic sustainability.

Acknowledgement: This research project was supported by the MANMOUNT (PID2019-105983RB-100/AEI/ 10.13039/501100011033) project funded by the MICINN-FEDER, and the SOLPYR (POCTEFA 2021-2027 (EFA045/01)) project funded by Interreg Poctefa and European Union.

Keywords: Mediterranean mountains; carbon sequestration; natural revegetation; extensive grazing; forest practices.

How to cite: Cortijos-López, M., Lasanta, T., Zabalza-Martínez, J., Muñoz-Rojas, M., Cammeraat, E., Sánchez-Navarrete, P., and Nadal-Romero, E.: Management of marginal landscapes in the Mediterranean mountains: Driving soil regulatory functions towards Global Change adaptation., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17958, https://doi.org/10.5194/egusphere-egu26-17958, 2026.

Although conservation tillage practices generally do not enhance soil organic carbon sequestration in temperate arable soils, they substantially modify the vertical distribution and composition of soil organic matter compared to conventional tillage. Yet, the implications of these tillage-induced alterations for soil organic matter stability remain poorly understood. Gaining a deeper understanding of soil organic matter stability is essential in the context of global warming and increasingly variable rainfall patterns.

Here, we investigated depth-dependent patterns of soil respiration and its temperature sensitivity under conventional tillage (CT) and reduced tillage (RT) as well as under current (100%) and reduced rainfall (50%) in a temperate cropland. Soils were sampled in January 2025 from a long-term tillage experiment established in 1970 in central Germany on a Haplic Luvisol under a humid temperate climate (mean annual air temperature 9.6 ± 0.7°C; mean annual precipitation ~610 ± 120 mm). The field trial follows a randomized block design with 16 plots, including eight managed under CT with mouldboard ploughing to 27–30 cm and eight under RT with rotary harrowing to 7–10 cm. Rainout shelters, designed to remove 50% of rainfall, were installed in 2022 in half of the plots. Soils for the incubation experiment were collected from four replicate plots per tillage  treatment and under 100% rainfall at four depths (i.e., 0–10, 10–20, 20–30, and 30–60 cm). Additional samples from rainfall-exclusion plots were collected in August 2025 and are currently being analysed.

Laboratory incubations were performed under controlled conditions using an automated Respicond system. Soils were sieved, adjusted to 60% water holding capacity, and incubated under stepwise temperature changes from 5 to 25°C, followed by a cooling phase back to 5°C, with temperature sensitivity analyses primarily based on the second incubation phase.

Preliminary results under normal rainfall conditions showed that respiration rates were highest in surface soils and declined with depth, while the deepest layer (i.e., 30–60 cm) showed comparatively low and less temperature-responsive respiration. Depth patterns differed between tillage systems: reduced tillage enhanced respiration in the topsoil, whereas conventional tillage showed higher respiration at intermediate depths (10–30 cm), reflecting contrasting vertical distributions of SOC and organic matter fractions. No significant tillage effects on respiration were observed below the plough layer.

Across both tillage practices, temperature sensitivity declined significantly with soil depth, indicating weaker relative temperature responses in subsoils. Mean temperature sensitivity values did not differ between CT and RT when averaged across depths. Normalisation to organic matter fractions showed lower temperature sensitivity of MAOM-associated respiration compared to POM-associated respiration, consistent with differences in substrate quality and energetic constraints on decomposition.

Overall, tillage primarily redistributed organic matter within the topsoil, while subsoil carbon exhibited lower temperature sensitivity, suggesting reduced responsiveness to short-term warming with important implications for modelling soil carbon–climate feedbacks. Ongoing analyses will assess how two years of reduced rainfall modify the vertical distribution and temperature sensitivity of soil organic matter.

How to cite: Iddris, N. A.-A., Apostolakis, A., Hanczaryk, J. S., Tyystjärvi, V., Schimmel, H., Bauke, S., and Meijide, A.: Depth-dependent temperature sensitivity of heterotrophic soil respiration under long-term tillage and reduced rainfall, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19558, https://doi.org/10.5194/egusphere-egu26-19558, 2026.

Organic nitrogen (ON) represents the majority of nitrogen compounds found in soil systems, although often overlooked in favour of the minority of more bioaccessible inorganic nitrogen compounds. One of the main pools of soil ON is mineral-associated organic matter (MAOM), in these complexes organic compounds chemically and physically interact with soil mineral surfaces. Understanding ON cycling in soils is paramount to ensuring sustainable management of soil resources and predicting pollutant nitrogen outgassing.  A key aspect of this, is understanding ON vulnerability to microbial breakdown. Previous work has shown how organic matter in MAOM complexes can persist in soils for centuries, protected from microbial degradation. However, more recent research has highlighted that organic matter can be mobilised from soil mineral surfaces and become accessible to microbes once more. MAOM complexes therefore can represent a far more dynamic pool of nitrogen than previously thought and the drivers of their formation need more robust characterisation. Our research focused on abiotic soil properties as potential drivers of ON adsorption to soil mineral surfaces, as well as the influence of time and the identity (chemistry and size) of investigated nitrogen compounds. Soil mineralogy data, from XRF analysis, as well as soil texture and pH data were coupled with adsorption assays and mixed effects modelling. This revealed the dominant influence of adsorbate identity on total adsorption and rate of adsorption to soil mineral surfaces. The presence of aluminium and iron, both common in soils and reactive with organic matter, and soil pH also had significant influences on nitrogen compound adsorption. These findings add to the growing body of literature on the drivers of MAOM complex formation, support the use of newer adsorption potential proxies, and highlight the importance of considering organic matter identity when predicting nitrogen recalcitrance in soils.  

How to cite: Vaughan, A., Purchase, M., and Mushinski, R.: MAOM Formation: Abiotic Drivers of Soil Organic Nitrogen Adsorption to Soil Mineral Surfaces, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7560, https://doi.org/10.5194/egusphere-egu26-7560, 2026.

High organic carbon (OC) in agricultural sandy soils (>85% sand) is generally unexpected under prevailing paradigms of organic matter (OM) formation and stabilisation, which emphasise mineral protection. Nevertheless, so-called “black sand” soils are widespread in north-western Europe and store exceptionally large amounts of OC. Much of this OC resides in the finest soil fractions and is chemically enriched in alkyl C, likely reflecting historic heathland vegetation, but it remains unclear whether this enrichment is associated with minerals or occurs within largely organic microstructures. To address this, we combined particle-size fractionation, solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, optical photothermal infrared (O-PTIR) microscopy, and nanoscale secondary ion mass spectrometry (NanoSIMS) to investigate OM organisation and composition at the microscale. Fine fractions stored a disproportionate amount of OC, showing pronounced alkyl enrichment and a decline in O/N-alkyl C. NanoSIMS revealed that OM occurred as finely divided domains rather than intact particles. OM-dominated areas expanded with increasing OC, and while some OM was associated with Al-rich domains, much showed no clear mineral association, indicating that OM accumulation is not solely driven by organo-mineral interactions. O-PTIR revealed alkyl-rich microdomains reflecting the chemical signature of historic vegetation. Aliphatic signals increased with OC content and OM coverage, indicating that SOC in black sand soils is stored as micrometre-scale, alkyl-rich micro-particulate OM. This challenges mineral-centric concepts of SOC stabilisation and highlights the importance of OM–OM interactions and land-use legacy in controlling carbon storage in sandy soils.

How to cite: Colocho Hurtarte, L. C., Sandt, C., Höschen, C., Kögel-Knabner, I., Lugato, E., Urbanski, L., and Schweizer, S. A.: High organic matter accrual in black sand soils linked to microscale aliphatic hotspots, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20000, https://doi.org/10.5194/egusphere-egu26-20000, 2026.

Molecular diversity has been proposed as a critical factor controlling soil organic matter (SOM) persistence. However, in contrast to dissolved organic matter, molecular diversity of solid phase organic matter remains largely unexplored. Here, we show the molecular diversity features of solid phase organic matter through a direct mass spectrometric scan of particulate and mineral associated organic matter (POM and MAOM) that show a strong relationship with carbon turnover rates, collected from a long-term grassland recovery experiment after 0, 23, and 43 years. We found that the highest molecular diversity (Hill Number = 1603±124) existed in SOM that had the slowest carbon turnover rate. Molecular diversity of MAOM exhibits greater correlation (R² = 0.95, p < 0.001) with SOM persistence than that of POM. Molecular diversity became increasingly enriched from top to subsoil horizons (360% increase), consistent with a breaking down of large molecules into a range of low- to high-molecular-weight molecules. Soil depth and total iron content were main factors impacting the diversity change of POM and MAOM, highlighting the combined control of microbial decomposition and mineral interaction in shaping molecular features of solid phase organic matter. Together, these results suggest that molecular diversity may not operate as a limiting factor for carbon utilization by decomposers but as an ecosystem property that incorporates organo-mineral interactions.

How to cite: Yang, Z. and Sun, T.: Solid Phase Molecular Diversity Enhances Soil Organic Matter Persistence, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21126, https://doi.org/10.5194/egusphere-egu26-21126, 2026.