This session is dedicated to studies exploring the dynamic interactions, underlying mechanisms, and implications of organo-mineral interactions at multiple scales, as well as their spatial and temporal heterogeneity within the soil system. It includes research on SOM formation pathways (e.g., plant-, rhizosphere-, microbial-, and pyrogenic-derived), its storage in aggregates, and its association with mineral surfaces, as well as their responses to management practices and global change drivers. Furthermore, studies on nutrient and contaminant behavior, greenhouse gas fluxes, carbon storage, mineral transformations, and related processes—using laboratory, field, modeling, or innovative methodological approaches that enhance our understanding of soils and sediments in biogeochemical cycles—are part of this session. We aim to improve our mechanistic understanding of SOM dynamics and discuss new insights and approaches.
Orals: Wed, 6 May, 14:00–18:00 | Room 0.11/12
Soil organic carbon (SOC) is a key regulator of soil functioning and ecosystem services, fundamental to nutrient cycling, soil structure, and long-term carbon sequestration1. Within this pool, mineral associated organic matter (MAOM) represents the most persistent fraction, formed through interactions between microbially processed organic inputs and reactive mineral surfaces. We aim to advance the understanding of soil carbon dynamics across scales, from laboratory scale MAOM formation to ecosystem level patterns in managed agricultural soils. Specifically, we seek to identify which characteristics of the clay fraction, including clay minerals and metal oxides, govern MAOM formation under contrasting management regimes. To achieve these objectives, at the laboratory scale, we investigated interactions between organic matter inputs and mineral substrates in Loess soil using a controlled incubation experiment with defined mineral filled mesh bags, comparing straw amended and unamended soils. Following incubation, MAOM was characterized using nanoscale analysis, including thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and elemental analysis. At the field scale, we assessed long-term agricultural systems under conventional and conservation management, incorporating organic inputs, service crops, and quantified SOC partitioning across Vertisol soil fractions and depths. Despite differences in scale and soil, consistent trends emerged, revealing enhanced association of organic carbon with mineral fractions in response to organic inputs1. In the laboratory incubation, carbon accumulation was highest on goethite, lower on montmorillonite, and negligible on quartz, despite montmorillonite’s higher surface area. This pattern was further supported by greater mass loss during thermal digestion, indicating mineral specific enhancement of MAOM formation2. At the field scale, conservation agriculture, which enhances organic inputs, showed a greater proportion of SOC associated with mineral fractions relative to particulate pools. While the clay sized fraction dominated MAOM storage, a measurable fraction of MAOM was also detected in the sand sized fraction, indicating additional carbon stabilization pathways under long-term management3. Linking these scales allows laboratory derived mechanisms to be interpreted in field conditions and, conversely, using field-scale patterns to refine mechanistic understanding of MAOM formation.
References
1Mayer, M. et al. Dynamic stability of mineral-associated organic matter: enhanced stability and turnover through organic fertilization in a temperate agricultural topsoil. Soil Biol. Biochem. 184, 109095 (2023).
2Kirsten, M. et al. Iron oxides and aluminous clays selectively control soil carbon storage and stability in the humid tropics. Sci. Rep. 11, 5076 (2021).
3Li, Y. et al. Conservation tillage facilitates the accumulation of soil organic carbon fractions by affecting the microbial community in an eolian sandy soil. Front. Microbiol. 15, (2024).
How to cite: Mishael, Y., Kaner, N., and Siegel, I.: Linking MAOM Formation from Lab to Field Scale - Conventional vs. Conservation Agricultural Management , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22470, https://doi.org/10.5194/egusphere-egu26-22470, 2026.
Mineral-organic associations are central to soil carbon persistence and nutrient cycling, yet their vulnerability to climate change remains uncertain. In particular, the mechanisms underlying their formation and disruption in the rhizosphere are insufficiently resolved. Here we examined how anticipated shifts in precipitation influence the disruption and (neo)formation of mineral-organic associations in the wheat rhizosphere, and how mineral crystallinity influences this processes. To investigate this, we conducted a 12‑week pot experiment with winter wheat (Triticum aestivum L.), using agricultural soil amended with mineral-organic associations formed by 13C‑labeled microbial necromass adsorbed onto 57Fe‑labeled iron oxides of contrasting crystallinity (ferrihydrite vs. goethite). Plants were exposed to three precipitation regimes reflecting projected Central European climate patterns: optimal irrigation compared to intermittent droughts or floodings. Precipitation exerted notable and opposing effects on necromass-ferrihydrite associations: disruption (and subsequent 13C mineralization) was reduced under intermittent drought (0.8x), but intensified under intermittent flooding (1.4x) compared with optimal precipitation. Necromass-goethite associations, by contrast, were largely stable across precipitation regimes. 57Fe Mössbauer spectroscopy and nanoSIMS imaging revealed Fe mineral transformations, especially under intermittent flooding conditions, and preliminary nanoSIMS data indicate rapid (neo)formation of mineral-organic associations. Together, these findings show that root-driven transformations of mineral-organic associations, particularly those comprised of poorly crystalline mineral phases, are sensitive to changing precipitation patterns, suggesting enhanced vulnerability of this carbon pool under future climate scenarios.
How to cite: Jamoteau, F., Frei, J., Van-der-Loo, E., Muehe, E. M., Thompson, A., ThomasArrigo, L., Carlos Colocho Hurtarte, L., Kocsis, L., Spangenberg, J., and Keiluweit, M.: Simulated climate extremes alter dynamics of mineral-organic associations in the rhizosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9420, https://doi.org/10.5194/egusphere-egu26-9420, 2026.
Mineral associated organic matter significantly contributes to durable carbon sequestration in soils. Besides organic compounds adsorbed on mineral surfaces, organo-mineral associations in the form of coprecipitates are increasingly cited in the literature as a key mechanism of interaction between organic matter and soil minerals. Coprecipitates thus emerge as essential soil bodies in controlling carbon dynamics in soils. Their transitional structural/nature as well as their precise formation and maturation/transformation mechanisms remain poorly understood.
Over the past 10 years, we have aimed to: (1) model the molecular structure of these coprecipitates (Basile-Doelsch, et al., 2015; Tamrat, et al., 2018; Tamrat, et al., 2019) (2) observe and characterize them down to the nanoscale (Jamoteau, et al., 2023; Jamoteau, et al., 2025a), (3) test their stability against mineralization by microorganisms (Jamoteau et al., 2025b), and (4) explain their formation mechanisms in soils in relation to microbial activity. Our work relied on both synthetic samples (laboratory coprecipitates) and natural andosols, and we employed complementary approaches combining physicochemistry, microfluidic systems, respirometry, electron microscopy (SEM, TEM), and spectroscopy (EXAFS, STXM, EDX, EELS) to characterize the nature, the structure and the dynamics of coprecipitates.
This contribution will provide a synthesis of the studies conducted on these organo-mineral structures called "nanoCLICs" for nanosized coprecipitates of inorganic oligomers with organics (Tamrat, et al., 2019). We demonstrate that nanoCLICs could represent, within soil constituents, an ultimate boundary between biotic and abiotic components. NanoCLICs could thus play major roles in the biogeochemical dynamics that control soil functioning.
Basile-Doelsch, I. et al. Environ. Sci. Technol. 49, 3997-3998, doi:https://doi.org/10.1021/acs.est.5b00650 (2015).
Jamoteau, F. et al. Environmental Science & Technology 57 (49), 20615-20626, DOI: 10.1021/acs.est.3c06557 (2023)
Jamoteau, F. et al. Environ. Sci. Technol. 57, 20615-20626, doi:10.1021/acs.est.3c06557 (2025a)
Jamoteau, F. et al. SOIL, 11, 535–552, 2025, doi.org/10.5194/soil-11-535-2025 (2025b)
Tamrat, W. Z. et al. Geochimica et Cosmochimica Acta 229, 53-64, doi:https://doi.org/10.1016/j.gca.2018.03.012 (2018).
Tamrat, W. Z. et al. Geochimica et Cosmochimica Acta 260, 15-28, doi:https://doi.org/10.1016/j.gca.2019.05.043 (2019).
How to cite: Basile-Doelsch, I., Jamoteau, F., Cam, N., Doelsch, E., Girard, T., Rose, J., Campos, A., Vidal, V., Pailles, C., Levard, C., Gassier, G., Thuries, L., Borschneck, D., Duvivier, A., and Chaurrand, P.: What are the organo-mineral associations called 'nanoCLICs'?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9164, https://doi.org/10.5194/egusphere-egu26-9164, 2026.
The stability of soil organic carbon (SOC) relies on its association with reactive minerals. However, the co-evolutionary feedback between mineral transformation and SOC persistence remains a key uncertainty. A typical example is that while iron oxides stabilize SOC, their inevitable transformation into more crystalline phases is often assumed to weaken SOC protection. Here, we examine how polygalacturonic acid (PGA), a typical SOC, actively modulates ferrihydrite transformation, and its feedback on carbon persistence, under varying PGA loading (C/Fe ratios) and Fe(II)-induced redox conditions. Our goals are to test whether SOC stability decreases as typically assumed and to reveal the underlying mechanisms of this mineral-organic interplay.
We find that goethite, the transformation product of ferrihydrite, does exhibit lower carbon protection capability due to its reduced surface area. Surprisingly, in ferrihydrite-PGA complexes, carbon protection is maintained across carbon loadings through two distinct pathways: at high carbon loading, PGA suppresses ferrihydrite transformation, forming a stable organo-mineral association; at low carbon loading, transformation proceeds but protection is sustained by the residual surface area of the evolving mineral. Moreover, we identify an important negative feedback: the degradation product, galacturonic acid (GA), more strongly inhibits mineral transformation than PGA itself, suggesting that partial degradation actively reinforces the stability of the remaining carbon.
Our results demonstrate that carbon saturation governs two functional pathways: a direct, static organo-mineral stabilization pathway under high saturation, and a resilient, dynamic stabilization pathway under low saturation. Critically, this study reveals that SOC and its degradation products can actively regulate mineral transformation, thereby influencing their own long-term persistence. This microscopic feedback illustrates a self-regulating capacity in soil systems, suggesting that such intrinsic negative feedbacks may enhance soil carbon resilience under a dynamic environment.
How to cite: Wu, J., Pjevac, P., and Kraemer, S. M.: Coupled Dynamics of Mineral Transformation and Mineral-Associated Organic Matter (MAOM) Degradation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21317, https://doi.org/10.5194/egusphere-egu26-21317, 2026.
At the surface, most soil organic C starts in plant roots, with belowground inputs five times more likely to persist than aboveground plant biomass. Microbes swarm these roots, driving blooms of activity and predation and releasing necromass, enzymes, and protein- and polysaccharide-rich extracellular polymeric substances (EPS) that aggregate soil and lock in C. But in deeper soil horizons, C is acted upon via distinct metabolic pathways. Indeed, deep soils are the cradle of soil formation--at the bedrock-soil interface (“regolith”), mineral weathering changes soil pH and releases nutrients and secondary mineral forming ions (Si, Al, Fe), impacting soil structure, microbial composition and activity, and the turnover time of soil organic matter. While deep soil horizons (>30 cm) are often considered biologically quiescent, deep soil C is highly sensitive to environmental change and comprises the majority of the global soil C pool. Our deep soils research has identified many active microorganisms at depth: “dark autotrophs” with genes for non-photosynthetic CO2 fixation, archaeal ammonia oxidizers, symbiotrophic fungi, and evidence of mineral weathering that forges secondary minerals, setting the stage for long-lasting mineral-associated organic matter (MAOM). We have also tested the durability of deep soil carbon. Roots of hardy perennial grasses that penetrate down to a meter (or more) introduce a net flux of radiocarbon-young recently fixed carbohydrates, but these new resources do not seem to accelerate decomposition (priming), and the added carbon is often short-lived. Persistence of deep root carbon does not appear to correlate with common soil properties and environmental factors (silt+clay, cation exchange capacity, pH, precipitation, temperature). However, certain soil organic matter components do have markedly distinct turnover times. When we used compound specific 14C analyses along a soil depth profile, we found significant differences in the 14C age of distinct organic molecules (acid insoluble>bulk soil carbon> chloroform-extracted microbial biomass, total lipids and amino acids>phospholipids and respired CO2). These findings suggest that in deep soils, organic matter consumers appear to preferentially use young C transported from the surface (perhaps as roots or dissolved organic carbon) rather than recycling older soil organic carbon. These data also suggest that molecular structure may play a role in soil organic matter stability in the oligotrophic habitat of deep soils.
How to cite: Pett-Ridge, J., Slessarev, E., Nuccio, E., Min, K., Kuhn, A., Banfield, J., Wood, T., Grant, K., Finstad, K., Morrison, K., and Mcfarlane, K.: Deep Soil Carbon: Suprises We Find When We Keep Digging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16625, https://doi.org/10.5194/egusphere-egu26-16625, 2026.
Soil organic carbon (SOC) is a key component in the soil ecosystem. SOC can be stabilised against microbial mineralisation by several mechanisms. One mechanism is the formation of mineral-associated organic carbon (MAOC), in which the sorption of SOC to mineral surfaces protects it from microbial decomposition. Another mechanism is the thermodynamic limitations on microbial activity. When oxygen is limited in the environment, microbial activity is constrained to alternative respiration pathways with lower mineralisation rates. Soil moisture influences both of these mechanisms. However, it remains unclear how the interaction and the relative importance of both mechanisms vary across soil moisture gradients.
To study the mechanisms behind SOC stabilisation in alpine environments, we selected three transects covering a moisture gradient highlighted by vegetation type changes from snowbed to grassland habitats. We assessed SOC content and thermal stability with RockEval analysis. We measured soil pH, texture and total elemental composition, and monitored soil temperature and moisture in situ. We found that SOC content increased from the snowbed to the grassland habitat. Soil moisture monitoring revealed a strong gradient with wet to potentially waterlogged conditions in the snowbed habitat and drier conditions in the grassland habitat.
To assess the potential of SOC stabilisation through MAOC formation along the soil moisture gradient, we quantified the distribution of carbon in three density fractions: the free light fraction, the occluded light fraction and the heavy fraction. To further investigate MAOC formation, we quantified the abundance of reactive Fe and Al phases that preferentially form associations with SOC. We expect the relative proportion of carbon in the free light fraction and the occluded light fraction to be higher in the snowbed where thermodynamic limitations are stronger. Inversely, we expect the relative proportion of carbon in the heavy fraction to be higher in the grassland where MAOC formation dominates.
We are currently assessing the possibility for SOC stabilisation due to thermodynamic limitations using electron accepting capacity as a proxy for the availability of terminal electron acceptors and electron donating capacity as an indicator for microbial anaerobicity. We expect thermodynamic limitations to be more important in the wetter snowbed, while SOC stabilisation in the MAOC dominates in grassland.
How to cite: Dienes, B., Mendoza, O., and Aeppli, M.: Mineral and thermodynamic controls on soil organic carbon stabilisation along a soil moisture gradient in the Swiss Alps, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16556, https://doi.org/10.5194/egusphere-egu26-16556, 2026.
Predicting soil organic carbon (SOC) stocks in forest ecosystems is crucial for assessing forest C balance, but the relative importance of key drivers, including soil mineral properties, litter inputs, and climate, remains uncertain. Here, we linked SOC stocks measured to 100 cm depth at 556 old-growth Swiss forest sites (350 to 2000 m a.s.l.) to soil properties, net primary production (NPP), climate (mean annual precipitation, MAP: from 700 to 2100 mm; mean annual temperature, MAT from 0 to 12°C), and forest type. We compared measured SOC stocks with stocks simulated by Yasso20 model, commonly used for reporting SOC stock changes in greenhouse gas inventories. Since Yasso20 accounts only for litter inputs and climatic conditions, deviations between modelled and measured stocks can reveal the significance of organo-mineral interactions that we hypothesize to be crucial for SOC stocks.
Total SOC stocks exhibited distinct regional patterns, with highest values in the Southern Alps, characterized by soils rich in Fe and Al oxides. On average, SOC stocks simulated by Yasso20 aligned with measured SOC stocks (13.7 vs 13.2 kg C m-2). In soils with pH ≤ 5, SOC stocks and model deviations were driven by exchangeable Fe, while in soil with pH > 5, exchangeable Ca was the dominant controlling factor. Beyond soil mineral properties, MAP emerged as an important driver of SOC stocks, while NPP remained unrelated to SOC stocks.
Our study demonstrates that soil mineral properties play a dominant role for SOC stocks across Swiss forest soils. Incorporating mineral-driven SOC stabilization into models can thus improve our ability to predict SOC stocks and its long-term dynamics.
How to cite: Guidi, C., Gosheva-Oney, S., Didion, M., Flury, R., Walthert, L., Zimmermann, S., Oney, B., Niklaus, P., Thürig, E., Viskari, T., Liski, J., and Hagedorn, F.: Drivers of soil organic carbon from temperate to alpine forests: a model-based analysis of the Swiss forest soil inventory, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14364, https://doi.org/10.5194/egusphere-egu26-14364, 2026.
Calcium (Ca) is increasingly recognized as a key factor in promoting carbon persistence in semi-arid soils, which are typically characterized by low soil organic carbon content, alkaline pH, and Ca abundance. However, direct evidence or a clear mechanistic understanding of Ca contribution to carbon persistence remains limited, due partly to the lack of established techniques for determining Ca species in soils, especially Ca-bearing minerals. We therefore aimed to investigate the relationship between Ca species and mineral-associated organic matter (MAOM) fraction (>1.7 g cm-3, <53 µm) in semi-arid agricultural soils of India using two non-destructive methods.
Specifically, we used X-ray powder diffraction (XRPD) and Ca K-edge X-ray absorption near-edge structure (XANES) spectroscopy to characterize Ca species across six surface soils from agricultural fields in south India, exhibiting a wide range of soil Ca content (6.0 - 79.2 g kg-1) and MAOM-C content (1.5 - 7.4 g kg-1) with alkaline pH (8.0 – 8.9). Ca-bearing mineral concentrations, namely Ca-plagioclase, carbonates (calcite and dolomite), and clay minerals (smectite and vermiculite), were determined by XRPD. The contributions of Ca-plagioclase and carbonates to soil Ca were calculated by multiplying their concentrations by Ca contents estimated from the ideal mineral formula. Exchangeable Ca was operationally estimated by assuming that it is dominantly associated with the interlayer sites of smectite and vermiculite. Furthermore, the Ca species were determined by XANES using linear combination fitting with standards, namely Ca-plagioclase (anorthite), calcite, and exchangeable Ca (montmorillonite). The Ca content of each species was calculated by multiplying the Ca ratio from XANES fitting and the soil Ca content measured by X-ray fluorescence (XRF).
The sum of the estimated Ca content from XRPD was highly correlated with the soil Ca content by XRF, with high accuracy (y = 0.81x, p<0.001, r = 0.98). Each Ca ratio estimated from XRPD showed a similar trend to that obtained from XANES, with one exception. Given that the XRPD fitting can generally utilize a large library of standards with distinct diffraction patterns, the differences between XRPD and XANES may reflect the representativeness of the selected XANES standards for Ca species in the soils. These results suggest that the non-destructive XRPD approach is effective for quantifying Ca-bearing minerals in semi-arid soils and, when combined with XANES, it offers complementary information on Ca speciation. MAOM-C was positively correlated with exchangeable Ca (p<0.05, r = 0.90) and carbonates (p<0.05, r = 0.84) according to XRPD-based Ca estimates. On the other hand, XANES-based Ca analysis showed that MAOM-C was positively correlated with calcite (p<0.05, r = 0.82), and weakly, though not significantly, correlated with exchangeable Ca (p=0.12, r = 0.70), indicating that XRPD-based Ca showed stronger relationships with MAOM-C. There was no significant relationship between MAOM-C and Ca-plagioclase in both XRPD- and XANES- Ca estimates. Our results suggest that both exchangeable Ca and carbonates might be more strongly associated with MAOM-C accumulation compared with Ca-plagioclase in the semi-arid agricultural soils. The current approach of distinguishing soil Ca species would help to elucidate the mechanisms underlying Ca-MAOM associations in soils.
How to cite: Seki, M., Nakao, A., Yang, P.-T., Chen, C.-L., Kurokawa, K., Wagai, R., Miyazaki, H., Jegadeesan, M., Kannan, P., Tanaka, H., Sugihara, S., and Yanai, J.: Speciation of Ca-bearing minerals and its relation to MAOM in semi-arid agricultural soils: a combined XRPD - XANES approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8919, https://doi.org/10.5194/egusphere-egu26-8919, 2026.
Different species of willow (Salix sp.) were historically used to improve slope stability and as the first step in afforestation on sandy soils. Willows can be grown from cuttings that harbour microbial symbionts (bacteria, ectomycorrhizal, and arbuscular mycorrhizal fungi) and promote successful plant establishment.
In an indoor pot experiment, we studied successional changes in bacterial and fungal communities following planting of willow cuttings in clean sand. Physical, chemical and microbiological parameters of root-affected sand were monitored during the growth of willows. Samples were taken from different pots that were disassembled after 30, 60, 90, 150, and 180 days of growth. Sand aggregation around roots was observed in pots sampled at days 150 and 180. Sequencing of gene libraries and qPCR for bacterial 16S rRNA and fungal ITS2 marker genes were performed on DNA extracted from loose root-affected sand and sand aggregates.The concentrations of soil total and water-extractable organic carbon and several metals (Ca, Mg, Mn, Cr, Cu, Fe, and Zn) were measured by ICP-OES and ICP-MS.
Concentration of water-extractable organic carbon in root-affected sand increased twofold from day 30 to day 150. Bacterial communities exhibited a clear pattern of increasing gene abundance and alpha-diversity over time. Proteobacteria were dominant in all samples, while Bacteroidota, Planctomycetota, Chloroflexota abundances increased in samples of day 150. However, no clear trends could be observed for the taxonomic structure of fungal communities, as the distribution of fungal dominants in sand samples was more scattered.
Sand aggregates differed from loose sand samples in terms of Ca and Mg concentration (40-200-fold higher), as well as the number of fungal and bacterial marker genes. Bacterial communities in sand aggregates were dominated by plant-associated bacteria, such as Sphingobium sp., while Firmicutes were significantly reduced, compared to those in loose sand. Some endophytic and plant-pathogenic fungi were found in aggregates. SEM analysis showed that crystals were formed between aggregated sand grains. So presumably, calcium carbonate precipitated due to root respiration affecting local conditions, while bacterial biofilms on sand grains acted as crystallization points. Alternatively, oxalic acid could have been exuded by willows and either precipitated as calcium oxalate or converted by oxalotrophic bacteria. Bio-induced precipitation of carbonates around willow roots enhances slope stability and carbon sequestration.
Our findings show that the growth of willow cuttings may alter the properties of rooted sand not only through root reinforcement, but also through bio-induced sand aggregation that occurs after five months.
This work is part of the project Soil Is Alive (SoIA), which was granted by the Carlsberg Foundation as part of the Consolidator Excellence Grant, Semper Ardens: Accelerate.
How to cite: Zhelezova, A., Innocent Otim, G., Trapp, S., and Rocchi, I.: Bacterial succession and bio-induced carbonate precipitation following willow growth, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9256, https://doi.org/10.5194/egusphere-egu26-9256, 2026.
Microbial processes drive the turnover of soil organic matter by regulating both matter and energy flows. Although carbon cycling has been extensively studied, microbial energy flows and their modulation under heterogeneous soil conditions remain insufficiently explored. Our work develops a thermodynamically consistent framework for quantifying microbial energy turnover in soils. The framework combines calorespirometric heat flow measurements with carbon flow (CO₂ evolution, substrate consumption and biomass formation), using an enthalpy-based balance approach and newly developed calorespirometric instruments.
Although our primary focus is on thermodynamic constraints and calorimetric quantification, these methods offer a promising route to investigate the consequences of soil heterogeneity. For instance, variations in soil aggregations, water content, redox conditions or the C/N ratio can create spatially distinct microhabitats, that alter local energy turnover and metabolic efficiency. By linking heat and carbon flows to these heterogeneous microenvironments, our approach provides a pathway to assess the energetic impacts of soil heterogeneity across scales.
In this presentation, we introduce a conceptual framework and preliminary data on the use of calorespirometry to link heat production rates with carbon flows, providing a window into microbial energy dynamics in heterogeneous soils. Our work highlights the potential of thermodynamic measurements to complement the structural and biogeochemical characterizations of soil heterogeneity, providing new insights into the energetic constraints that shape microbial activity.
How to cite: Maskow, T., Yang, S., Du, Y., Kästner, M., Di Lodovico, E., Fricke, C., and Miltner, A.: Thermodynamic Insights into Microbial Energy Turnover in Heterogeneous Soil Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21617, https://doi.org/10.5194/egusphere-egu26-21617, 2026.
Trophic interactions are key in the shaping of soil microbial communities, and therefore in their central role on soil function and biogeochemical cycles. The physical features of soil pore space greatly impact microbial activity as well as soil trophic interactions. Smaller pore necks (< 2-3 µm) are believed to provide soil bacteria with shelter from larger predators (mainly protists and nematodes). Microfluidic devices are increasingly used to address ecological questions at the microscale because they enable precise and customizable fabrication of pore-space geometries. This allows pore size and architecture to be treated as controlled experimental variables, while simultaneously permitting high-resolution microscopic assessment of microbial growth and activity, as well as spatially resolved chemical analysis. Here, we show current work on the use of microfluidics to manipulate trophic interactions and to assess their role in the cycling of organic matter. We present the use of a multi-depth soil chip where selected pore space areas are reduced to a 1-2 µm height and 1 µm width (predator filters), allowing for refugia with significantly reduced protist abundance. We show that, while most of these predator filters achieve reduced predation pressure by protists in open areas beyond, it is not uncommon for smaller protists to overcome them. In open spaces where protist predation is significantly reduced, preliminary results suggest increased microbial biomass –and necromass– accumulation. Spaces with occluded entries also tend to show a slower colonization rate and seem to favour certain bacterial taxa, based on morphological differences. This work and its future developments contribute to our understanding of how pore architecture shapes trophic interactions at the microscale and how these impact soil organic matter accumulation.
How to cite: Behncké Serra, A., Zou, H., and Hammer, E. C.: Protist predator exclusion in a soil chip ecosystem impacts bio- and necromass accumulation?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14347, https://doi.org/10.5194/egusphere-egu26-14347, 2026.
Soil structure creates spatial heterogeneity that shapes ecosystem functions, including water retention and root colonization. Chernozems – soils characterized by exceptionally stable aggregation resulting from millennia of root-soil co-evolution – offer a unique model to investigate how aggregate-scale pore architecture controls plant responses to drought. Using soil microcosms (4 × 10 cm, ~80 g soil) with aggregates from Native Steppe and Arable Chernozems, we established six experimental treatments (3 aggregate sizes × 2 soil types) with three replicates each. Root-soil dynamics were tracked through repeated X-ray computed tomography (Neoscan N80, Belgium) at 16 µm resolution. Imaging was synchronized with plant developmental stages – germination, first leaf, and third leaf stage at permanent wilting point – yielding a total of 54 soil tomograms for analysis.
Preliminary processing of the data reveals distinct pore network architectures across aggregate size classes. Small aggregates exhibited low CT-visible porosity (24%) with high solid phase connectivity (6.60 mm⁻³), while medium aggregates showed moderate porosity (39%) with lower connectivity (0.64 mm⁻³), and large aggregates had the highest porosity (49%) but the lowest connectivity (0.51 mm⁻³). This structural gradient directly controlled root colonization: solid phase connectivity showed a strong negative correlation with root volume growth (r = −0.76), suggesting that matrix mechanical cohesion, rather than pore characteristics alone, limits root expansion. Medium aggregates – which naturally dominate in undisturbed steppe soils – provided optimal conditions for root development, with 90% greater root surface expansion compared to small aggregates. Root sphericity decreased 3–4 times more in medium aggregates (−0.14) than in small aggregates (−0.04), indicating greater architectural plasticity critical for water acquisition. Importantly, our preliminary results also show that medium aggregates provided the greatest drought resistance: plants in these microcosms reached the permanent wilting point latest, suggesting that this aggregate fraction optimizes both root development and water availability over time.
These findings demonstrate that native Chernozem aggregate structure represents an optimized spatial configuration balancing root accessibility with water retention. The strong coupling between aggregate-scale heterogeneity and root response suggests that tillage-induced disruption of natural aggregate distributions may compromise this evolutionary optimization. Our approach – combining high-resolution CT with growth stage-synchronized imaging – offers a framework for quantifying how spatial heterogeneity translates into ecosystem-relevant soil functions. Data processing is ongoing, and final results will include expanded replication and additional root morphometric parameters.
How to cite: Yudina, A., Abrosimov, K., Timofeeva, M., and Kochneva, M.: Linking aggregate-scale pore structure to plant water acquisition: A 4D X-ray CT study of wheat roots in Chernozem, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8194, https://doi.org/10.5194/egusphere-egu26-8194, 2026.
Small-scale drivers of soil organic matter (SOM) mineralization can have important implications for larger scale soil carbon dynamics. SOM affects soil aggregation and structure formation. In turn, the soil pore network can impact SOM mineralization as it governs soil aeration and determines the physical accessibility of SOM to microbes. However, it remains elusive if structural or chemical soil properties determine the SOM turnover in soil aggregates. The aim of this study was to assess how SOM mineralization is affected by soil structure and chemical properties on the aggregate scale. For this purpose, we combined microbial respiration essays with 3-D X-ray micro-computed tomography (CT) scans of single aggregates. We expected that microbial respiration is driven by the co-location of SOM and soil pores and their connectivity but further influenced by the quality of organic matter and its stabilizing complexation with soil minerals. For this purpose, we measured the microbial respiration in soil aggregates (4-6 mm) from soils of two long-term field trials from central and southern Sweden with differing soil textures (sandy loam and clay loam) and organic matter sources and qualities (bare fallow, mineral fertilization, straw addition). Basal respiration rates were measured for moist single aggregates using MicroRespTM (µg CO2-C g-1 SOC h-1). We characterised the internal pore networks of aggregates using X-ray micro-CT with a voxel edge length of 5 µm, to assess the influence of pore size distribution, pore volume, pore surface area and connectivity on microbial respiration rates. In addition, we mapped particulate organic matter to compare its spatial distribution in relation to the pore network as an indicator of physical accessibility to microbes. For estimating the chemical accessibility of SOM we determined its composition using solid state 13C nuclear magnetic resonance spectroscopy and its potential chemical complexation with amorphous aluminium hydr(oxides). The highest respiration rates were observed for aggregates from straw-amended soils, even after normalization for carbon content. Our results will contribute to a better understanding of the small-scale mechanisms of SOM turnover that affect larger-scale organic carbon stocks in soils.
Acknowledgements: The study was funded by FORMAS grant no. 2022-00225.
How to cite: Heinze, W. M., Pihlap, E., Herrmann, A. M., Nunan, N., Simonsson, M., and Larsbo, M.: How does pore structure affect microbial respiration on the aggregate-scale?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9439, https://doi.org/10.5194/egusphere-egu26-9439, 2026.
Soils are inherently heterogeneous, with spatial variability emerging from interactions among plants, soil microbes and fauna, physicochemical properties, and local climatic and geological conditions. Plants perceive this patchiness and adjust their strategies accordingly; for instance, by increasing investment in arbuscular mycorrhizal fungi (AMF) to access water or nutrients that are scarce or unevenly distributed. These adaptive responses, in turn, influence biogeochemical processes. As agriculture shifts toward practices that reduce environmental impacts, and as extreme climatic events such as drought become more frequent, soil heterogeneity is expected to intensify. This trend highlights the need for crop varieties capable of maintaining performance in increasingly heterogeneous soil environments.
After outlining key drivers of soil heterogeneity, we present two studies demonstrating how plant investment in AMF under water or nutrient limitation shapes belowground carbon dynamics and nutrient acquisition. In the first study, using maize mutants defective in AMF colonization combined with isotopic tracing, we show that AMF enhance plant–microbe interactions by increasing carbon transfer to both AMF and saprotrophic microbes. This higher carbon flow promotes microbial transformation of plant-derived carbon into forms that may contribute to persistent soil organic matter. In the second study, we show that ryegrass compensates for the low solubility of a circular fertilizer by increasing investment in AMF, resulting in phosphorus uptake comparable to that achieved with conventional fertilizer.
Together, these findings illustrate the capacity of crops to buffer resource heterogeneity- whether driven by management or climate change- through symbiotic investment, with cascading consequences for biogeochemical cycling. A key challenge ahead remains to capture soil heterogeneity across scales to better predict its impacts on plant performance and ecosystem processes.
How to cite: Vidal, A., Steiner, F., Colombi, T., Cooper, H. V., Bhosale, R. A., Mueller, C. W., Ferron, L., and van Groenigen, J. W.: Drivers of soil heterogeneity and their impacts on biogeochemical cycles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9226, https://doi.org/10.5194/egusphere-egu26-9226, 2026.
Ferrihydrite (Fh) is a ubiquitous, metastable Fe oxyhydroxide that strongly influences the cycling and persistence of mineral-associated organic matter (MAOM) in redox-dynamic soils. Under anoxic conditions, thiol-bearing organics can promote reductive dissolution and generate surface Fe(II) that catalyzes transformation to more crystalline Fe phases, yet how silicates and organic functionality jointly regulate these processes remains unclear. Here, we combine batch incubations of Si–Fh coprecipitates (Si/Fe = 0–0.18) with comparative experiments using representative low-molecular-weight ligands (glutathione, cystine, and glutamic acid) to determine how Si and molecular structure govern adsorption, Fe(II) availability, transformation kinetics, and mineral products. Increasing Si/Fe suppressed aqueous Fe(II) release and increased retention of surface-associated Fe(II)/Fe(III), stabilizing Fh for weeks at Si/Fe = 0.18 and shifting transformation pathways at lower Si to mixtures of lepidocrocite and a porous goethite phase enriched in Si and OM. In parallel, ligand adsorption was highly mineral-phase specific (Fh ≫ lepidocrocite) and depended on size and functional-group richness, which in turn modulated Fe(II)-catalyzed transformation selectivity: cystine most strongly suppressed progression beyond lepidocrocite, whereas glutathione and glutamic acid permitted hematite formation under higher Fe(II), consistent with coupled effects of surface passivation and aqueous complexation of reactive Fe species. Electron microscopy and elemental mapping revealed preferential OM retention in amorphous Fh and selective incorporation of Si and OM into porous goethite, while lepidocrocite showed limited OM association. Together, these results identify Si/Fe and ligand functionality as geochemical “switches” that control Fe mineral evolution and determine whether MAOM is excluded, stabilized, or redistributed during anoxic transformation, with direct implications for carbon persistence in ferruginous soils.
How to cite: Zheng, Y., Asraf, B., Kovarik, L., Kukkadapu, R., and Engel, M.: Silicate passivation and organic ligand functionality steer ferrihydrite recrystallization and MAOM retention, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16689, https://doi.org/10.5194/egusphere-egu26-16689, 2026.
In the context of climate change, the long-term storage of organic carbon (OC) in soils and sediments is increasingly important, as these systems can act as both carbon sinks and potential sources under changing environmental conditions. Rivers transport 0.9 to 1.9 Pg of carbon per year from terrestrial sources into the ocean, with a large proportion assumed to be transformed, mineralized, or permanently stored. However, due to the dynamic nature of fluvial sediments, stored OC is heterogeneous in both source and composition. Consequently, the influence of sediment properties such as grain size, elemental composition, and depositional conditions on the molecular stabilization of OC remains poorly understood, and OC-matrix heterogeneity complicates molecular characterization.
Here, we investigate the molecular composition of OC in sediments from four depth profiles of a hydrologically inactive meander of the Oder River, where sediments have been deposited over long periods and include sediment types of varying ages. OC fractions were directly analyzed from milled sediments by laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). To our knowledge, this represents the first application of LDI-FT-ICR-MS to fluvial sediments, enabling direct molecular characterization of complex organic matter without solvent extraction or chemical pre-treatment. This preserves fragile and high-molecular-weight compounds and provides unique insights into matrix-bound OC pools. In addition, total organic carbon (TOC), stable carbon isotopes (δ¹³C), and radiocarbon (¹⁴C) were used to assess sources, transformation, and residence times.
LDI-FT-ICR-MS detected between 900 and 2,000 molecular formulae across sediment types. Molecular composition differed systematically between sediment matrices: the aromaticity index (AI) was higher in fine-grained sediments such as loam, clay, and peat-like deposits (AI up to ≈0.65), whereas coarse-grained sands showed lower AI (≈0.45). Sands exhibited higher double bond equivalents (DBE ≈10) and mean m/z values, indicating less aromatic, more aliphatic structures. The O/C ratio was lowest in peat-like sediments (≈0.25) and higher in sands (≈0.40), reflecting differences in oxidation state and carbon sources. Molecular variability decreased with depth and was lowest in very fine-grained clay and loam, suggesting enhanced stabilization and long-term persistence of OC.
These results highlight the central role of sediment properties in OC stability and provide insights into molecular mechanisms of carbon sequestration in river systems, with direct relevance for long-term carbon storage and climate protection strategies.
How to cite: Mählmann, I., Guo, J., Ruben, M., Frenzel, J., Mollenhauer, G., Hovius, N., Sachse, D., and Lechtenfeld, O.: Influence of Physical and Mineralogical Sediment Properties on the Molecular Composition and Stability of Organic Carbon in Fluvial Sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16569, https://doi.org/10.5194/egusphere-egu26-16569, 2026.
To accurately assess nutrient dynamics and contaminant transport in soil ecosystems, understanding colloidal transport is crucial, but its role in the nutrient cycle has not yet been sufficiently explored. In this study, we examined the composition, isotopic enrichment, and transport of free and occluded colloids isolated from a ⁵⁷Fe-labeled agricultural soil. Elemental analyses showed differences in elemental composition between the different types of colloids. Free colloids were richer in Ca, while occluded colloids were higher in Fe and K. Isotopic analyses showed, that ⁵⁷Fe enrichment was concentrated in the fine free fraction (<20 µm; up to +690 ‰), while occluded colloids demonstrated only minor shifts (+90 ‰). Column experiments under saturated flow conditions showed no detectable ⁵⁷Fe breakthrough, with the isotopic enrichment confined to the upper 1 cm of soil, consistent with strong retention and dilution of the label. This work also highlights the potential and limitations of stable metal isotope labeling for tracing natural colloids in soils. Mass-balance calculations demonstrated that ⁵⁷Fe tracer detectability is governed primarily by isotopic enrichment rather than total Fe added, and an enrichment of approximately 0.255 g ⁵⁷Fe kg-1 soil would be required for dependable detection of colloids above analytical precision.
How to cite: De Matteis, A., Cacerez, J. C., Siebers, N., Weihermueller, L., and Berns, A.: Free and occluded colloidal transport dynamics: a novel approach using stable Fe-isotope labeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17365, https://doi.org/10.5194/egusphere-egu26-17365, 2026.
Soil organic carbon (SOC) accumulation is a slow but crucial process in sandy post-mining soils (sPMS), frequently limiting recultivation success. At the same time, acid mine drainage in post-mining landscapes generates high quantities of iron- and organic-rich residues (iron hydroxide sludge, IHS), which poses a severe environmental threat. At present, IHS are, due to a lack of viable usage options, typically landfilled. Recycling IHS as soil ameliorants could enhance SOC accumulation and stabilization by introducing highly reactive minerals to sandy substrates. However, concerns remain regarding both the fate of organic carbon (OC) initially bound to IHS and the material’s potential impact on nutrient availability as well as on the mobility of potentially toxic elements.
To investigate the sorption behavior of IHS, batch adsorption experiments were conducted using natural dissolved organic matter (DOM) of differing degrees of aromaticity with a set of mineral samples, including two IHS with contrasting properties, an sPMS and a synthetic goethite. Sorption was tested under different pH conditions, and the stability of bound organic matter was assessed by desorption experiments. Combined chemical and spectroscopic analyses provided mechanistic insight into interfacial processes.
Adsorption experiments revealed pronounced differences in OC sorption among the tested materials. While DOM sorbed most strongly to goethite surfaces not pre-occupied with organic matter, IHS accumulated substantially more OC than sPMS. At low DOM concentrations, IHS partly released initially bound OC. Increasing DOM aromaticity enhanced OC uptake and initial affinity across all sorbents and accentuated sorption differences between the two IHS. The IHS characterized by higher surface area, lower initial OC, and lower pH behaved as a stronger sorbent but, surprisingly, exhibits a lower dithionite-citrate-bicarbonate-extractable iron content and a lower share of oxalate-extractable iron than the less strongly sorbing IHS. Phosphorus was almost completely removed from solution when exposed to IHS, whereas sulfur was released into solution. Contrary to general concerns, arsenic was not mobilized from IHS, while zinc was released from one IHS across a wide pH range, with increasing mobilization under acidic conditions. To a large extent, the OC freshly sorbed from the DOM onto goethite and IHS was not re-solubilized after repeatedly exposing the materials to solutions containing no OC.
The findings indicate that IHS may serve as a locally available substrate to support OC sequestration in sPMS. However, the effectiveness of this process is governed by both sorbent and DOM composition, as well as by DOM concentration, and is also accompanied by other effects on the immobilization or release of substances, potentially affecting the reclamation success. Our results highlight the importance of careful selection of IHS with favorable chemical and mineralogical properties, as well as detailed investigation of their behavior under near-natural conditions, to enable targeted reuse as sustainable soil ameliorants within circular economy frameworks.
How to cite: Harlow, E., Mikutta, R., Kaiser, K., Herrmann, J., and Stein, M.: Organic carbon stabilization by acid mine drainage–derived iron hydroxide sludge: Evidence from adsorption experiments and implications for soil amelioration, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19337, https://doi.org/10.5194/egusphere-egu26-19337, 2026.
Posters on site: Wed, 6 May, 10:45–12:30 | Hall X3
Elevation gradients provide a natural framework for studying environmental change over short geographical distances. As the altitude increases, the temperature declines, the variability in air humidity increases, and the growing season shortens. Furthermore, even under otherwise comparable environmental conditions, the slope aspect affects the microclimate of the ecosystems. These combined factors strongly influence ecosystem processes, including microbial activity and the rate of litter decomposition in mountainous forest ecosystems and thus affecting carbon and nutrient cycling.
To test the hypothesis on the relative roles of seasonality, elevation, and microclimate in controlling litter decomposition rates in forest ecosystems, we conducted a field experiment in Rila Mountains (Bulgaria) along an altitudinal gradient, throughout different stages of the vegetation period, and across contrasting slope exposures. We applied an updated Tea Bag Index (3.0) protocol as described in Middelanis et al. (2023) ensuring a time-series mass loss data by excavating the buried tea bags three times within a 90-days field experiment per season. The study was set along 100 m altitudinal gradients between 1500-1800 m a.s.l on north-facing and south-dominated slopes during three biologically active seasons – from early spring to late autumn. Enzyme activities and soil pH were measured at each sampling event (i.e., tea-bag incubation and excavation), thus providing repeated measurements that captured seasonal and microclimatic variation. In addition, soil physicochemical properties (soil organic carbon, total nitrogen, texture, bulk density, skeleton content, and ICP-based geochemistry) and stand mensuration were obtained to provide the sampling plots' characterization. Meteorological data, including air temperature and precipitation variables, were analyzed to characterize the regional meteorological conditions during the studied period. This integrated approach facilitated the analysis of the effects of multiple factors influencing litter decomposition in mountain forests.
The study found that the labial fraction and the decomposition constant differed notably between tea types, with consistently higher decomposition rates and decomposable fractions for green tea compared to rooibos tea across all sites and seasons, reflecting differences in substrate quality. The decomposition rate varied between sampling plots, exposure sites, and seasons. No clear pattern in decay rate was identified across the elevation gradient during the study period, with the exception of the spring season, when a decline in decomposition rate was observed with increasing elevation. Strong seasonal patterns were exhibited, with maximum decomposition rates occurring in summer at south-facing sites, whereas north-facing sites demonstrated equal or higher decomposition rates in autumn. These trends are further supported by statistically significant variations in the seasonal activity of microorganisms observed between the two surveyed areas with contrasting slope aspects, as well as by significant disparities in enzyme activity across seasons. The study demonstrated that litter decomposition in mountain forests is shaped by multiple interacting factors. It provided a good basis for comparing the relative influence of seasonality, elevation, and microclimatic conditions, highlighting the importance of their combined effects. However, further long-term experiments are needed to refine these insights and better capture the temporal variability of decomposition processes.
How to cite: Stoeva, L., Kirova, L., Kolev, K., Zhiyanski, M., Boteva, S., Angelova, B., and Sashova, I.: Seasonal and altitudinal variability of litter decomposition across slope aspects in the Rila Mountains, Bulgaria, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3107, https://doi.org/10.5194/egusphere-egu26-3107, 2026.
Arsenic (As) contamination in flooded soils is strongly governed by redox-driven iron (Fe) cycling at the soil–water interface (SWI), yet few remediation strategies explicitly exploit this dynamic process. Here, we propose and test a novel remediation mechanism based on organic matter (OM)–induced formation and manual removal of floating iron films. Using As-contaminated soils from Foshan, China (185.66 μg As g⁻¹ soil; 22.09 mg Fe g⁻¹ soil), a 10-week flooding experiment was conducted with different OM amendments, including cassava starch (0.05% and 0.1%) and milled straw (0.1% and 0.2%), alongside an unamended control. Continuous monitoring of dissolved oxygen and redox potential revealed that OM addition markedly enhanced reducing conditions at the SWI, promoting the reductive dissolution of Fe(III) minerals and the release of dissolved As. As redox gradients migrated upward, Fe(II) was re-oxidized in the overlying water, leading to the formation of floating iron films that effectively scavenged As from the water column. Periodic manual removal of the iron films resulted in substantial As export from the system. Depth-resolved soil extractions further demonstrated a redistribution and net depletion of labile As and Fe fractions near the SWI. These results demonstrate that coupling OM-stimulated iron reduction with targeted removal of secondary iron phases offers a promising, process-based strategy for arsenic remediation in flooded soils.
How to cite: Shu, X. and Chen, Z.: Organic matter–induced formation and removal of floating iron films as a mechanism for arsenic remediation at the soil–water interface, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3683, https://doi.org/10.5194/egusphere-egu26-3683, 2026.
In the context of accelerating climate change, understanding how soil forms in some of the most extreme environments on Earth, such as the cold deserts of Antarctica, is critical. As ice-free areas expand, new substrates are exposed to pedogenesis, a process involving various biogeochemical reactions such as the accumulation of organic matter and the movement of substances within the soil profile. In order to gain a better understanding of these microscale pedogenic processes, it is important to comprehend how and why chemical and biological heterogeneity emerges in such harsh conditions.
In this study, we combined microaggregate fractionation, chemical analyses, and correlative light and electron microscopy (CLEM) to identify early indicators of soil formation and microbial components in Antarctic soils. Bulk soils were fractionated into three microaggregate size classes (53–250 µm, 20–53 µm, < 20 µm), each further separated into free and occluded fractions, yielding six microaggregate fractions in addition to the bulk soil. All fractions underwent chemical analyses to determine elemental composition and key soil properties. CLEM was applied to detect and visualize microbial and organic components within the soil matrix. Energy-dispersive X-ray spectroscopy (EDX) complemented CLEM by revealing the spatial distribution of major elements and mineral phases.
Results showed marked differences among microaggregate classes and between free and occluded fractions, highlighting chemical and biological microscale heterogeneity overlooked by bulk analyses. CLEM revealed organic matter and potential microbial structures within the soil matrix, while EDX highlighted patchy elemental distributions. Notably, the <20 µm fraction displayed distinct chemical characteristics and structural features, suggesting a critical role in early pedogenic differentiation.
These findings indicate that initial soil formation in Antarctic cold deserts emerges at microaggregate-related spatial scales and is closely associated with microbial and organic components. Microscale approaches such as microaggregate analysis combined with CLEM and EDX are therefore essential for understanding the earliest stages of pedogenesis, not only in polar regions but also in other extreme terrestrial environments and analogous extraterrestrial settings, such as Martian cold deserts.
Keywords: Antarctic soils, Early pedogenesis, Soil microaggregates, Soil organics, CLEM
How to cite: Khosravani, P., Carlo Fischer, F., Wagner, D., and Scholten, T.: Microaggregate-scale heterogeneity as an indicator of initial soil formation in Antarctic cold desert soils, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13020, https://doi.org/10.5194/egusphere-egu26-13020, 2026.
Soil microbes drive terrestrial biogeochemical cycles, yet their fine-scale spatial distribution and occupancy of pore space remain poorly understood. Using a multidisciplinary approach, we resolve microbial colonization of soil pores at a micrometer resolution and assess how cell density shapes potential microbial interactions. We derive a general scaling law that converts X‑ray microtomography outputs into micrometer-scale estimates of habitable surface area. Across diverse soils, we found that microbial abundance scaled positively with habitable surface area, and that bacteria occupied a substantially larger fraction of pore surfaces than previously recognized. In addition, the diameter of soil pores was found to modulate the potential for bacterial interactions, which likely span phylogenetic lineages. These findings revise prevailing views of microbial colonization of soil pores and have direct implications for conceptualizing microbial interactions including modelling of microbially mediated processes.
How to cite: Schmidt, H., Schlüter, S., Felde, V., Raynaud, X., Pollak, S., Pjevac, P., Halisch, M., Braunmiller, H., Koebernick, N., Handschuh, S., Zeller-Plumhoff, B., Guseva, K., Richter, A., and Nunan, N.: Microbial cell density and interaction potential in soil are governed by the extent of habitable pore surfaces, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9843, https://doi.org/10.5194/egusphere-egu26-9843, 2026.
Soils hold the largest terrestrial reservoir of organic carbon (OC), and understanding the factors that are essential for soil OC formation, stabilization and transformation is crucial for predicting carbon-climate feedbacks. Mineral-associated organic matter (MAOM) is widely recognized as a dominant stabilization pathway for soil OC. However, the extent to which mineralogy - and the elemental makeup of mineral assemblages - regulates MAOM formation and persistence remains insufficiently resolved. Here we compare OC composition of six soil profiles developed on contrasting parent materials (sandstone, shale, and dolomite) in the Sudeten Mountains of central Europe. Bulk soil was fractionated into matrix-free particulate organic matter (POM), and MAOM using combined density and grain size separation.
In shale- and dolomite-hosted profiles, MAOM accounts for the majority weight, whereas sandstone-hosted profiles contain a greater proportion of POM. With depth, the relative weight contribution of MAOM decreases and POM increases in all profiles. Regarding the total organic carbon (TOC), sandstone-derived profiles generally contain less OC than shale- and dolomite-derived soils. TOC declines with depth in each fraction, with consistently higher OC content in MAOM than in POM. Similar with the weight distribution, the relative OC contribution of MAOM decreases while POM increases, despite the overall OC depletion down-profile.
To probe stabilization mechanisms beyond concentration patterns, ramped pyrolysis/oxidation (RPO) is being applied to quantify thermal stability of OC among mineralogies. A positive correlation between TOC and average activation energy presents in most profiles except for one sandstone-based profile. This is contrary to the general expectation that low-TOC samples should contain higher average activation energy (i.e., a negative TOC-stability relationship). The positive correlation in our samples suggests that specific compound classes or organo–mineral associations, rather than OC content alone, may drive apparent thermal stability; ongoing FT-ICR-MS will target compositional drivers. In addition, the standard deviation of energy decreases down-profile, indicating a decrease in OC heterogeneity.
To enable cross-mineral comparison, the OC loading of different fractions is also being normalized to mineral specific surface area (SSA). Upcoming ICP-OES measurements of elemental composition will test links between MAOM abundance and key elements, with the goal of identifying mineralogical and elemental controls on MAOM stabilization across contrasting parent materials.
How to cite: Guo, J., Ruben, M., Mählmann, I., Frenzel, J., Vonk, J., Frings, P., Lechtenfeld, O., Mollenhauer, G., Hovius, N., and Sachse, D.: Parent Material Shapes Carbon Quantity and Composition Across Soil Profiles: Insights from MAOM and POM Fractionation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9802, https://doi.org/10.5194/egusphere-egu26-9802, 2026.
Adsorption of dissolved organic matter (DOM) onto minerals plays a critical role in the global carbon cycle, influencing carbon stability and sequestration across terrestrial and aquatic ecosystems. Currently, it remains unclear if mineral properties or DOM composition are more relevant for predicting adsorption. We tested this by quantifying the maximum adsorption capacity (Qmax) of five mineral materials (podzol Bs horizon, agricultural topsoil, glacial stream sediment, kaolinite-dominated clay, synthetic goethite) for five DOM sources (humic lake, peat, leaf litter, algae, and pyrogenic organic matter). Adsorption characteristics were determined using a modified Langmuir model. In addition, aliquots of three mineral samples were treated with sodium hypochlorite to remove pre-existing organic matter, enabling assessment of adsorption capacity onto bare mineral surfaces. Qmax values spanned 31–28,630 mg C kg-1, exceeding previously reported ranges and showing that both DOM composition and mineral properties variably control adsorption capacity. Even strongly adsorbing minerals such as goethite and clay showed large variation across DOM sources, being highest for peat and lowest for algae. Likewise, DOM from different sources differed in their adsorption affinities for the different mineral surfaces. Treatment with sodium hypochlorite increased DOM adsorption, depending on material type and mineral characteristics, such as hydrous aluminum and iron phases. In summary, carbon adsorption onto minerals depends on the characteristics of both the minerals and the organic matter. This suggests that soil models that do not consider the characteristics of organic matter are limited in accurately describing adsorption and predicting carbon sequestration potentials in soils and aquatic ecosystems.
How to cite: Abbasi, M., Groeneveld, M., Kaiser, K., Kalbitz, K., Tranvik, L., and Kothawala, D.: Dissolved Organic Matter Composition and Mineral Characteristics both Control Adsorption Processes , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13417, https://doi.org/10.5194/egusphere-egu26-13417, 2026.
Dissolved organic matter (DOM) is a key component of soil systems and plays an important role in soil solution chemistry and organic matter transport. This study focuses on the comparison of DOM obtained by two different approaches: soil solution sampling and water extraction from soil. The aim is to characterise and compare dissolved organic matter collected from the same site and from identical depths of the soil profile.
Soil solution samples were collected using lysimeters installed at depths of 20, 40, and 60 cm. Water-extractable dissolved organic matter (WEDOM) was obtained from soil samples collected at the same depths and locations. The study site is a permanently grassed area managed by annual mowing, where the cut biomass is left on the surface without further intervention. This management represents a stable system with continuous organic matter input.
The physicochemical characterisation of DOM was based on a set of complementary analytical methods. Basic properties were determined by measuring pH and electrical conductivity. Dynamic light scattering (DLS) was used to assess particle size distributions, while zeta potential measurements were applied to evaluate surface charge and colloidal behaviour. UV–Vis spectroscopy was used to calculate selected absorbance coefficients related to aromaticity and the degree of humification.
By combining field-based lysimeter sampling with laboratory water extraction and multiple analytical techniques, this study provides a methodological framework for the comparison of different DOM fractions along the soil profile.
Acknowledgement
This work was supported by The NATO Science for Peace and Security Programme, project Nr. G6296. https://land-security.org/.
How to cite: Mullerova, K., Enev, V., and Pekar, M.: Dissolved Organic Matter in a Grassland Soil Profile: A Pilot Study Based on Lysimeter and Water Extraction Approaches, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10659, https://doi.org/10.5194/egusphere-egu26-10659, 2026.
Black carbon (BC) is produced by incomplete biomass combustion and serves as a key electron shuttle in subsurface environments, where it coexists with reduced solid organic matter (SOMred). However, the role of BC’s abundant redox moieties in mediating electron transfer from SOMred to O2 during redox fluctuations remains unexplored. In this study, four BC types were prepared from distinct biomass precursors (pine wood and zein) at different pyrolysis temperatures (500°C and 800°C). BC enhanced hydroxyl radical (•OH) production by 1.2–1.8 fold compared with SOMred alone. Notably, a two-electron transfer pathway dominated •OH formation in both systems. BC amplified •OH production mainly by promoting electron release from the solid phase of SOMred. Characterization and model experiments revealed that graphite crystallites accelerated electron transfer, while quaternary N groups increased electron release from SOMred, as demonstrated by electrochemical analysis and DFT calculation. This •OH-enhancement process further facilitated As(III) removal. These findings highlight BC’s significant potential to mediate solid-phase electron transfer in SOM-rich environments.
How to cite: Sun, J., du, Y., chen, Y., ma, T., and wang, Y.: Rapid Electron Transfer at the Organo–OrganicInterface: Black Carbon-Mediated Electron Shuttling Enhances ROS Generation during Solid Organic Matter Oxidation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11867, https://doi.org/10.5194/egusphere-egu26-11867, 2026.
This study investigates the relationship between aggregate micro-environments and microbial metabolic costs to quantify how soil physical structure constrains microbial physiology. Specifically, the objective was to determine whether aggregate size and structure impose distinct energetic trade-offs on nitrogen-cycling bacteria. Disturbed silty clay soil samples classified as Plinthic Acrisols and characterized by low pH (3.99 ± 0.02) and soil organic carbon of 2.59 ± 0.01 g kg-1were taken under natural grassland in Yiyang city, Hunan Province, China. Natural aggregates sieved at 2-5 mm (large macroaggregates, LM) and 0.5-1 mm (hereafter small macroaggregates, SM) were inoculated with exogenous urease-producing bacteria (UPB) and incubated at 20 and 30 degrees Celsius (three replicates, each) for 7 days. The rate of SOC mineralization, urease activity, total extractable extracellular polysaccharides, and UPB populations were determined, and the temperature sensitivity (Q10) of SOC mineralization was calculated. Afterward, LM and SM structures were characterized by pore-size distribution determined by mercury intrusion porosimetry (MIP), and by structural stability assessed using laser diffraction.
Results revealed that LM offered superior protection for UPB colonization compared to SM. Specifically, at 20 degrees Celsius, UPB population abundance in LM was 235.56 × 103 CFU/g, whereas in SM was significantly lower at 196.66 × 103 CFU/g. This higher biomass in LM supported a substantial C mineralization rate of 21.28 mg·kg-1·d-1. Notably, LM exhibited a lower Q10 (0.96) compared to SM (1.19). MIP analyses refined this understanding, revealing that LM possesses a higher volume of habitable mesopores compared to SM. While this specific pore range facilitates extensive bacterial colonization, the accumulation of extracellular polysaccharides within the tortuous and constricted pore throats likely creates a bio-physical barrier that restricts diffusion. This physical architecture explains the paradox of high biological density yet low temperature sensitivity. In conclusion, large macroaggregates function as low-cost metabolic niches, where tortuous pore structures maximize bacterial survival but constrain metabolic flexibility through physical diffusion limits.
How to cite: Lu, Z.-C., Zhou, J., Dal Ferro, N., Morari, F., and Wu, X.: Micro-Architectural Constraints on Urease Activity: How Aggregate Structure Modulates Microbial Trade-offs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11069, https://doi.org/10.5194/egusphere-egu26-11069, 2026.
Iceland has some of the world's most active dust sources. Volcanic activity shapes the nature of its ecosystems and soils, which mainly classify as Andisols. The dust deposited in Iceland consists of glass of basaltic origin, which releases Al, Fe, and Si during weathering. Areas close to dust hotspots, such as proglacial sites, can receive up to 1 kg/m2 of dust per year. Naturally, these vast deposition rates influence soil composition and dynamics. In our study, we investigated the effects of dust deposition on soil properties in Icelandic birch woodlands, which are the only native woodlands. We selected ten study areas in old birch woodlands (60+ years old) throughout the country and classified each area according to its dust deposition rate, with 1 indicating very low (< 50 g/m2/yr) and 6 indicating extremely high deposition (500-1000 g/m2/yr). Soils were sampled to a depth of 30 cm. The birch woodland soils were all typical Andisols, with high carbon content and the presence of clay minerals, such as allophane and ferrihydrite, and metal-organic complexes. The results showed a great variability in the examined soil properties between areas of different dust categories, with clear trends. The carbon content and stocks in the top 30 cm were highest in areas far from dust sources and lowest in areas close to them. As the dust falls on the surface and slowly integrates with the soil, the carbon content in the soil dilutes; however, it also invokes carbon burial. We estimated that up to 26 g/m2/yr of carbon gets buried in areas close to dust hotspots (category 6). Thus, despite the carbon stocks being “low” in the top 30 cm of soil (~ 4 kg/m2), the overall carbon stocks across the entire soil profile may be greater than in areas with lower dust deposition rates due to carbon accumulation and burial. The same dilution effect was observed on clays and metal-organic complexes. Interestingly, the dust deposition correlated with the Al/Si ratio of allophane, with a lower Al/Si ratio in areas with higher dust deposition rates. These areas also had a higher soil pH and contained less active aluminium, which explains the lower allophane Al/Si ratios. All in all, dust deposition had a positive impact on the birch woodland soils, as it rejuvenates the soil, bringing fresh basaltic materials to the surface, raising the soil pH and CEC. We encourage further studies on dust deposition effects, especially in deeper soils (> 30 cm depth). This study highlights the importance of understanding the impact of dust deposition on soil dynamics, as it plays a crucial role in Icelandic ecosystems.
How to cite: Sanchez, S., Thorsson, J., and Arnalds, O.: Dust deposition influences properties of Icelandic birch woodland soils, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12548, https://doi.org/10.5194/egusphere-egu26-12548, 2026.
Arsenic (As) is a toxic trace metalloid found naturally in the earth’s crust. Due to weathering of arsenic-bearing minerals, volcanic activity or anthropogenic activities, high quantities of As can be moved to the surface and into the surrounding soils. Leaching of arsenic from polluted areas to nearby water bodies poses a risk to local water supplies.
In polluted soils, arsenic is mainly present in its inorganic forms arsenite (AsIII) and arsenate (AsV). AsV is less toxic and easily immobilized by poorly crystalline iron (Fe) and manganese (Mn) oxides while AsIII is more toxic and mobile. Arsenate is dominantly present under oxic conditions, meanwhile AsIII usually prevails under anoxic conditions. Manganese oxides are known to rapidly oxidize AsIII to AsV under anoxic conditions, however, the presence of aqueous FeII inhibits their oxidation capacity [1]. Iron oxides such as magnetite are strong sorbents of arsenite, yet AsIII oxidation only occurs in the presence of aqueous FeII [2]. In nature, Mn and Fe often substitute for one another but the reactivity of mixed minerals towards As under anoxic conditions remains largely unknown.
Here, we studied arsenite oxidation during sorption to jacobsite ((MnII,MnIII)FeIII2O4) by reacting 1g/L jacobsite with 26µM AsIII at pH 7 (50 mM MOPS buffer) under anoxic conditions for 28d. The mineral was reacted pure or after equilibration for one hour with 1mM aqueous FeII, MnII and CaII before spiking with AsIII to investigate the role of competing divalent cations. Aqueous samples were collected over time to measure the As and cation concentrations with ICP-MS. The oxidation state of As, Fe and Mn in the solid phase was measured with their respective K-edge XANES after 6h, 48h, 7d and 28d.
After 6h, ca. 85% of the AsIII was removed from solution in the cation-free, MnII- and CaII-reacted treatments, while 99% of the arsenite was removed in the FeII-reacted treatment. Results from linear combination fitting (LCF) of the As K-edge XANES spectra showed the near complete oxidation of AsIII to AsV within 7d in the cation-free (97% AsV) and CaII-reacted treatment (93% AsV). In comparison, AsIII oxidation was slower for the jacobsite equilibrated with MnII (83% AsV after 7d), but still near complete after 28d (96% AsV). Equilibration of jacobsite with aqueous FeII significantly inhibited As oxidation and after 28d only 40% of the AsIII was oxidized to AsV. Results from the LCF of the Mn K-edge XANES spectra revealed reduction of structural MnIII by 15% after the reaction with aqueous FeII which might cause the reduced oxidation capacity of jacobsite.
Our results demonstrate the strong As sorption capacity of jacobsite under anoxic conditions. However, the presence of other redox-active elements in soil solution may hinder the oxidation of arsenite on the jacobsite surface. These results have implications for remediation approaches seeking to remove As from contaminated soil to provide safe water supplies.
[1] Pettit Mock, R. et al. 2019. ACS Earth Space Chem. 3 (4), 550-561.
[2] Gubler, R. & ThomasArrigo, L. 2021. J. Hazard. Mat. 402, 123425.
How to cite: Lacina, M., Bouchet, S., and ThomasArrigo, L.: Rapid arsenite oxidation by jacobsite under anoxic conditions: The role of divalent cations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19828, https://doi.org/10.5194/egusphere-egu26-19828, 2026.
Soil profile stratification under long-term organic fertilization: responses of soil fertility, structure, and microbiomes in a lucerne-based rotation
Long-term organic fertilization influences soil fertility, structure, and microbial communities, but the vertical distribution of these effects is not well studied, particularly in dry sub-humid regions where topsoil stratification can be pronounced. We evaluated how contrasting organic fertilization systems shape soil functioning and lucerne (Medicago sativa L.) performance within a long-term organic rotation in eastern Austria. Four fertilization systems were compared: FS1 (GM; stockless, two-year lucerne green manure), FS2 (GM+MC; stockless, GM plus municipal compost), FS3 (FU+FYM; livestock, lucerne forage-use plus farmyard manure), and FS4 (FU+BD; livestock, lucerne forage-use plus biogas digestate). Soil was sampled at 0–15 cm (topsoil) and 15–30 cm (subsoil); aggregate stability was assessed in the surface layer (0-5 cm). Across all systems, soil depth was the main driver of chemical, physical, and microbial patterns. SOC and TN, plant-available P and K, pore volume, and bacterial and fungal gene copy numbers decreased from 0–15 cm to 15–30 cm, whereas pH and bulk density increased with depth. Depth also strongly structured bacterial and fungal community composition, with fungal communities showing clearer responses to fertilization system than prokaryotic communities. Management effects were most evident in the topsoil: GM+MC and FU+FYM showed higher topsoil P and especially K, and tended to improve soil structure compared with GM and FU+BD. Despite these soil differences, lucerne dry-matter yield did not differ among fertilization systems, while the first cut consistently yielded more than the second cut. Overall, long-term organic fertilization primarily modified topsoil fertility and physical condition, whereas depth-driven gradients governed whole-profile patterns in soil properties and microbiome composition. These findings underline the need to explicitly account for soil profile stratification when designing lucerne-based organic systems under dry sub-humid conditions.
Key words: Bacterial community composition, compost (municipal compost), organic fertilization, soil organic carbon.
How to cite: Salehi, A., Gorfer, M., Surböck, A., Strohmeier, S., Seidel, S., Gregur, P., Berger, H., and Gollner, G.: Soil profile stratification under long-term organic fertilization: responses of soil fertility, structure, and microbiomes in a lucerne-based rotation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20346, https://doi.org/10.5194/egusphere-egu26-20346, 2026.
Dynamics of soil organic carbon (SOC) are controlled by microscale heterogeneity in substrate distribution, microbial accessibility, and mineral diversity. Still, how the microscale organization of low molecular weight (LMW) compounds influences microbial accessibility and soil energy cycling remains poorly understood. Mass spectrometry imaging (MSI) techniques, that are widely applied in biomedical research, offer new opportunities to visualize the distribution of molecular diversity in complex environmental matrices at the micrometer scale. Here, we explore the feasibility of desorption electrospray ionization mass spectrometry imaging (DESI-MSI), a surface-based ambient ionization technique, for spatially resolved detection of LMWs in soil. Sample preparation was guided by soil-specific constraints, including controlled water content and short incubation times, to minimize molecular redistribution and microbial alteration of existing metabolites. Using embedded samples of a range of different agricultural soils differing in management, we obtained two-dimensional distribution maps of amino acids, sugars, and fatty acids at a spatial resolution of 50 µm. Our preliminary measurements show localized micrometer-scale enrichments and spatially segregated distributions of detected compounds (sucrose, palmitic acid, glutamic acid, leucine, etc.), suggesting the presence of distinct molecular patterns at the microscale. Certain soil samples notably contain biochar particles, providing a chance to explore the distribution of LMWs across sorptive carbonaceous surface. These results demonstrate the ability to detect spatial patterns of LMWs across soil structures using DESI-MSI. Ongoing work aims to improve spatial resolution and identify molecular co-locations with specific organic and mineral soil components. Mapping metabolites at the microscale with spatial metabolomics provides new insights into how soil energy and carbon dynamics are organized.
How to cite: Ahmad, S., Zborovsky, L., Strittmatter, N., Minceva, M., and Schweizer, S.: Mapping Carbon Compounds Relevant for Soil Energy Cycles at the Aggregate Scale, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14317, https://doi.org/10.5194/egusphere-egu26-14317, 2026.
Posters virtual: Mon, 4 May, 14:00–18:00 | vPoster spot 2
EGU26-7583 | Posters virtual | VPS16
Adsorption of heavy metals to chia seeds' mucilageMon, 04 May, 14:09–14:12 (CEST) vPoster spot 2
Plant roots actively modify the physical properties of the soil in their area by secreting mucilage. Chia seed mucilage (CSM) is used as a model for plant root exudates primarily because of its similarities in physicochemical properties to natural root mucilage and its easy extraction in substantial, consistent quantities for laboratory experiments to study plant-soil-water relations. CSM can form highly viscous solutions at low concentrations and exhibits excellent properties, including water-holding capacity, surface tension, and emulsion stabilization. Most previous studies focused on chia seed mucilage as a conceptual model to describe the effect of mucilage on soil hydraulic properties, solute movement and gas diffusion in soil. However, the interactions between CSM and heavy metals have not been studied yet. Here, we showed the role of CSM as a bio-adsorbent for the removal of heavy metals and contaminants. Due to its sensitivity, non-destructivity, and simplicity, molecular fluorescence spectroscopy has been used to provide qualitative and quantitative information on the interaction between natural dissolved organic matter and metal ions. CSM was extracted from hydrated chia seeds and characterized using fluorescence Excitation-Emission Matrices combined with the Parallel Factor Analysis (EEM-PARAFAC) method. The binding interactions of CSM fluorescent components with heavy metals were quantified using fluorescence quenching titration and the Stern-Volmer model. Competitive binding studies were also conducted using one heavy metal as the quenching agent in the presence of competing heavy metal ions. Unconstrained PARAFAC modelling with two to four components was performed separately on 61 EEMs obtained from different concentrations of CSM samples, and the final component scores were determined through core consistency analysis, split-half analysis, and examination of the explained variance percentages. Protein-like (tryptophan & tyrosine) substances were the main fluorescent components identified by EEM-PARAFAC. The quenching titration results showed that the fluorescence intensity of CSM fluorescent components decreased with increasing heavy metal concentration under various environmental conditions. This strong quenching effect implies the binding ability of CSM to heavy metals and its significance in understanding metal toxicity, bioavailability, and transport in soil and natural waters.
How to cite: abrha, K. A. and Arye, G.: Adsorption of heavy metals to chia seeds' mucilage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7583, https://doi.org/10.5194/egusphere-egu26-7583, 2026.
