Conceptually, soil organic matter is currently being separated into particulate and mineral bound organic matter (POM, MAOM) due to their distinct formation, composition, stability, and respective drivers. However, several aspects of MAOM are still either not fully understood or actively debated. This applies to the roles of microorganisms in the formation and breakdown of MAOM under changing environmental conditions and the relative accumulation of plant versus microbial-derived carbon on mineral surfaces. Furthermore, the response of MAOM to global change and hence potential of soils to store carbon storage remains uncertain.
Addressing these knowledge gaps requires innovative experimental approaches as well as the application of cutting-edge methods such as stable isotope techniques, high-resolution spectroscopy, and molecular-level analyses. This session brings together contributions from researchers at the forefront of employing or developing novel tools to decipher C, N, and P cycling in soils with special focus on their control by the formation and stability of MAOM.
Clay minerals are recognized for their capacity to protect organic matter (OM) from microbial degradation, thereby playing a significant role in soil carbon sequestration. However, recent studies have yielded conflicting results regarding the maximum saturation limit of mineral-associated organic carbon in agricultural soils, highlighting the need to better understand the underlying fine-scale processes.
This study aimed to investigate the maximum capacity of different clay minerals to stabilize OM and to evaluate the saturation limits of mineral surfaces. A controlled laboratory experiment was conducted using three clay types with distinct specific surface areas: kaolinite, montmorillonite, and sepiolite. Microcosms were prepared using a sand-clay mixture (80% sand, 20% clay) with increasing proportions of green waste compost (GWC) at 1%, 5%, 10%, 25%, and 50%. Weekly CO₂ emissions were monitored over six months. At the conclusion of the incubation period, interactions between clay minerals and OM were examined via scanning electron microscopy (SEM). Bacterial and fungal abundances were quantified using quantitative PCR (qPCR), and microbial catabolic activity was assessed with Biolog EcoPlates™. Furthermore, OM thermal stability was evaluated using Rock-Eval® pyrolysis, while biological stability was assessed through the temperature sensitivity of microbial respiration (Q10).
CO₂ emission data indicated the lowest release in treatments with sepiolite, followed by montmorillonite and kaolinite, with the highest emissions observed in control treatments without clay minerals. The extensive specific surface area of sepiolite significantly suppressed microbial activity. Stabilization effects of clay minerals on OM mineralization were measured at compost levels of up to 5%, 10%, and 25% for kaolinite, montmorillonite, and sepiolite, respectively, beyond which the saturation of mineral surfaces occurred.
SEM analysis demonstrated that OM persisted predominantly as particulate organic matter (POM) in the absence of clay minerals, while mineral-associated organic matter (MAOM) was detected in treatments containing clay minerals. Microbial biomass and activity patterns closely aligned with CO₂ emission trends, indicating that clay minerals constrained microbial access to OM depending on clay type and saturation capacity. Rock-Eval® pyrolysis revealed lower hydrogen and oxygen indices in OM incubated with sepiolite and montmorillonite, suggesting enhanced thermal stability. These results were positively correlated with the increased biological stability, as reflected by Q10 values. This study underscores the pivotal role of clay minerals in stabilizing OM, with stabilization efficiencies and saturation thresholds varying significantly among clay mineral types.
How to cite: Mikajlo, I., Barré, P., Baudin, F., Robain, H., and Z. Lerch, T.: Influence of Clay Mineralogy on Organic Matter Stabilization Potential, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5259, https://doi.org/10.5194/egusphere-egu25-5259, 2025.
Soil organic matter (SOM) contributes to a large number of ecosystem services and represents an important global carbon store. In grassland ecosystems, domestic grazing by large herbivores can alter organic carbon storage in soils greatly either directly (defoliation, trampling and defecation), or indirectly (change in plant species composition, quality of organic matter, nutrient cycling, soil temperature and moisture). Due to these changes and potential interactions, grazing may not only affect the amount of SOM but also its formation pathways, which both have implications for the distribution of particulate organic matter (POM) and mineral-associated organic matter (MAOM). Most studies conducted so far concern grasslands in temperate and continental regions of low to intermediate soil organic carbon (SOC) stocks, but less is known for cold and moist regions with intrinsic high SOC levels.
Our study takes part in a unique long-term experiment in grasslands of the oceanic alpine region of Setesdal, Norway (elevation ~850-1050 m and annual precipitation 1170-1760 mm). The region has nutrient poor granitic parent material, deep, moist and acidic organic horizons and a long history of grazing. The distribution of SOC with respect to particulate - (POC) and mineral-associated organic carbon (MOC), stability mechanisms and radiocarbon dating will be analysed in grazed (44-88, sheep/km2), short term non-grazed (23 years exclusion) and long term non-grazed fields (more than 60 years exclusion). Recent studies show that grazing induced shifts in plant species composition have led to low quality litter with low decomposition rate, while long term grazing-excluded fields have more nutrient rich vegetation. We hypothesise that grazing will increase overall SOC and relative POC content compared to long-term excluded fields where there will be an overall faster turnover and higher relative MOC content.
Here we present empirical results on the stocks and fractions of SOC that can inform how grazing impacts organic matter formation and stability in a cold, oceanic climate. More than 200 soil samples have been analysed for total C, N, texture, bulk density and pH, and a selection are currently being analysed for SOC fraction distribution (POC and MOC) and their 14C age. SOC stocks vary greatly (45 to 442 tonnes/ha) due to large variations in soil depth (9-37 cm) and soil type. Initial results suggest only small differences in total SOC stocks between the grazing treatments, and a lower MOC/POC ratio in the grazed areas. Preliminary results of 14C analysis indicate that although POM is considered a labile fraction of SOM, it can be preserved for hundreds of years due to climate-induced low decomposition rate. Investigating MAOM and POM in light of historic and current grazing pressure shows how land use and vegetation directs SOM formation pathways, and how this may affect carbon preservation in the long term.
How to cite: Kok, N. H. L., Tau Strand, L., Furey, G. N., Mulder, J., Austrheim, G., Speed, J. D. M., and Martinsen, V.: How grazing impacts mineral association and stability of organic matter in oceanic alpine soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12414, https://doi.org/10.5194/egusphere-egu25-12414, 2025.
Soil organic matter bound to soil minerals contribute to long-term soil carbon and nutrient sequestration by protecting organic matter from rapid microbial decomposition. However, the binding between minerals and organic matter can be weakened by plant root exudates enhancing the potential for additional CO2 emissions. Root exudates contain low molecular weight organic acids that might promote carbon and nutrient release by stimulating microbial decomposition of soil organic matter (priming) or directly by weathering of soil minerals. The vulnerability of mineral-associated organic matter is particularly significant at higher latitudes where global warming is accelerating the thaw of carbon-rich permafrost soils and where changes in vegetation distribution due to warming can already be observed. Differences in root exudation among plant types and their effect on soil carbon and nutrient cycling remain however unknown. Here, we explore differences in root exudation between functionally different tundra plants to assess how the release of specifically organic acids contributes to soil organic matter destabilization in the Arctic. We analyzed total carbon, organic acids as well as other primary metabolites in root exudates from Betula glandulosa, Alnus viridis, and Eriophorum vaginatum with liquid chromatography–mass spectrometry and showed that exudation rates differed significantly between B. glandulosa and E. vaginatum. Organic acids contributed less than 2% to total organic carbon exudation and measured exudation rates were much lower than typically simulated in laboratory incubations that test organic acid effects on soils. These observations question to what extent previous laboratory findings describe processes relevant in natural systems. Based on our observational data, we designed a soil incubation experiment comparing how priming and soil mineral destabilization by organic acids influence carbon, nitrogen and phosphorus cycling in the mineral-associated organic matter fraction (MAOM) and the intact bulk soil. Organic acid mixtures at two concentrations were compared: A commonly applied concentration corresponding to 1% of soil organic carbon (SOC) and a lower concentration representing seven days of observed organic acid exudation (0.001% of SOC). Preliminary data emphasize that the more realistic low acid treatment did not stimulate microbial CO2 production compared to the control without acid addition while in contrast the high acid treatment led to an overstimulation of microbial CO2 production of about 80-100%. We will connect these observations to data on CO2 sources and changes in soil nitrogen and phosphorus, to assess the impact of changes in vegetation distribution on mineral-bound organic matter in thawing permafrost soils.
How to cite: Wegner, R., Sauerland, L., Plassmann, M., Gaita, S. M., Monteux, S., Oburger, E., Mikutta, R., and Wild, B.: Does a shift in vegetation type in high-latitude soils enhance soil organic matter destabilization from mineral-organic associations by organic acid exudation?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8014, https://doi.org/10.5194/egusphere-egu25-8014, 2025.
Hydrolases are a main group of enzymes that catalyze soil organic matter (SOM) decomposition, thus influencing the fate of organic carbon and nutrient release rates in soils. The rate of enzyme-catalyzed processes depends on the pool size and lifespan of enzymes. Both are strongly affected by soil genesis and mineralogy, yet this remains poorly documented by experimental data. In this study, we added three pure enzymes (β-glucosidase, acid phosphatase, and leucine aminopeptidase) to three soil horizons from a podzolic chronosequence with contrasting pedogenic characteristics: BC horizon: mainly primary minerals; Ae: quartz and organic matter enriched; and Bf: organo-metallic complexes and iron oxide enriched. Although the addition of pure enzymes increased enzyme activities by 1.3–2.3 times, only 7–22% of enzymes remained active one day after their addition into soil, moreover the active portion of added enzymes dropped to 5–12% over one week. The decay of enzymes followed the first-order model with rates ranging from 0.047 to 0.104 day–1. The lack of enzyme stabilization processes in BC horizon mainly comprised of primary minerals with lower specific surface area and reactivity led to greater activity loss of acid phosphatase compared to horizons enriched with organic matter (Ae) and/or pedogenic iron subproducts (Bf). The adsorption of leucine aminopeptidase on the surface of iron oxides in Bf horizon decreased enzyme activity but prolonged the persistence of enzyme activity. However, the catalytic efficiency of enzymes adsorbed on the surface of iron oxides was lower than that of enzymes associated with organic matter (Ae) or existed in a free form (BC). Our findings highlight the need to (i) further investigate the relationship between enzyme activity and SOM decomposition rate, especially if soil minerals reduce enzyme catalytic efficiency, and (ii) carefully consider incorporating soil genesis into enzyme activity-based models to improve the predictions, for example, of SOM decomposition.
How to cite: Wang, C. and Cornelis, J.-T.: Soil genesis and mineralogy alter the stability and activity of hydrolytic enzymes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7490, https://doi.org/10.5194/egusphere-egu25-7490, 2025.
Soil microbes produce extracellular enzymes that play a crucial role in organic matter degradation. However, the adsorption of these enzymes to clay mineral surfaces could impede the functioning of extracellular enzymes. Microbes are thought to counteract this adsorption of extracellular enzymes by producing more extracellular polymeric substances (EPS) under adverse conditions. This led to the hypothesis that enzyme adsorption to clay minerals could stimulate EPS production, potentially mitigating the negative effects of adsorption on enzyme activities.
To test this, we conducted a mini-incubation experiment with artificial sterile soil and Bacillus subtilis to investigate i) whether different clay minerals increase EPS production by B. subtilis and ii) whether increasing EPS contents preserve the activity of extracellular enzymes in the presence of clay minerals. Artificial soils were prepared (%/w) with sand (75%), silt (15%), and either an expandable clay mineral, montmorillonite (MT) or non- expandable, kaolinite (KL) clay (10%) and then sterilized using gamma radiation. We included a control treatment containing an additional 10% sand in place of clay minerals. The soil samples (20 g) were supplemented with LB-Lenox media (20 g/l) as a substrate, inoculated with B. subtilis, and incubated in three replicates for 3 days at 25 °C. CO2 production was monitored with GC measurements at 24-hour intervals. On day 3, soils were destructively sampled and analyzed for amylase enzyme activity and colony-forming units (CFU). EPS was extracted and quantified for protein and polysaccharide content. All procedures were carried out under sterile conditions, and data collected were normalized to the number of bacterial cells added to each treatment.
The results showed a significant (p < 0.05) higher level of EPS production (EPS-protein and EPS-polysaccharide) in the soil amended with MT compared to the control and KL-amended soil. There was no significant difference between the EPS production in control soil and KL-amended soil. Clay minerals did not significantly influence amylase activities (p < 0.05), contradicting previous reports of reduced enzyme activities due to mineral adsorption. However, there was a significant reduction in the CFU in soil with clay minerals compared to the control. This might be an indication of struggle for nutrient availability by B.subtillis and hence the need for EPS production. The observation of high EPS production in the presence of high MT content with no adsorption effects on enzyme activities may be due to a near-steady enzyme-mediated degradation of organic matter in the soil, which is crucial for C cycling. Future experiments should clarify the effect of clay minerals and bacterial EPS production on extracellular enzymes activities in other biogeochemical cycles like the N and P cycle.
How to cite: Olagoke, F. K., Ratering, S., Schnell, S., Siemens, J., and Mulder, I.: Effect of soil minerals on the production of extracellular polymeric substance by Bacilli subtilis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12751, https://doi.org/10.5194/egusphere-egu25-12751, 2025.
Soil microbial carbon use efficiency (CUE) represents an important driver of soil organic C formation and turnover. The balance between anabolic and catabolic processes are known to regulate SOC formation through microbial growth and stabilization of microbial residues, and SOC mineralization, respectively. CUE is largely affected by environmental factors but also regulated by the availability of organic substrates and electron acceptors. This is particularly the case in soils that are temporarily subjected to shifts in redox conditions, as those that occur when rice paddy soils are flooded or drained. In such soils, microbial CUE may be affected by electron donor availability (e.g. presence or absence of labile crop residues) as well as electron acceptor availability as O2 becomes limiting and other oxidized species like nitrate and FeIII minerals are reduced. These changes in metabolic activities may also be accompanied by a change in microbial communities which are more adapted to changes in redox conditions and use their resources more efficiently affecting the overall community CUE.
The aim of this work is to explore the effects of short-term changes in soil redox conditions (i.e. from aerobic to anaerobic) on the microbial physiology of a rice paddy soil, by unravelling the effects of management-related differences in electron donors and acceptors on microbial growth, respiration and CUE, as well as their dependence on changes in microbial community composition. For this we set up a microcosm experiment where a paddy soil was incubated for 17 d with a factorial combination of (i) redox conditions (oxic vs. anoxic), (ii) with or without rice straw, and (iii) with or without added nitrate. Soils were destructively sampled after 4, and were analysed for DOC, dissolved nitrate and FeII , and microbial biomass C (MBC). Soil aliquots were incubated with D2O for 48 h to measure rates of microbial respiration (CO2 and CH4) and growth by tracing isotope incorporation into phospholipid fatty acid (PLFA) biomarkers, to calculate CUE.
Our preliminary results showed that under anoxic conditions nitrate was rapidly consumed within 4 d while Fe(III) was reduced at a later stage particularly where easily degradable rice straw was added. This could mean that in the first few days the microorganisms are not facing C deficiencies, however, as the anoxic conditions persist the C input enhances microbial activity leading to an increase in Fe(II) in solution. This was also seen in the MBC, as the presence of rice straw enhanced MBC, irrespective of the other treatments (i.e. redox conditions and NO3- addition). Redox-driven changes in community level microbial physiology (growth, respiration and CUE) as well as changes in the active microbial community (PLFA-based) will provide further insights on the role of changing redox conditions and management on key parameters related to soil carbon accrual.
How to cite: Ehlinger, A., Canarini, A., Richter, A., and Said-Pullicino, D.: Shifts in microbial CUE as a function of available organic C resources and electron acceptors under changing soil redox conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20184, https://doi.org/10.5194/egusphere-egu25-20184, 2025.
Tree mycorrhizal type is increasingly recognised as a key determinant of the quantity and quality of soil carbon stocks from local to global scales. However, direct evidence linking mycorrhizal associations to the age and persistence of soil carbon pools remains lacking. Here, we leverage radiocarbon (14C) analysis to investigate the mean age of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions across temperate forests dominated by arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) tree species. Our findings reveal significant interactions between mycorrhizal type and soil depth: EcM-dominated soils exhibit younger carbon in the surface organic horizon (higher ∆14C) but older carbon in the deeper mineral horizons (lower ∆14C) compared to AM-dominated soils, in both the POC and MAOC fractions. These patterns suggest that EcM associations may suppress surface-layer decomposition, whereas AM-dominated systems promote recent root-derived carbon inputs at greater depths, creating a comparatively broader carbon age gradient under EcM trees. Moreover, carbon persistence mechanisms likely differ between fractions. Specifically, mean age of POC is mainly driven by substrate quality (e.g., C:N ratios), whereas mean carbon age of MAOC is closely tied to microbial processing, as indicated by δ15N enrichment. These findings provide novel insights into how mycorrhizal-mineral interactions can shape soil carbon storage and persistence and inform forest management strategies aimed at climate change mitigation.
How to cite: Liu, S., Bradford, M. A., and Ward, E. B.: Radiocarbon evidence of the role of tree mycorrhizal type in modulating mean soil carbon age , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14043, https://doi.org/10.5194/egusphere-egu25-14043, 2025.
Soil organic carbon (SOC) is a key player in global carbon cycling and has primary effects on soil quality and functioning. There is a general interest in modeling SOC content and understanding the factors controlling its accumulation and stability. The mid-IR spectra that provide fingerprints of soil chemical composition are well recognized in modeling and predicting SOC contents, which is generally done using different types of empirical multivariate analyses. This work suggests, for the first time, the decomposition of soil mid-IR spectra using nonnegative multivariate curve resolution (MCR) with an alternating least squares (ALS) algorithm [1]. The advantage of the nonnegative MCR-ALS decomposition is that it allows the expression of soil mid-IR absorbance in terms of contributions from chemically meaningful components, following the Beer-Lambert law. Hence, this combination of IR spectroscopy and the nonnegative MCR-ALS decomposition proposes a new analytical approach to decipher soil compositions and elucidate the components controlling soil functions. Potentially, the nonnegative MCR-ALS decomposition can identify chemically individual components or groups of constituents maintaining constant proportions in a series of samples. Based on this decomposition, a simple mechanistic model is developed to link the identified MCR-ALS components with their contribution to the whole SOC content [1]. This approach has been used to examine the SOC of soil samples collected in the north and south of Israel, from different depths and under different land uses. Four components including a carbonate-rich constituent and three others representing clay-organic matter associations were capable of quantitatively describing 99.7% of the variance of soil mid-IR spectra. SOC modeling using these four components suggested a SOC content threshold affecting modeling performance such that SOC content below 1.0 % w w-1 could be modeled with RMSD of 0.18% w w-1. The emergence of this threshold is currently related to mechanisms of how different SOC fractions become "mirrored" in mid-IR spectra. This threshold could be useful to distinguish between different types of SOC, i.e., those tending to tightly interact with mineral surfaces and those having weak connections with minerals, if at all. The perspectives in extending the whole approach for a wide range of SOC contents are also discussed.
[1] Borisover, M., Lado, M., & Levy, G. J. (2025). Modeling Soil Organic Carbon Content Using Mid-Infrared Absorbance Spectra and a Nonnegative MCR-ALS Analysis. Soil & Environmental Health, 3(1) 100123.
How to cite: Levy, G., Borisover, M., and Lado, M.: A new mechanistic approach to link soil chemical composition and organic carbon content: decomposing mid-IR spectra with multivariate curve resolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4686, https://doi.org/10.5194/egusphere-egu25-4686, 2025.
How to cite: Haddad, L., Vincent, A., Giesler, R., and Schleucher, J.: Unveiling Soil Organic Phosphorus Dynamics Using one- and two-dimensional 31P NMR, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20112, https://doi.org/10.5194/egusphere-egu25-20112, 2025.
Phosphorus is an essential element for all organisms. A sub-optimal supply of agricultural soils may influence yields, soil quality and health, carbon sequestration and nutrient turnover processes. Changes in soil P levels can influence microbial communities, altering key pathways in the nitrogen (N) and carbon (C) cycles and affecting greenhouse gas emissions. In this laboratory incubation experiment, we examine the effects of three long-term P fertilization levels in an Irish grassland soil on N and C transformation processes, associated greenhouse gas fluxes and plant growth using stable isotope techniques (15N and 13C). This research is part of the EJP SOIL project “ICONICA” (Impact of long-term P additions on C sequestration and N cycling in agricultural soils).
The soil samples were obtained from the low, medium and high P fertilizer treatments of the long-term P-fertilization experiment on grassland at Johnstown Castle, Ireland. The soil was sieved, filled into plant pots and sown with three maize seeds per pot. All pots received 5 days after sowing the same amount of glycine and ammonium nitrate (NH4NO3), only differing in the isotopic labels of N and C, respectively: The 13C-labelled glycine was applied together with NH415NO3, the 15N-labelled glycine together with the unlabelled NH4NO3, and the unlabelled glycine together with 15NH4NO3. Plant, soil and gas samples were taken 0, 1, 3, 7 and 10 days after label application by glycine-NH4NO3 addition and were analysed for (15)NH4+-N, (15)NO3--N, organic (15)N, organic (13)C contents as well as for nitrous oxide ((15)N2O), carbon dioxide ((13)CO2), and methane (CH4) fluxes.
We aim to deepen our understanding of the complex relationships between the nitrogen-carbon-phosphorus cycles and their impacts on plant growth under varying levels of phosphorus application. The development of the Ntrace analysis tool into the CNtrace analysis tool is expected to enhance insights into these interactions and transformation processes. Key hypotheses include increased plant biomass, elevated CO2 emissions and reduced N2O emissions at high P-levels. Further analyses and interpretations are ongoing.
How to cite: Dannenberg, L., Jansen-Willems, A., Clough, T., Bhople, P., Bondi, G., Müller, C., and Kleineidam, K.: Impact of Long-Term Phosphorus Fertilization on Nitrogen and Carbon Cycle Dynamics, Greenhouse Gas Fluxes and Plant Growth in an Irish Grassland Soil, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19679, https://doi.org/10.5194/egusphere-egu25-19679, 2025.
The carbon to nitrogen (C: N) ratio is essential for regulating soil nutrient content balance, directly influencing crop yields and microbial activity. Spatially explicit monitoring of this ratio in temperate regions remains important to fully understand its benefits. Therefore, we sought to understand the influencing factors of the C: N and map its spatial variability at 250 m/pixel across Germany. We applied n = 1687 surface soils obtained from the 2015 Land Use and Coverage Area Frame Survey (LUCAS) database coupled with key environmental covariates, including soil, climatic, human-related, topographic, and remote sensing data. A cubist machine learning model was used to relate the C: N ratio with these environmental covariates. Our analysis revealed that pH, elevation, latitude, and silt were among the top four important covariates, accounting for 76.6 % of the total variance in the C: N ratio. The Cubist model demonstrated acceptable predictive capabilities, with a root mean square error (RMSE) of 2.55 and a relatively low bias of 0.02. Our C: N prediction map indicated that the northwestern region of Germany exhibited high C: N ratio values ranging between 15 and 24. This range suggests conducive conditions that support soil microbial activity and greater nutrient availability. Furthermore, this region has high precipitation and NDVI values, corroborating our earlier point. Our findings emphasize the importance of soil, topography, and human activity in influencing the C: N ratio in temperate regions like Germany. Thus, understanding their roles in soil stoichiometry is crucial for developing effective land management strategies to enhance soil health and agricultural productivity.
Keywords: Soil pH, Human Footprint, Nutrient Dynamics, Cubist Model, Climate Mitigation
How to cite: Khosravani, P., Kebonye, N. M., Baghernejad, M., Moosavi, A. A., Mousavi, S. R., and Scholten, T.: Identifying the Spatial Drivers of Soil Carbon-Nitrogen Stoichiometry in Germany, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-262, https://doi.org/10.5194/egusphere-egu25-262, 2025.
Forests occupy about 40% of Canada, with managed forests totalling 225 million hectares. While Canadian forests have historically acted as an essential carbon sink for Canada, intensifying disturbances have drastically decreased the carbon sink provided by trees. However, most carbon is found belowground in Canadian forests, with forest floors and mineral soils containing more than three times the amount of carbon stored in trees. About 28,800 million tonnes of carbon are sequestered in mineral soils of Canadian forests alone, corresponding to 10% of stocks found in forest soils globally. Even slight variations in these extensive carbon stocks can have a profound impact not only on the carbon balance of Canadian forests but also on the global carbon cycle. Despite their importance, there is still great uncertainty about the mechanisms controlling soil carbon persistence in Canadian forests.
Our research project aims to address this major knowledge gap by quantifying soil carbon formation and persistence across the major forested ecozones of Canada. We have established a nationwide network of experimental sites to compare key soil types under different tree species representative of Canadian-managed forests. We will measure the decadal sensitivity of soil carbon to environmental shifts, including global change, harvesting and fire. We will clarify the linkages between carbon persistence and soil biodiversity. Overall, this research project will establish the foundational scientific knowledge required to improve current predictions of soil carbon response to environmental shifts in Canadian forests. This will, in turn, allow for a meaningful inclusion of forest soil carbon in Canadian climate policies, including global commitments under the United Nations Framework Convention on Climate Change.
How to cite: Quideau, S., Norris, C., Adesanya, T., Carodenuto, S., Diochon, A., Karst, J., Laganiere, J., Poirier, V., and Simpson, M.: Carbon under Canadian forests- why soils matter, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2181, https://doi.org/10.5194/egusphere-egu25-2181, 2025.
In soils, inositol hexaphosphate (phytate) is a recalcitrant form of organic phosphorus (OP), making it a critical component of phosphorus (P) cycling in terrestrial ecosystems. Understanding phytate turnover and the factors influencing it is therefore essential. However, the lack of multiple stable P isotopes has hindered investigations of phytate dynamics under natural conditions over extended periods. To address this, we propose a novel technique for determining the carbon isotopic composition (δ13C) of inositol in phytate at a compound-specific level. For this purpose, phytate was extracted from soil, and purified via ion exchange chromatography, followed by dephosphorylation, derivatization, and analysis using GC-MS and GC-C-IRMS. Pure compounds were also analyzed to assess protocol efficiency, identify isotopic fractionations, and apply isotopic corrections due to derivatization. Phytate extracted from soil samples was identified using GC-MS chromatograms. Replicate analyses of the pure compounds showed that the protocol is highly reproducible. The proposed method was able to identify, quantify, and measure the δ13C values of inositol in phytateseparately from other sugar molecules such as glucose and fructose. The δ13C values showed high reproducibility, with values varying by less than 0.5‰, and with no detectable isotopic fractionation during sample preparation. The δ13C values of phytate in soil samples reflected the dominant vegetation type (C3 or C4) at the study site. This study introduces a novel approach to measuring the δ13C values of inositol in phytate from environmental samples, offering new opportunities for investigating and quantifying OP dynamics using stable carbon isotopes.
How to cite: Sarangi, V. and Spohn, M.: A novel GC-C-IRMS method for determining the carbon isotope composition of inositol hexaphosphate (phytate): A step towards unveiling soil organic phosphorus cycling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3209, https://doi.org/10.5194/egusphere-egu25-3209, 2025.
Plant- and microbial derived carbon (C) are the main components of soil organic carbon (SOC), however, their relative contributions to soil fractions with different stability remain uncertain. Particulate organic carbon (POC) and mineral associated organic carbon (MAOC) are considered to have different formation mechanisms and different stabilities. Here, we compared two perennial cropping systems (festulolium and grass-clover) with an annual cropping system (maize), to investigate their effects on soil POC and MAOC, and quantify the contribution of plant- and microbial derived C to the two C fractions.
The results showed that the two perennial crops had higher POC and MAOC than maize at 0–20 cm soil depth, with higher proportions of POC in SOC. Microbial necromass was linked to the perennials’ higher POC, as festulolium and grass-clover showed higher fungal and bacterial necromass in POC at 0–20 cm. In contrast, maize showed significantly higher microbial necromass C in MAOC than festulolium and grass-clover. Total microbial necromass C accounted for only 30% of POC and 31% of MAOC across all systems, suggesting that plant-derived C could dominate these two C pools. However, no statistical differences were detected in the lignin phenols content in POC and MAOC at 0–20 cm. Our results challenge the conventional assumption that necromass C dominates MAOC, highlighting the significance of plant-derived C in POC and MAOC, which could have a greater influence on soil C sequestration than previously thought.
How to cite: Shang, Y., Liang, Z., Abalos, D., and Olesen, J.: Enhancing plant-derived carbon is key to building stabilized soil organic carbon with perennial crops, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3212, https://doi.org/10.5194/egusphere-egu25-3212, 2025.
Nutrient loss from cropped fields, especially phosphorus (P) loss, is a major environmental concern due to its negative impacts downstream. Phosphorus use efficiency (PUE) is an important metric in evaluating crop response to P-fertilizer applications and improving P runoff loss from cropped fields. The objective of this study is to demonstrate the development of PUE assessments from cropped fields and apply those metrics to provide geospatially explicit PUE across the fields. The work was conducted at a research farm near Riesel, Texas, USA, where the fields are managed with various levels of agronomic conservation adoption. Precision agricultural technologies are used to record planting, fertilizing, and harvesting (total yield and grain quality), and soil spatial variability assessment. Five sampling points were located in each field where intensive soil and crop measurements were recorded. Grain yield and quality data from the harvesting combine were compared to ground-truthed data from the sampling points in each field to determine crop P removal in grain. These data were then used to predict and map the PUE across the fields using a machine-learning technique. This study showcased an efficient way of monitoring the spatial aspects of PUE in agricultural fields that may improve fertility strategies, thereby leading to improved farm profitability and less P runoff from cropped fields.
How to cite: Adhikari, K., Smith, D. R., and Hajda, C.: Monitoring spatial dynamics of phosphorus use efficiency in cropped fields, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3348, https://doi.org/10.5194/egusphere-egu25-3348, 2025.
Soil organic matter (SOM) turnover plays a major role in the global carbon cycle, but also influences soil health and fertility. Accurate modeling of SOM dynamics requires a comprehensive understanding of the interconnected biological and physical processes occurring at small spatial scales. The availability of carbon (C) and nitrogen (N) resources governs microbial growth, while microbial dynamics, in turn, affect the distribution of C and N within soils. The stabilization of microbial necromass within soil aggregates or its association with mineral surfaces and the role of living and decaying roots as sources of carbon require closer attention to evaluate their role for soil aggregation. This aggregation process in turn impacts the degradation of (potentially occluded) organic matter. We present a mechanistic model at the pore scale, which includes a microbial model that takes into account the turnover and C/N ratios of different organic matter sources. It is combined it with a cellular automaton model for simulating dynamic soil structural reorganization. The disturbance of soils following the input of organic matter of different quality, i.e. with distinct decomposition rates and/or C/N ratios is examined and compared for bulk soil and rhizosphere environments to assess their effect on soil organi carbon and nutrient storage capacity. We further elucidate the spatial and temporal dynamics of carbon use efficiency (CUE). In summary, our approach provides valuable insights into the complex processes that govern soil carbon cycling.
How to cite: Ray, N., Rötzer, M., Prechtel, A., Lehndorff, E., and Scheibe, A.: Combining microbial modeling and soil structure dynamics for an improved understanding of soil organic carbon and nitrogen turnover, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3854, https://doi.org/10.5194/egusphere-egu25-3854, 2025.
Despite the importance of soil organic phosphorus (OP) for plant nutrition, its dynamics in ecosystems remain elusive due to the lack of multiple stable P isotopes. Here, we developed a method that isolates the soil OP pool to measure its carbon isotope signature. To develop the method, we, first, tested three extractants (0.5 M H2SO4, HCl, and NaOH), which are commonly used for soil OP extraction, to evaluate their capacity to preferentially extract OP. Next, we isolated OP from the extract by coprecipitation and adsorption, and for this purpose, we evaluated different pH treatments (ranging from pH 1.5 to 10), and iron- or aluminum hydroxide additions. Our results show that H2SO4 extracted the largest amount of soil OP, and these extracts had the lowest organic carbon-to-organic phosphorus (OC:OP) ratio compared to HCl and NaOH extracts. The pH adjustments of the extracts to pH 4 – 7.5 removed ≥ 95% of the extracted OP from the solution. The molar OC:OP ratio of the precipitates was the lowest (11 – 16) at pH 7.5, showing a strong preferential OP removal due to the pH alteration of the extracts. Metal hydroxide addition (combined with pH treatment) did not further improve the preferential OP precipitation. Finally, we determined the carbon isotope ratio (δ13C) of the isolated OP pool. Overall, the method developed here provides a simple and effective approach to determine the carbon isotope ratio of the soil OP pool, which opens new avenues to study soil OP dynamics.
How to cite: Tian, Y. and Spohn, M.: A method to determine the carbon isotope ratio of soil organic phosphorus, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4587, https://doi.org/10.5194/egusphere-egu25-4587, 2025.
The largest and most persistent portion of soil organic carbon (OC) is stored with mineral-associated organic matter (MAOM). Advancing our understanding of the processes and drivers involved in the formation and stabilization of MAOM is thus key to improving predictions of the mitigation potential of soils and their response to global change. While it is known that MAOM content and stability change with soil depth, our mechanistic understanding of the factors driving these changes is still incomplete. We expected that mineral type, as a determinant of a soil’s OC sorption and stabilization capability, and vegetation cover, as a modifier of organic inputs into the soil, would shape depth patterns in the content and stability of MAOM. We exposed pristine goethite (iron oxide) or illite (phyllosilicate clay) for five years in 24 forests (15 deciduous and 9 coniferous) and 23 grasslands (14 on mineral soils and 9 on organic soils) in three regions across Germany. Minerals were placed at 5 cm and 30 cm depths at all 47 sites, and additionally at 0 cm depth (i.e., at the boundary between the organic surface layers and the mineral soil) at forest sites. After recovery, the OC content of the field-exposed minerals was determined by dry combustion and the composition of the newly formed MAOM was determined using X-ray photoelectron spectroscopy (XPS). Stability of MAOM was indicated by the mineralizability of OM associated with the field-exposed mineral samples, by measuring the release of carbon dioxide per gram OC in laboratory incubations. Results show, on average, three times more MAOM formation in topsoils than subsoils at sites on mineral soils but we did not find any effect of depth at sites on organic soils. Changes in MAOM formation across depth reflected organic inputs from the overlying soil and were more substantial for coniferous forests than other vegetation covers, especially directly beneath the organic surface layer. We observed more substantial depth changes in MAOM content for goethite than illite. There was a consistent decrease in MAOM mineralizability (i.e., increase in stability) with soil depth for illite but not for goethite. Interestingly, the mineralizability of goethite-associated OM from forests was higher in subsoils than in topsoils. Mineralizability of MAOM was negatively correlated with the share of highly oxidized compounds (i.e., carboxylic/carbonyl C) across depth for goethite but not illite, suggesting different mechanisms underlie the depth-dependent changes in the stability of OM associated with the two minerals. Overall, our study evidence that depth patterns in the amount and stability of MAOM in soils are shaped by mineral type and vegetation cover. This insight can help guide process-oriented grouping of soils for improved prediction of soil OC content and stability across depth at larger scales.
How to cite: Bramble, D. S., Schöning, I., Ulrich, S., Mikutta, R., Kaiser, K., and Schrumpf, M.: Depth-dependent changes in the amount and stability of newly formed mineral-associated organic matter in temperate soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7250, https://doi.org/10.5194/egusphere-egu25-7250, 2025.
Soil N2O emissions are laborious and difficult to quantify and upscale to larger areas as they vary greatly in space and time. Nitrogen isotopes are gaining increasing attention in research as a potential tool for improving landscape-scale assessments of N2O emissions. This is because biochemical processes in the nitrogen (N) cycle discriminate against the heavier N isotope, 15N, resulting in distinct isotopic signatures in the product and residual substrate. Maps of the spatial distribution of isotopes, isoscapes, offer a promising approach for identifying spatial variability in N processes. However, integrating soil N isotopes into ecosystem models requires a better understanding of the drivers behind their spatial variation.
Moist depressions in crop fields are known to be N2O hotspots and thus represent key sites for exploring soil 15N patterns associated with N2O emissions. This study focused on the spatial patterns of soil N isotopes across two rolling fields in Zealand, Denmark. A total of 148 topsoil samples (0-10 cm) were collected along multiple topographic transects and analyzed for soil N and carbon content and isotopic composition, together with soil texture (at selected locations). Correlations between parameters were assessed using Spearman’s Rank Correlation Coefficient.
Emitted N2O is expected to be depleted in 15N relative to its source substrate. Based on this, we hypothesized that soil N in N2O hotspots (moist depressions) would show higher 15N enrichment compared to adjacent soils, due to greater losses of 15N depleted N. Contrary to this hypothesis, the results showed a significant positive correlation between soil δ15N and elevation, with the lowest δ15N values observed in the depressions. This suggests that processes other than N2O emissions play an important role in shaping the isotopic patterns. The study revealed substantial spatial variability in soil δ15N (3.8-9.8‰) underscoring the importance of sample location in determining isotope fractionation patterns and highlighting the need for further investigation to refine the application of isoscapes.
How to cite: Matthiesen, M., Rasmussen, C., and Ambus, P.: Can Isotopic Maps Reveal Soil N2O Hotspots?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9852, https://doi.org/10.5194/egusphere-egu25-9852, 2025.
Afforestation is used as a climate change mitigation measure and for reduction of nitrate losses to ground- and surface water. Temporal development of soil carbon (C) storage and nitrogen (N) retention since conversion from cropland to forest are not well understood. Stable isotope composition provides footprints of soil processes and can be utilized to give insight on soil C and N processing in a changing soil environment from cropland to forest. With this in mind, the present study investigates vertical and temporal development of soil δ13C and δ15N of the forest floor (FF) and underlying mineral soil profiles in an afforestation chronosequence. The chronosequence is confined to four oak forest stands established in 1970, 1977, 1988, and 2009 that have been sampled in 1998, 2011, and 2022 in the same plots resulting in an age span from forest age 2 to 52 years plus an additional pre-conversion cropland and a 200-year-old forest serving as an old growth reference. The samples used were collected in six soil depths including the FF to a maximum of 50 cm depth and were analyzed for C, N, and their stable isotope composition. Over time, the forest soil develops a natural stable isotope gradient with depth for both C and N. The cropland soil has the highest N content and exhibited a high and homogeneous δ15N through 40 cm soil depth (8.2 - 9.3 ‰). Despite a continuous C3 plant cover, a small decrease in δ13C from input plant residue can be distinguished after afforestation, resulting in a depleted FF compared to the cropland soil. 10 years after afforestation a soil δ13C and δ15N profile has developed with a depleted signal in the FF and the top 5 cm of the mineral soil. The depth gradients develop further over time towards the old growth reference. Both gradients develop as a result of a new isotopically depleted FF and following depletion of the top layers of the mineral soil due to mixing of new organic matter. From these footprints, an accumulation of new C in the mineral soil can be hypothesized despite another study having shown this specific forest’s mineral soil to be a C source to the atmosphere during the initial 40 years after afforestation. Additionally, a degree of C turnover is apparent from the steady state of the C isotope composition at the old growth reference where in most cases there is no significant difference in δ13C between sampling campaigns at either depth or between the three topsoil layers of the mineral soil at either campaign. Finally, only minimal amounts of N may have been lost via denitrification, as this process is typically associated with significant fractionations that would have resulted in soil δ15N enrichment, which is not observed in the top mineral soil.
How to cite: Ravn, L. E., Yu, M., Ambus, P., and Gundersen, P.: Development of carbon and nitrogen stable isotope abundances in afforested soil profiles of abandoned cropland over five decades , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10346, https://doi.org/10.5194/egusphere-egu25-10346, 2025.
Biogenic iron (oxyhydr)oxides (BIOS) are iron–organic carbon (OC) coprecipitates formed by Fe(II)-oxidizing bacteria that can be found in diverse terrestrial and aquatic environments, such as acid mine drainage, wetlands, and river systems. These biogenic iron minerals may contribute to effectively store OC as a “rusty sink”, where 21.5 ± 8.6% of the total OC is associated with iron minerals in sediments (Whitaker et al. 2021, Lalonde et al. 2012). The properties of BIOS (OC content, association via sorption or coprecipitation, surface area) are determined by the environmental conditions and the microorganisms they are formed by, influencing its effectiveness as a rusty sink. BIOS are rich in OC, which affects its physical properties (e.g. surface area, charge, reactivity) (Whitaker et al. 2021, Sowers et al. 2019). Thus, the bioavailability of the OC associated with BIOS (OCBIOS) might impact its properties as a carbon sink. Preliminary tests show that Fe(III)-reducing bacteria are not able to oxidize OCBIOS as the sole electron donor when coupling to Fe(III) reduction. However, in suboxic and anoxic environments, fermenters are known to play an important role in the degradation of OC and could therefore potentially use the OCBIOS.
Given the importance of BIOS as a carbon sink, we enriched a consortium of fermenting microorganisms from sediments of Lake Constance with different carbon sources (simple to recalcitrant) to test their ability to degrade OCBIOS. We measured gas emissions (CO2, CH4,H2), volatile fatty acid concentration, and DOC to follow the degradation of OC and the production of metabolites. Initial results indicated differential production of gases and organics depending on the amended carbon sources. We are determining the changes in the composition of the microbial community with 16S rRNA Illumina Sequencing. In the next step, we will test the ability of these fermentative enrichment cultures to degrade the OCBIOS of the photoferrotroph Rhodopseudomonas palustris TIE-1 (Jiao et al. 2005) and the nitrate-reducing Fe(II)-oxidizing culture KS (Straub et al. 1996).
Whitaker, A. H., Austin, R. E., Holden, K. L., Jones, J. L., Michel, F. M., Peak, D., Thompson, A., & Duckworth, O. W. (2021). The structure of natural biogenic iron (oxyhydr)oxides formed in circumneutral pH environments. Geochimica et Cosmochimica Acta, 308, 237–255.
Lalonde, K., Mucci, A., Ouellet, A., & Gélinas, Y. (2012). Preservation of organic matter in sediments promoted by iron. Nature, 483(7388), 198–200.
Sowers, T. D., Holden, K. L., Coward, E. K., & Sparks, D. L. (2019). Dissolved Organic Matter Sorption and Molecular Fractionation by Naturally Occurring Bacteriogenic Iron (Oxyhydr)oxides. Environmental Science & Technology, 53(8), 4295–4304.
Jiao, Y., Kappler, A., Croal, L. R., & Newman, D. K. (2005). Isolation and characterization of a genetically tractable photoautotrophic Fe(II)-oxidizing bacterium, Rhodopseudomonas palustris strain TIE-1. Applied and environmental microbiology, 71(8), 4487–4496.
Straub, K. L., Benz, M., Schink, B., & Widdel, F. (1996). Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. Applied and Environmental Microbiology, 62(4), 1458–1460.
How to cite: Lorenz, J., Tenelanda-Osorio, L., Kappler, A., and Mansor, M.: Degradation of the organic carbon associated with Biogenic Iron (Oxyhydr)oxides by fermenting cultures enriched from Lake Constance, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12614, https://doi.org/10.5194/egusphere-egu25-12614, 2025.
Wildfires, which impact millions of hectares globally each year, are among the most significant natural disturbances to terrestrial ecosystems [1]. Their effects on soil organic matter (SOM) and soil organic carbon (SOC)—critical indicators of soil health and ecosystem resilience [2,3]—are profound yet not fully understood. This study investigated the recovery of SOC and its molecular composition over a year following wildfires of varying severity (high, moderate, and low/moderate) in the Caldera de Taburiente National Park, La Palma, Spain. Using evolved gas analysis/mass spectrometry (EGA/MS) and thermogravimetry (TG), we analyzed quantitative and qualitative changes in SOM, focusing on the distribution of active and stable carbon pools across two soil depths (0–2 cm and 2–5 cm). The results revealed that high-severity fires caused substantial SOC losses, particularly in the topsoil layer (0–2 cm), with minimal recovery observed within one year. Moderate- and low/moderate-severity fires preserved a greater proportion of active carbon promoting faster and more consistent recovery patterns [4]. The subsurface layer (2–5 cm) showed greater resilience, exhibiting minimal SOM changes regardless of fire severity, likely due to its thermal insulation capacity. Molecular analyses by EGA/MS indicated that fire severity shaped SOC recovery dynamics and influenced the relative abundance of organic compound families, such as lipids, lignins, polysaccharides, and nitrogen compounds. High-severity fires led to the accumulation of more recalcitrant compounds, while lower severities retained higher proportions of labile fractions, accelerating soil regeneration [4]. These findings underscore the critical role of fire severity and soil depth in determining SOC recovery rates and composition. The study highlights the value of combining EGA/MS and TG as a rapid, reliable approach to assessing fire-induced changes in SOM, offering practical insights for post-fire soil health assessment and ecosystem management.
References:
[1] Pausas, J.G., Keeley, J.E., 2009. A Burning Story: The Role of Fire in the History of Life, BioScience 59, 593–601.
[2] González-Pérez, J.A., González-Vila, F.J., Almendros, G., Knicker, H., 2004. The effect of fire on soil organic matter—a review. Environ. Int. 30, 855–870.
[3] Jiménez-Morillo, N.T., De la Rosa, J.M., Waggoner, D., et al., 2016. Fire effects in the molecular structure of soil organic matter fractions under Quercus suber cover. Catena 145, 266–273.
[4] Jiménez-Morillo, N.T.; Almendros, G.; De la Rosa, J.M.; et al., 2020. Effect of a wildfire and of post-fire restoration actions in the organic matter structure in soil fractions. Sci. Total Environ. 728, 138715.
Acknowledgements: This work received support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the research project FIRE2C (ref. CNS2023-143750). N.T. Jiménez-Morillo acknowledges the “Ramón y Cajal” contract (RYC2021-031253-I) funded by MCIN/AEI/10.13039/501100011033 and the European Union “NextGenerationEU”/PRTR. The authors would like to thank the Caldera de Taburiente National Park (La Palma, Spain) for the sampling permits and logistic assistance during the experiment.
How to cite: González-Pérez, J. A., Correa, G., de la Rosa, J. M., Lorenzo, J. H., Miller, A. Z., and Jiménez-Morillo, N. T.: Rapid assessment of post-fire soil health recovery using thermal and molecular analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16762, https://doi.org/10.5194/egusphere-egu25-16762, 2025.
Soil organic matter (SOM) is the largest terrestrial carbon (C) pool and conceptually, it can be divided into mineral-associated organic matter (MAOM) and particulate organic matter (POM), with MAOM is regarded as more persistent than POM. MAOM is mainly formed by organic compounds attaching to the surfaces of reactive minerals, thus rendering the organic compounds less accessible to decomposers and their enzymes. Recent studies proposed that high-quality litter maximizes the synthesis of microbial products and residues, which then increasingly contribute to the formation of MAOM. However, the extent to which microbial or plant residues contribute more to MAOM formation is still under debate and may vary between mineral types. The accumulation of organic carbon and nutrients at mineral surfaces in the course of MAOM formation also changes the substrate availability to microbes, and thus, their community composition and traits, such as enzyme activities and carbon use efficiency (CUE). In order to better understand if MAOM formation is more efficient from microbial than plant-derived substrates, and how mineral types influence this process, we setup a one month incubation experiment with microbial and plant-derived substrates (bacterial residue (Bacillus subtilis), C:N = 3.7; fungal residue (Aspergillus niger), C:N = 12.5; maize litter (zuckermais yucon chief), C:N = 15.4) in presence of three minerals of different reactivity (pure quartz, SSA = 0.2 m2 g–1; quartz with 20 wt.% goethite, SSA = 15.4 m2 g–1; quartz with 20 wt.% illite , SSA = 34.6 m2 g–1) in a factorial design. A common inoculum from a German agricultural soil was added to each of these substrate–mineral mixtures. We found that both substrate quality and mineral type significantly influenced MAOM formation and microbial properties. Maize litter had the highest MAOM formation efficiency, followed by fungal residues and bacterial residues. We also found that the presence of minerals with higher reactivity reduced decomposition rates and increased MAOM formation efficiency (except for bacterial residue). Mineral type also affected the microbial community composition and its functioning, with higher enzyme activities in presence of goethite than illite and pure quartz. Accordingly, we found no evidence for preferential stabilization of microbial-derived over plant-derived residues on any of the tested minerals, particularly not for bacterial residues, which decomposed fastest and with the smallest CUE measured at the end of the experiment. Mineral type strongly influenced the microbial habitat with higher sorption leading to overall reduced decomposition in the presence of highly reactive (pristine) mineral surfaces.
How to cite: Yuan, Y., Kaiser, K., Mikutta, R., Kölbl, A., Goldmann, K., and Schrumpf, M.: How do minerals and organic substrates interact in controlling the efficiency of the formation of mineral-associated organic matter?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19684, https://doi.org/10.5194/egusphere-egu25-19684, 2025.
Conservation agricultural management (CA) including organic fertilization and crop rotations has been adopted to enhance organic carbon (OC) storage in arable soils mainly via increasing OC input to soils. However, it remains unclear how CA can affect OC in mineral-associated OM (MAOM) which is important for long-term C storage. For example, CA would potentially influence organo-mineral associations in MAOM by changing soil pH and/or the input of base cations (e.g. Calcium (Ca2+)). In the present study, to estimate OC bound to metal cations and reactive mineral phases, we first assessed soil extraction with pyrophosphate-dithionite (PD, pH 7.5) by measuring PD extractable metals and co-dissolved OC in soil and comparing with conventional extraction techniques (pyrophosphate, acid oxalate and dithionite-citrate). We then examined the extent to which long-term (> 20 yrs) CA enhances OC in MAOM and OC bound to metal cations and reactive mineral phases compared to inorganic fertilization (control) in arable topsoils under temperate climate.
Soils were sampled from eight long-term experimental sites with different soil managements (CA vs control) and under contrasting soil mineralogy (i.e. Andisol group (n=3) and non-Andisol group (n=5)) located in Japan, Canada, and France. Density fractionation (cut-off density: 1.8 g cm-3) with sonication (475 J mL-1) was conducted to isolate particulate organic matter (POM) and MAOM fractions in these soils. The nature of OC in MAOM fraction was then assessed by PD and other chemical extraction techniques.
As for the assessment of selective dissolution techniques, PD at near-neutral pH appeared to dissolve aluminum (Al) and iron (Fe) extractable by the conventional extractions, which suggests that PD extraction is a practical method to approximate OC bound to metal cations and reactive mineral phases. As for the effects of soil managements, POM-C was effectively enhanced by CA managements for both soil groups, whereas CA effectively enhanced MAOM only for non-Andisoil group. Among C pools in MAOM PD-extractable OC, which contributed 20±7 SD % of MAOM-C, was not enhanced significantly by CA. In the presentation we also plan to discuss relative importance of extractable metals (Al, Fe, and Ca) in OC present in organo-mineral associations of the studied soils.
How to cite: Fukumasu, J., Gregorich, E., Yang, P.-T., Kajiura, M., Chenu, C., and Wagai, R.: Effects of long-term soil managements on the nature of OC in organo-mineral associations in temperate arable soils: selective dissolution approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19912, https://doi.org/10.5194/egusphere-egu25-19912, 2025.
Dutch peat meadows, once carbon sinks, now contribute nearly 4% of the country’s CO2 emissions, releasing 6.9 Mton CO2-eq annually due to historical drainage and conversion to agricultural lands. Drainage exposes the organic-rich peat soils to oxygen, leading to increased microbial activity, organic matter decomposition, and associated CO2 emissions, thereby adding to global warming.
We hypothesize that clay addition to peat meadows will reduce soil respiration, as previously observed in mineral soils. The reduction is caused by 1) retarding oxygen diffusion 2) reducing enzyme activity by immobilization of enzymes 3) protecting substrate from microbial degradation through binding and/or physical protection.
We conduct long-term field- and lab incubation experiments, as well as short term laboratory experiments to gain mechanistic insights in the mitigating effects of clay addition on peat degradation. A wide array of clay types sourced from sedimentary marine and fluvial deposits in the Netherlands, is tested on their emission reduction potential. In the field, flux chambers measurements in the clay amended plots provide continuous CO2 emissions from peat soil and vegetation. Laboratory tests involve long-term laboratory incubations under controlled conditions, as well as Soxhlet analyses and potential enzyme activity.
To gain a better understanding of the influence of clay minerals on enzymes, ongoing laboratory experiments focus on how different clay minerals affect soil enzyme immobilization under substrate-saturated conditions.
Preliminary results, to be presented at the conference, provide more detailed insights about the specific interactions between clay particles and organic carbon in peat soils compared to mineral soils. These findings contribute to a deeper understanding of the mineral dynamics in this organic-rich environment.
How to cite: Hansen, J., Keuskamp, J., van Agtmaal, M., and Hefting, M.: The Effect of Clay Addition on Soil Respiration Dynamics in Peat Meadows, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19946, https://doi.org/10.5194/egusphere-egu25-19946, 2025.
