Soils serve as critical carbon sinks, playing a vital role in mitigating global warming and ensuring global food security. However, rapidly changing climatic and environmental conditions, such as extreme weather events, threaten the soil’s capacity to act as carbon sinks. Land use changes, particularly those driven by agricultural intensification, can alter hydrological regimes and carbon dynamics in landscapes. Understanding these dynamics by measuring GHG fluxes and investigating soil properties is crucial for designing sustainable land management practices that promote both environmental stability and climate resilience, ensuring soils continue to play a critical role in combating climate change.
Currently, little is known of how soil carbon responds to extreme events such as floods and droughts and how their repeated impacts shape carbon storage and loss and ultimately affect the carbon balance.
This study examined the impact of altered hydrological conditions, driven by the conversion of native vegetation to cropland, on carbon dynamics and carbon loss pathways. The aim is to identify patterns of methane and carbon dioxide emissions from naturally and recently inundated soils and their key driving factors.
We conducted in situ gas measurements using mobile trace gas analysers and a mobile smart chamber in the heavily agricultural Argentinian Pampas and Espinal ecoregions. Additionally, we collected soil samples from 0-30 cm depth for chemical analysis (including total and dissolved organic carbon) and measured soil temperature, electrical conductivity, soil moisture, and pH.
The results show complex interactions and dependencies between methane emissions and environmental variables. Methane fluxes are more than 5 times higher in saturated areas (median = 49.98 μg/m²) compared to dry areas (median =-8.34 μg/m²), primarily influenced by water table depth and soil moisture. Contrary to expectations, soil salinity, measured as electrical conductivity, exhibited a positive effect on methane production, reaching a threshold around 55 mS/m, suggesting possible tolerance or adaptation mechanisms of methanogens. Carbon dioxide emissions showed a reduction of almost 50 % in drier areas, primarily driven by soil moisture, highlighting the strong impact of moisture on carbon dynamics.
The findings highlight the critical role of hydrological conditions, particularly flooding, in driving methane fluxes from soils, emphasizing the need for targeted management practices to mitigate carbon loss and adapt to changing climatic conditions. Additionally, the effect of soil salinity on methane production underscores the importance of considering salinity in future research and management strategies.
How to cite: Nevermann, S., Jobbagy, E., Nosetto, M., Houspanossian, J., Diez, F., Ostle, N., and Rufino, M.: Carbon dynamics in agricultural soils: Insights into methane fluxes under changing hydrological conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16120, https://doi.org/10.5194/egusphere-egu25-16120, 2025.
The established paradigm assumes that drainage may decrease the vast soil organic carbon (SOC) reservoir in global wetlands. Yet drainage can also promote SOC stabilization by fostering the accrual of metal-bound organic carbon (bound OC) upon oxygen exposure. Here, this emergent mechanism is tested for the first time at a regional scale, using literature data and a nationwide, pairwise survey of drained wetlands across China. We show that long-term (15–55 years) drainage largely increased metallic protection of SOC (bound OC%) in non-Sphagnum wetlands, but consistently decreased bound OC% in Sphagnum wetlands following replacement of the ‘rust engineer’ Sphagnum by herbaceous plants. Improved SOC stock estimates based on 66 soil profiles reveal that bound OC increases can compensate for the loss of unbound SOC components in non-Sphagnum wetlands with substantial accrual of reactive metals. Metallic stabilization of wetland SOC is hence a widespread but overlooked mechanism that is heavily influenced by vegetational shifts. Incorporating this novel mechanism into models will improve prediction of wetland SOC dynamics under shifting hydrological regimes.
How to cite: Liu, C., Zhao, Y., and Feng, X.: Metallic protection of soil carbon: Divergent drainage effects in Sphagnum vs. non-Sphagnum wetlands, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2284, https://doi.org/10.5194/egusphere-egu25-2284, 2025.
Soil organic carbon (SOC) decomposition underpins soil-atmosphere carbon exchange and is regulated by climate change-mediated variation in soil redox conditions. Soil anoxia, commonly occurring following precipitation, soil flooding and erosion events, is assumed to preserve SOC. Yet, water saturation may also increase SOC decomposition relative to unsaturated conditions, and contradictory findings among previous studies remain unexplained. Here, using incubation experiments on 20 soils collected across a 24° latitude gradient in China, we show that 70% of the soils showed higher or similar anoxic decomposition rate of SOC compared to oxic treatment after 2–3 weeks, suggesting fast SOC loss under anoxia. Variation in alternative terminal electron acceptors shows that fast anoxic decomposition was primarily driven by iron (Fe) reduction, which accounted for up to 90% of anoxic CO2 production. Meanwhile, positive relationships among water-extractable organic carbon (OC), ferrous Fe, and SOC decomposition rate suggest release of readily metabolized substrates following Fe reduction, providing substrates for anoxic metabolism and potentially leading to the loss of OC protected by Fe (Fe-bound OC; a slow-cycling OC pool under oxic conditions). Mass balance calculation confirms that Fe-bound OC loss was similar to elevated anoxic SOC decomposition in magnitude, and random forest modeling indicates that soils rich in reducible Fe and SOC most likely experience elevated SOC decomposition under anoxia. Overall, our findings demonstrate that fast anoxic decomposition of SOC is a potentially important pathway that may stimulate SOC loss under climate change-mediated intense hydrologic regimes, particularly for soils rich in reducible Fe and SOC.
How to cite: Feng, X., Liu, T., Wang, X., Wang, S., Zhu, E., and Hall, S. J.: Iron-driven fast decomposition of soil carbon under anoxia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2314, https://doi.org/10.5194/egusphere-egu25-2314, 2025.
Carbon dynamics in peatlands are regulated by biogeochemical processes, including heterotrophic respiration, where microorganisms utilize organic carbon (OC) as an electron donor and respire CO2. In the absence of oxygen, ferric iron (FeIII) is an important electron acceptor. However, the presence of FeIII minerals can also modify carbon dynamics by adsorbing OC or occluding OC in microaggregates, thus limiting the mineralization of OC. Along with its speciation and heterogeneity across soils, the overall impact of iron (Fe) on OC mineralization in mineral-rich peatlands remains unclear. To investigate this complex role of Fe and its impact on OC mobilization, we designed model anoxic soil incubations, where multiple Fe species were added to mimic soil heterogeneity. To this end, reactive Fe species (ferrihydrite; Fh, a ferrihydrite-silicate coprecipitate; FhSi in which Si:Fe = 0.05 mol/mol, FeIII-peat complex; FePeat) and more stable Fe species (goethite; Gt) either in pure forms (Fh, FhSi, and Gt) or mixtures of the two (95/5% Gt/Fh; GtFh, 95/5% Gt/FePeat; GtFePeat) were added to an ombrotrophic peat, increasing the Fe content of the soil from 0.1% to 6% (w/w). The incubations were prepared anoxically in crimp-sealed vials and lasted for 70 days. Two incubation series were established to allow for (1) measurements of the headspace CO2 concentrations (over 60 days) and (2) sampling of the soil slurry after 4, 17, 35, and 70 days. The latter was used to follow trends in pH, Eh, dissolved organic carbon (DOC) and Fe speciation in the aqueous-phase, and amounts of OC and Fe mobilized from the solid-phase in sequential chemical extractions: 0.5 M HCl (sorbed Fe), hydroxylamine-HCl (short-ranged-ordered Fe oxyhydroxides), and 6 M HCl (crystalline Fe hydroxides).
The results show that the addition of Fe species changes the carbon dynamics. The addition of reactive Fe species (Fh, FhSi) promoted CO2 production and resulted in higher concentrations of aqueous Fe, suggesting reductive dissolution of the minerals as they served as extra electron acceptors for microbial respiration. In contrast, in the Gt treatment, the goethite addition alone did not affect CO2 production until the 20th day, after which CO2 production was first inhibited and then promoted (after 42 days) compared to a control treatment which received no Fe additions. However, when small fractions (5%) of reactive Fe species were added alongside goethite (GtFh and GtFePeat), CO2 production was up to 1.5-2.2 times higher than in the Gt treatment. Yet, the lowest DOC concentrations were measured in Fh and FhSi, suggesting that the ferrihydrites re-adsorbed the released OC. Furthermore, while fractions of extractable Fe in Fh and FhSi did not change significantly over the incubation, a strong increase in 6 M HCl extractable Fe in all goethite-containing treatments suggests that, although less reductive dissolution occurred, mineral recrystallization may have occurred.
These results highlight the complex impacts of exogenous Fe species on carbon dynamics and shed light on the vulnerability of peatlands as carbon sinks in the context of climate change, where changes in groundwater geochemistry, including Fe content and watertable fluctuation, may be expected.
How to cite: Tseng, Y.-H., Zelano, I., and ThomasArrigo, L.: Impacts of exogenous Fe species on C dynamics in peat soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10400, https://doi.org/10.5194/egusphere-egu25-10400, 2025.
The release and translocation of organic matter and metal cations, particularly Al and Fe, from topsoil and their immobilization in the subsoil is the key pedogenetic process active in podzols. In the Fichtelgebirge mid-mountain range, Northern Bavaria (Germany), strong acidic conditions have been reported since the 1980s. Independent of the dominant parent material, the soils are characterized by distinct podsolization reflected by elevated concentrations of Fe and Al in the soil solutions, the subsoil horizons, and even in groundwater. Previous research on these matured podzols revealed organometallic complexes as primary forms of Fe and Al in those soils. We hypothesize that the transport fate of Fe and Al is governed by the dynamics of organic matter rather than the dissolution and mobilization of Fe and Al-bearing minerals.
In this experimental pedogenesis study, we conducted a series of soil column experiments using materials from Fichtelgebirge topsoil (Ae) and subsoil (Bs) horizons to investigate the role of organometallic complexes as well as the interplay of (im)mobilization of Al, Fe, and organic matter. Over a period of more than 4 months, we analyzed the effect of different inflow solutions, including artificial rainwater (ARW) and organic matter-enriched ARW, on the composition and properties of the effluent at high temporal resolution combining complementary instrumental analytical techniques.
The results showed that flow variations and changes in ionic strength during tracer application significantly increased the mobilization of particles, elements, and organic matter. During the application of high ionic strength influent solution, we observed the release of iron species from cation exchange sites, leading to a decrease in aggregate stability and particle release, thus peptization. In contrast, aluminum is released in association with organic matter after conditions of low ionic strength re-established, clearly showing that Fe and Al follow distinct release dynamics. Furthermore, calcium ions replaced protons during the addition of ARW finally increasing effluent pH. However, the short-term application of organic matter compensated for this leaching by providing additional protons. This renders the supply of organic matter from litter a decisive source of protons and main contributor to soil acidification and podzolization.
How to cite: Eid, R., Ritschel, T., Guhra, T., Papsdorf, R., and Totsche, K.: Coaction of dissolved organic matter and electrolyte fluctuations on Al and Fe dynamics in matured podzols – what matters more? , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18172, https://doi.org/10.5194/egusphere-egu25-18172, 2025.
Content and composition of soil organic matter (SOM) is linked to soil ecosystem services like nutrient storage. Information on the molecular composition of SOM in terms of functional groups (e.g. carboxyl; C=O, and carboxylate groups; COO-) can be obtained by spectroscopic approaches like Fourier Transform Infrared (FTIR) spectroscopy. Complex interactions between SOM, metal cations as well as soil minerals affect dynamics and reactivity of SOM, and cause -at the molecular level-changes in the strength of the bonds between C and O. The latter affects the wavenumbers (WN) and intensities of COO- absorption bands in FTIR spectra. Such changes may become challenging when using approaches like partial least square analysis or machine learning to interpret FTIR spectra of soil samples with respect to SOM content or soil properties, because such approaches are based on fixed ranges of WN (from textbooks), or sets of single WN determined by statistical approaches.
The aim of current study is to enhance the mechanistic understanding of the interactions between SOM, cations and minerals by studying the cation/mineral effects on spectral data of organic matter in terms of absorption intensity and WN of band maxima characteristic for C=O and COO- groups compared to the spectral data of organic matter itself.
Humic acids (HA; as a model substance for SOM) as well as mixtures of HA with Fe3+ were prepared at a 4:1 (HA:Fe3+) stoichiometric ratio in absence or presence of finely ground minerals (quartz, illite, and montmorillonite; to simulate soil solid surfaces). The mixtures were freeze-dried, and characterized by FTIR spectroscopy using KBr technique. After smoothing, baseline correction, and “subtraction procedure” to minimize the bands typical for soil minerals that might overlap with of humic acid, the FTIR spectra were interpreted for the intensity of C=O and COO- bands, both related to polysaccharide (COC) band intensity as a reference. The spectra of HA-Fe3+ mixtures compared to those of HA showed an increase in intensity (although the SOM amount did NOT change) and a shift in WN of COO- band maxima (WN 1620-1550 cm-1). For the HA-Fe3+-mineral mixtures, the changes in both, intensity and shift, were significantly higher compared to HA-Fe3+ mixture with the strongest effect for HA- Fe3+ - montmorillonite mixture. Compared to the COO- band, effects on C=O band (WN 1750-1719 cm-1) were weaker.
Comparing FTIR spectra of HA-Fe3+, and HA-Fe3+-mineral mixtures with that of HA shows that SOM-cation as well as SOM-cation-mineral interactions may affect the spectral data of OM. This finding suggests FTIR analysis to offer a possibility for reflecting on OM-cation/mineral interactions. However, the changes observed for spectral properties of HA-Fe3+ -mineral mixtures compared to those of HA suggest that SOM-cation/mineral interactions also becomes of relevance when interpreting FTIR spectra of soil samples with respect to soil properties because the automated approaches used to do so are mostly based on predefined WN ranges and assumes band intensities to reflect on the amount.
How to cite: Bhattarai, N. and H. Ellerbrock, R.: Effects of cations and minerals on FTIR spectra of humic acid, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3605, https://doi.org/10.5194/egusphere-egu25-3605, 2025.
One of the key parameters defining the healthy functioning of soil is its structure. The organization of mineral particles, organic matter (OM), water, and air into a complex matrix of soil aggregates plays a particularly important role in long-term carbon (C) storage, as C compounds can be ‘hidden’ within the aggregate structure, shielding them from decomposers. Soil aggregation is a dynamic process influenced by physical, chemical, and biological factors; however, the individual and combined effects of these factors on the formation and turnover of aggregates are not well understood.
The aim of this study was to examine the incorporation of fresh litter inputs with differing physicochemical properties, including their carbon-to-nitrogen (C/N) ratio—maize (C/N = 12) and straw (C/N = 103)—into aggregates formed de novo from mineral soil, with or without the presence of microbiota. Using rare-earth element oxides, we labeled structures formed during a four-week incubation with a single litter type and traced their incorporation into newly formed aggregates after mixing the soils and incubating them for a subsequent seven-week period.
We found that, regardless of quality, litter was the most important factor driving soil aggregation during the initial stages of the process. The presence of both litter types together further enhanced aggregate formation. Contrary to our hypothesis, and likely due to the short time frame of the experiment, neither microbial abundance nor community composition significantly affected overall aggregation. However, further visualization of the different litter-associated structures across the cross-sections of the aggregates from various size fractions using synchrotron radiation-based X-ray fluorescence nanospectroscopy (SR-nanoXRF) enabled us to estimate potential influence of the microbes via their preferred litter type. Specifically, contrary to our expectations the bulk analysis showed that bacteria-favoured low C/N ratio maize litter had a stronger effect on both overall aggregation and the formation of macroaggregates, which we initially hypothesized would be supported by the high C/N ratio straw litter preferred by fungi. However, further analysis of the XRF intensity maps confirmed an increasing incorporation of straw-associated soils into >250 μm structures, likely facilitated by fungal growth and hyphal enmeshing. Phospholipid fatty acid analysis further corroborated this, showing a relatively higher abundance of fungi in macroaggregates in straw-containing soil.
We also implemented semi-variogram analysis on the XRF maps, which allowed us to estimate the size and distribution of straw- and maize-associated structures within the aggregates. We found that while microaggregates were more commonly formed from individual litter-associated structures, larger aggregates (> 250 μm) were newly made from de-aggregated soil.
In conclusion, our study provides insights into the initial stages of aggregate formation following litter additions and the development of associated microbial communities. The spatial analysis enabled by SR-nanoXRF allowed us to visualize internal aggregate structures, shedding light on processes that cannot be fully understood through bulk analysis alone.
How to cite: Pucetaite, M., Persson, P., Parker, J., Johansson, U., and Hammer, E.: Visualization of soil aggregates via X-ray fluorescence nanoscopy provides new insights into primary aggregation processes induced by litter inputs, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15205, https://doi.org/10.5194/egusphere-egu25-15205, 2025.
Forest soils are substantial sinks for carbon, yet the effectiveness of management practices such as strict conservation to sequester more soil organic carbon is unknown. A complication is the increase in forest disturbance due to climate change, key to which is the loss of canopy cover. We sampled paired control-canopy gap plots in drought-sensitive Norway spruce stands on Stangosols developed on Buntsandstein in the Black Forest National Park (2000 mm MAP, 5.5°C MAT). Three pairs are located within the management zone with sanitation logging of beetle infestations (gaps < 5 years old with little deadwood), and three more are in the core zone without logging since 1911 (gaps < 15 years old with moderate deadwood).
In the management zone, canopy gaps had warmer and slightly wetter forest floors (+0.45 °C and +1.7 % Vol), but thickness, carbon stocks and C/N ratios in the forest floor did not differ between canopy treatments. In the core zone by comparison, circa 50 Mg ha-1 more carbon was found under closed canopies than canopy gaps as well as both canopy types in the management zone. Density fraction revealed most changes occurred in the free and occluded light fractions, which constituted circa 50 and 40% of SOC, respectively, in the core zone.
Lignin-derived phenols were extracted with cupric oxide oxidation to trace the source of soil organic matter (SOM). Lignin markers in the forest floor came mostly from coniferous wood and at times was less oxidized in canopy gaps. Additionally, subsoil horizons exhibited surprisingly little lignin oxidation regardless of canopy treatment, resulting in lignin-derived phenols constituting up to circa 20% of SOM. This applied to other CuO-extractable phenols, which in subsoil accounted for a further 10% of SOM.
The increase in core zone SOC stock is of a similar magnitude to accumulated deadwood if averages of 10 m3 ha-1 year-1 annual growth increment and 33% mortality are assumed. Wood-derived lignin absent in canopy gaps likely underwent less-oxidative photodegradation and leaching both deeper into mineral soil and laterally into catchments. Sequestration of wood-derived particulate SOM is thus possible in moist, acidic sandy soils, but such SOM is sensitive to disturbance-driven microclimatic changes.
How to cite: Stutz, K., Benz, L., Kaiser, K., Glaser, B., and Popa, F.: Century of forest conservation sequesters wood-derived particulate organic matter, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18803, https://doi.org/10.5194/egusphere-egu25-18803, 2025.
Soil organic matter (SOM) is an essential soil component for land sustainability and ecosystem functioning in addition to its potential for soil carbon (C) storage. This study evaluate the effects of tillage (moldboard plow (MP) and no-tillage (NT)) and commercial fertilizer as limestone ammonium nitrate (28-0-0) at three rates (0, 100, and 200 kg N ha–1) on SOC and SOM fractions (particulate organic matter-C (POM-C) and mineral-associated organic matter-C (MAOM-C)). The study was established in 2008 on the National University of Lesotho Campus Farm, Roma Valley of the Maseru District in Lesotho, southern Africa. Soil samples were collected from 0–5, 5–10, 10–15, and 15–30 cm depths. Under NT, the SOC and POM at 0-15 cm were 54 and 40% higher than 15-30 cm depth, respectively. The MP had 17 and 35% higher SOC and POM at 0-15 than 15-30 cm depth. The highest N-rate (200-N) increased POM by 28.8% for MP and 22.6% for NT than the100-N rate. The C:N ratio was highest with coarse-POM and lowest with MAOM at both tillage practices. The NT managed soil conserved the majority of SOC within MAOM fraction. In contrast, the MP conserved the majority of SOC within fine-POM fraction which made it more susceptible to loss particularly through wind erosion. The MP managed soil exhibited SOC losses from the MAOM indicating that the MAOM is readily destabilized and lost its associated C. The data generated from this study shows a unique distribution of SOC among the SOM fractions that was partially influenced by land management. These findings suggest the need for conservation efforts to reduce SOM losses and improve land sustainability and SOM conservation.
How to cite: Mikha, M. and Marake, M.: Land Management Influenced Soil Carbon Distribution in Lesotho, Southern Africa, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1352, https://doi.org/10.5194/egusphere-egu25-1352, 2025.
Posters on site: Mon, 28 Apr, 08:30–10:15 | Hall X3
Soil carbon sequestration is suggested as a mechanism to remove CO2 from the atmosphere; however, uncertainty persists regarding the residence time of the stored carbon. Different land uses on the same soil series were selected to compare the amount and properties of soil carbon present and to evaluate the soil-sorbed carbon by loss on ignition estimation and X-ray photoelectron spectroscopy. Surface horizon soil samples were collected from a second-growth cedar forest, mowed grassland, hybrid poplar forest, perennial strawberry field, and an annually cropped wheat field at Totem Field at the University of British Columbia in Vancouver, British Columbia. Soils were analyzed using loss on ignition (LOI), X-ray diffraction (XRD), and angle-resolved X-ray photoelectron spectroscopy (XPS). Results indicate high variability in soil properties and carbon storage across different land uses. Specifically, perennial vegetation exhibited lower soil bulk density and higher soil carbon content compared to agriculturally managed fields, correlating with differences in soil pH. XPS indicated major differences in the amount of C-C and C=O bonds and minor differences in the amount O-C=O and Pi-Pi bonds associated with soil in the different land uses. This study contributes valuable insights that help to inform the relationship between land use practices and soil carbon storage potential.
How to cite: Fausak, L., Diaz-Osorio, F., Reinesch, A. C., and Lavkulich, L.: Application of X-ray Photoelectron Spectroscopy (XPS) to Assess Soil Organic Matter Under Different Land Uses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-57, https://doi.org/10.5194/egusphere-egu25-57, 2025.
Soil minerals such as iron (Fe) oxyhydroxides, are commonly associated with organic matter (OM) and silicon (Si) in complex, heterogeneous mineral frameworks that passivate the mineral surface. Thus, both Si- and OM-associated Fe minerals typically exhibit slower redox-induced transformation kinetics and follow distinct transformation pathways. Interestingly, certain OM coatings can also actively mediate Fe mineral transformations, leading to chemically modified structures and altered OM distribution patterns. However, the balance between the passive and reactive roles of OM, the potential synergistic or antagonistic effects of Si and OM on Fe mineral reactivity, and the impacts of mineral transformation on OM association and distribution remain poorly understood.
In this study, we investigated Fe(II)-catalyzed transformations of ferrihydrite with varying Si/Fe ratios in the presence of small organic compounds with different functional group compositions, under both oxic and anoxic conditions. Abiotic reductive dissolution of ferrihydrite was observed upon incubation with cysteine but not with glutathione, despite both compounds containing redox-active thiol groups. In the presence of cysteine under anoxic conditions, aqueous Fe(II) catalyzed the transformation of ferrihydrite into lepidocrocite, with 50% and 65% transformation observed within 6 and 21 days, respectively, as confirmed by XRD, Mössbauer spectroscopy, and TEM.
In contrast, Si-ferrihydrite (Si/Fe = 0.09) displayed a slower transformation extent (33%) and rate, with transformation products appearing only after 10 days of anoxic incubation. Moreover, both goethite and lepidocrocite formed, indicating that Si also influences the mechanism of mineral transformation. Surprisingly, while no mineral transformation of Si-ferrihydrite was detected by XRD or Mössbauer spectroscopy after 6 days, TEM imaging revealed the presence of a more crystalline and porous intermediate phase. This unidentified phase exhibited bifurcated and non-aligned growth patterns, suggesting it may serve as a precursor to more crystalline structures. These findings provide key insights into how Si and OM co-effects influence Fe mineral evolution pathways.
TEM imaging of 6-day incubated minerals (under anoxic conditions) also revealed differences in the affinities of organic coatings following ferrihydrite transformation. A distinct preference for pristine ferrihydrite over newly formed lepidocrocite was observed. However, in the case of Si-ferrihydrite, a uniform coating was maintained. This highlights how Fe mineral transformations may affect the affinity and distribution of associated OM, influencing its stability and persistence in the environment.
How to cite: Zheng, Y., Asraf, B., and Engel, M.: How Si and Organic Matter Shape Ferrihydrite Transformation Pathways, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5071, https://doi.org/10.5194/egusphere-egu25-5071, 2025.
The global increase in food demand and the effects of climate change require ever higher area irrigation of arable land. On the other hand, the concentrations of organic micropollutants (OMPs) in irrigation water sources are also steadily increasing. However, the effects of OMPs on the microbiota and, thus, on soil organic matter (SOM) stability are still poorly understood.
Consequently, our primary objective is to investigate the effects of pharmaceutically active compounds (PhACs) and their metabolites (Carbamazepine -CBZ, Trans-carbamazepine - TCBZ, 17α-ethynylestradiol - EE2, 17β-estradiol - βE2, 17α-estradiol - αE2, Estrone - E1, Estriol - E3, Diclofenac-sodium - DFC, 5-Hydroxydiclofenac - 5-HDFC), an antifungal compound (difenoconazole - DCL) and an antibiotic (Ciprofloxacin - CPX) on the stability of soil organic matter. Soils were incubated after seven treatments: control, PhACs, CPX, DCL, CPX+PhACS, DCL+PhACs, CPX+DCL+PhACs.
We have selected a soil type (Phaeozem) for our research, which is typical of the Earth's arable lands. The soil studied has not been documented to have been tilled for several decades, so past human impacts are now negligible. A series of 90-day incubation experiments were performed, in which soils were pretreated with defined concentrations of OMPs.
Quantitative and qualitative parameters of soil organic matter were determined using a CHNS elemental analyser, an NDIR CN analyser, a UV-Vis spectrophotometer (SUVA254 and phosphatase and hydrolase activities), and a spectrofluorometer (Coble peaks and PARAFAC analysis). Microbial communities were also monitored (16S and 18S RNA sequencing). Since the data were not normally distributed, the Kruskal-Wallis test was used for statistical data analysis.
The ratio of labile fractions of SOM was primarily estimated based on the dissolved fraction ratio (DOC/SOC), aliphatic components, and protein-like components. The microbial activity was estimated based on enzyme activities.
The soil chemistry and microbiological parameters studied were significantly different for all treatments. The proportion of labile fractions decreased over time in all treatments. Contrary to our initial assumption, the proportion of labile fractions decreased most significantly in the combined treatment. The microbial communities changed in different patterns during the incubation.
Our results show that the presence of OMPs in irrigation water affects the ratios of labile and stable fractions SOM and also influences the rate of decomposition.
Hungarian Scientific Found funded the study, project no. K142865 and DKOP-23-03.
How to cite: Szalai, Z., Bauer, L., Dévény, Z., Jakab, G., Filep, T., Zacháry, D., Vancsik, A. V., Maller, C., Vajna, B., and Szabó, L.: Effects of organic micropollutants on the stability of soil organic matter: the hidden effect of the irrigation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8146, https://doi.org/10.5194/egusphere-egu25-8146, 2025.
Soil organic matter (SOM) content is an essential indicator of soil fertility, functionality, and health. Increasing SOM content also improves the hydrological conditions of the soil and, therefore, is a helpful tool against climate change-affected extreme precipitations and droughts. Moreover, the carbon amount additionally stored in the soil may support the reduction of greenhouse gas concentration in the atmosphere. SOM is primarily stabilized by minerals being resistant to decomposition. Many land use and agrotechnical-related initiations have succeeded in increasing SOM in the surface layer. However, half of the SOM is globally stored in the subsoil (>30cm). The primary way of SOM migration to the subsoil is through water-solved transport via infiltration and leaching. Nonetheless, we have only limited knowledge of potential SOM increases in the subsoil. The present study aimed to investigate the roles of soil mineral types on dissolved SOM stabilization. Quartz, illit, muscovite, goethite standards, and a mixture of them (model soil) were treated with the IHSS Suwannee River fulvic acid standard III. The adsorbed Carbon content was measured by mass spectrometry, and the SOM composition was measured by X-ray photoelectron spectroscopy (XPS). SOM changes in the liquid phase due to adsorption were investigated using high pressure liquid chromatography (HPLC). Results indicated that the adsorption order of the minerals was quartz<muscovite<<illite<<goethite. The adsorbed carbon amount was not linked directly to the minerals' surface area, indicating the highlighted role of mineralogy. The model subsoil adsorbed less SOM compared to the prediction based on the single mineral adsorption results, indicating mineral aggregation and active surface decrease. HPLC results of the remaining SOM compounds in the solution after adsorption showed inverse linkage to the solid surface related XPS results, proving the parallel applicability of the two approaches. HPLC indicated four SOM peaks, of which three had lower measured than predicted adsorption for the model soil. Quartz preferred to adsorb hydrophobic organic matter, whereas goethite showed a hydrophilic preference. Overall, the results proved the importance of mineral composition on organic matter compound preference and stabilization. This work was supported by the National Research, Development and Innovation Fund of Hungary [project no. K 142865].
How to cite: Jakab, G., Dévény, Z., Vancsik, A., Sulyok, A., Frey, K., Karlik, M., Király, C., and Szalai, Z.: The role of minerals in fractionated soil organic matter stabilization in the subsoil, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9221, https://doi.org/10.5194/egusphere-egu25-9221, 2025.
The majority of soil carbon resides in mineral-associated organic matter (MAOM) in most soils. MAOM is assumed to be relatively inert to environmental change because it is protected from microbial activity thus contributing substantially to soil carbon storage. However, plants and associated microbes may destabilize MAOM through mineral dissolution and exchange reactions in the rhizosphere, potentially causing soil carbon loss. Here, we quantified the magnitude of MAOM destabilization in the rhizosphere in response to extreme precipitation dynamics expected with climate change. To do this, we followed the fate of labeled MAOM (13C microbial necromass adsorbed to iron minerals: goethite or ferrihydrite) in the wheat (Triticum aestivum L.) rhizosphere during a 12-week pot experiment subjected to precipitation regimes mimicking the status quo (optimal conditions) as well as extreme intermittent droughts and flooding as is expected in central Europe with climate change. We found that MAOM destabilization was significantly greater under flooding compared to optimal and drought conditions. MAOM destabilization was particularly accentuated in MAOM initially bound to poorly crystalline ferrihydrite compared to MAOM initially bound to more crystalline goethite. Our results suggest that MAOM bound to poorly crystalline minerals (ferrihydrite) may be particularly vulnerable to destabilization in the rhizosphere during intense precipitation events, contributing to soil carbon loss. With the increasingly extreme nature of precipitation events, understanding the underlying MAOM destabilization mechanisms in the rhizosphere is essential for better predictions of the soil carbon response to environmental change.
How to cite: Frei, J., Jamoteau, F., Van der Loo, E., Bosco-Santos, A., Mendoza, O., Ballu, A., Kocsis, L., Spangenberg, J., ThomasArrigo, L., Muehe, E. M., and Keiluweit, M.: Episodic drought and flooding impacts on the destabilization of mineral-associated organic matter in the rhizosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10893, https://doi.org/10.5194/egusphere-egu25-10893, 2025.
However, calcareous soils cover more than 30% of the Earth's surface, research on the mechanisms of soil organic carbon (SOC) stabilization mostly focus on acidic soils. The SOC stabilization in calcareous soils is less frequently studied.
The present study investigates the carbon fractions with different stability in 29 Hungarian calcareous soil samples. The soil samples have varying organic (0.12 - 19.38 %) and inorganic (0.00 - 5.39 %) carbon content and represent different soil types (Cambisol, Arenosol, Gleysol, Solonchak, Regosol and Chernozem), land uses (forest, grassland, arable and marshy meadow) and depths (0-220 cm).
Physical fractionation was applied in order to represent the different organic matter fractions with varying stabilities. Six particle size fractions were separated during the fractionation: > 200 µm, 50-200 µm, 20-50 µm, 2-20 µm, 0-2 µm and particulate organic matter (> 200 µm). The mass, organic and inorganic carbon content and nitrogen content of the fractions were determined. The chemical characterisation of the fractions was determined using FT-IR spectroscopy. For the detection of relative changes in the spectra and the chemical characterization of the different particle size fractions, an aromaticity index (A1610 cm-1/A2920 cm-1) and relative absorbances (2920, 1740, 1680, 1610, 1525, 1270, 1160 and 1050 cm-1) were calculated.
Principal component analysis showed great differences between the six particle size fractions in terms of their chemical properties (characteristic organic compounds, aromaticity and C/N ratio).
This research was funded by the SIC-SOC-DYN “Organic and inorganic carbon dynamic in calcareous soils” project of the 1st external Call within the EJP SOIL program, the National Research, Development and Innovation Fund, Hungary [Project N° 2019-2.14-ERA-NET-2022-00037 and FK 142936] and the French National Research Agency [Project N° ANR-22-SOIL-0003-01].
How to cite: Zacháry, D., Filep, T., Jakab, G., Király, C., Szalai, Z., and Chevallier, T.: Chemical characterization of particle size fractions of calcareous soils from Hungary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11338, https://doi.org/10.5194/egusphere-egu25-11338, 2025.
Deciphering controls on soil organic carbon (SOC) is fundamental for predicting SOC distribution and sequestration potential. Mineral-associated organic carbon (MAOC) is recognized as the most stable, making it vital for modeling long-term SOC dynamics. However, its controlling factors and pathways remain unclear, particularly in tropical and subtropical soils. We used topsoil from 66 sites (non-volcanic, mainly natural vegetation) from India (arid and high pH), Tanzania, Cameroon, Japan (Okinawa), Indonesia (humid and low pH) with a wide range of soil pH (3.9 to 9.2) and moisture condition (represented by effective precipitation, EP = precipitation – potential evapotranspiration, −1740 to 2850 mm). Soils were categorized as strongly acidic (pH ≤ 5.5), weakly acidic (5.5 < pH ≤ 7), and alkaline (pH > 7). Soil fundamental physicochemical properties and mineral components that may influence MAOC, including active Al/Fe (oxalate-extractable Al and Fe: Alo + Feo), clay, and exchangeable Ca and Mg (Caex + Mgex), were determined. MAOC was measured as the carbon content in the fine heavy fraction (FHF, > 1.7 g cm−3 and <53 µm) obtained through density and particle size fractionations. The ratio of MAOC to FHF was used to indicate the carbon stabilization ability of FHF. Correlation analyses examined the influence of climate, vegetation, and soil properties on MAOC. Structural equation modeling (SEM) quantified the contributions of factors correlated with MAOC to its accumulation.
The fractionation results showed that MAOC accounted for 76 ± 14% of SOC, confirming it as the primary fraction regulating overall SOC. Correlation analysis identified pH, EP, net primary productivity, and active Al/Fe and clay contents as significant factors affecting MAOC. Notably, no significant relationship was found between MAOC and Caex + Mgex, even in alkaline soils, suggesting that Ca and Mg ions play a minimal role in SOC stabilization. SEMs revealed active Al/Fe content as the primary factor across all pH categories, regulating most of MAOC (β > 0.48, R2 > 0.80 for all, strongly acidic, and weak acidic soils; β = 0.62, R2 = 0.61 for alkaline soils). Direct impact of EP was the secondary factor. Introducing clay content as a parallel factor to active Al/Fe reduced quality metrics (e.g., P < 0.05) of SEMs for all pH categories and showed no significant contribution to MAOC, indicating its less importance even in alkaline soils. Interestingly, the carbon stabilization ability of FHF was comparable in strongly and weakly acidic soils but significantly lower in alkaline soils. This difference is likely due to the lower active Al/Fe content in alkaline soils, where the intensified drying enhanced crystallization. The lower slope of MAOC to Alo + Feo in alkaline soils further highlights the reduced carbon stabilization ability of active Al/Fe, likely due to the lowered positive charge in alkaline conditions and decreased hydroxyl groups from enhanced crystallization. In summary, active Al/Fe controlled MAOC, which constituted most of SOC, while soil pH and moisture conditions regulated its abundance and carbon stabilization ability, and higher moisture levels also directly enhanced MAOC.
How to cite: Lyu, H., Tokunaga, A., Ashida, K., Hartono, A., Kilasara, M., David Mvondo Ze, A., Funakawa, S., Watanabe, T., and Sugihara, S.: Control of active Al and Fe on mineral-associated organic carbon regulates soil organic carbon distribution in acidic to alkaline tropical and subtropical soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15057, https://doi.org/10.5194/egusphere-egu25-15057, 2025.
The stability of soils is determined by the presence of (in-)organic aggregation agents, including inorganic cements and polyvalent cations, as well as organic binding and bridging agents. These components strengthen the resistance of soils and their (micro-)aggregates against mechanical and physicochemical stress caused by fluctuating water and changes in the composition of infiltrates. Consequently, the availability, distribution and stability of these aggregation agents directly influences particle (im-)mobilisation and translocation (lessivation), a key process in the formation of diagnostic soil horizons, particularly in the context of pedogenesis on loess substrates. Given that soils developed from loess are agriculturally valuable but sensitive to environmental stress, we explored the susceptibility of topsoils from Regosols and Luvisols to particle release under osmotic and hydraulic stress. We conducted a series of water-unsaturated column experiments at controlled boundary conditions over a duration of seven months and recorded the response of those soils to physicochemical and hydraulic stress at high temporal resolution.
Our experimental pedogenesis study revealed that both soil types tolerated hydraulic stresses well during drainage or ponding but responded sensitively to varying ionic strengths. Following an initially electrostatically induced particle immobilization during a pulse of high ionic strength, we observed a significant particle release driven by peptization. Particularly in Luvisol columns, a notable release of hydrophobic organic material, originally stabilized within aggregates, was observed. In contrast, the effluents of Regosol were primarily characterized by carbonate dissolution products. A large proportion of the released calcium ions was immobilized in the solid phase and contributed to the formation of cation bridges, while inorganic carbon became increasingly enriched in the effluents. In Luvisol, progressive depletion of calcium at cation exchange sites and limited availability of carbonates were also observed, which increased soil susceptibility to environmental stresses, resulting in an irreversible loss of (in-)organic aggregation agents. This induces the translocation of iron, aluminum, phosphorus, and predominantly hydrophobic organic matter via organo-mineral associations into deeper soil horizons.
Our experiments show that with ongoing pedogenesis, common stresses such as alternating water supply, ponding, or drainage have a significantly lower impact on the composition of mobile soil inventory compared to disruptive stresses, such as rapid increases in ionic strength, e.g., due to intense wetting-drying cycles, road salt application, or fertilization. Therefore, stress induced by chemical gradients appears to play a more critical role in the extent of lessivation than mechanical stress resulting from flow. Our results also suggest a need for gentle and sustainable soil management practices to preserve the stabilizing function of aggregation agents in soil.
How to cite: Guhra, T., Ritschel, T., Van Overloop, L. F., and Totsche, K. U.: Resistance of loess-derived soils to environmental stress: lessivation revisited, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16707, https://doi.org/10.5194/egusphere-egu25-16707, 2025.
Carbon dioxide (CO2) assimilation into organic carbon through photosynthesis is widespread, but the biogenic conversion of CO2 into inorganic carbon compounds is often overlooked. One biogenic pathway, facilitated by oxalogenic plants, fungi, and oxalotrophic bacteria, is known as "The Oxalate Carbonate Pathway" (OCP) (Rowley et al., 2017). The process entails the plant uptake of soil calcium, the transformation to calcium oxalate (CaOx) crystals within plant tissues, and their return to the soil via tissues decomposition or as exudes, where CaOx is subsequently catabolised and stored as calcium carbonate in the soil (CaCO3).
Afforestation/reforestation with plants that perform OCP holds significant global CDR implications. For example, Oxisols that are free of carbonates (the Amazon basin and other ecoregions featuring tropical weathered acidic soils) cover >750m ha, with the potential for gigaton scale carbon removal (i.e. 1-2 tCDR/ha/year as CaCO3, could yield 0.75-1.5 Gt/year). Also, OCP in alkaline karst environments may contribute to delaying the return of CO2 to the atmosphere.
During the OCP, bicarbonate (HCO3-) is also produced, and is the dominant carbon/mole species between soil pH 6-10, with carbonate precipitation from pH 8.3, and above. The carbon removal efficiency and fate of the CDR product thus depends on the environmental conditions.
Notably, OCP-based biomineralization has not yet been covered by existing MRV methods. Existing enhanced weathering frameworks, have paved foundations for quantifying carbon removal through bicarbonate ion flushing into the ocean (Mills et al., 2024), and the Microbial Carbon Mineralization methodology quantifies carbonate minerals stored in soil (Andes and Ecoengineers, 2023). The OCP pathway represents a potential carbon removal approach that combines carbonate mineral storage in soils and bicarbonate ion flushing to the ocean that will require a combination of quantification methods and a new methodology.
This mesocosm study aims to elucidate the mechanisms governing CaOx decomposition and its impact on soil pH, soil oxalates, and the relative contribution of CaCO3 precipitation and HCO3- flushing on net carbon removal in the system. Furthermore, it seeks to assess impact on soil organic carbon and potential SOC destabilization risks.
We present early results from a benchtop trial, comparing two soil types from the dry tropics of Yucatàn (slightly basic-rendzic Leptosols vs alkaline soils), and litter (CaC2O4.H20-treatment vs KBr-control) on inorganic carbon dynamics. The soil mesocosms received distilled water and were run under ambient conditions, for 20 days, to track pH, carbonate content, bicarbonate flushing, and soil organic carbon during decomposition. Using titrations, we estimated CO2 removal as soil-based carbonates, and as flushed HCO3- in effluent. Potential implications for leveraging the OCP for carbon removal will be discussed.
How to cite: Altinalmazis-Kondylis, A., Álvarez-Rivera, O. O., Estrada-Medina, H., Troein, E., Flores, M., and Chang, E.: Exploring inorganic carbon dynamics in soil via the Oxalate-carbonate pathway: A methodological approach for monitoring carbon dioxide removal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17143, https://doi.org/10.5194/egusphere-egu25-17143, 2025.
High-resolution imaging approaches revealed heterogeneous soil microstructure with latent spatial patterns that are interrelated with biogeochemical matter cycles in soils. Previous reports of the patchy distribution of soil organic matter (OM) at the microscale demand for extensive of the spatial coverage and arrangement of OM across mineral surfaces in soils. Here, we present a meta-analysis based on nanoscale secondary ion mass spectrometry (NanoSIMS) measurements and a machine-learning segmentation providing insights into diverse soil samples. We identified the coverage of OM across mineral surfaces to be correlated with the bulk soil OM content, showing the expanding but limited arrangement of OM. We employed a lacunarity approach to evaluate the spatial heterogeneity based on the heterogeneous distribution of C and N. This analysis reveals the heterogenous patterns across OM size and its C and N composition within OM, indicating different compositional and structural properties with the OM. In our contribution, we provide novel pathways to evaluate the heterogeneous arrangement of OM in soils across mineral surfaces at the microscale.
How to cite: Hu, Y., Zollner, J. M., Höschen, C., Werner, M., and Schweizer, S. A.: Quantifying the spatial heterogeneity of soil organic matter and minerals at the microscale based on NanoSIMS meta-analysis , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17384, https://doi.org/10.5194/egusphere-egu25-17384, 2025.
Amorphous phases are intermediate weathering products that result from primary mineral dissolution and transform into crystalline secondary phases like clays during rock weathering. Thus, amorphous phases are important indicators of the onset of beginning soil formation at depth. Yet, their abundance in deeper sections of the weathering zone, the saprolite, is rarely investigated. We analysed five granitoid drill cores from a climate gradient in the Chilean Coastal Cordillera by means of sequential extractions to study amorphous phases and to determine the concentrations of the elements they contain. Here, amorphous phases are operationally-defined based on the sequential extraction method. Scanning electron microscopy (SEM) of selected samples was used to investigate the morphology and chemistry of such phases.
Unexpectedly, we find non-negligible concentrations of extractable elements even in visually classified fresh bedrock. A comparison to primary mineral element composition and dissolution rates suggests that likely micro-scale pre-weathering of primary minerals has primed release of these elements. Higher in the profile, when sufficient fluid flow enables solubilisation of elements from pre-weathered minerals, loss of these elements into the dissolved phase (quantified by elemental loss balances) is the main process driving the evolution of the weathering profile. At the surface, their concentration strongly correlates with rainfall, but this correlation is diminished at depth. Amorphous phases were mainly observed along grain boundaries of biotite, in etch-pits on plagioclase, and in the dissolution structure of amphiboles and they are enriched in O, Si, Al, and Fe.
We use the concentrations to assign different zones in the weathering profile. These zones show distinguishable weathering processes that are reflected in the concentration of extractable Al and Fe: in soil and subsoil both dissolution and formation of reactive secondary weathering products are most important. In saprolite, dissolution and mobilisation into the dissolved phase are the main mechanisms. In bedrock, pre-weathering of primary minerals likely induced by post-magmatic processes, hydrothermalism, groundwater flow or gaseous O2 diffusion releases elements into amorphous phases such that water flow higher in the profile mobilises these elements during denudation. The onset of soil formation in the means of primary mineral dissolution is hence located much deeper than expected.
How to cite: Krone, L. V. and von Blanckenburg, F.: Amorphous phases - A new indicator for deep weathering, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17890, https://doi.org/10.5194/egusphere-egu25-17890, 2025.
Soil organic carbon (SOC) is critical for soil quality and agroecosystem sustainability and soil moisture is a key component in the response of SOC turnover to climate variations. Several processes are involved in the regulation of SOC decomposition by soil moisture, but O2 transport in large structural pores that drain near saturation is particularly important in wet soils, which may become more common with increasing flood risk under climate change. These effects are poorly understood because standard incubation experiments disrupt soil structure by sieving. We therefore investigated the effects of soil structure on C mineralisation under wet conditions in order to improve models of SOC turnover that take into account soil structure and soil management.
We measured CO2 emissions from soil cores of contrasting structure in laboratory incubations and derived different parameter sets of moisture response functions. This was done at pressures ranging from saturation to -600 cm, using intact or sieved soils from conventional tillage and no-till treatments. An analytical stochastic model of C mineralisation under different climatic conditions was developed and run using the contrasting parameter sets derived from the incubation data, and the consequences of neglecting soil structure were quantified by differences in model predictions.
The functions describing the response of C emission rates to soil moisture had different shapes for soils of contrasting structure (i.e. between sieved and intact cores or between the two different tillage treatments). The optimum degree of saturation for C emission rates (i.e. where rates were maximal) was closer to one for the more structured soils: 0.90 for intact no-till cores, 0.85 for intact till cores and 0.70 for sieved cores. In addition, sieving increased C mineralisation rates at saturation. Differences between tillage treatments were also evident in the drier range, with C emission rates decreasing more rapidly as the soil was drained from the optimum degree of saturation to the driest pressure head of - 600 cm for the soil from the conventional tillage treatment.
Predictions of C emission rates with the analytical model parameterized using the response curves from sieved or intact soil cores diverged rapidly with increasing rainfall. These differences increased to a plateau as soil conditions became wetter for both tillage treatments, but were always higher for the no-till treatment.
We conclude that neglecting soil structure or changes in soil structure in dynamic predictions of soil organic carbon stocks in response to climatic variations can lead to significant errors. We suggest that a revision of the static view of moisture response functions of C mineralisation is needed. More efforts should be made to establish theoretical or empirical links between soil structural characteristics (in particular the occurrence and distribution of structural pore space) and the parameters of the response function.
How to cite: Coucheney, E., Manzoni, S., Casali, E., Koestel, J., Lewan, E., and Jarvis, N.: The role of soil structure for the moisture response function of carbon mineralization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19046, https://doi.org/10.5194/egusphere-egu25-19046, 2025.
N2O reduction is a key process controlling and mitigating the highly heterogeneous N2O emission from soils. We developed a new method (15N2O reduction tracing: 15N2O RT) to quantify potential gross reduction rates of N2O by incubating only a few grams of soil samples, spiking single-labeled 15N2O tracer as the direct substrate for N2O reduction, and analyzing its direct product (the 15N/14N ratio of N2). In the present study, we proved two concepts of the method: (a) direct determination of N2O reduction potential and (b) downscaling of soil mass to identify N2O reduction hotspots. First, the mass balance of 15N between N2O consumption and N2 production was confirmed (recovery rate: 107% ± 10%) using pure cultures of complete denitrifying bacteria. Second, we applied our method to soil profiles at a secondary forest (O, 0–5 cm A1, 5–20 cm A2 horizons), no-tillage agricultural plot (O, A1, A2), and conventional tillage plot (0–20 cm Ap horizon). Their N2O reduction potentials under a controlled soil water potential (−1 kPa) and 0.1% 15N2O air varied across orders of magnitude: higher in the shallower, carbon-rich horizons (O–A1). Our method allowed the direct comparison between the N2O reductions and copy numbers of nosZ (the functional gene responsible for N2O reduction), which revealed no clear relationship across the studied samples. Instead, the variation in N2O reduction potential co-varied with the soil total carbon (C) content, C/N ratio, and 16S rRNA gene copy number, suggesting C substrate control on the N2O reduction. By further reducing the required soil mass, the current method may help disentangle N2O production and reduction hotspots at a macroaggregate scale (approximately > 2 mm diameter) to clarify mechanisms behind the heterogeneous N2O dynamics in soils.
Keywords: nitrous oxide, isotopic labeling, incubation, N2O reducers, soil profile scale
How to cite: Shingubara, R., Nakajima, Y., Uno, H., Shimada, H., Jinno, J., Ito, K., Matsumura, E., Hara, S., Minamisawa, K., and Wagai, R.: A direct method to determine gross N2O reduction potential: Downscaling soil mass to constrain the reduction hotspots, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19467, https://doi.org/10.5194/egusphere-egu25-19467, 2025.
Association of organic matter (OM) with mineral phases have been identified as one important process to explain organic carbon sequestration in soil. Associations include cationic bridging between OM and mineral surfaces, OM adsorption on mineral surface, and OM co-precipitation with minerals. Some recent studies underlined the crucial role of poorly crystalline aluminium (Al) and iron (Fe) oxides to sequestrate organic carbon. These findings have been mostly observed so far at a global or continental scale. Moreover, the links between soil mineralogy and geochemistry and carbon are still unclear, given the time span needed to unveil them.
The aim of the study is to verify if relationships between geochemical parameters and soil carbon content are still noticeable at the scale of the French European territory, in temperate forests covering a wide range of soil types. Taking advantage of a double inventory of soil organic carbon at a 15-year interval, we further aim to assess if a relationship exists between soil geochemical parameters and C dynamics over a decade. In this forest monitoring network of 102 sites across mainland France (RENECOFOR) three soil depths were analysed at each campaign: 0–10, 10–20, 20–40 cm. The particulate organic carbon (POC) fraction proportion of the 0–10 cm soil depth was known for 53 sites for the second campaign. The mineral-associated organic carbon (MAOC) content could be therefore inferred.
The soil carbon content was linked to the oxalate-extractable Al and Fe (Alox and Feox) at each of the three depths. The relation with the MAOC content of the 53 sites subset was also highly positively correlated. The cation content (exchangeable calcium and magnesium, Caex and Mgex) showed a positive effect on carbon content for a subset of the sites. These had a higher pH and were mainly located on alkaline parent rocks. However, the stock change was mainly unaffected by neither minerals nor cation content, or by other soil characteristics.
How to cite: Mezerette, F., Derrien, D., Mao, Z., Ngatchou-Wandji, J., and Saint-André, L.: The influence of minerals and cations contents on carbon sequestration in temperate French forests’ soils, and their effects on carbon dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20053, https://doi.org/10.5194/egusphere-egu25-20053, 2025.
The application of crushed basalt to soil receives increasing attention as its chemical weathering can promote carbon dioxide removal. However, its impact on the persistence of organic matter (OM) in soil remains poorly understood due to the complex mineral compositions of basalt particles and their interactions with OM during rock weathering. We hypothesized that the mineral-OM aggregation is promoted by easily weatherable primary minerals together with secondary minerals during basalt weathering. To elucidate if the formation of mineral-OM aggregates was mineral-selective, we characterized the basalt-POM aggregates formed in a 6-month leaching experiment using basalt-plant residue-quartz sand mixtures. The aggregates were isolated with density fractionation, and the mineral compositions and distributions were examined with quantitative X-ray diffractometry (qXRD), X-ray absorption near-edge structure (XANES), micro XANES (μ-XANES), and scanning electron microscopy (SEM). The results showed that the meso-density fraction (MF; 1.8 – 2.4 g cm-3) of the fresh basalt was initially enriched with smectite and amorphous minerals than the bulk basalt as found with qXRD, which might have initiated the mineral-POM aggregation at the early stage of the 6-month incubation. The SEM images showed patchy coatings of POM on the basalt particles, implying the presence of preferential binding sites. We found preferential incorporation of plagioclase and pyroxene into the MF aggregate relative to other primary minerals present in basalt with increasing amorphous mineral phase. Using μ-XANES, we found Fe(III) secondary minerals, presumably from smectite or amorphous minerals, located on the surface of basalt particles and thus appeared to act as binders between basalt and POM particles. However, smectite could be initially present and incorporated in MF during basalt weathering.
This study demonstrated the importance of not only the secondary minerals but also the easily weatherable primary minerals for promoting fresh organic matter stabilization under wet-and-dry cycles. The stability of the OM in these meso-density aggregates remains unclear. Further study is needed to evaluate the physical structures of mineral-OM aggregates as well as the biodegradability of the OM therein for determining carbon stability.
How to cite: Yang, P.-T., Kurokawa, K., Nakao, A., Kogure, T., and Wagai, R.: Contribution of primary and secondary mineral phases to organo-mineral aggregation during crushed basalt weathering in the presence of fresh plant residue, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20079, https://doi.org/10.5194/egusphere-egu25-20079, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 3
EGU25-1091 | ECS | Posters virtual | VPS14
Organo-mineral interactions in the floodplain govern the stability of buried organic carbon in continental marginsTue, 29 Apr, 14:00–15:45 (CEST) | vP3.19
The global carbon cycle is largely controlled by the drawdown of atmospheric CO2 by plants and preservation of organic carbon at the continental margins. In the context of Himalayan rivers, previous studies explored the fate of terrestrial organic carbon (Corg)without much consideration of its preservation within the floodplains. We undertook a novel approach to investigate the spatio-temporal preservation of Corg in floodplain paleosols, which form an intermediate between the source of Corg and their subsequent deposition in the continental margin. Towards this, we sampled five 35 m long sediment cores spanning the entirety of the Ganga River Floodplain (GRF). Carbon isotopic composition of Corg and soil carbonates (SC) (δ13Corg and δ13CSC) and oxygen isotopic composition of SC (δ18OSC) along with soil texture, Al and Fe oxides (Alox and Feox) were used as predictors (n=158) of Corg preservation. The Random Forest Regression (RFR) model with the built-in feature importance tool was used to disentangle the dominant predictor of Corg across all the study sites. Our results suggest that in the upper stretch of GRF, Corg is low and preservation was predominantly controlled by the vegetation type (C3/C4) with grasslands accruing more Corg than forests. In contrast, in the lower stretch of GRF, the preservation was dominantly controlled through the formation of Alox and Feox organo-mineral complexes, with the resultant Corg being one-order higher compared to upper stretches. Previous studies suggested that rapid burial predominantly acted as a major controlling factor on the sustenance of Corg in Bay of Bengal. However, our results along with similar Al/Si vs. Corg correlations within the lower GRF compared to the previously reported values from riverine suspended load and shelf sediments suggest that the floodplains transformed the labile Corg into stable organo mineral aggregates at lower stretch of GRF before it was deposited into the Bay of Bengal. We suggest that protection of Corg in floodplain is an importantstep towards its preservation at continental shelf. In the context of the Himalayan river system and the amount of Corg effectively preserved, the role of floodplains has profound implications for the global carbon cycle.
How to cite: Adhya, S. P. and Sanyal, P.: Organo-mineral interactions in the floodplain govern the stability of buried organic carbon in continental margins, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1091, https://doi.org/10.5194/egusphere-egu25-1091, 2025.
