Orals: Wed, 6 May, 08:30–12:28 | Room 0.11/12
Redox cycles, geochemistry, and pH are recognized as key drivers of subsurface biogeochemical cycling in the critical zone but are typically not included in land surface models. These omissions may introduce errors when simulating carbon cycling and greenhouse gas emissions in systems where redox interactions, and pH fluctuations are important, such as coastal regions where sulfate concentrations associated with saltwater influence can drive biogeochemical contrasts across salinity gradients or upland systems where redox-active micronutrients contribute to litter decomposition. Here, we coupled the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with geochemical reaction network simulator PFLOTRAN, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, iron oxide reductive dissolution and dissolved iron oxidation, sulfate reduction, sulfide oxidation, methanogenesis, methane oxidation, and pH dynamics and simulated biogeochemical cycling and methane production across coastal gradients of salinity and elevation. Model simulations were parameterized using laboratory incubations and literature values and evaluated using measured porewater concentrations and surface gas emissions from wetland field sites across coastal regions of the United States. In addition, we demonstrate that interactions between manganese redox cycling and nitrogen availability can influence litter decomposition and organic matter cycling in temperate forest ecosystems. These results demonstrate how directly simulating biogeochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling, and highlight the value of porewater biogeochemical data for evaluating process-based biogeochemical models.
How to cite: Sulman, B., O'Meara, T., and Herndon, E.: Modeling Redox Biogeochemistry Influences on Carbon Cycling at Site to Continental Scales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3962, https://doi.org/10.5194/egusphere-egu26-3962, 2026.
Rhizosphere processes such as root exudation play a major role in carbon cycling by influencing microbial activity. Although the effect of root exudation, both positive and negative, in terrestrial systems has been widely studied, the corresponding effect in coastal systems is unknown. This gap in knowledge is particularly critical because coastal systems sequester 111.4 Tg C /year, a large fraction of which can be attributed to the vegetation. In this study, we characterized the root exudates of the dominant plant species in the pioneer zone, Salicornia spp. and Spartina anglica, and examined how vegetation affects the sediment biogeochemistry in this zone. Field site analysis revealed a high influence of vegetation on microbial sediment respiration as in situ CO2 emissions from vegetated sediments were 3.6-fold and 4.2-fold higher for Salicornia spp. and Spartina anglica plots, respectively, than CO2 release in unvegetated sediment. The sediment content of organic carbon and nitrogen and porewater ammonium concentrations (an indicator of organic matter degradation) were however not elevated in the vegetated sediment. Analysis of root exudate composition revealed that fumarate, acetate, and formate accounted for 30–38% of total root-released carbon in both salt marsh species, indicating a stronger environmental than species-specific influence on root exudation. Further, in a microcosm experiment, we evaluated the impact of root exudation on organic carbon cycling in rhizosphere sediments of the two dominant plant species. We focused on the coupling of organic carbon oxidation to sulfate (SO42-) and ferric iron (Fe(III)) reduction. The addition of model root exudates to the rhizosphere sediment of Salicornia spp. and Spartina anglica resulted in a 2.4-fold and 1.3-fold increase of CO2 emissions compared to the controls, respectively. The CO2 release even exceeded added organic carbon oxidation, indicating potential microbial “priming” and enhanced organic carbon mineralization in the sediment. Organic carbon addition caused increased sulfate reduction but had no significant effect on iron reduction, emphasizing the dominance of sulfate reduction for organic carbon oxidation in salt marsh sediments. The rapid microbial response to organic carbon addition highlights the stimulation of microbial activity by root exudation. Thus, root exudates are an important component in predicting the stability of salt marsh carbon sinks and global carbon cycling.
How to cite: Joshi, P., Raab, F., Kainz, N., Mollenkopf, M., and Kappler, A.: Impact of root exudates on microbial carbon cycling in salt marshes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13356, https://doi.org/10.5194/egusphere-egu26-13356, 2026.
While Earth’s critical zone is defined from the canopy to the base of aquifers, the role of groundwater in the greenhouse gas (GHG) budget remains under-represented. Groundwater acts as a massive biogeochemical engine where redox-driven processes play a vital role in nutrient cycles. In agricultural systems where nitrate leaches from the soil layer, the subsurface provides a critical ecosystem service through denitrification. However, this redox-driven process converts nitrate to N2 by oxidizing reduced materials such as organic carbon and pyrite, thereby producing dissolved inorganic carbon (DIC). Despite its potential to act as an anthropogenic source of CO2, the climatic implications of groundwater denitrification have not been quantitatively investigated on a large scale.
In this study (recently published in Biogeosciences, 2025), we investigated the DIC increase driven by denitrification across Danish aquifers. Using a national groundwater chemistry dataset and machine learning techniques, we identified eight different redox clusters spanning from oxic to methanogenic conditions. The spatial architecture of these redox clusters was found to be primarily governed by the hydrogeological framework. By combining the clusters with the subsurface structural information, we predicted the predominant denitrification processes at the redox interface, where nitrate is fully reduced to N2. Our results revealed that about 76% of the area is driven by pyrite oxidation, while the remainder is driven by organic carbon decomposition.
By coupling these findings with a national nitrogen model and the process-specific stoichiometry of N and C, we estimated that groundwater denitrification in Denmark releases approximately 104kt of CO2 annually. Current IPCC guidelines for GHG accounting cover liming, urea, and other carbon-containing fertilizers as anthropogenic CO2 sources from agricultural systems. However, our findings indicate that groundwater denitrification generates a CO2 flux equivalent to nearly half of the emissions from agricultural liming in Denmark (246 kt CO2 eq. yr-1), which is currently the predominant source. Crucially, groundwater denitrification exclusively mobilizes “old” geological carbon pools—organic carbon and carbonates—that have been stably stored for millennia. Because these pools are effectively non-renewable on human timescales, this represents a net addition of carbon to the active cycle. We conclude that a quantitative understanding of the coupling of C and N in the deep critical zone must be investigated across diverse global climatic and geological conditions.
How to cite: Kim, H., Koch, J., Hansen, B., and Jakobsen, R.: Mapping the redox architecture of the critical zone for quantifying CO2 emissions from groundwater denitrification, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18325, https://doi.org/10.5194/egusphere-egu26-18325, 2026.
Nitrate contamination of water bodies driven by agricultural activities remains a widespread environmental concern. In groundwater-fed surface waters, nitrate concentrations are governed by the transport of N-excess through groundwater systems. Denitrification plays a central role in mitigating this transport by reducing nitrate to inert N₂ gas in anoxic zones. The anoxic zone in an aquifer is separated from the oxic zone by a sharp boundary, i.e., the nitrate reduction front (NRF). This NRF can be retrieved from sediment colour changes, shifting from oxidised hues (yellow, brown, or red) to reduced colours (grey, green, or black), provided there are sufficient Fe-bearing minerals and redox colours are not masked by high clay or organic matter contents. Hydrochemical data from multilevel observation wells provide a more integrated signal of redox conditions, yet precise NRF delineation remains challenging since usually, groundwater sampling is not done for consecutive depth intervals. Integrating lithological and hydrochemical information therefore offers a more robust approach.
This study presents a two-stage machine learning (ML) framework to predict groundwater NRF positions across Flanders. Firstly, a logistic regression model (ML1) was developed to estimate oxidation probabilities for individual borehole layers using lithological characteristics (colour, texture, stratigraphy) and relative depth within the aquifer as predictors. The model reproduced the redox conditions (reduced or oxidized) in 72% of borehole layers, which were assessed from the hydrochemistry (dissolved oxygen, [Fe2+] and redox potential) of the associated filters for groundwater monitoring.[JV2] [ES3] The derived probabilities were used to assess the likelihood that a boundary between two borehole layers is located at the NRF leading to a likelihood-depth profile. A separate likelihood profile was developed using the hydrochemistry data from the filters of a multilevel borehole. Multiplication of these two profiles yielded the most probable NRF position, resulting in NRF estimates at 1,902 locations.
Secondly, these positions were used to train a gradient-boosted regression tree model (XGBoost) to predict the NRF depth for any location across Flanders (ML2i). The training data were the NRF depths at the 1,902 multilevel observation wells.
How to cite: Khan, A. H. A. N., Shirali, M., Van Ho, N., Ahmadi, Z., Vanderborght, J., Smolders, E., Serral Asensio, E., and Diels, J.: Spatial prediction of the groundwater nitrate reduction front across Flanders with borehole lithology and geochemistry data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21275, https://doi.org/10.5194/egusphere-egu26-21275, 2026.
Groundwater accounts for up to 30% of our drinking water resources, with over 2.5 billion people worldwide relying on its purity and availability. However, geogenic arsenic contamination in groundwater poses a serious threat to this limited resource, potentially endangering the health of over 200 million people worldwide.
Arsenic (As) contamination in aquifers is a common issue along the Red River Delta located in Vietnam. As-bearing minerals such as Fe(III) (oxyhydr)oxides are essential to immobilizing As and preventing it from contaminating groundwater. Emerging evidence in past years have linked anaerobic oxidation of methane (AOM) to the reductive dissolution of Fe(III) minerals leading to the greater release of As into groundwater. Further investigation is needed to elucidate the specific microorganisms involved and their underlying microbial mechanisms, along with the broader relevance of this process in other aquifers.
To gain deeper insight, two drilling field campaigns were performed in villages Dan Phuong and Van Phuc along the Red River. Groundwater and sediment samples were taken for geochemical and molecular biology analysis. From the geochemical analysis, we have produced chemical profiles of potential available electron donors (CH4, NH4+, H2, CO2, DOC) and electron acceptors (Fe(III), SO42-, NO3-, NO2-) along sediment depth in order to confirm the relevance of CH4 in this system. At Van Phuc and Dan Phoung CH4 was found to be the dominant electron donor with a maximum concentration of 0.06 and 0.015 mmol kg-1 respectively. At the Van Phuc site, novel microbial trapping devices were inserted into one of the previous drilling wells and collected 4 months later for the enrichment of organisms specialized in Fe(III) reduction and AOM. Obtained enrichment cultures are being used to measure rates of Fe(III) reduction and CH4 oxidation with labelled 13C under different growth conditions. In summary, we have found CH4 to be the primary driver of Fe(III) reduction at certain depths at both field sites. Further work will focus on the sequencing of sediment and groundwater samples to develop a profile for the present and active microbial community. With the combination of the geochemical and sequencing results we hope to confirm the relevance of Fe(III) reduction and AOM influence on As mobilization.
How to cite: Green, H., Zhu, J., Duyen, V. T., Abramov, S., Van Li, A., Viet, P. H., Kappler, A., and Kleindienst, S.: Impact of anaerobic methane oxidation coupled to iron(III) reduction on arsenic mobilization in aquifers along the Red River delta in Vietnam, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1135, https://doi.org/10.5194/egusphere-egu26-1135, 2026.
Anthropogenic phosphorus (P) loading is a key driver of eutrophication and deoxygenation of coastal
marine ecosystems. Vivianite, an authigenic Fe(II)-P mineral, can act as a major sink for P in coastal
sediments. In brackish and marine systems, vivianite formation is typically observed below the sulfate
methane transition zone (SMTZ), where Fe2+ is not scavenged to form Fe sulfides. However, vivianite
has also been detected in euxinic coastal systems, highlighting the need to better understand P cycling
under these conditions.
Here, we investigated how seasonal variations in bottom water redox conditions impact P recycling and
burial in a seasonally euxinic marine coastal basin (Scharendijke basin, Lake Grevelingen, the
Netherlands) using water column chemistry, porewater geochemistry, solid-phase extractions and
mineralogical analyses (light microscopy, micro-XRF and SEM-EDS).
Our data reveals cryptic Fe-P cycling in the surface sediments, with transformation of Fe-oxide bound
P to vivianite occurring above the SMTZ during spring and summer, when bottom waters shift from
oxic to euxinic. We show that dissimilatory Fe reduction supplies porewater Fe2+ that reacts with
phosphate to form vivianite, which we confirmed by a combination of light microscopy, micro-XRF
and SEM-EDS. High sedimentation rates promote the rapid burial of these vivianite-rich layers below
the SMTZ, preventing later exposure to sulfide and ensuring permanent P burial.
Our results indicate that vivianite formation and preservation in sediments overlain by seasonally
euxinic bottom waters reflect an interplay between Fe availability, sulfide production, phosphate supply,
and sedimentation rate. Porewater profiles observed in the Baltic Sea suggest that vivianite formation
above the SMTZ may occur across a range of sedimentary settings. These findings highlight a
previously unrecognized pathway of P transformation in surface sediments of brackish and marine
coastal systems.
How to cite: Piso, L., van Helmond, N. A. G. M., Haukelidsaeter, S., Zygadlowska, O. M., Klomp, R., Lenstra, W. K., Jetten, M. S. M., and Slomp, C. P.: Cryptic Fe-P cycling in sediments of a seasonally euxinic coastal system, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17183, https://doi.org/10.5194/egusphere-egu26-17183, 2026.
The biogeochemical cycling of carbon is closely intertwined with iron processes. At aquatic redox interfaces, Fe(II) oxidizes and rapidly undergoes hydrolysis, coprecipitating as Fe(III) with natural organic matter (NOM) and other ions to form short-range-ordered (SRO) Fe minerals that can protect carbon from biotic and abiotic transformations. Relevant Fe-NOM research is dominated by studies of organic carbon fate, yet comparatively little attention has been given to the influence of organic carbon chemistry on iron mineral structure and properties. Organic ligands in NOM bind with Fe and are known to interfere with Fe(III) polymerization, with the extent of interference depending on ligand type and concentration. In comparison to their “pure” counterparts, SRO Fe(III) (oxyhydr)oxide minerals associated with NOM may have altered redox potentials (Eh) due to changes in Fe speciation, coordination environment, and/or particle size. However, it remains unclear if a link exists between coprecipitate mineral properties (e.g., range of order), Eh, and macro-scale redox processes (i.e., extents and rates of redox reactions). In this project, we investigate and link bulk and atomic-scale structural properties of different coprecipitates to their redox properties measured via mediated electrochemistry. Coprecipitates were synthesized by titrating Fe(III) solutions in the presence of 4 model organic ligands at varying Fe:ligand molar ratios. Model ligands were chosen as NOM binding analogs based on binding strength (log K) and type (carboxylate vs phenolic). High resolution X-ray diffraction (HR-XRD) analysis of coprecipitates synthesized with carboxylate ligands show a decrease in coherent scattering domain both with increasing ligand concentration and number of carboxylate functional groups, indicating that carboxylates decrease Fe(III) crystallinity in a systematic fashion. These results were confirmed via Mössbauer spectroscopy (MBS), which showed a decrease in blocking temperature with increasing ligand and/or carboxylate content. Interestingly, while coprecipitates synthesized with catechol followed this trend at high ligand ratios, lower ligand ratios promoted the transformation towards lepidocrocite and spinel-like phases, suggesting that the electron-donating properties of catechol steer early Fe(III) transformation pathways more rapidly than carboxylate ligands. Electrochemically, the Fe(III)-NOM coprecipitates were more reducible (i.e., greater reduction extent and rate) than ligand-free ferrihydrite controls. When paired with results from HR-XRD and MBS, this finding suggests that coprecipitate redox reactivity is controlled by crystallinity. Ongoing work is investigating the role of particle size in this relationship. Overall, these preliminary mechanistic results may help link the importance of reactive iron phases to carbon dynamics (persistence vs. mineralization) in the environment. Future microbial reduction experiments will be employed to understand how coprecipitate thermodynamics influence biological redox reactivities.
How to cite: Hudson, J., Oker, L., Ilin, A., Valenzuela, E., van Grinsven, S., Joshi, P., Thompson, A., Haderlein, S., and Kappler, A.: Redox properties of Fe-OM aggregates: Linking structure to redox reactivity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13788, https://doi.org/10.5194/egusphere-egu26-13788, 2026.
Dissimilatory iron reduction (DIR) processes play a critical role in regulating the transformation of dissolved organic nitrogen (DON), yet the structural-dependent biogeochemical behaviors of DON from different sources remain poorly understood. Using a suite of complementary analytical techniques, we systematically compared the mineralization pathways and molecular transformations of nitrogen (N)-containing compounds in two representative dissolved organic matter (DOM): (1) dissolved black carbon (DBC) derived from pyrolysis (representing combustion-sourced DON from fire-affected ecosystem) and (2) leached dissolved organic carbon (LDOC) derived from compost (representing biogenic-sourced DON from undisturbed ecosystem), as sole electron donors during DIR. Batch experiments demonstrated that DBC produced three times more ferrous iron than LDOC and achieved a substantially higher mineralization ratios of N-containing compounds (41% versus 23% for LDOC), with ammonium being the sole detectable N mineralization product. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with N near-edge X-ray adsorption fine structure (NEXAFS) analysis revealed that DBC degradation specifically targets polycyclic aromatic components rich in aromatic N, especially those in 5-membered rings (e.g., pyrrolic N), which account for nearly half of DON loss. In contrast, LDOC preferentially removes labile amide N from lignin-like components and enriches recalcitrant aromatic N from polycyclic aromatic components. The number of N-containing molecular formulas decreased by 51.6% in DBC but only by 10.2% in LDOC. Thermodynamic calculations confirm that aromatic N, particularly pyrrolic N, dominates the degraded DBC pool. These electron-rich Lewis basic structures form stable coordination complexes with iron that facilitate electron transfer and activate adjacent carbon bonds for oxidative ring-cleavage. The estimated electron flux derived from the degradation of pyrrolic N alone accounted for ~11% of the total ferrous iron produced in DBC, underscoring the critical role as a redox-active molecular "ignition points". In contrast, LDOC mineralization followed a conventional carbon-centered anaerobic pathway. This study elucidates the contrasting mineralization behaviors of biogenic-sourced and combustion-sourced DON driven by DIR, revealing fundamental structure-reactivity relationships that govern N biogeochemical cycling.
How to cite: Zhang, Z., Cui, X., and Zhu, D.: Structural Diversity Controls Dissolved Organic Nitrogen Mineralization Driven by Dissimilatory Iron Reduction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15866, https://doi.org/10.5194/egusphere-egu26-15866, 2026.
Redox processes within the Earth’s critical zone tightly couple organic carbon turnover with nutrient transformations and the mobility of redox-sensitive trace metals in groundwater. In livestock-dominated settings, manure and wash-water can impose substantial dissolved organic carbon (DOC) loads on shallow water resources, yet the coupled relationships among DOC, redox conditions, nitrate occurrence, and trace-metal behavior across interacting water compartments remain insufficiently constrained. This study evaluates these linkages at an animal husbandry site in Querétaro, Mexico, using a multi-proxy dataset (n = 6) spanning lagoon water, well water, irrigation water derived from stored groundwater, on-site tap water, and two university campus end-use waters included as external context. DOC concentrations were elevated across all waters (48.87–190.50 mg L⁻¹), with lagoon water defining a strong organic-loading endmember. Redox conditions ranged from reducing in lagoon water (ORP −17 mV; DO 2.31 mg L⁻¹) to oxidizing in other compartments (ORP 218–291 mV; DO 3.63–5.20 mg L⁻¹). DOC showed a strong inverse relationship with ORP (r = −0.93), while nitrate increased with ORP (r = 0.80) and decreased with DOC (r = −0.93), consistent with carbon-fueled oxygen demand and diminished nitrate persistence under lower redox potential. Trace metals exhibited element-specific responses: Zn and Cr increased with DOC (r = 0.97 and 0.83) and decreased with ORP (r = −0.98 and −0.96), indicating enhanced metal mobility under DOC-rich, reducing conditions. Lagoon water also displayed the highest electrical conductivity and markedly elevated Zn and Cr, supporting its role as a concentrated source reservoir. In contrast, Cu concentrations did not scale with DOC across compartments, suggesting additional source controls such as distribution-system influences. Overall, the findings identify livestock-derived DOC as a first-order driver of redox gradients that structure nitrate patterns and trace-metal behavior across both natural and engineered water systems.
How to cite: Tripathi, S. and Kumar, M.: Livestock-derived organic carbon drives redox gradients shaping nitrate and trace-metal behavior across interacting waters in Querétaro, Mexico, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15966, https://doi.org/10.5194/egusphere-egu26-15966, 2026.
Plants are an essential part of the terrestrial C cycle, and it is commonly assumed that they acquire C solely from the atmosphere. However, evidence from plant physiology research indicates that plants can also take up dissolved C via their roots. This concept, however, is not commonly acknowledged within the soil science community. If such an uptake mechanism existed, it could significantly impact C cycling studies involving plants. Here, we aimed at quantifying plant C uptake from solution and soil in two pot experiments. In a first experiment, seedlings of Plantago lanceolata and Picea abies were grown in hydroculture enriched with 13C-labelled glucose or L-lysine. After four weeks, 2.0-5.5% of the applied 13C was recovered in the plants, along with a shift in the δ13C signal of the plant tissues from -31‰ up to +78 ‰. Here, our findings suggest a larger uptake of lysine as compared to glucose. In a second experiment, plants were grown for eight weeks in both sandy and silty soil amended with 13C-labelled soil organic carbon (SOC). After harvesting, we found up to 0.74% of the available tracer substance to be recovered in the plant tissues. Our findings show that the pathway of root C uptake from soil exists, but this mechanism has minor relevance in soil, likely due to competition with microbes for available C. However, it indicates the need for adjustments in our understanding of plant-soil interactions with respect to SOC dynamics, especially in experimental setups where 13C is added to soil: A decrease in soil 13C cannot be attributed exclusively to mineralization processes.
How to cite: Meyer, N., Giese, L., Karhu, K., Helmisaari, H.-S., and Bucka, F. B.: Plant uptake of soil carbon: an overlooked flux in carbon cycling?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22459, https://doi.org/10.5194/egusphere-egu26-22459, 2026.
Wetlands represent critical yet vulnerable carbon reservoirs, whose stability is threatened by increasing redox fluctuations driven by climate change and human activities. Here, we investigated the mechanisms controlling soil organic carbon (SOC) mineralization during anaerobic-aerobic transitions and under warming conditions, using three black soils from Northeast China with contrasting land-use histories. Our results showed that anaerobic CO2 emissions increased by 29% - 44% under warming and governed by synergistic iron-organic carbon-microbe interactions, inducing the destabilization of iron-bound organic carbon. Upon oxygenation, short-term aerobic CO2 pulses were primarily driven by anaerobic legacy effects (e.g., preserved enzymes and reductants-derived •OH) rather than renewed microbial respiration, being more vulnerable to warming (25% - 31%) than prolonged oxygenation (10% - 17%) in soil A and soil B. Sterilization experiments showed that preserved enzymes contributed substantially more to aerobic CO2 pulses (52%) than •OH-mediated oxidation (27%) in soil A. A random forest model identified •OH and anoxic hydrolases as key predictors of short-term aerobic CO2 release (56.6% explained variance). Mechanistically, •OH played a dual role: promoting oxidative activity while simultaneously inhibiting anaerobic hydrolases. These findings establish that SOC mineralization potential and its temperature response are fundamentally determined by intrinsic soil properties, and anaerobic processes, traditionally viewed as C stabilization, may paradoxically drive C loss during redox fluctuations that intensify under climate warming.
How to cite: Wang, Y.: Redox-Driven Carbon Loss in Black Soils Under Climate Warming: The Overlooked Role of Anaerobic Legacy Effects , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18489, https://doi.org/10.5194/egusphere-egu26-18489, 2026.
Organic amendments have shown profound performance to enhance soil carbon (C) sequestration and crop productivity. However, the synergistic effects of various organic amendments on maintaining/improving pH buffering capacity (pHBC) remain unclear. Here, based on a 42-year ongoing field experiment, we explored the response of pHBC and crop productivity to synergistic incorporation of plant C input (green manure with or without straw) vs. diverse C input (livestock combined with plant C). Mineral fertilization caused pH decline by 0.12-0.39 units, compared with diverse C input. Plant C input showed no significant effect on pHBC, whereas diverse C input, e.g., synergistic incorporation of green and livestock manure, increased soil pHBC by 31-40% compared with control. This primarily attributed to the rise in Ex-Ca2+ and soil organic matter, particularly humic substance compositions. Furthermore, the protonation of organic anions produced from humic acid dissociation also enhanced pHBC. Critically, the mitigation of soil acidification establishes a critical foundation for improving crop yields, average outperforming control and plant C input by 32% and 12%, respectively. These findings highlight that the diverse C input based on soil amendments as agronomically optimal and sustainable strategy to mitigate soil acidification and sustain crop productivity in acid soil area.
How to cite: Liu, L., Liu, K., Colinet, G., Jeroen, M. J., and Zhang, W.: Diverse organic amendments mitigate soil acidification by improving soil organic matter in acid soil area over four decades, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16672, https://doi.org/10.5194/egusphere-egu26-16672, 2026.
The unique geological structure combined with human activities cause serious rocky desertification in fragile karst regions, which restricts regional social and economic developments. Vegetation restoration is the key practice of comprehensive administration of rocky desertification, but this process is extremely slow, especially in some special karst geomorphic units. Nitrogen (N) element has been suggested to be a critical limiting factor for vegetation growth, but the characteristics of soil N supply and plant N acquisition remain largely unknown in karst regions. This hinders our better understanding of vegetation restoration of karst rocky desertification as well as its restoration effects. We chose natural succession sequences with different vegetation restoration stages in karst peak-cluster depression and faulted basin regions. Vegetation survey and data collection were conducted, and the N/phosphorus (P) ratio, N content and δ15N values of plant leaf were used to reflect the degree of plant N limitation. In addition, 15N labeling techniques were employed to investigate soil N transformation rates, available N supply capacity and N acquisition characteristics of the dominate plant species during vegetation succession. We found that plants were severely limited by N in the early stages of vegetation restoration, which was more seriously in the karst fault basin. As vegetation recovered, plants were no longer limited by N but by P. This difference was mainly attributed to the changes in soil N supply capacity and plant N utilization strategies. In the early stages of vegetation restoration, the rates of soil N supply processes including mineralization and nitrification was weak and inorganic N was mainly ammonium. In the later stages, soil inorganic N supply capacity increased significantly, resulting in higher inorganic N content dominated by nitrate. In such N condition, plants can adjust their own root functional traits to develop different N utilization strategies. Plants develop larger specific root length and specific surface area in the early stages to increase ammonium utilization, but plants improve nitrate utilization in the later stages. Overall, our results unraveled the mechanism underlying reduced plant N limitation following vegetation restoration through increasing soil inorganic N supply and adjusting plant N utilization strategy. The present study provided a scientific basis for ecological restoration and reconstruction of karst rocky desertification.
How to cite: Wen, D. and Zhu, T.: The mechanism underlying plant nitrogen limitation following vegetation restoration in karst regions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2962, https://doi.org/10.5194/egusphere-egu26-2962, 2026.
Legumes play a crucial role in vegetation restoration by mediating atmospheric nitrogen (N) inputs and enhancing soil N availability, especially in degraded ecosystems. A deeper understanding of the mechanisms through which legumes accelerate vegetation restoration will help provide a reference for the government to formulate ecological restoration strategies and enhance ecological service functions. In degraded ecosystems, low soil N availability intensifies plant-microbe competition for N, thereby impairing vegetation restoration processes, even under long-term monoculture afforestation. Although legumes are known to influence soil N availability through biological N fixation and the stimulation of microbial N transformations, the integrated mechanisms underlying these effects remain poorly understood. Here, we investigated eight native legume species differing in nutrient utilization strategies following nine years of natural restoration in a degraded karst ecosystem of southwest China characterized by severe rocky desertification. Four non-legume species served as controls. We found that four legume species, including Lespedeza juncea, Indigofera mengtzeana, Sophora davidii, and Indigofera pseudotinctoria, exhibited nutrient-acquisitive traits, whereas the remaining four legume species of Dalbergia hupeana, Bauhinia brachycarpa, Bauhinia comosa, and Solanum viciifolia showed nutrient-conservative strategies. Integrated analyses of plant leaf N/phosphorus ratios and vector-threshold angles revealed that both plant and soil microbial growth associated with nutrient-acquisitive legume species were no longer N-limited, whereas severe N limitation persisted under nutrient-conservative legume species. These contrasting patterns were primarily explained by the changes in the rates of soil free-living N fixation and inherent N transformation processes that control inorganic N production. Specifically, soils associated with nutrient-acquisitive legume species exhibited significantly higher rates of free-living N fixation, gross N mineralization, and gross ammonium immobilization, corresponding with reduced plant and microbial N limitation. Structural equation modeling further indicated that nutrient-acquisitive legume species enhanced inorganic N supply capacity by increasing soil energy and substrate availability, microbial biomass, and the abundance and activity of free-living diazotrophs, thereby effectively alleviating ecosystem N limitation. Beyond elucidating species-specific pathways and mechanisms through which legumes alleviate N limitation, our results provide critical guidance for species selection and management in the ecological restoration of degraded karst ecosystems.
Keywords: Degraded ecosystem; Free-living N fixation; Legume; Plant and microbial N limitation; Soil inorganic N supply
How to cite: Liu, L., Zhu, T., and Müller, C.: Nutrient-acquisitive legume species stimulate soil free-living N fixation and organic N mineralization to alleviate N limitation in the degraded ecosystem, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2965, https://doi.org/10.5194/egusphere-egu26-2965, 2026.
Acidic soils constitute a major constraint to agricultural productivity in Estonia, particularly in the southern regions, affecting approximately 54.5% of the country’s agricultural land. Soil acidity adversely influences nutrient availability, base saturation, and crop performance, making liming a key management practice for sustainable crop production. This study investigates the effectiveness of Corestone, a newly developed limestone-based liming material produced by grinding limestone overburden excavated during oil shale mining operations, in enhancing the chemical properties of acidic soils and improving nutrient uptake in oilseed rape (Brassica napus L.) under controlled experimental conditions.
A pot experiment was conducted using four chemically distinct acidic soils treated with Corestone at two application rates corresponding to 50% and 100% of the calculated lime requirement. Winter oilseed rape cultivar ‘Fenja’ was grown on the treated soils. Soil pH and plant-available phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca) were determined before and after the experiment. Aboveground biomass production and nutrient concentrations in plant tissue were also analysed.
Liming with Corestone resulted in a consistent increase in soil pH across all soils, with application at 100% of the lime requirement leading to neutral or near-neutral pH values. Soil calcium concentrations increased significantly following liming, particularly in the most acidic soil with the highest lime demand. Although the Ca:Mg ratio improved after treatment, optimal ratios were not fully achieved. Responses of plant-available phosphorus varied among soils, whereas potassium concentrations generally declined following liming. Soil magnesium concentrations increased in most treatments.
No statistically significant differences in rapeseed biomass were observed between liming rates; however, a clear positive trend in biomass production was evident, except in one soil. This response may be attributed to reduced micronutrient availability under elevated pH conditions. Plant tissue analysis revealed consistently low phosphorus and potassium concentrations, while calcium and magnesium concentrations were relatively high. Positive correlations were observed between changes in soil and plant phosphorus and magnesium contents. In contrast, increasing soil calcium concentrations and rising soil pH were associated with higher plant phosphorus but lower plant calcium concentrations, indicating complex nutrient interactions following liming.
Overall, the results demonstrate that Corestone is an effective liming material for alleviating soil acidity, while emphasizing the importance of soil-specific nutrient dynamics when optimizing liming strategies for oilseed rape cultivation.
How to cite: Paat, A., Tõnutare, T., and Shanskiy, M.: Liming-Induced Changes in Phosphorus Availability and Soil–Plant Interactions in Acidic Soils, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10229, https://doi.org/10.5194/egusphere-egu26-10229, 2026.
Biocrusts occur globally in ecosystems where plants are typically sparse, allowing sunlight to reach the soil surface, especially in natural drylands. Important ecosystem services have been attributed to them. Soil metabolomics is an emerging and powerful approach connecting soil chemistry, biology, and ecology. Understanding the role of biocrusts and their metabolic expressions is crucial for explaining how nutrient cycling adapts under arid conditions. Thus, in this study, a total of 56 plots, 20 m x 20 m in size, were surveyed in an arid natural area. For each plot, biocrusts and bulk soils (1-10 cm depth) were described through untargeted metabolomic analysis (LC−QTOF), and fertility properties. Multi and univariate approaches revealed both functional adaptation and spatial heterogeneity linked to biocrust development. A total of 59 differentially expressed metabolites (DEMs) were identified. Biocrusts concentrated a variety of metabolites related to intense biological activity and carbon and phosphorus cycling, as well as stress tolerance. Meanwhile, the bulk soils below serve as a reservoir for more persistent organic compounds and benefit from the nutrients and organic products released by the biocrust. Furthermore, the metabolomic fingerprints of the soils were found to be highly correlated with soil fertility properties. For example, our results reinforce the role of P (and Mn-associated processes) as key components of biocrust-driven biogeochemical cycling in arid soils. Definitely, this study demonstrates the power of untargeted metabolomics in revealing functional chemical expression and offers tools for managing the fertility of degraded soils by promoting healthy biocrusts.
How to cite: Criado-Navarro, I., Ledesma-Escobar, C. A., Priego-Capote, F., Eisenhauer, N., García-Velázquez, A., Salazar-García, R., Castillo, P., and Archidona-Yuste, A.: Soil metabolomics reveals biocrust-mediated carbon and nutrient cycling in arid ecosystems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12358, https://doi.org/10.5194/egusphere-egu26-12358, 2026.
The ecological functioning of subsurface environments—including soils, lake and marine sediments, and aquifers—is largely governed by redox processes mediated by complex microbial communities inhabiting porous media. The composition and spatial organization of these communities emerge from the interplay between porous geometry, porewater flow, microbial interactions within and across populations, and microscale geochemical heterogeneities. Identifying the biological and environmental controls on microbial community structure is therefore crucial for understanding and predicting the functions of subsurface ecosystems. However, progress remains limited by the difficulty of observing microbial communities and characterizing their cell-scale geochemical environment within opaque porous matrices.
Recent advances in microscale imaging technologies offer new opportunities to overcome these challenges. Microfluidic devices—transparent platforms that reproduce the pore structure and flow conditions of soils and sediments under controlled laboratory settings—have emerged as powerful tools for investigating subsurface biogeochemical dynamics. When combined with fluorescently tagged bacteria, microfluidics enables non-invasive, real-time visualization of microbial populations and their self-organization in response to physicochemical gradients or microbial interactions. In parallel, microfluidic integration with transparent optical sensors, such as optodes and luminescent nanoparticles, has been shown to allow mapping of microscale physicochemical gradients, e.g., oxygen concentrations, driven by microbial activity coupled with advection and diffusion processes.
Despite these advances, the simultaneous imaging of microbial community dynamics and geochemical gradients remains challenging. Existing luminescent sensors typically emit in the visible range of the spectrum, overlapping with the emission wavelengths of commonly used fluorescent protein tags. This spectral interference has so far prevented the concurrent detection of fluorescently tagged microorganisms and sensor signals within the same microfluidic platform.
Here, we present a novel microfluidic platform integrating a transparent oxygen optode emitting in the near-infrared (NIR) region of the light spectrum. Spectrofluorometric characterization demonstrates that this NIR-emitting optode eliminates spectral interference with most used fluorescent tags, enabling simultaneous imaging of microbial populations and oxygen dynamics at the microscale.
We demonstrate the potential of this approach by investigating the progressive colonization of sandy sediment under flowing conditions by an aerobic microbial community and the associated formation of microscale oxygen gradients. The community consists of two bacterial strains representing distinct phenotypes—elongated and rounded cell shapes—engineered to express mScarletI and GFP, respectively. The results reveal clear differences in spatial organization and cluster morphology between the two strains, consistent with previous observations of shape-dependent colonization under flow. Moreover, the data suggest that distinct cell morphologies differentially influence local oxygen gradients, highlighting a direct link between microbial physical traits and microscale redox dynamics.
Beyond this proof-of-concept application, the proposed methodology is highly versatile. Spectral analyses indicate that up to four microbial populations could potentially be imaged simultaneously alongside oxygen dynamics. Furthermore, rapid advances in luminescent sensor chemistry are expanding the range of physicochemical parameters that can be mapped in the NIR. Finally, the platform is compatible with complementary microscale analytical techniques, such as SIMS or synchrotron-based methods, enabling integrated investigations of microbial activity, geochemical gradients, and mineralogical transformations in subsurface environments.
How to cite: Ceriotti, G. and Borisov, S. M.: Simultaneous real-time imaging of oxygen gradients and microbial community spatial organization in confined environments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3536, https://doi.org/10.5194/egusphere-egu26-3536, 2026.
Estimating how carbon and nitrogen pools respond during the transition from conventional to organic management is critical for ensuring overall system sustainability. Organic crop production, characterized by its restricted use of synthetic inputs and focus on resources present in the agro-ecosystem, is widely considered a sustainable alternative to conventional crop production. However, one of the most challenging aspects of organic crop production in western Canada is its reliance on the practice of tillage for weed and nutrient management, which can elicit significant process responses for both carbon and nitrogen. A proposed alternative to intensive tillage is to integrate crops and livestock in order to promote improved nutrient cycling. While both are considered effective management techniques, their impact on carbon and nitrogen cycling during the organic transition period from no-till conventional practices remains poorly understood. This research aims to examine how the introduction of tillage and an integrated crop-livestock system alter edaphic conditions which govern nutrient dynamics in soil.
A field research project comparing intensive tillage and integrated crop-livestock during the organic transition period is currently underway in a humid continental climate in western Canada. Intact soil cores were collected in the third growing season of the trial and underwent a laboratory incubation experiment which analysed carbon dioxide (CO2) and nitrous oxide (N2O) fluxes. Following the incubation, soil cores were analysed with a hydraulic property analyser, and available nitrogen and dissolved organic carbon were determined. In combination, these measurements provide a comprehensive understanding of nutrient pool responses to variable soil conditions created by contrasting management techniques.
Results suggest that the intensive tillage system exhibited higher losses in the form of N2O and CO2 emissions (p <0.05), as a consequence of higher nutrient concentrations (p <0.05) and water-filled pore space (p<0.05). Since N2O and CO2 are potent greenhouse gasses, increased emissions under intensive tillage highlight important climate implications of soil management choices. Additionally, greater nutrient losses reduce the pool of nitrogen available for crops, potentially undermining soil fertility during the transition to organic production. These findings underscore how strongly management practices shape nutrient availability during this critical transition period.
How to cite: Marchesan, A., Hernandez Ramirez, G., and Kubota, H.: Investigating carbon and nitrogen cycling during the transition to organic agriculture with a laboratory incubation study , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-343, https://doi.org/10.5194/egusphere-egu26-343, 2026.
Enhancing soil organic carbon (SOC) in croplands is fundamental to climate‑smart agriculture (CSA), yet the mechanisms by which labile and recalcitrant carbon (C) inputs build and stabilize SOC remain unclear. Crop residues supply labile C that improve microbial activity and particulate organic matter (POM) formation, whereas biochar is considered a recalcitrant, negative‑emission amendment. This study used ¹³C‑labelled maize crop residues and its derived biochar, applied at 1.5% w/w (1.46 atom% ¹³C) to trace amendment-derived C into different soil C pools such as particulate organic C (POC), mineral-associated organic C (MAOC), microbial biomass C (MBC), and microbial respiration. Soil was sampled (Grabenegg, Austria) from two long-term management practices: one fertilized with mineral NPK since 1954 (SM1) and one receiving the same NPK regime plus wood‑derived biochar since 2022 (SM2). In a 44‑day laboratory incubation we quantified how the laboratory amendment of 13C-crop residue and 13C-biochar, alone and interacting with the long-term soil management practices, affect microbial utilization and C stabilization.
The results showed that field biochar treated soil (SM2) had higher SOC (by 51%) than non-biochar amended field soil (SM1), indicating that longer-term biochar application enhanced soil C stocks. In the short-term, at day 44 of the incubation, SOC under laboratory applied biochar was highest in both soils (SM1: 21.1g C kg-1; SM2: 26.8 g C kg-1), followed by crop residue (SM1: 15.2 g C kg-1; SM2: 21.1 g C kg-1), compared to soil with no laboratory C amendment (SM1: 11.8 g C kg-1; SM2: 17.8 g C kg-1). This highlights that in the short-term SOC gains were higher for biochar (SOC gains of 9.0-9.3 g C kg-1) than for crop residue (SOC gains of 3.0-3.3 g C kg-1). In addition, both soils showed a strong positive relationship between SOC and POC (R² = 0.783) but not with MAOC, indicating that SOC increases were largely driven by changes in the POM. The 13C tracing will allow to partition and follow the amendment-derived allocation of C into microbial biomass, respiratory use, and the transfer from the POC into the MAOC pool by microbial turnover (data evaluation ongoing). Overall, biochar proved to be the most effective CSA amendment for enhancing SOC in both long-term and in the short-term.
How to cite: Bibi, S., Hafiza, B. S., Wanek, W., Vlasimsky, M., Vezzone, M., Nakamya, J., Heiling, M., Dercon, G., Sandén, T., Hood-Nowotny, R., and Spiegel, A.: Short-term carbon dynamics from ¹³C‑labelled maize residue and its derived biochar in long‑term NPK and NPK plus biochar amended soil, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17172, https://doi.org/10.5194/egusphere-egu26-17172, 2026.
Methane (CH4), a potent greenhouse gas, is predominantly produced in wetland soils through biological processes. Recent studies reveal that reactive oxygen species (ROS) can abiotically generate CH4 via oxidative demethylation of organic compounds, yet the environmental significance of this pathway remains unexplored. Here, we investigate the potential for reactive oxygen species (ROS)-driven CH₄ formation across diverse wetland soils during redox fluctuations. Using sterilized soils from 14 Chinese wetlands amended with a model methyl donor, we identified a linear relationship between hydroxyl radical (•OH) accumulation and CH₄ production, yielding 91 nmol·L⁻¹ CH₄ per nmol·L⁻¹ •OH. Mechanistic validation with citric acid and sodium citrate demonstrated that ligand-mediated iron chelation and acidification work together to enhance this pathway by preventing iron precipitation. Natural biomaterials such as fish remnants and rice litter acted as methyl donor hotspots, contributing approximately 50% of total CH4 emissions during oxygenation. These findings establish ROS-driven CH4 production as a pervasive abiotic pathway under ambient conditions. Our results underscore the necessity of reevaluating water management strategies in wetlands, where fluctuating water levels may inadvertently amplify abiotic CH4 fluxes.
How to cite: Liu, Z. and Chen, Z.: Abiotic Methane Production Driven by Soil Reactive Oxygen Species, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6964, https://doi.org/10.5194/egusphere-egu26-6964, 2026.
For reasons of simplification and economy, the availability of nutrients and trace elements has been tried to approach via one-step extractions, which ideally rely on field experiments utilizing conventional fertilization, crops and varieties. However, particularly in the case of the main nutrient phosphorus, the commonly used extraction methods show significant weaknesses in reflecting actual plant availability. Zehetner et al. (2018) correlated 14 P-extraction methods to 50 different soils and obtained a maximum Pearson correlation coefficient of 0,365 between extracted P and crop yields.
A special sequential leaching sequence has been designed initially for sediments to assign the phosphate anion to exchangeable, Fe, Ca, Al, and humic-bound fractions, because they reflect different mobilization pathways. Fractionation is carried out in the following steps: 1. exchangeable (NH4Cl/NaOH pH 7), 2. Fe-bound (Na2S2O4/NaHCO3 pH 7), 3. Humics + Al-bound P (1M-NaOH), 4. Ca/Mg-bound P (0,5M HCl) and 5. residual P (boiling 1M-NaOH). Humics and its element loads are determined by alkaline extraction, which also contains exchangeables versus OH- and soluble hydroxo-complexes, and residuals from living cells. If the step with dithionite is omitted, the Fe-bound P moves to the subsequent fractions. At the same time, the NaHCO3-extract indicates the proportion of exchangeable P relative to HCO3- ;comparable to Olsen-P.
8 manure fertilized and 8 plant-residue fertilized organically farmed soils were fractionated using sequential extraction to quantify the different forms of phosphorus. The availability of other nutrients was also determined in the respective fractions. For comparison, the phosphorus content in the CAL extract was also analysed. In NaOH, precipitation of hydroxides and hardly soluble salts competes with soluble humic and hydroxo-complexes. Hardly alkali mobile cations might underestimate their part bound to humics, like for Mn, Mg, Ca, Sr and Ba.
As a result, the humics fraction turns out to act as a main carrier of phosphate and trace elements like Cu, and is underestimated by CAL or Olsen (NaHCO3)-extraction.
With respect to P released by CAL, dithionite released about double for samples classified as deficient (< 25 mg/kg), whereas it indicated slightly less amounts than P-CAL for 40-50 mg/kg.
For other nutrients was observed: K-CAL showed two separate linear relationships for manure and for plant residue fertilized soils versus NH4Cl exchangeable K. Mg-CAL correlated well with Mg-NH4Cl (ρ=0,851), unless dolomite was present. Exchangeable Li was higher with NH4Cl than with CAL, but well correlated (ρ=0,983).
M. J. Hedley,J. W. B. Stewart,B. S. Chauhan B.S.C.: Changes in Inorganic and Organic Soil Phosphorus Fractions Induced by Cultivation Practices and by Laboratory Incubations. Soil Science Society of America Journal 46(5), 970-976 (1982)
R. Psenner, R. Pucsko, M. Sager; (1984); Die Fraktionierung organischer und anorganischer Phosphorverbindungen von Sedimenten - Versuch einer Definition ökologisch wichtiger Fraktionen (Fractionation of organic and inorganic phosphorus compounds in lake sediments); Arch. Hydrobiol./Suppl. 70, 111-155
F. Zehetner, R. Wuenscher, R. Peticzka, H. Unterfrauner: Correlation of extractable soil phosphorus (P) with plant P uptake: 14 extraction methods applied to 50 agricultural soils from Central Europe. Plant Soil Environ. 64(4), 192-201 (2018)
How to cite: Sager, M., Bonell, M., and Risse, S.: Sequential leaching to investigate speciation of phosphorus and other elements in organically farmed soils, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7395, https://doi.org/10.5194/egusphere-egu26-7395, 2026.
Manganese (oxyhydr)oxides are abundant redox-active minerals that regulate carbon and nutrient cycling in the environment. Predicting the environmental reactivity of these oxides remains challenging due to the structural diversity and varying Mn oxidation states. We quantified the reduction kinetics of three geochemically relevant manganese oxides—birnessite, manganite, and hausmannite—using extracellular electron shuttles with varying redox potentials to systematically modulate the thermodynamic driving force for electron transfer. Rate-Gibbs free energy (ΔrG) relationships for individual manganese oxides could be established using our previously developed approach used to characterize iron oxide reduction. While ΔrG correlated with reduction kinetics for individual oxide phases, it failed to explain trends across different minerals. To address this challenge, we used the Pourbaix free-energy difference (Δ𝚿). It allowed us to predict reactivity across all three Mn oxides without requiring detailed knowledge of exact reaction stoichiometry, which is often unknown in natural systems. We further developed a coupled kinetic–mass transport model that we demonstrated that the three oxides share similar mass-transfer coefficients while their intrinsic electron-transfer rate constants differ significantly. Classical nucleation theory was applied to contextualize these differences, indicating that the balance between surface and bulk energies controls the dissolution barrier. Our work provides a predictive framework applicable to a variety of redox-active minerals, facilitating the modeling of redox fluxes in complex geochemical environments where mineral complexity previously hindered accurate predictions.
How to cite: Liu, X., Pothanamkandathil, V., Schwab, L., Mao, S., and Aeppli, M.: Predicting rates of manganese oxide reduction from thermodynamic driving forces and structural properties, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9339, https://doi.org/10.5194/egusphere-egu26-9339, 2026.
Symptoms of eutrophication are increasingly evident in remote clearwater lakes. To identify the sources and dynamics of phosphorus release, we measured total phosphorus (TP) in sediments and soluble reactive phosphorus (SRP) fluxes across the sediment–water interface at 54 locations in a deep temperate lake. Once renowned for its clear waters, Lake Stechlin has experienced a fourfold increase in water column TP over the past decade. SRP fluxes from sediments generally increased with water depth across all three lake basins, although there were significant variations in SRP concentrations, up to threefold, among sampling locations at the same depth. Notably, the lake´s total mean SRP flux in June (1.12 mg m-2 day-1) was higher than that determined in October (0.74 mg m-2 day-1). This result can be attributed to the substantial contribution (about 31 % of total SRP release) of shallower sediments (0-20 m), which are not affected by by seasonal anoxia. Our findings highlight a notable spatial variability of SRP fluxes and underscore the importance of considering often overlooked shallow sediments when assessing P dynamics in lakes.
How to cite: Holdt, J., Gonsiorczyk, T., Reimer, A., Gessner, M. O., and Thiel, V.: Spatial patterns of sediment phosphorus contents and release in a deep clearwater lake undergoing unexpected eutrophication, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9375, https://doi.org/10.5194/egusphere-egu26-9375, 2026.
Redox transformations within continental weathering profiles exert a first-order control on the redistribution and export of redox-sensitive elements, yet the mechanisms governing element retention versus release in semi-arid critical-zone settings remain poorly quantified. Here we examine a calcrete-capped weathering profile developed on Early Cretaceous andesitic lavas (∼125–122 Ma) in the Chaoyang Basin, North China, to investigate how vertically structured redox conditions influence molybdenum (Mo) behavior during weathering. The ~16 m-thick profile displays a pronounced redox stratification, comprising an oxidized, carbonate-cemented calcrete cap overlying progressively reduced, Fe-rich saprolite above fresh bedrock. Integrated iron speciation, bulk geochemistry, and mineral-scale observations reveal the presence of a shallow Fe-reduction front and a deeper ferruginous zone that acts as a transient sink for trace metals. Molybdenum concentrations increase markedly below the calcrete–saprolite boundary, while bulk-rock and pyrite δ98Mo values as low as −3.6‰ indicate sequestration of an isotopically light Mo pool at depth. Mass-balance considerations suggest that enhanced Mo mobilization from the upper oxic zone slightly outweighs retention in the reduced saprolite, resulting in a modest net Mo export from the profile. These observations support a vertically organized, two-stage redox filtering system in which climatic wet–dry cycling promotes Mo release near the surface, whereas deeper ferruginous conditions temporarily retain isotopically light Mo. By modulating both the magnitude and isotopic composition of riverine Mo fluxes, such continental redox architectures provide an important upstream control on marine δ98Mo signals used to reconstruct past ocean redox conditions. Our results highlight how redox heterogeneity in the terrestrial critical zone shapes trace-element cycling from land to ocean.
How to cite: Lin, Q., Pogge von Strandmann, P. A. E., Sun, M.-D., and Xu, Y.-G.: Vertical redox stratification in a Barremian critical-zone weathering profile revealed by extreme Mo isotope fractionation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9594, https://doi.org/10.5194/egusphere-egu26-9594, 2026.
Floodplains are integral parts of river systems, often characterised by periodic flooding. Increased river flooding caused by climate change potentially has significant impacts on oxidation-reduction processes in floodplain soils, altering soil oxygen availability, microbial populations, carbon and nitrogen cycling, and greenhouse gas (GHG) emissions. Flooding can also cause the deposition of metal-contaminated sediments on floodplain soils. During flooding, redox-driven changes in metal speciation can alter metal solubility and bioavailability, thereby affecting soil microbial composition and GHG emissions. Although the effects of flooding on redox-sensitive soil processes are well documented, few studies have examined the combined effects of flooding and metal contamination on soil GHG emissions.
To address this gap, we conducted an outdoor mesocosm experiment using intact floodplain soil cores collected from sites with contrasting flood histories (frequently vs rarely flooded), with varying levels of background metal concentrations. Soil cores were inundated for seven weeks to simulate a flood event. GHG fluxes were measured twice weekly before, during, and after the flood event. Two closed-chamber approaches were employed: CO₂ and CH₄ concentrations were measured in real-time for three minutes per chamber using an ultraportable greenhouse gas analyser, while discrete chamber gas samples were collected at four time points over a one-hour enclosure period for N₂O analysis by gas chromatography. A wide range of redox-sensitive soil parameters (oxygen concentration, pH, dissolved organic carbon, metals, and anions) were measured in soil pore water weekly. Soil subsamples collected before and after flooding will be used for metagenomic analysis to assess changes in microbial community composition and the abundance of functional genes associated with key redox processes, including GHG emissions. By integrating redox geochemistry and metagenomic analyses, this study aims to provide mechanistic insight into how flooding and metal mobility regulate microbial functions and soil GHG emissions.
How to cite: Islam, M., Hodson, M. E., Keane, B., and McNamara, N. P.: Flooding, metal mobility, and microbial controls on greenhouse gas emissions from floodplain soils, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10376, https://doi.org/10.5194/egusphere-egu26-10376, 2026.
Soil fertility is a key determinant of agricultural productivity and sustainable land management. Among essential nutrients, phosphorus (P) plays a critical role in crop growth and yield formation, yet its availability in soils is highly variable and strongly controlled by soil physicochemical properties and management practices. Reliable assessment of plant-available P is therefore essential for accurate fertilizer recommendations and environmentally sound nutrient management. The Mehlich 3 (M3) extraction method is widely used as a multi-element soil test to estimate plant-extractable nutrients, including P, K, Ca, Mg, and micronutrients, across a broad range of soil types. However, interpretation of M3-extractable P remains challenging due to differences in extraction efficiency among soils with contrasting texture, mineralogy, pH, and organic carbon content.
Phosphorus in soil solution is in dynamic equilibrium with solid-phase P pools, and the size and replenishment of extractable P depend on both solution chemistry and soil solid-phase properties. The aim of this study was to investigate changes in extractable P over six consecutive Mehlich 3 extractions and to evaluate how soil texture and chemical properties influence this process. Sequential extractions were conducted on soils differing in texture and chemical composition, allowing assessment of P release dynamics beyond a single extraction event.
The results demonstrate that Mehlich 3–extractable P does not always fully represent the pool of plant-available phosphorus in soils. Sequential extraction patterns revealed substantial differences in P release among soils, indicating varying capacities of solid-phase P to replenish the soil solution. Soil organic carbon, calcium, magnesium, and clay content were identified as key factors controlling the relative proportions of sequentially extracted P. These findings highlight the importance of soil-specific controls on P extractability and suggest that a single Mehlich 3 extraction may be insufficient to characterize soil P availability in certain soil types.
From a practical perspective, the results indicate that assessment of soil P reserves, in addition to standard Mehlich 3 P measurements, could improve fertilizer recommendations and support more efficient and environmentally sustainable phosphorus management.
How to cite: Tõnutare, T., Tõnutare, T., Varend, V., Krebstein, K., and Kõlli, R.: How Reliable Is Mehlich 3 for Estimating Plant-Available Phosphorus? Evidence from Sequential Soil Extractions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11553, https://doi.org/10.5194/egusphere-egu26-11553, 2026.
Climate change is altering precipitation patterns, which can stimulate carbon (C) and nitrogen (N) cycling processes in terrestrial ecosystems, potentially leading to increased soil greenhouse gas (GHG) emissions. However, a systematic understanding of how soil GHG fluxes respond to both increased precipitation (IP) and extreme precipitation (EP) across diverse global ecosystems is still lacking. To address this knowledge gap, we conducted a global meta-analysis based on data extracted from 49 published studies. Our objectives were to quantify the effects of IP and EP on fluxes of CO₂, CH₄, and N₂O, and to explore the key driving factors behind these responses. The results revealed that: (1) IP significantly enhanced soil CO₂ emissions by 10.2% and N₂O emissions by 61.7%, but had no significant effect on CH₄ fluxes. In contrast, EP significantly increased emissions of N₂O (by 61.8%), CO₂ (by 13.3%), and CH₄ (by 3.2%). (2) Ecosystem type mediated the GHG response under IP treatment (P < 0.01). Among grasslands, forests, and farmlands, the forest ecosystem showed the highest response ratios for CO₂ (30.4%), N₂O (61.8%), and soil respiration (37.5%), while grasslands exhibited the lowest responses. (3) Variation in CO₂ flux was primarily associated with soil dissolved organic carbon and microbial biomass carbon (both P < 0.001), whereas changes in N₂O flux were most strongly linked to soil NH₄⁺-N content (P < 0.001). This study synthesizes global experimental data to clarify the distinct impacts of IP and EP on GHG emissions, highlighting the critical role of ecosystem-specific traits and soil biogeochemical properties. Our findings provide an integrated perspective for predicting soil-climate feedbacks under future precipitation regimes.
How to cite: Zhang, M., Li, C., Wang, W., Tong, X., and Wang, K.: Responses of greenhouse gas emissions to increased precipitation events in different ecosystems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12156, https://doi.org/10.5194/egusphere-egu26-12156, 2026.
Kauri (Agathis australis), a long-lived conifer endemic to northern New Zealand, is threatened by kauri dieback disease caused by the soilborne oomycete Phytophthora agathidicida. The pathogen causes root damage and dieback, which may impair water and nutrient uptake and may alter soil processes. Field-based evidence on how nutrient availability varies with stand health status and season remains limited.
We investigated plant-available nutrient supply in kauri dominated forests across the Waitākere Ranges (New Zealand), using 6 plots (24 subplots) established across three sites (Cascades, Piha and Huia) including plots which show strong evidence of kauri dieback symptoms (symptomatic) and plots without strong dieback symptoms (asymptomatic). Plant-available nutrients (NO3--N, NH4+-N, PO43--P), base cations (Ca, Mg, K), micronutrients (Fe, Mn, Cu, Zn, B, S), and trace/toxic metals (Pb, Al) were measured using Plant Root SimulatorTM (PRS) probes, which provide an integrated, field-based measure of nutrient supply rates. Sampling was conducted from March 2024 to February 2025, with probes deployed and retrieved every three months.
Our findings show that health status had an effect on Cu and Pb supply with lower rates in symptomatic plots which may reflect a reduction in rhizosphere activity and organic matter cycling that decreases the exchangeable fraction of these strongly complexing metals. Season had an effect on most nutrients with lower supply rates during austral summer (Dec–Feb) likely reflecting drier-soil conditions limiting transport (reduced diffusion) combined with greater plant demand. NO3--N showed no seasonal or health status related effect, consistent with its high mobility and very low rates. In austral winter (Jun–Aug), NH4+-N supply increased, likely due to reduced plant uptake and suppressed nitrification under cool, wet conditions. Overall, these results indicate that seasonal controls dominate plant-available nutrient supply across the Waitākere kauri forests, while health status is associated with more targeted shifts in specific elements and there is minimal evidence of health status × season interactions for most nutrients. This study underscores the importance of repeated, in-situ measurements for interpreting nutrient dynamics in disease-impacted ecosystems.
How to cite: Yang, S., Struijk, M., and Schwendenmann, L.: Effect of season and health status on nutrient dynamics in diseased Agathis australis forests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14344, https://doi.org/10.5194/egusphere-egu26-14344, 2026.
“Redox-Dependent Molecular Signatures of Carbon Transformation and Persistence in incubated Dissolved Organic Matter”
Rania Mobarak1, Carsten Simon1,Klaus Holger Knorr2, Maximilian P. Lau3, Oliver J. Lechtenfeld1
- BioGeoOmics, Department of Environmental Analytical Chemistry, Helmholtz Centre for Environmental Research-UFZ, 04318 Leipzig, Germany.
- Institute of Landscape Ecology- ILÖk, University of Münster, 48149 Münster, Germany
- Interdisciplinary Environmental Research Centre, Technische Universität Bergakademie Freiberg ,09599 Freiberg, Germany
Microbial transformation of dissolved organic matter (DOM) under anoxic conditions exerts a fundamental control on carbon persistence, redox coupling, and energy transfer in aquatic and peatland ecosystems. Although previous studies have demonstrated that anoxic microbial processing favors the accumulation of chemically reduced organic matter, the mechanistic links between DOM transformation pathways, electron acceptor availability, and microbial activity remain poorly constrained.
Here, we investigated DOM dynamics under controlled anoxic conditions using incubation experiments with natural DOM sourced from three ecosystems characterized by contrasting redox histories: (i) the hypolimnion of a holomictic lake, (ii) the monimolimnion of meromictic lake, and (iii) the permanently anoxic peat pore waters. Microbial inocula originating from each respective system were added to the DOM incubations, alongside parallel abiotic controls to disentangle biologically mediated from abiotic transformation processes. Anoxic conditions were established via N₂ purging and maintained throughout the 90-day incubation period, with all filtration and subsampling conducted inside an anoxic glove box to prevent oxygen intrusion.
Temporal changes in dissolved organic carbon (DOC) concentrations were monitored in conjunction with key inorganic electron acceptors, including nitrate, dissolved iron, and sulfate, to evaluate coupled carbon turnover and redox dynamics. Across all incubations, we observed pronounced initial changes in DOC and electron acceptor concentrations, followed by more gradual transformation phases, indicative of sustained microbial metabolism under anoxic conditions. Distinct temporal trajectories among ecosystems highlight the strong influence of prior redox exposure, electron acceptor availability, and intrinsic DOM quality on anoxic organic matter transformation pathways.
Ongoing molecular-level characterization using liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry (LC-FT-ICR-MS) will resolve changes in DOM molecular composition, oxidation state, and energetic characteristics in relation to microbial metabolism and terminal electron-accepting processes. Together, this integrative approach provides mechanistic insight into how redox conditions regulate DOM reactivity and carbon persistence in anoxic environments, with implications for predicting carbon storage and redox-mediated feedbacks under shifting oxygen and hydrological regimes.
How to cite: Mobarak, R.: Redox-Dependent Molecular Signatures of Carbon Transformation and Persistence in incubated Dissolved Organic Matter, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16165, https://doi.org/10.5194/egusphere-egu26-16165, 2026.
Soil microorganisms play a central role in regulating terrestrial ecosystem functioning, yet their long-term responses to sustained atmospheric nitrogen (N) enrichment remain unclear. Here, we compiled a global dataset of 6,255 paired observations from 308 field-based N addition experiments to assess how ecosystem functionality and microbial properties respond across gradients of N input rate and experimental duration. Across ecosystems, N enrichment increased ecosystem functionality by 17.6% but reduced microbial biomass by 4.2%, with both effects strengthening under higher N inputs and longer exposure times. Spatially explicit meta-forest modelling revealed that long-term N enrichment elicited stronger ecosystem and microbial responses globally, with grid-scale variation primarily controlled by soil properties—especially soil pH, sand content, and bulk density—rather than by N deposition rates alone. Notably, we detected a temporal reversal in the relationship between microbial biomass and ecosystem functionality: positive under short-term N enrichment but increasingly negative over time. This shift likely reflects a transition in microbial life-history strategies, characterized by the replacement of oligotrophic (K-selected) taxa with copiotrophic (r-selected) taxa, leading to altered resource-use efficiency and declining microbial biomass. Our findings demonstrate that microbial biomass and life-history strategy shifts are critical determinants of long-term ecosystem functioning under sustained N enrichment, highlighting the dominant role of soil constraints in shaping ecosystem responses at the global scale.
How to cite: Wang, J., Li, K., Zhou, Z., Wu, J., Bo, M., Shen, X., and Zou, J.: Temporal shifts in microbial life-history strategies control ecosystem functioning under sustained nitrogen enrichment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17085, https://doi.org/10.5194/egusphere-egu26-17085, 2026.
Diffusion is a key mechanism controlling nutrient transport and availability in terrestrial ecosystems. This process is controlled by interactions among soil chemical, biological, and physical properties, through which biotic and abiotic factors jointly influence the diffusion of organic nutrients (e.g., amino acids and organic carbon and phosphorus compounds) as well as inorganic ions (anions and cations). Thus, identifying the key soil factors controlling the diffusion flux of different nutrient forms is important. However, most previous studies have examined the diffusion fluxes of individual nutrients and fewer studies have considered multiple organic nutrient forms together, but only in a few soils at maximum.
In this study, 63 soils covering different geologies and land management, with a wide texture and pH range, were collected across Austria. After transport to the laboratory, intact soil cores were brought to field capacity and after 48 hours subjected to microdialysis measurements of solute diffusion rates over 60 hours. Dialysate samples collected during the experiment were analyzed for amino acids (20 proteinogenic amino acids) as well as inorganic anions (Cl-, NO3-, SO42-, PO43-) and inorganic cations (NH4+, K+, Mg2+, Ca2+). In parallel, a duplicate set of soil cores were analyzed for soil physicochemical and biological properties, including soil pH, texture, pore size distribution, water content, soil organic carbon, total nitrogen, and total phosphorus, exchangeable cations, microbial biomass and respiration, and soil enzyme activities.
Across the 63 soils, soil pH values ranged from 3.9 to 8.2, and soil textures from sandy to clayey. Solute flux determination has been finished and final soil physicochemical properties are currently under analysis. We expect that (i) differences in soil texture lead to changes in pore structure and pore size distribution, as well as the continuity of water films and diffusion path lengths under field capacity, shorter path lengths at finer soil textures promoting nutrient diffusion in soil. (ii) Negative charges on clay minerals and soil organic matter increase cation adsorption and exchange, reduce cationic solute concentrations in the soil solution, and thereby slow down effective cation diffusion. (iii) Soil pH affects the surface charge of soil organic and mineral particles, which in turn affects the adsorption and release of inorganic ions and amino acids, thereby influencing their mobility and availability.
To test these predictions, we will conduct an integrated data analysis of solute properties (e.g., mass, pKa, charge, hydrophobicity, solubility, %C, %N, %O, other structural properties) and soil properties (see above). We will test diffusive fluxes of all solutes quantified across the 63 soils with principal component analysis. In parallel, we will use LASSO (Least Absolute Shrinkage and Selection Operator), multiple linear regression, and structural equation models to understand direct and indirect controls on solute and nutrient fluxes by diffusion. By integrating the diffusion behavior of multiple organic and inorganic nutrients across a wide range of soils differing in soil physical, chemical and biological properties, this study will greatly improve our understanding of nutrient transport in heterogeneous soils, identify key drivers and thereby help clarify the processes influencing soil nutrient availability.
How to cite: Wen, M. and Wanek, W.: Biotic and Abiotic Controls on Soil Nutrient Diffusion Flux Based on Microdialysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17408, https://doi.org/10.5194/egusphere-egu26-17408, 2026.
We need to radically transform our food and agriculture system to stay within the planetary boundaries. Sustainably increasing plant production while substantially reducing animal production unveils the need of alternatives to both chemical-based conventional and animal-based organic agriculture. In stockless farming systems, legume–grass mixtures provide no direct economic return; therefore, their proportion in crop rotations is often reduced. As a result, stockless farming is often considered challenging in terms of long-term soil fertility and plant nutrition, although research on this topic is rare. To address the specific challenges faced by veganic and stockless organic farming, a long-term field experiment was established in 2017 in Hesse, Germany, in which one veganic and two stockless organic farm systems differing in crop rotation, each combined with three fertilization systems, are compared to a mixed farm system with three livestock density levels. All systems also include an unfertilized control treatment, in which legume–grass mixtures are mulched. This experiment provides a unique basis for analyzing the long-term effects of different management and fertilization strategies on soil biogeochemical processes in detail.
Here, we aimed to study the effects of (stockless/veganic) crop rotation and organic fertilization on microbial biomass and enzyme kinetics. For this purpose, we collected soil samples from the topsoil (0-30 cm) after the harvest of potatoes in all 16 treatments in the second crop rotation of the experiment (8 years after experimental setup). We analyzed the soil microbial biomass by chloroform fumigation extraction, as well as the enzyme kinetics (Vmax, Km) of β-1,4-glucosidase (C cycle), β-1,4-N-acetylglucosaminidase, leucine aminopeptidase (N cycle), and phosphomonoesterase (P cycle) by microplate assays with fluorogenic substrates.
First results indicate that a stockless farming system, aiming at maximizing economic returns by prioritizing high-value root crops and cereals, shows a higher microbial N limitation than stockless farming systems aiming at increasing soil fertility or following veganic growing principles. Enzyme kinetics are generally more influenced by fertilization treatments than by farm types. Especially in the vegan farm type, Vmax of β-1,4-glucosidase, β-1,4-N-acetylglucosaminidase, and phosphomonoesterase differed significantly between the fertilization treatments (comparing compost, cut and carry, and silage).
Taken together, microbial biomass and enzyme kinetics are dependent on fertilization type, but less on farm type/crop rotation. Stockless/veganic and mixed farms do not differ significantly at first glance, especially if the stockless/veganic farms work towards maintaining and/or increasing soil fertility.
How to cite: Schwerdtner, U., Peglow, N., Möller, M., Bruns, C., Athmann, M., and Pausch, J.: Effects of crop rotation and organic fertilization on microbial biomass and enzyme kinetics in a long-term stockless/veganic field experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21029, https://doi.org/10.5194/egusphere-egu26-21029, 2026.
The chemical speciation of phosphorus (P), iron (Fe), and manganese (Mn) in Arctic marine sediments is critical for evaluating nutrient availability, redox dynamics, and carbon preservation in rapidly changing polar environments. In Baffin Bay, intensifying climate-driven changes in sea-ice cover, primary productivity, and terrestrial inputs have increased the significance of microorganisms in mediating mineral transformations and diagenetic pathways that control the mobility of these elements. This study examines the relationship between microbial community composition and the speciation of P, Fe, and Mn in sediment cores collected along a depth gradient from shallow coastal zones to deep basins. Through the integration of solid-phase extractions, porewater chemistry, sediment geophysical properties, and microbial community profiling, microbially driven redox transitions that regulate elemental release, retention, and burial are identified. The results demonstrate that distinct microbial assemblages, particularly iron-reducing, sulfate-reducing, and manganese-oxidizing taxa, are associated with zones of intensified elemental cycling, thereby influencing the distribution of reactive and mineral-bound P, Fe, and Mn. These findings emphasize the role of microbially mediated geochemical processes in controlling nutrient cycling and sedimentary stability in Baffin Bay and offer a framework for predicting future biogeochemical responses to ongoing Arctic environmental change.
How to cite: Avetisyan, K., Dittrich, M., Zaferani, S., Hassan, M., and Jantunen, L. M.: Chemical Speciation of Phosphorus, Iron, and Manganese in Arctic Sediments of Baffin Bay: A Microbial Community Perspective, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22125, https://doi.org/10.5194/egusphere-egu26-22125, 2026.
