This session aims to improve our understanding of organic matter processing at the interface of terrestrial and aquatic ecosystems. We solicit contributions dealing with amounts, composition, reactivity, and fate of DOM and POM and the stoichiometry of its constituents (i.e., C, N, P, S) in soils, lakes, rivers, and the ocean as well as the impact of land use change and climatic change on these processes. For example, when assessing carbon dynamics across the terrestrial-aquatic continuum, it is important to recognize the key role of peatlands and peat restoration efforts as sources of organic matter for streams and rivers, as well as the contribution of mineral soil horizons to C fluxes at the catchment scale. Contributions addressing lateral fluxes of sediment and carbon induced by soil erosion or permafrost thaw are also welcome. We aim to bring together scientists from various backgrounds, but all devoted to the study of dissolved and/or particulate organic matter using a broad spectrum of methodological approaches (e.g. molecular, spectroscopic, isotopic, 14C, other tracers, and modeling).
In my talk, I propose that stoichiometric imbalances between microbial metabolic needs and carbon (C) : nitrogen (N) : phosphorus (P) ratios affect reactive macronutrient flows between ecosystems and in landscapes, much like how stoichiometric imbalances of macronutrients affect organism growth and nutrient cycling at smaller scales. More specifically, I hypothesize that the mismatch between microbial C : N : P ratios and biologically reactive macronutrient ratios modulates macronutrient retention and export. When microbial C : N : P matches nutrient availability, reactive macronutrients should be retained or transformed, reducing downstream transport. Conversely, stoichiometric imbalances between microbial C : N : P and reactive macronutrient C : N : P lead to excess reactive macronutrients being exported to downstream ecosystems
These stoichiometric imbalances are strongly modified by dissolved organic matter (DOM) quantity and especially by DOM composition, which defines the microbial reactivity of DOM. With laboratory microcosm and stream mesocosm experiments, colleagues and myself provide first mechanistic evidence for the importance of DOM composition for the stoichiometric modification of macronutrient flows. Furthermore, comparing global published C : N : P data from soils, lakes, and marine ecosystems, we find evidence that microbial activity uniformly modulates reactive DOM and macronutrient ratios across environments, affecting macronutrient cycling and flows, with probable secondary effects on ecosystem functioning and eutrophication.
The proposed concept links small-scale mechanistic understanding to ecosystem-scale patterns of macronutrient cycling in inland-water ecosystem networks. This cross-scale perspective highlights the need for integrated stoichiometric experimental and monitoring research to better understand reactive macronutrient cycling and flows, with high potential for improved macronutrient management.
How to cite: Graeber, D.: Dissolved organic matter composition may be a key modifier of ecosystem-scale macronutrient reactivity and flows across the terrestrial - aquatic continuum, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1926, https://doi.org/10.5194/egusphere-egu25-1926, 2025.
Dissolved organic matter (DOM) is a major source of macronutrients in freshwaters, yet has variable and poorly understood bioavailability to bacteria and other organisms. Because intrinsic variation in bioavailability is caused by chemical structures of organic nutrients, DOM composition data should improve predictions of bioavailable resource pool sizes. We hypothesized that bioavailable organic carbon (C) and nitrogen (N) fractions are made up of freshly produced humic- and protein-like DOM, respectively, whereas bioavailable phosphorus (P) is linked to microbially-derived DOM with potential organophosphate content and/or to chemical structures associated with DOM-Fe-phosphate complexes. These ideas were tested in eight, unproductive and organic matter-rich stream and lake sites, where we performed C, N and P bioassays with bacteria in combination with analyses of DOM composition using fluorescence excitation-emission matrix (EEM) analysis. Bioavailable C followed the predicted patterns, with strong links to fluorescent features indicating recently produced DOM. Surprisingly, bioavailable N was poorly related to DOM composition, including protein-like fluorescence, and was instead driven mainly by the amount of inorganic N. Bioavailable P was best linked to microbially-derived organic components. The standard nutrient variables explaining most of the bioavailable total dissolved C, N and P, respectively, were dissolved organic carbon, dissolved inorganic nitrogen and total phosphorus. In addition, DOM composition variables made significant unique contributions to explaining the variance in bioavailable C (19%), N (13%) and P (18%). Overall, DOM composition analysis is a promising tool to improve prediction and develop our understanding of bioavailable macronutrients in organic matter-rich freshwaters.
How to cite: Berggren, M., Rulli, M. P. D., Bergström, A.-K., Sponseller, R. A., and Hensgens, G.: Does DOM composition help explain bioavailable macronutrient concentrations in organic matter-rich freshwaters?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8762, https://doi.org/10.5194/egusphere-egu25-8762, 2025.
Coastal ecosystems are increasingly influenced by the lateral transport of organic matter, where pigmented dissolved organic carbon (DOC) contributes to water darkening and affects nutrient dynamics. These changes coincide with rising dissolved organic phosphorus (DOP) inputs, which have implications for eutrophication and carbon cycling. However, it is unclear how the bioavailable DOP (BDOP) pool responds to the individual and interactive ecosystem-level effects of water darkening, increased DOC, and higher inorganic nutrient concentrations. To explore these interactions, we conducted bioassays to estimate BDOP in a fully factorial mesocosm experiment manipulating the supply of inorganic nutrients, labile DOC (glucose) and pigmented compounds causing darkening. Results showed that while labile DOC had limited influence on bioavailable BDOP, nutrient enrichment increased BDOP in clear water. In darkened waters, added inorganic phosphorus persisted largely in its inorganic form, reflecting decreased conversion to BDOP. These findings reveal the complex interplay between light availability, organic matter inputs, and phosphorus bioavailability. By highlighting the impact of water darkening on nutrient and carbon dynamics, this study underscores the need for integrated management approaches to mitigate eutrophication and support ecosystem resilience across the terrestrial-aquatic continuum.
How to cite: P. D. Rulli, M., Garnier, A., Huss, M., Sponseller, R. A., Bergström, A.-K., Younes, H., Bell, O., and Berggren, M.: Exploring the effects of nutrients, carbon, and water darkening on coastal phosphorus bioavailability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17388, https://doi.org/10.5194/egusphere-egu25-17388, 2025.
Worldwide, farming activities exert strong impacts on the amount and molecular composition of dissolved organic matter (DOM), which constitutes an important vector of organic nitrogen (ON) transport from soils to the aquatic environment (Graeber et al., 2015). However, there are major knowledge gaps on the drivers of ON loss to water courses. In Denmark, stream data from the Danish national monitoring program (NOVANA) shows that total ON currently accounts for nearly 20 % of the annual total N loading to Danish coastal waters. In a recently initiated research project ‘orgANiC’ we are investigating the loss and fate of ON forms in five smaller agricultural catchments across Denmark (Petersen et al., 2021).
We are measuring dissolved ON (DON) and particulate ON as well as dissolved organic matter (DOM) and particulate organic matter (POM) in various source waters (soil water and groundwater), pathways (tile drains and surface runoff), and receiving streams using a comprehensive array of sampling technologies. In soil water we utilize suction cups taking weekly composite water samples, in groundwater we sample from near-surface (app. 1-5 m below surface) screens in boreholes using the Montejus principle, and in tile drains, surface runoff from fields and streams we are taking both grab samples and automated ISCO samples. These are activated when the hydrograph levels and hydrograph gradients exceed certain thresholds, determined from analysis of the long-term hydrograph data.
We are performing both indirect (total N minus inorganic N) and direct analysis of DON (size exclusion chromatography) on water samples from the different hydrological compartments. The loss of particulate ON (PON) is also monitored in tile drainage water, surface runoff and streams as these three hydrological paths are believed to be of increasing importance with the observed increase in extreme weather conditions. In the presentation we will share our current insights into the challenges of indirect DON measurements across different hydrological pathways by comparing it with direct measurements of DON and PON. We will also demonstrate how the concentrations and composition of ON fractions vary across the agricultural catchments under investigation as they represent different soil types, climate conditions and agricultural management (crops, fertilization, etc.).
References
Graeber, D., I. G. Boëchat, F. Encina-Montoya, and others. 2015. Global effects of agriculture on fluvial dissolved organic matter. Scientific Reports 5: 16328. doi:10.1038/srep16328.
Petersen, RJ, Blicher-Mathiesen, G, Rolighed, J, Andersen, HE & Kronvang, B 2021, 'Three decades of regulation of agricultural nitrogen losses: Experiences from the Danish Agricultural Monitoring Program', Science of the total Environment 787: 147619. https://doi.org/10.1016/j.scitotenv.2021.147619
How to cite: Kronvang, B., Petersen, R. J., Rolighed, J., Thorsen, M., Frederiksen, R. R., Larsen, S. E., Hasselholt, A., Hansen, B., Kim, H., Goldhammer, T., Graeber, D., and Zak, D.: Impact of agriculture and water paths on organic nitrogen loss to Danish headwater streams, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11554, https://doi.org/10.5194/egusphere-egu25-11554, 2025.
Soil organic matter (SOM) plays a vital role in most soil functions related to agriculture. It is a building block of soil structure, it buffers pH and nutrient availability, and it supports the soil food web. Up to now in agricultural practices, including agriculture labs, SOM has only been characterized as one pool. Recently, more attention has been on the fractionation into particulate organic matter (POM) and mineral associated matter (MAOM) in relation to SOM dynamics. MAOM will be the most stable pool of SOM and mineralization is probably dominated by rhizosphere processing, and therefore controlled by plant nutrient demand. Based on the idea that microbial biology plays a key role in both the formation and degradation of MAOM, we propose that adjusting agricultural management to optimize the build-up of MAOM is the way forward in minimizing nutrient losses to surface waters.
In this study we measured POM and MAOM, based on size fractionation, in pairs of agricultural plots with contrasting soil management. Furthermore we followed mineralization rate via continuous measurements of EC, moisture content and soil temperature, and based on ion-binding resin bags placed at 10, 30 and 60 cm depth.
Overall, texture is a strong predictor of the amount of MAOM, but on top the application of compost appears to have a positive effect, both on grass- and cropland. But we have indications that in some cases our MAOM fractions are dominated by fine POM, probably caused by the practice of incorporation of organic manure into the soil. Initial results show that nitrogen leaching is more associated with POM than with MAOM.
How to cite: Smits, M.: Mineral associated organic matter in practice, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12630, https://doi.org/10.5194/egusphere-egu25-12630, 2025.
Manure addition increases amounts of soil organic matter (SOM), water-extractable organic matter (WEOM), microbial biomass, and microbial activity. Mass balances have shown that soil organic C build-up is paralleled by a comparatively low retention of the added manure C, which also declines substantially with time. The implications for SOM’s molecular composition are not fully understood, but imply transformation of manure-derived organic matter as a main driver of C accumulation. We studied four long-term manured soils (24-118 years) to unravel potential mechanisms of manure turnover and SOC build-up on the molecular level. Soils were sampled a year after the last manure application.
Bulk SOM and manure were studied directly via solid-state laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). The LDI-FT-ICR-MS results indicated that manure increased SOM's energetic potential by +0.9 ± 0.2 kJ/mol C (1.5 ± 0.4%), and this trend was confirmed by bulk elemental analysis (+5.4 ± 2.8 kJ/mol C; 12.6 ± 6.5%). The addition of manure changed the composition of SOM components corresponding to 3–16 % of the total ion abundance compared to the controls, with the higher proportions found in longer running field trials. However, marker compounds directly related to manure explained only 2–12% of the molecular changes, while markers unrelated to the original manure signatures explained 67–84%. Long-term manure addition resulted in increased saturation, oxidation, and molecular weight, and decreased aromaticity of SOM as compared to unfertilized soils. Accumulated molecules had a higher energetic potential and, despite being chemically similar to the original manure, a higher mass, suggesting that manure-derived building blocks were used for the microbial synthesis of larger molecules. Molecules with lower energetic potential disappeared in manured soil samples, mirrored by a higher oxidation state of WEOM. Consequently, we also found higher water-extractable organic C yields (normalized to soil organic C) in manured samples.
To reveal potential sources of these oxidized compounds, WEOM was studied by liquid-state FT-ICR-MS coupled with liquid chromatography, and compared to representative necromass extracts (plant, fungal, bacterial). Our results indicated a clear shift towards a more bioavailable, complex, necromass-dominated but oxidized WEOM fraction in manured soils. This finding markedly differs from the tendency towards more strongly reduced SOM, which was determined by solid-state measurements. The overlap with necromass FT-ICR-MS signatures suggested a dominant bacterial control of the changes in WEOM properties and also resulted in a stronger imprint of oxidized plant markers. Yet, the dominant fraction (83% of ion abundance) explaining the shift in oxidation state was not associated to any necromass type. This indicates an oxidation of the existing SOM reserves (“priming”).
Together, the combination of solid- and liquid-state FT-ICR-MS techniques provided complementary insight, demonstrating how manure addition affects the long-term SOC balance mirrored by SOM and WEOM composition. The comparison with potential endmembers (necromass extracts, manure) provided valuable insight into pathways of SOM turnover and will allow to identify novel process markers for future studies.
How to cite: Simon, C., Stumpf, K., Kaiser, K., Lorenz, M., Maskow, T., Miltner, A., Mulder, I., Thiele-Bruhn, S., and Lechtenfeld, O.: Long-term manuring of soil results in divergent responses of dissolved and particulate organic matter on the molecular level, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12124, https://doi.org/10.5194/egusphere-egu25-12124, 2025.
The residence time of organic carbon (OC) in terrestrial reservoirs, particularly soils and freshwater systems, plays a crucial role in modulating the dynamics of the global carbon cycle. Radiocarbon (14C) is an invaluable tool for tracing the time since the biosynthesis of organic matter, enabling the quantification of carbon residence times in these terrestrial pools. While the majority of carbon fixed through terrestrial primary productivity rapidly returns to the atmosphere, a stabilized fraction of OC escapes (re-)mineralization. This OC may subsequently be exported from terrestrial ecosystems and buried in marine and terrestrial sedimentary sequences over longer timescales, effectively sequestering atmospheric CO2.
Mineral association has been identified as a key mechanism of this stabilization. Consequently, source-specific biomarkers targeting terrestrial, mineral-associated OC are of particular interest for tracking especially resistant OC species. In our study, we apply compound-specific 14C analysis on leaf wax fatty acids (n-alkanoic acids). These long-chain fatty acids (C24+) are exclusively produced by vascular plants. Moreover, their highly hydrophobic nature promotes mineral association, making them ideal molecular markers of stabilized soil OC that can be traced through export and burial.
We employ a source-to-sink approach, targeting mineral soil profiles, fluvial sediment, and lake sediment within two Alpine sediment routing systems: the Alpine Rhine and Alpine Rhone catchments. Additionally, we analyze selected depths from well-dated deltaic sediment cores spanning the past 120 years to estimate catchment-averaged transit times of long-chain fatty acids and to assess temporal variability in these trends.
Initial results indicate significant pre-aging of OC in soil profiles, Δ14C from -100‰ to below -500‰, combined with rapid and efficient fluvial export of our target compounds. Sediment core data reveal millennial-scale catchment transit times for long-chain fatty acids. Further, they show the impact of anthropogenic disturbances, which have led to an increase in the age of exported soil OC across the investigated period.
How to cite: Mittelbach, B., Calvarese, D., Moreno Duborgel, M., Rhyner, T., Wartenweiler, S., White, M., Blattmann, T., Haghipour, N., Wessels, M., Dubois, N., and Eglinton, T.: Tracing the export of terrestrial biospheric carbon from source-to-sink through molecular 14C analyses in two large Alpine catchments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15543, https://doi.org/10.5194/egusphere-egu25-15543, 2025.
Land use is a primary driver of the spatial distribution of soil organic carbon (SOC) and significantly influences the terrestrial carbon cycle. This study used the SWAT-C model to simulate the export of SOC, dissolved organic carbon (DOC), and particulate organic carbon (POC) in the Wu River Basin, analyzing the effects of land use on SOC spatial distribution. Model calibration with 2012–2017 total organic carbon (TOC) data achieved Nash-Sutcliffe efficiency values above 0.7, confirming reliability. The simulated results showed an average annual TOC export of 17.3 kgC/ha, with DOC and POC contributing 10.38 kgC/ha and 6.9 kgC/ha, respectively. Bare land had the highest POC export (66.7 kgC/ha), followed by dry cropland (32.3 kgC/ha), while urban areas and coniferous forests exhibited the highest DOC exports (15.1 and 12.4 kgC/ha, respectively). SOC storage was highest in rice field (313 tonC/ha) and lowest in bare land (175 tonC/ha). Sub-watersheds dominated by bare land and dry cropland recorded TOC exports exceeding 21 kgC/ha, marking them as future SOC export hotspots. These findings highlight the significant influence of land use on SOC distribution and provide a scientific basis for ecosystem service preservation, and sustainable watershed management.
Key words: Soil organic carbon, SOC storage, SWAT-C model, land use, Taiwan
How to cite: Lin, G.-Z. and Chiang, L.-C.: Evaluating the impact of land use on soil organic carbon spatial distribution by SWAT-C model – a case study of the Wu River Basin, Taiwan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7919, https://doi.org/10.5194/egusphere-egu25-7919, 2025.
From a limnological perspective, dissolved organic matter (DOM) can originate from allochthonous sources on the landscape or from autochthonous sources within the water body itself. In many streams and lakes, allochthonous organic materials contributing to the DOM are derived from terrestrial plants, plant litter, and soil organic material, which all include some products of microbial growth and decay. The many streams in the McMurdo Dry Valleys (MDV) provide an opportunity to understand the biogeochemistry of DOM derived solely from microbial phototrophs and heterotrophic bacteria because of the absence of plants on the barren landscape and the abundant perennial microbial mats in the stream channels. Analysis of the 20-year record of dissolved organic carbon concentrations in the streams indicates that biogeochemical processes in microbial mats and the hyporheic zone support chemostasis for DOC in these streams. Even though the stream DOC concentrations are typically quite low, about 1 mg C/L or less, we were able to use fluorescence spectroscopy to chemically characterize the DOM samples from a broad array of meltwater streams. Many streams had a distinct “humic-like” signature and some presence of an “amino-acid like” signature. In contrast, a short dilute stream that does not support mats and primarily receives DOM from the surface of the glacier had an “amino-acid like” and only a weak “humic-like” fluorescence signature. The presence of a “humic-like” signature may indicate a source from organic matter pools in the hyporheic zone which accumulate due to advection of microbial mat material from the channel. Autochthonous organic matter pools may also influence DOC concentrations in temperate streams. In addition, stream DOM may represent a labile DOM source to the lakes that contributes to supporting the mixotrophic phytoplankton communities.
How to cite: McKnight, D. and Zeglin, L.: Dissolved organic matter biogeochemistry in the McMurdo Dry Valleys, Antarctica: varying chemical quality of microbially-derived DOM in glacial meltwater streams , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14768, https://doi.org/10.5194/egusphere-egu25-14768, 2025.
Glaciers impact carbon cycling in downstream ecosystems by releasing diverse and bioavailable dissolved organic carbon (DOC). However, our understanding of organic carbon (OC) dynamics in Icelandic glaciers remains limited, as most studies have focused on other glacial regions and often lack seasonal-scale resolution.
In this study, we investigate the bioavailability of glacial OC from Icelandic streams using incubation experiments. We sampled Virkisá on a seasonal scale (a total of 72 incubation experiments) and supplemented these samples with additional data from the glacial streams Skaftafellsá, Svínafellsá, Kvíárjökull, and Fjallsá for comparison. In the glacial stream Virkisá, DOC concentrations were highest in spring at the onset of the melt season (0.18 ± 0.11 mg/L) and lowest in autumn (0.08 ± 0.02 mg/L). Notably, we observed not only seasonal variability in DOC concentrations but also in the bioavailability of glacial OC. At the glacier outlet, DOC bioavailability was consistently negative throughout the year (-18.18%), indicating DOC production during incubation experiments. Similarly, negative BDOC values (ranging from -1.44% to -24.1%) were confirmed in four other glacier-fed streams during summer, discharging from the ice cap Öræfajökull. However, further downstream, incubation experiments revealed seasonal shifts: negative bioavailable DOC (BDOC) values in spring (-18.04% at 900 m from the glacier outlet) and positive values in summer (55.55% at the same site), likely reflecting increased biological activity and DOC consumption during summer.
Overall, BDOC values showed a positive correlation with distance from the glacier. At the furthest sampling point, 3000 m from the glacier outlet, BDOC averaged +8.21% in spring and 57.02% in summer. These findings challenge previous reports of high glacial OC bioavailability and underscore the need for a more in-depth understanding of the chemical and biological processes in glacier-fed streams, particularly at a seasonal scale—a factor often neglected in studies due to the difficult accessibility of glaciers during winter.
How to cite: Wild, A.-K., Fasching, C., Baum, J., and Chifflard, P.: Bioavailability of dissolved organic carbon in Icelandic glacial streams changes seasonally and with distance from the glacier, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13097, https://doi.org/10.5194/egusphere-egu25-13097, 2025.
Transport of biodegradable organic carbon (bDOC) across land-water interfaces supports the ecological and biogeochemical functioning of northern freshwater ecosystems. Yet, we know little about how the generation and supply of terrestrial bDOC to boreal headwaters is regulated by the physical, biological, and hydrological properties of the riparian interface. We used 7-, 14- and 28- day bDOC incubations on eight occasions during the northern growing season to assess how terrestrial and aquatic bDOC concentrations differ along flowpaths connecting riparian soils to a headwater stream. We found that bDOC quantity declined along the transition from land to water, and that riparian soils had higher concentrations of bDOC compared to aquatic landscape components. Additionally, these differences corresponded to changes in the optical and chemical properties of the dissolved organic matter pool. Further, the generation of bDOC in riparian soils varied across interface types and reflected hydrogeomorphically determined differences in soil organic matter storage, groundwater level dynamics and soil microbial activity. In particular, the potential transfer of bDOC from soils to groundwater appeared largely regulated by the degree of contact between soils and lateral subsurface flowpaths. Riparian interfaces with near-constant opportunity to deliver resources laterally to streams by shallow, preferential groundwater flowpaths were found to have a relatively poor capacity to generate bDOC within local soils. At the same time, groundwater within these same interfaces had higher concentrations of bulk DOC and bDOC, likely due to connections with larger contributing hillslopes which serve as important support systems to streams during baseflow periods. Collectively, our results show that boreal headwaters are comprised of a continuum of interface types that differ in capacity to generate bDOC in near-stream soils, and in opportunity to mobilize and convey bDOC laterally. Ultimately this leads to wider variability in when and where within the broader stream network these inputs may be most important to aquatic ecosystems.
How to cite: Reidy, M., Berggren, M., Lupon, A., Laudon, H., and Sponseller, R.: Riparian zone heterogeneity influences the production and fate of biodegradable dissolved organic carbon across land-water interfaces , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1244, https://doi.org/10.5194/egusphere-egu25-1244, 2025.
Recent studies highlight a concerning reality: wildfires are becoming more frequent and intense, particularly in northern high-latitude regions where temperatures are rising fastest. Boreal forests, vital carbon (C) reservoirs, play a key role in long-term C storage and climate regulation. However, climate change-driven increases in wildfire frequency, intensity, and severity threaten to turn these soils from C sinks into sources, disrupting soil biogeochemical cycles and hindering forest recovery and ecosystem resilience. Fire significantly alters soil organic matter (SOM) and C cycling processes, particularly impacting soil dissolved organic matter (DOM). In boreal forests of Northern Europe, low-intensity surface fires are common, but their short-term effects on soil DOM dynamics remain poorly understood. We aimed to investigate the short-term effects of a low-intensity surface fire on post-fire DOM properties and dissolved organic carbon (DOC) content in boreal forest soils.
Fieldwork was conducted in a dry Scots pine boreal forest of Eastern Finland (Ruunaa, North Karelia) that underwent a prescribed restoration fire on June 30th, 2022. The burning resulted in a non-stand replacing surface fire of low intensity and severity. To capture short-term post-fire responses, we compared DOC content, δ¹³CDOC, and DOM absorbance properties in soil water and throughfall collected from burned and unburned control plots during the first growing season following the burning (from July to October 2022). DOM was analyzed for changes in concentration and isotope composition with a coupled elemental analyzer and mass spectrometer (EA-IRMS), while changes in DOM chemical composition were characterized using UV-visible absorbance spectrophotometry.
Our results indicated that soil DOC contents declined immediately after the fire in burned plots compared to control ones, accompanied by slight enrichment of burned soils DOM in ¹³C. These findings suggest reduced availability of labile C substrates following SOM and biomass combustion, fire-induced reduction of the microbial biomass, and introduction of newly formed pyrogenic carbon (PyC), which has a lower proportion of lignin-derived ¹³C. Additionally, the soil DOM from burned soils showed slightly higher degrees of aromaticity and molecular weights, indicating a shift towards more aromatic and recalcitrant compounds, suggesting the presence of a more stable C pool in the soil water of fire-affected soils.
Our findings emphasize the crucial role of low-intensity surface fires in influencing DOM dynamics and provide vital insights for understanding the post-fire soil C cycling and ecosystem recovery in boreal forests of Northern Europe. Understanding these dynamics is crucial for improving C balance models in these forests and equipping policymakers and forest managers with the tools needed to enhance resilience in one of the planet’s most vital ecosystems.
How to cite: Rebiffé, M., Kohl, L., Köster, E., Keinänen, M., Berninger, F., and Köster, K.: Low-Intensity Surface Fires and Dissolved Organic Matter: Unraveling Post-Fire Carbon Dynamics in Northern European Boreal Forest Soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12105, https://doi.org/10.5194/egusphere-egu25-12105, 2025.
Export of dissolved organic carbon (DOC) from catchments to streams has increased in the last decades in many catchments across the Northern hemisphere. Mobilisation of DOC from riparian soils and wetlands is highly dependent on discharge and is triggered by storm events. In many cases a very strong correlation between DOC and Fe concentrations during storm events can be observed in the streams suggesting joint source areas and mobilisation mechanisms. In this contribution we will discuss causes and mechanisms of Fe transfer from catchments into aquatic systems. Analyses of Fe species from a 40-years sample archive from the Große Ohe Catchment in the Bavarian Forest National Park indicated that between 60 and 100 % of the dissolved Fe determined were in the reduced form Fe(II). Thus, a substantial amount of redox equivalents will thus be exported from catchments, and the implications for e.g. the oxygen budget of streams will be discussed.
How to cite: Peiffer, S., Hopp, L., Kölbl, A., Beudert, B., and Lechtenfeld, O.: The role of dissolved organic carbon for the export of iron from catchments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12453, https://doi.org/10.5194/egusphere-egu25-12453, 2025.
Boreal and subarctic wetland soils accumulated at least 550 Gt of organic carbon (OC) over the last 10,000 years, a large part of which is associated with Fe and Al (hydr)oxides as coprecipitates and via adsorption processes. Mobilisation of some of these pools via dissolved organic carbon (DOC) from soils to streams could be enhanced by reduction of ferric iron - triggered by rising water tables and oxygen depletion - via two distinct processes. Fe reduction can (i) directly release coprecipitated OC if iron (hydr)oxides are reductively dissolved and (ii) release OC by desorption from mineral surfaces if pH increases with Fe reduction, which is referred as to indirect (redox driven) mobilisation here. Both redox driven direct and indirect mobilisation likely occur under relatively wet and warm conditions such as during rewetting in the vegetation period. However, the relative importance of reductive dissolution of Fe-OC associations versus desorption of OC and its controlling factors are still unclear under field conditions as they were only investigated in the lab so far. Therefore, the relative importance of direct versus indirect mobilisation of OC and its controlling factors was studied for twelve catchments of the Krycklan research site in boreal Sweden. From long term monitoring data on stream discharges, DOC and Fe, molar DOC/Fe ratios of riparian soil waters released into the stream during rewetting of catchments in summer were computed using Generalised Additive Models. From these ratios, the relative importance of desorption for total DOC mobilisation via Fe reduction was calculated assuming a constant DOC/Fe ratio for direct mobilisation, i.e. the ratio at which OC and Fe occur in coprecipitates. DOC/Fe ratios were found to be positively correlated with average DOC concentrations in streams (coefficient of linear correlation of ρ = 0.78), and with the fraction of forest covered by spruce (ρ = 0.81). Higher reactive Fe/Al contents and hence larger mineral surfaces may be linked to spruce forest promoting intense weathering of soil’s primary minerals. Both high DOC in porewater (DOC in the stream as a proxy) and large mineral surfaces (spruce cover as a proxy) are required for desorption (indicated by relatively high DOC/ Fe ratios) to happen. If direct release of DOC with Fe reduction was accompanied by additional indirect mobilisation via a pH dependent desorption, up to twelve times more DOC was released for the same amount of Fe (hydr)oxides being reduced - compared to direct mobilisation via dissolution of iron (hydr)oxides alone. Mobilisation processes driven by Fe reduction and subsequent pH increase may intensify with climate change by enhanced drying and wetting cycles in boreal systems such as Krycklan.
How to cite: Selle, B., Knorr, K.-H., Lidman, F., Hortmann, A., Škerlep, M., and Laudon, H.: Redox and pH driven mobilisation of dissolved organic carbon from boreal wetlands, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13310, https://doi.org/10.5194/egusphere-egu25-13310, 2025.
Ditching of peatlands has been used extensively in Scandinavia with the purpose to promote tree growth. Studies show that concentrations of dissolved organic matter (DOM) are higher in waters exported from ditched peatlands compared to pristine systems, suggesting that ditching may contribute to browning observed in surface waters in forested regions.
After ditching and when trees are established, the peat will be colonized by ectomycorrhizal (EM) fungi, which supply the trees with nutrients. We hypothesize that EM-fungi will mobilize DOM to the soil water while mining the peat for nutrients and saprotrophic fungi will become more active when the peat gets aerated, which will also result in mobilization of DOM. In the current project we are exploring the link between fungal communities and DOM mobilization in a peatland gradient, spanning from pristine conditions with high water level and lack of trees, to strongly drained conditions with low water level and established pine forest. Along this gradient, soil water was sampled from ground water tubes. Water and peat samples were analyzed for organic matter concentrations and the fungal community was characterized by metabarcoding.
DOM concentrations in the soil water were increasing towards the ditch - where the water level was lower and the tree growth higher - as was the fungal biomass. While these results are in line with our hypothesis, the results on fungal community composition will provide important information to assess the link between fungal processes and DOM mobilization.
This study will bring much needed information on succession of fungal communities with different decomposition strategies along peatland ditching gradients and potential links to DOM mobilization and surface water browning.
How to cite: Myridakis, A. I., Wallander, H., Floudas, D., and Kritzberg, E.: Peatland ditching as a driver of dissolved organic matter mobilization - the role of fungal communities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20215, https://doi.org/10.5194/egusphere-egu25-20215, 2025.
Coastal wetlands are vital to global carbon cycling because they can store large amounts of Soil Organic Carbon (SOC). These ecosystems are influenced by complex interactions between salinity, flooding frequency and vegetation, which affect the formation, stabilization and mobilization of Dissolved Organic Carbon (DOC). Stabilization mechanisms, including mineral association and aggregation, are critical for long-term SOC storage, with Mineral-Associated Organic Matter (MAOM) being the dominant mechanism. However, the mechanisms driving DOC mobilization in estuarine marshes, particularly spatial and seasonal variabilities and the effects of climate and vegetation, remain poorly understood.
This study addresses these gaps by examining how seasonal fluctuations driven by biotic factors impact DOC concentrations in marsh soils along salinity and flooding gradients. As a part of 12 months field study, pore-water samples are being collected monthly using suction cups in nine marsh zones along the Elbe Estuary, representing a salinity gradient (salt, brackish, and freshwater marshes) and flooding gradients (pioneer, low, and high zones) at depths of 10 cm and 30 cm. The collected samples are analyzed for Non-Purgeable Organic Carbon (NPOC), anions, and Iron (Fe) concentration. Preliminary results revealed that NPOC concentrations were consistently higher in salt marshes compared to brackish and freshwater marshes. Pioneer zones exhibited the highest NPOC concentrations, particularly at 30 cm depth, highlighting the interaction of site and elevation as key factors driving spatial variability. Seasonal trends showed elevated NPOC levels during summer, followed by declines in autumn, likely driven by increased organic matter decomposition during warmer periods. Our results indicate a negative correlation between NPOC and Fe concentrations, suggesting that redox-driven mechanisms, such as Fe reduction, play a critical role in DOC stability and release. In conclusion, DOC mobilization in the Elbe Estuary is strongly influenced by salinity and flooding gradients, with higher concentrations in salt marshes and during summer. Understanding DOC dynamics in tidal marshes is essential for predicting the impacts of climate change on carbon cycling within estuarine ecosystems. As global sea levels rise and salinity gradients shift, this research provides important baseline knowledge to inform strategies for protecting the carbon sinks of coastal wetlands.
How to cite: Ashfaq, S., Neiske, F., Becker, J. N., and Eschenbach, A.: Carbon Cycling in Estuarine Marshes: A Focus on DOC Stabilization and Mobilization Pathways, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12677, https://doi.org/10.5194/egusphere-egu25-12677, 2025.
Posters on site: Thu, 1 May, 08:30–10:15 | Hall X1
Dissolved organic carbon (DOC) plays a fundamental role for the aquatic ecosystem and the global carbon cycle. It also interferes with drinking water treatment processes. Its removal is costly and depends on its quantity and quality, i.e. its concentration and molecular composition. Riverine DOC concentrations have increased in Europe and North America in recent decades, primarily driven by reductions in acid deposition. Currently, changing climatic conditions such as increasing temperatures, heavy rainfall events and droughts are gaining importance in determining DOC concentrations. However, the specific mechanisms by which climate variability drives riverine DOC concentrations and its chemical composition at different time scales are not sufficiently understood. Therefore, reliable forecast about future developments are challenging. In forested headwater catchments, where riparian soils are major sources of DOC export, riparian soil moisture might be paramount to determine DOC quantity and quality. Soil moisture is driven by climate variability and controls subordinate and interdependent processes that can shape DOC quantity and quality. However, limited data of soil moisture from forested headwaters and specifically from their riparian zones are available. In this context, we will study the upper Rappbode catchment in the Harz mountains, which drains into Germany’s largest drinking water reservoir. We will relate high-frequency soil moisture observations at multiple depths (vertical dimension) at different riparian profiles with differing wetness characteristics (topographic dimension) to the corresponding DOC quantity and quality over temporal scales, including short-term, seasonal/annual, and long-term by modeling. We hypothesize that currently and in future patterns of soil moisture in the vertical and topographic dimension play a pivotal role as drivers of the temporal dynamics of DOC quantity and quality in riparian soils and subsequently in the corresponding surface waters. Initial results from our sampling campaigns highlight differences in riparian soil water chemistry between the locations of different wetness characteristics, but also distinct vertical heterogeneities. We will present further findings and results that improve the understanding of how soil moisture drives riverine DOC quantity and quality, with special consideration of vertical heterogeneities in the riparian profiles. With a refined understanding of DOC dynamics, more reliable forecasts can be made to derive targeted adaptation strategies for safe drinking water supplies and to better assess future impacts on aquatic ecosystems and the global carbon cycle.
How to cite: Burkhardt, P. D., Musolff, A., and Ledesma, J. L. J.: How do vertical and topographic riparian soil moisture patterns shape headwater dissolved organic carbon dynamics?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5582, https://doi.org/10.5194/egusphere-egu25-5582, 2025.
Permafrost soils store about twice as much organic carbon as the atmosphere. In the future, certain permafrost regions will develop anoxic soil conditions due to thaw-induced soil subsidence and waterlogging. Under these conditions, methane (CH4) emissions due to decomposition of newly thawed organic carbon will likely increase. The net release of CH4 from soil depends on the availability of more energetically favorable electron acceptors than CO2, which could on the one hand suppress methanogenesis and on the other hand act as an electron acceptor for anaerobic CH4 oxidation. Since many common inorganic electron acceptors (sulfate, nitrate) are present only in low concentrations in permafrost soils, we hypothesize that natural organic matter (NOM) and/or ferric iron (Fe(III)) are more abundant and can act as significant electron acceptors. However, to which extent NOM fractions such as dissolved organic matter (DOM) and particulate organic matter (POM) as well as Fe(III) minerals influence methane production and methane oxidation in permafrost soils is unknown. In this project, we therefore aim (i) to characterize the redox-active moieties of DOM and POM fractions from permafrost soils, (ii) to quantify the effect of these NOM fractions on methanogenesis suppression and/or CH4 oxidation, and (iii) to identify the microorganisms that are able to oxidize CH4 coupled to NOM or Fe(III) reduction by performing enrichment culture experiments. To achieve this, we collected and isolated NOM from a thawing permafrost peatland in Sweden (Stordalen Mire, Abisko) across multiple thaw stages. We analyzed the changes in electron accepting and donating capacity of NOM fractions across permafrost thaw stages via mediated electrochemical reduction and oxidation, respectively. Enrichments targeting anaerobic CH4-oxidizers were set up using an inoculum from partly thawed and fully thawed permafrost thaw stages, amended with poorly crystalline Fe(III) minerals, AQDS (a model compound for redox-active moieties in NOM) and POM. In the future, microcosm experiments with isolated NOM fractions and 13C-labeled CH4 or 13C-labeled CO2 will be performed in order to quantify the influence of NOM on methane oxidation or methanogenesis suppression, respectively. Spectroscopic, isotope-tracing and molecular biology techniques will be used to track the reduction of amended electron acceptors, concentration of labeled gases and the change in abundance of targeted microorganisms. Overall, this work will help to assess the role of NOM and Fe(III) in influencing CH4 cycling in thawing permafrost peatlands.
How to cite: Voggenreiter, E., Valenzuela, E., van Grinsven, S., and Kappler, A.: Role of natural organic matter and iron(III) for methanogenesis and methane oxidation in thawing permafrost soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1459, https://doi.org/10.5194/egusphere-egu25-1459, 2025.
Climate change has intensified the mobility of dissolved organic matter (DOM) from land into aquatic ecosystems leading to increased brownification and hypoxia. Constructed wetlands (CWs) offer a potential mitigation strategy but optimal wetland design with respect to DOM removal remains underexplored. This study examined how depth and water residence time (WRT) affect DOM processing in experimental CWs during summer and fall. Organic matter was added to mimic brownification, and DOM changes were tracked using fluorescence spectroscopy and microbial activity measurements. A key finding was that labile DOM degrades rapidly within the first two days. At longer WRT shallow CWs released terrestrial-like fractions potentially increasing downstream brownification, while deep CWs showed sustained DOM degradation and slower internal production, potentially reducing downstream brownification. Based on spectral ratios it was found that microbial processes dominated DOM degradation, although photodegradation played a significant role during summer. Strong correlations between bacterial processes and DOM composition, highlight the critical role of labile carbon in driving microbial activity. Bacterial production correlated strongly with labile DOM fractions (Peaks T and M), while bacterial respiration, correlated with both labile and humic-like DOM fractions. Our results suggest that CWs can be optimized as tools for mitigating climate change impacts and improving water quality, ensuring long-term ecological sustainability. In addition, our findings advocate for integrating shallow and deep systems in series to maximize carbon removal, minimize brownification, and adapt to seasonal variability.
How to cite: Sjöstedt, J., Jones, K., Borgert, J., and Liess, A.: Carbon removal mechanisms and microbial dynamics in constructed wetlands of differing depths, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5834, https://doi.org/10.5194/egusphere-egu25-5834, 2025.
Mountain glaciers are vanishing worldwide because of climate change, triggering cascading downstream effects. Today, glaciers are recognized as stores of dissolved organic matter (DOM), which once released, can support the microbial metabolism and food webs in glacier-fed streams. This glacier-derived DOM is often reported to be ancient and highly bioavailable. However, our understanding of how such DOM may change in the future, as mountain glaciers continue to melt, remains limited.
We aimed to determine whether the quantity and quality of DOM in glacier-fed streams are shifting as glaciers retreat. Leveraging DOM data from the Vanishing Glaciers project and using a space-for-time substitution approach, we investigated how both DOM quantity and quality may change across a wide range of glacier-fed streams worldwide. We analyzed optical properties of DOM sampled as close to the glacier snout as possible in 181 glacier-fed streams draining the world’s major mountain ranges. Dissolved organic carbon (DOC) concentrations in these streams were very low (median: 146.3 ppb, interquartile range (IQR): 99.4-211.7 ppb). Parallel Factor Analysis (PARAFAC) identified six major DOM components, highlighting a dominance of proteinaceous compounds in the glacier-fed streams. Furthermore, by integrating additional optical measures, such as fluorescence (median: 1.5, IQR: 1.3-1.7), humification (median: 0.4, IQR: 0.2-0.5) and biological (median: 1.6, IQR: 1-2.3) indices, we will characterize DOM composition and potential sources. These data will be compared to glacier coverage, stream water stable isotopes, major ions, the mineralogical composition of suspended sediments and benthic chlorophyll a. Our unique large-scale dataset allows us to improve current understanding of DOM dynamics and related carbon cycling in glacier-fed aquatic ecosystems, which are now changing at an unprecedented pace because of climate change.
How to cite: Hou, J., Peter, H., Deluigi, N., LIanos-Paez, O., and Battin, T.: Climate-change impacts on dissolved organic matter in glacier-fed streams, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6561, https://doi.org/10.5194/egusphere-egu25-6561, 2025.
Total organic nitrogen (TON) constitutes almost 20% of the total nitrogen (TN) riverine loadings to Danish coastal waters. Thus, knowledge about the TON concentrations in streams and its spatial variation is essential to accurately assess the importance of TON for TN loadings to coastal waters and thereby achieving a more precise basis for calculation of the sources of TON in catchments.
We used environmental monitoring data from 390 stream stations across Denmark for the period 2018-2021to calculate indirectly measured annual and seasonal average TON concentrations (~1,500 samples) along with a wide range of predictor variables. TON samples showed a mean annual TON concentration in Danish streams amounting to 0.70 mg L-1 with a standard deviation of 0.31 mg L-1 and revealed a relatively high spatial variability.
We trained a machine learning model to learn spatial and temporal patterns in our TON data set for prediction of spatially distributed annual and seasonal average TON concentrations in Danish streams in ungauged basins. Furthermore, we utilized quantile regression to estimate the uncertainty on model predictions, and we utilized quantile regression in combination with the Shapley additive explanations (SHAP) approach to investigate how the importance and influence of predictor variables vary across TON’s entire distribution.
The annual TON concentration is modelled with a root-mean-squared error of 0.20 mg L-1. The new national annual average TON concentration model is largely driven by the mean elevation (negative), the percentage of agricultural land (positive), the percentage of tile drained areas (positive), and the percentage of lakes (positive).
The predicted annual average TON concentrations were generally higher than the measured average annual TON concentrations, with an overall mean of 0.84 mg L-1, probably because catchments in the training data generally had higher mean elevations (DEM) than the prediction catchments as many ungauged catchments are located near the coast
The developed model and national TON maps contribute to our understanding of annual TON concentrations in streams supporting national-scale land-use and water management.
How to cite: R. Frederiksen, R., E. Larsen, S., and Kronvang, B.: Modelling Total Organic Nitrogen Concentrations in Danish Streams using Machine Learning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9125, https://doi.org/10.5194/egusphere-egu25-9125, 2025.
It is generally known that one of the important objectives of EU countries is to improve the quality of water in water bodies. The quality of the water in the Gulf of Tallinn is poor. Nitrogen and phosphorus compounds in coastal water and stormwater discharges have been studied, but very little is known about the role of dissolved organic matter, chemical properties and relationship with pollutants. It is important to be aware of the role of carbon compounds as nutrients, the high content and inflows into the coastal sea can lead to the proliferation of algae and bacteria, the reduction of dissolved oxygen in water, etc. The impact of algae on the water quality in the Tallinn Bay is a significant problem and can also worsen the water quality of Pirita beach. The main concern of bad water quality has been considered eutrophication, which causes algal bloom near coastline of Tallinn Bay.
In recent years, stormwaters from Tallinn are believed to be the main cause of high nutrient levels. In present study the intention was to investigate different factors by measuring the concentrations of organic carbon, total, inorganic and organic phosphorus and nitrates in different locations of the coastal seawater. The concentrations of phosphorus and nitrate were determined by spectrophotometry, organic carbon by HPLC. Detailed characterization of dissolved organic matter was carried out in order to identify sources of organic matter that has entered the water. As a result, it should become clear whether, in addition to the study of nitrogen and phosphorus compounds in coastal water, it would be expedient and necessary to monitor and characterize natural organic matter.
The aims of present study were: to determine the organic carbon, phosphorus and nitrate in coastal seawater near the stormwater discharge outlets; to investigate the climatic factors (rainfall, temperature), and freshwater inflow (River Pirita); to compare the results with average nutrient levels in the Gulf of Finland; to assess the condition of Tallinn Bay according to legislation.
The study results indicated that nutrient levels in the coastal seawater of the Tallinn Bay area were remarkably higher than average nutrient levels in the Gulf of Finland. According to legislation, the status class of Tallinn Bay is mainly poor, based on total phosphorus data and bad or even worse, based on nitrate data. Stormwaters did not increase nitrate and total phosphorus contents substantially and they mainly affected total phosphorus concentrations near the discharge outlets. River Pirita was identified as the major source of nitrates, but not of phosphorus. Further studies are required to obtain a complete picture about nutrient flows to Tallinn Bay.
How to cite: Lepane, V.: The coastal seawater quality evaluation based on organic carbon, nitrogen and phosphorus data of Tallinn Bay, Estonia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10057, https://doi.org/10.5194/egusphere-egu25-10057, 2025.
Organic carbon drives key processes in estuaries and rivers like (micro)biological production, oxygen consumption, transport of pollutants, and the flocculation/agglomeration of suspended particulate matter. The OrgCarbon project aims for an in‑depth characterization of organic carbon in field samples by using both established and innovative methods. Oxygen consumption, microbial respiration, potential for sorption of pollutants, origin and composition of the organic matter will be determined. By testing a variety of cross-disciplinary methods, we aim to develop a standardized protocol for studying organic carbon in estuaries and rivers. The goal is to develop an easy-to-use and cost-effective protocol that can be implemented in existing monitoring programs. As a result, knowledge about the origin and degradability of organic carbon and thus oxygen consumption rates could, in future, be determined routinely and included in water quality management.
First results from the highly turbid Ems Estuary show strong gradients in dissolved organic carbon (DOC) and total organic carbon (TOC) along the salinity gradient. TOC, but also the ratio of DOC to particulate organic carbon (POC), increases along the gradient from marine to freshwater. Spectroscopic measurements and absorption indices (e.g., SUVA254, SR, S275-295) provided first insights into organic carbon origin and composition and are easy to use and inexpensive. Additional analysis of microbial respiration and enzyme activity will provide information on organic carbon degradability and its role for the oxygen budget of rivers and estuaries.
How to cite: Fiskal, A., Amann, S., Vogel, A., Rovelli, L., Borgsmüller, C., Dierkes, G., Wick, A., and Fischer, H.: Introduction to the project OrgCarbon: Organic carbon in rivers – characterization, origin, and degradability – first results from the Ems estuary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15664, https://doi.org/10.5194/egusphere-egu25-15664, 2025.
Mobilisation of dissolved organic carbon (DOC) links fluxes from terrestrial ecosystems via streams to the oceans. The increase in mobilisation that has been observed as a browning of headwaters during the last decades, resulted in ecosystem change of receiving waters and had implications for drinking water production and carbon storage. Riparian soils at the groundwater/ surface water interface are hotspots of biogeochemical transformations shaping water entering the streams. Preferential flow paths, where larger areas of the watershed drain through a distinct point to the stream, have been described as discrete riparian inflow points (DRIP). DRIPs have high watertables, mostly organic soils and strongly influence stream discharge and chemistry. They have been identified as major sources of DOC to streams, making them key areas for studying DOC mobilisation mechanisms. High watertables connect highly conductive and organic rich top soil layers to streams, but also influence redox conditions in the ground. If oxygen and nitrate availability decreases, ferric iron gets reduced and could release DOC previously bound to iron (oxy) hydroxides. Reduction processes consume protons and thus increase pH, in turn increasing solubility for negatively charged organics.
We hypothesized that redox induced mobilisation of DOC plays an important role especially after drying and rewetting cycles occuring after warm and dry summers with the onset of late summer rains. During snowmelt, we hypothesized redox induced mobilisation to be less important due to cold conditions and a large fraction of surficial flow paths. In this study, data from sampling campaigns in a small forested headwater stream with adjacent riparian wetlands (DRIPs) located in the Krycklan Catchment Study in boreal Sweden, conducted during snowmelt 2024 and two late summer seasons in 2023 and 2024, is presented. Samples were analysed for DOC quantity and quality, iron speciation and concentration, oxygen saturation and pH, among others. We show that stream- and groundwater have distinct chemical properties. The role of riparian soils as source areas of solutes differs between seasons with a more diluting effect during peak discharge at snowmelt and concentrations being transport limited in summer and autumn. In groundwater, DOC and iron are co-mobilised with higher concentrations under reducing conditions. Oxygen saturation changes with watertables depending on whether they exceed ground level, resulting in different effects of watertable changes depending on small scale topography. We find some indication of DOC mobilisation due to redox induced pH increase in some DRIPs especially during snowmelt. DOC concentrations are higher pre- and during early snowmelt in the stream, maybe due to release of older, more reduced groundwater before the diluting effect of freshly melted snow dominates.
In conclusion ground- and streamwater chemistry relate differently dependent on season. Small scale topography results in non-uniformal effects of elevated watertables and thus groundwater chemistry is to some degree site specific. However, iron and DOC are jointly mobilised especially under low oxygen availability. In spring water that might have been subject to reducing conditions in late autumn, might still be present in the groundwater and could be released early on during snowmelt.
How to cite: Hortmann, A., Knorr, K.-H., Selle, B., and Laudon, H.: Links of ground- and streamwater in discrete riparian inflow points in boreal Sweden – DOC mobilisation and the role of reducing conditions during snowmelt and summer, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16430, https://doi.org/10.5194/egusphere-egu25-16430, 2025.
In recent years, soil has emerged as a central focus in natural carbon sink research. Past studies have largely concentrated on how plants capture atmospheric carbon through photosynthesis and progressively store it in the soil as they grow. This process is known as the "vertical process" of soil organic carbon accumulation.
However, in subtropical monsoon climates, soils in hillside regions are often subject to water erosion, which causes soil organic carbon to accumulate not only vertically but also laterally through the transport of terrestrial materials. This lateral movement represents the "horizontal process" of soil organic carbon accumulation. At the watershed scale, understanding the horizontal transport and accumulation of soil organic carbon is essential for accurate carbon budget assessments.
The movement of soil organic carbon plays a vital role in soil carbon dynamics within terrestrial ecosystems. This study focuses on gaining a deeper understanding of soil erosion processes and soil carbon storage in watersheds. The primary aim is to develop a slope soil carbon sink assessment model to evaluate the spatial distribution of soil carbon sinks within watersheds. Additionally, the study seeks to validate the model and assess its feasibility for practical applications.
How to cite: Wu, S.-W., Huang, J.-H., Tu, F.-J., and Lin, C. Y.: Development of a Model to Estimate the Spatial Distribution of Soil Carbon Sinks in Watersheds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17013, https://doi.org/10.5194/egusphere-egu25-17013, 2025.
Comprehensive, high-resolution data on intermittent natural springs with low discharge are still rare, although they represent an important interface between terrestrial and aquatic environments, and form the basis of our water systems. Due to their connection to groundwater, springs have been considered quite stable in terms of both hydro-biogeochemistry and water quality. However, with climate change, spring systems are subject to significant hydrological dynamics, partly due to changes in water availability. Currently, spring discharges are decreasing or drying up during more frequent droughts. The amount of nutrients exported to headwater streams is closely linked to hydrological processes. For intermittent springs, a significant change in biogeochemistry with increased nutrient export can be expected due to the temporary cessation of groundwater inflow combined with longer residence times of organic matter in the surrounding soil substrate. However, little is known about the role of intermittent springs in carbon cycling and their role in downstream carbon and nutrient export.
In order to fill this research gap, this study aims to asses and quantify the seasonal variability of carbon and nutrient composition and fluxes of intermittent or highly variable discharge springs as a function of climatic, site and biogeochemical parameters. We investigate a range of spring areas (44 springs) spread across the German low mountain ranges of the Ore Mountains, Sauerland, Black Forest and Rhenish Slate Mountains.
We measure the export of organic carbon based on high resolution data in selected springs, and complement these measurements with nutrient (nitrogen and phosphorus) samples on a seasonal basis. In addition, we investigate the composition of dissolved organic matter (DOM) to identify contributing carbon sources.
First results show that the spring flow regime determines carbon and nutrient concentrations, modulated by the characteristics of the spring type. Our study emphasizes the sensitivity of springs to hydrological shifts, particularly in the balance between groundwater and surface water contributions. A shift favoring surface water inputs, can increase nutrient exports, likely due to enhanced surface runoff carrying nutrients from the surrounding landscape. Climatic changes, with extreme rainfall events are becoming more frequent and intense, may alter the balance between groundwater inputs and surface water runoff in springs may result in higher carbon and nutrient fluxes into receiving waterbodies.
How to cite: Feld-Golinski, A., Fasching, C., and Chifflard, P.: Seasonal changes of organic carbon and nutrient fluxes in intermittent spring catchments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18115, https://doi.org/10.5194/egusphere-egu25-18115, 2025.
Increasing concentrations of dissolved organic carbon (DOC) in tributaries threaten the water quality of drinking water reservoirs in Europe and North America. Understanding the key factors influencing DOC dynamics in streams is essential for effective water resource management. This study is part of a concerted effort to determine the major sources of DOC entering a reservoir and to identify the key biogeochemical processes within the terrestrial-aquatic continuum that affect DOC concentrations.
We conducted a four-year multi-scale observational study in a small, heterogeneous catchment (8.5 km²) in the western Ore Mountains, Germany. The research design combined low-resolution (biweekly) measurements of soil water variables (e.g., DOC, pH, Al, Fe) with high-resolution (15-minute) sensor-based monitoring of environmental variables (e.g., temperature, precipitation, soil water content) at representative locations within the catchment. End-member mixing analysis (EMMA) quantified the contributions of peat, forest floor, and mineral soil horizons as sources of DOC, based on previous findings by Charamba et al. (2024), who qualitatively identified these sources within the catchment. In addition, relationships between DOC concentrations and potential explanatory variables were analyzed using Spearman correlations and Random Forest modeling.
In total, 16.5 kg DOC/ha*a were exported from the catchment to the reservoir. EMMA showed that peat soils contributed to about 85 % of the DOC in a tributary adjacent to these soils, corresponding to the highest area-related DOC load of 53 kg/ha*a. Nevertheless, across the entire catchment, mineral soils were the dominant source of DOC, contributing the most to the total DOC load exported to the reservoir (78 %; 13 - 18 kg/ha*a), while forest floors made the smallest contribution. At the temporal level, the contribution of the forest floor to DOC runoff increased under high flow conditions, highlighting the dynamic nature of DOC translocation from different soil sources to stream. Preliminary results of the correlation analysis highlight the influence of soil water chemistry, particularly Al and pH in C-rich horizons, on stream water DOC concentrations. Environmental variables such as precipitation and soil moisture were only moderately correlated with DOC concentrations. Random Forest analysis provided limited insights into key predictors, highlighting the complexity of the catchment and the processes underlying DOC production and translocation. Our results suggest that even bi-weekly sampling intervals may be insufficient to capture the temporal variations in soil processes affecting stream DOC concentrations. The variable time lag between soil processes and their hydrological expression poses a significant analytical challenge. Future research should focus on integrating high-resolution sensor data of DOC concentrations and water fluxes from hydrological monitoring stations. To address the limitations of Random Forest, we will use structural equation modelling (SEM) to refine conceptual models and identify causal relationships. Significant Spearman correlations between DOC and environmental and soil water parameters guide variable selection. The refinement of our conceptual model by SEM will be the basis for process-based modeling to predict the future development of DOC concentrations and fluxes in heterogeneous catchments.
How to cite: Nestler, E., Houska, T., Krause, T., Vieira Carlini Charamba, L., Ehrhardt, A., Müller, I., Stephani, A., Kaiser, K., Knorr, K.-H., Lau, M., Jackisch, C., and Kalbitz, K.: Dissolved organic carbon in a drinking water catchment in the western Ore Mountains, Germany: How much, Where from, When and Why – first insights, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18316, https://doi.org/10.5194/egusphere-egu25-18316, 2025.
The storage of organic carbon on land and its transfer to the ocean via rivers plays a critical role in the global carbon cycle. As the second-largest river system on Earth, the Congo Basin is a key region for carbon storage and export, with extensive wetlands and tropical forests contributing to a significant aboveground organic carbon reservoir. Recent discoveries have identified the Cuvette Centrale, a low-gradient depression in the center of the Congo Basin, as the world’s largest tropical peat complex, storing approximately 29 petagrams of carbon belowground. Despite its importance, key processes governing the export of carbon from these peatlands to the Congo River network remain poorly understood. Previous studies have shown that despite its low sediment load, the Congo River has a high dissolved organic carbon (DOC) and particulate organic carbon (POC) export—around 2 Tg POC and 12.5 Tg DOC annually. Aged organic matter observed in offshore marine sediment cores suggests, peatlands may significantly contribute to carbon export, but direct evidence remains incomplete. Here, we present a data set comprising surface peat and soil, as well as water, suspended sediment, and river bank samples. These were collected from the surface and small water bodies (pools) in the peatlands, tributaries within and outside the Cuvette Centrale, and the Congo River mainstem. We measured stable carbon and hydrogen isotopes of plant waxes and bulk organic carbon and nitrogen concentrations and stable isotopes, as well as radiocarbon content on a subset of samples. Based on these data, we aim to investigate the significance and the pathways of carbon export from these peatlands and their respective contributions to riverine DOC and POC, alongside other sources such as standing vegetation and in-situ aquatic production. This study provides new insights into the role of the Cuvette Centrale peatlands in the Congo Basin’s carbon dynamics.
How to cite: Menges, J., Garcin, Y., Bouka, G. U. D., Abaye, C., Guardiola, M., Bouillon, S., Stroobandt, Y., Mollenhauer, G., Grotheer, H., Kasemann, S., and Schefuß, E.: Organic carbon pathways from the Cuvette Centrale peatlands to the Congo River network, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19961, https://doi.org/10.5194/egusphere-egu25-19961, 2025.
