Although the factors controlling N turnover in soils are relatively well established under laboratory conditions, transposing these relationships to the field and landscape scales remains a significant challenge. The absence of data-sets collected in-situ impedes the validation of N processes, such as mineralization and denitrification simulated via process-based models, thereby rendering their results at field and regional scales highly uncertain. However, current ecosystem management challenges require accurate predictions of N fate to enable sustainable management that minimizes environmental losses.
We invite contributions from the following fields:
• Methodological advances in measuring and modelling of soil N processes, spanning from the micro- to the landscape scale;
• Measurements of N fluxes including specific loss pathways under field or field-like conditions with a focus on identifying controlling factors;
• Comparative studies demonstrating/evaluating novel approaches to constrain N turnover such as incubation under He/O2 atmosphere, 15N-tracer technique, N2O isotopologue approaches or other innovative methods;
• Process-based modelling of soil N processes at various scales;
• Linking nitrogen transformation rates to the function and structure of the soil microbial community.
Orals: Wed, 6 May, 16:15–17:35 | Room 2.95
The nitrogen (N) cycle is a complex interplay of different processes including the mineralization of soil organic matter, uptake of N by plants, microbial N immobilization, and nitrification and denitrification. These key processes result in the formation of various N-containing species, some of which have detrimental environmental effects. Unfortunately, some processes yield the same molecules, for instance nitrous oxide, complicating clear source partitioning. Process-based biogeochemical models are increasingly being used to assess the fate of N species in the environment. However, to reflect the complexity of N cycling, these models often include numerous mathematical descriptions of processes that depend on pool sizes, reaction rate constants, soil temperature, soil moisture or oxygen concentration. It has remained a challenge to determine a sufficient set of these quantities in high temporal resolution or for a given specific measurement site, resulting in a scarcity of easily available validation variables compared to a surplus of modeled quantities.
The fundamental processes of isotopic fractionation and mixing result in the characteristic isotopic compositions of the various N species at natural abundance level. In this context, the natural abundance isotopic compositions can be used to determine relative contributions of different processes to – for instance – nitrous oxide production or as an integrating validation quantity, since the soil 15N enrichment reflects N loss pathways on a time scale of decades. Similarly, labelling studies using fertilizers enriched in 15N have been used to study the allocation of fertilizers to soil and plants or to determine N2 emission. This is a significant, yet poorly understood component of N budgets and is extremely challenging to measure.
Thus, the aforementioned characteristics of the 15N isotopic composition make it a powerful tool for improving our understanding of the N cycle and for testing process-based biogeochemical models. In this presentation, we provide an overview of different model types used in the context of the N cycle, focusing specifically on isotope mixing models and process-based isotope models. We demonstrate how natural abundance 15N and 15N tracing studies can be employed to interpret measurement data, identify model weaknesses, refine models and reduce uncertainty of modeled N2O emissions.
How to cite: Wolf, B., Chojnacki, L., Kraus, D., Smerald, A., Fuchs, K., Weber, C., Lewicka-Szczebak, D., and Kiese, R.: Overview of isotope modelling in the context of the Nitrogen cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11594, https://doi.org/10.5194/egusphere-egu26-11594, 2026.
Soil is a major source of reactive nitrogen (N) gases, such as nitric oxide (NO) and nitrous acid (HONO), that influence the atmospheric oxidative capacity by affecting near-surface hydroxyl radical (OH) and ozone (O3) concentrations. Microbial N cycling activities in soil, particularly nitrification and denitrification, are recognised as significant biological sources of these gases, with emissions being especially notable after fertilizer application. Soil water content is the main variable determining the rates of nitrification (aerobic) and denitrification (anaerobic); however, calculations relating these processes to N gas emissions are often based on empirical relations without explicitly accounting for soil physical processes. Here, we introduce a mechanistic model, mecHONO, that integrates soil N transformation processes with soil pore drying dynamics, subject to physical constraints of mass conservation and liquid-gas interfacial transport. The model specifically accounts for evaporative concentration changes that impact pore chemistry, and, consequently, the transformation of microbial N products and their partitioning into NO and HONO. Through its application to controlled experiments, the model elucidates interactions between microbial activity and soil evaporation dynamics. Our results provide insights into effective nitrogen fertilizer application and land management to optimize nutrient utilization while simultaneously minimizing soil-derived NOx and OH emissions.
How to cite: Kim, M., Maier, S., Melo Silva, L., and Weber, B.: mecHONO: a mechanistic model of reactive nitrogen gas emission from drying soils , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11665, https://doi.org/10.5194/egusphere-egu26-11665, 2026.
Nitrous oxide (N₂O), a potent greenhouse gas, contributes significantly to climate change, with agricultural soils being a major source. In sub-Saharan Africa (SSA), increasing fertilization to boost productivity is expected to elevate N₂O emissions, however data scarcity and regional variability challenge accurate predictions. Thus, quantifying these fluxes remains a major challenge for both science and policy. Here, we present a process-based modelling study of N2O emissions using the CN-model, recently introduced as a mechanistic tool for simulating carbon-nitrogen coupling in terrestrial ecosystems [1], extended here for soil nitrogen transformations and N2O emissions. We apply the CN-model to an experimental maize cropping site in Eldoret, Kenya, as part of the N₂O-SSA project, which investigates greenhouse gas emissions in sub-Saharan African agroecosystems. The site in Eldoret (Kenya), features two annual rainfed maize and potato cropping seasons, with varied nitrogen fertilization regimes (0, 50, 100, and 125 kg N ha-¹ yr-¹). Our analysis covers the 2024 growing period (April 2024-January 2025), during which high-frequency flux measurements of N₂O, CH₄, and CO₂ were collected. The CN-model simulates microbial nitrification and denitrification pathways, soil moisture interactions, and fertilization impacts, providing process-level insights into observed N₂O flux dynamics. Model outputs are evaluated against measured greenhouse gas fluxes to assess predictive performance and to explore the effects of nitrogen input levels, precipitation patterns, and cropping cycles. Simulations under both current and future climate scenarios are used to assess potential trajectories under alternative management practices. This modeling framework is critical for improving nitrogen budgeting by enabling more precise and efficient fertilizer use, reducing unnecessary nitrogen losses, and supporting climate-smart agricultural practices. Preliminary results show that the CN-model captures both background and event-driven emissions effectively, highlighting the sensitivity of N₂O emissions to rainfall timing and nitrogen inputs. This work illustrates the value of combining mechanistic modelling with targeted field observations in sub-Saharan African smallholder systems to better constrain N₂O budgets and inform mitigation strategies under a changing climate.
ACKNOWLEDGEMENT
This research was generously supported by the Swiss National Science Foundation (SNSF) under grant number 200021_207348.
REFERENCE
1. Stocker, B. D. & Prentice, I. C. CN-model: A dynamic model for the coupled carbon and nitrogen cycles in terrestrial ecosystems. bioRxiv, 2024.2004.2025.591063 (2024). https://doi.org/10.1101/2024.04.25.591063
How to cite: Tufail, M. A., Agredazywczuk, P., Ouma, T., Barthel, M., Otinga, A., Njoroge, R., Leitner, S. M., Zhu, Y., Oduor, C. O., Oluoch, K. C., Six, J., Stocker, B. D., and Harris, E.: Modelling Nitrous Oxide Emissions from Croplands in sub-Saharan Africa Using the CN-Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1169, https://doi.org/10.5194/egusphere-egu26-1169, 2026.
Under the East Asian monsoon, Karst forest hillslopes in SW China experience strong soil erosion and nutrient runoff, yet the influence of rainfall event on soil nitrogen (N) transformation pathways and nitrous oxide (N2O) emission from these fragile soil systems remains poorly constrained. We conducted in situ measurements of soil N2O fluxes along a forested hillslope at the Yaji karst experimental site (Guilin, SW China), covering two contrasting topographic positions with replicated plots sampled across five summer rainfall events. We quantified soil moisture and extractable substrates (mineral N and dissolved organic carbon), measured nitrate dual isotopes (δ15N and δ18O), the abundance of denitrification-related functional genes and N2O isotopocule site preference (SP). The FRactionation And Mixing Evaluation (FRAME) model was used to partition pathway contributions and to estimate the fraction of N2O reduction.
Our results demonstrated that the topographic position significantly shapes the spatial distribution of soil moisture and inorganic N substrate, leading to divergent N2O emission patterns. Soils at the footslope functioned as a biogeochemical hotspot characterized by the preferential accumulation of nitrate (NO3-) with elevated water-filled pore space (WFPS), which resulted in two-fold higher N2O fluxes on average compared to the upper hillslope. Superimposed on this spatial contrast, fluxes varied strongly among events. During intensive rainfall, near-saturated conditions led to a strong dampening of the N2O flux (r = -0.75). FRAME results indicate that rainfall shifts the balance between nitrification-associated and denitrification-associated N2O production, with the direction and magnitude varying by topographic position. FRAME further suggests that the fraction of N2O reduction to N2 tends to increase under rainfall-influenced conditions at the footslope but decrease at the upper slope.
These findings highlight that hillslope topography acts as a key landscape variable in explaining spatial heterogeneity of water and N substrate balance as well as N2O emission patterns. Our study underscores the importance of integrating topographic driven resource redistribution into greenhouse gas models for subtropical Karst landscapes.
Keywords: Karst hillslope; N2O dynamics; Denitrification; Isotopic signature
How to cite: Zou, N., Gan, Z., Wu, Z., Li, J., Zhu, T., and Yu, L.: Topographic differences constrain event-scale soil nitrogen and N2O dynamics on a subtropical karst hillslope: Insights from isotopic signatures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10873, https://doi.org/10.5194/egusphere-egu26-10873, 2026.
Soil carbon (C) and nitrogen (N) cycles are closely coupled, with the nature of organic C input to soils, from crop residues to recalcitrant biochar, strongly influencing microbial N mineralization-immobilization turnover (MIT) and associated N2O/N2 emissions. However, the extent to which different organic C inputs regulate MIT and thereby control soil N retention and greenhouse gas emissions in agricultural systems remains poorly understood. An incubation experiment using field soils from a long-term fertilization trial (NPK application since 1954, with or without biochar since 2022) on a loess-derived Cambisol soil from Northern Austria (Grabenegg) was carried out to evaluate the short-term effects of maize-derived crop residues and biochar amendment on MIT. 15N-labeled fertilizers (15NH4NO3, NH415NO3; 150 kg N/ha) were applied to quantify gross N mineralization and immobilization, gross nitrification and NO3− immobilization, fertilizer N retention, and N2O and N2 emissions. Microbial biomass N (MBN), mineral N pools (15NH4+, 15NO3−), and gaseous N fluxes (15N2O and 15N2) were measured using established 15N isotope tracing and mass spectrometric techniques, allowing to track crop residue and biochar amendment effects on the partitioning of N transformation pathways and N2O reduction to N2.
Preliminary results revealed amendment-specific effects. After one week of incubation, laboratory amendment with crop residues increased NH4+ availability by 26% (2.50 ± 0.47 mg N kg−1) in soil with long-term biochar, but slightly decreased it by 8% (3.14 ± 0.69 mg N kg−1) in soil without long-term biochar treatment, relative to unamended controls (2.00 ± 0.79 mg N kg−1; 3.42 ± 0.82 mg N kg−1). In contrast, lab amended biochar strongly decreased NH4+ availability (~99%) in both field soils (0.03 ± 0.01 mg N kg−1; 0.04 ± 0.04 mg N kg−1), indicating a consistent response across soils regardless of field biochar application. Gross N mineralization, derived using 15N isotopic techniques with 15NH4NO3, was strongly stimulated by crop residues during first week, increasing rates by 172% and 290% relative to controls in soils with and without long-term biochar treatment, respectively, whereas lab amended biochar caused moderate increases of 59% and 39%. Compared to biochar, crop residues enhanced gross N mineralization 1.7-fold and 2.8-fold in soils with and without long-term biochar treatment, highlighting the stronger stimulation of N mineralization by labile C inputs. These findings show highly amendment-specific responses of MIT, differentially affecting soil N retention, and the mitigation of agricultural greenhouse gas emissions.
Keywords: nitrogen cycling, mineralization-immobilization turnover, organic amendments, biochar, crop residue, N2O emissions, N2 emissions, 15N tracer, climate-smart agriculture
How to cite: Hafiza, B. S., Bibi, S., Wanek, W., Vezzone, M., Resch, C., Heiling, M., Pucher, R., Vlasimsky, M., and Dercon, G.: Linking organic carbon inputs to microbial nitrogen mineralization-immobilization turnover and nitrous oxide dynamics using 15N tracer techniques, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17366, https://doi.org/10.5194/egusphere-egu26-17366, 2026.
15N tracing is an important tool to study N transformations in soils, cultures and aquatic systems. The most commonly used methods for quantification of 15N in NH4+, NH2OH, NO2- and NO3- rely on conversion to N2O and subsequent analysis by GC-IRMS. The disadvantage of these methods is that all four compounds are converted to the same product and thus cannot be measured in one run. Here, we convert each N species into a unique derivatization product that can be distinguished by high resolution mass spectrometry. For application in soils, the workflow includes the following steps: (1) extraction, (2) derivatization, (3) solid phase extraction (SPE) and (4) UPLC coupled to ESI-MS, in our case a Q-Exactive Orbitrap. NH4+, NO2- and NO3- can be extracted together with 1 M KCl. NH2OH, however, is not stable under these conditions and requires a different, newly developed extraction procedure. Steps (2) and (3) are carried out separately for each N compound. For the derivatization we employ reagents commonly used for the spectrophotometric detection of the respective N species: ortho-phthalaldehyde for NH4+, quinoline-8-ol for NH2OH, and n-naphthylethylenediamine with sulfanilamide or sulfanilic acid for NO2- and NO3-, respectively. Before reversed phase separation by UPLC, the derivatization products need to be cleaned up by SPE (step 3) to remove salts originating from extraction and derivatization that would harm the mass spectrometer. After clean-up, the SPE eluates can either be measured individually to achieve maximum sensitivity or be combined to measure all four compounds in one run on the UPLC-MS system. So far, we have optimized steps (1) to (3) and are currently working on optimizing the UPLC-MS method for concurrent separation and (isotope) quantification of all four N forms.
How to cite: Heldwein, N., Kitzinger, K., and Wanek, W.: Combined measurement of 15N-NH4+, NH2OH, NO2- and NO3- via derivatization and UPLC-MS, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4977, https://doi.org/10.5194/egusphere-egu26-4977, 2026.
Posters on site: Wed, 6 May, 14:00–15:45 | Hall X1
Nitrous oxide (N2O) is one of the most relevant anthropogenic greenhouse gases, mainly produced in agricultural soils. Understanding the mechanisms responsible for N2O production and consumption is crucial for developing N2O mitigation strategies in croplands but field studies combining N2O flux measurements with isotopocule signatures and metagenomic analysis are very rare.
With the aim to clarify how isotopic signatures and functional genes can help explaining N2O fluxes in a cropland, we conducted a two years study, from February 2023 to January 2025, at the Reinshof experimental farm of the University of Göttingen in Germany (51.49° N, 9.93° E). The crop sequence was sugar beet, winter wheat and winter barley.
We measured N2O fluxes with eight manually-operated static chambers and gas chromatography and additionally collected samples for analysis of N2O isotopocule signatures. We applied FRAME, a three-dimensional model based on 15N site preference, bulk 15N and 18O isotopic signatures to distinguish between the source processes (bacterial denitrification, nitrifier denitrification, fungal denitrification, nitrification) and to estimate the reduction of N2O to N2.
We regularly monitored soil water content, mineral nitrogen (Nmin) and dissolved organic carbon (DOC). Additionally, every four months we collected soil samples for real-time quantitative polymerase chain reaction (qPCR)-based quantification of bacterial and fungal DNA, as well as functional genes involved in nitrification (ammonia-oxidizing archaea and bacteria amoA genes) and denitrification (nirK, nirS, and nosZ clade I and II).
We observed mean N2O fluxes of 19.8 µg N2O-N m-2 h-1. Individual chamber measurements ranged from -26.7 to 573.1 µg N2O-N m-2 h-1. The spatial variability between chambers within one day showed a high coefficient of variation of 123%. N2O fluxes increased after fertilization, rewetting and harvest while highest fluxes occurred after a freeze-thawing event. Cumulative fluxes showed that 0.97% of applied fertilizer N was emitted as N2O-N. We observed a significant positive effect of soil moisture on N2O fluxes, no significant effect of Nmin and a significant negative effect of DOC.
Preliminary results showed that bacterial denitrification was the dominant process responsible for N2O production. As N2O fluxes increased, the proportion of bacterial denitrification increased while the proportion of nitrification decreased. Following freeze-thawing, there was more bacterial denitrification than after the fertilization events and very little fungal denitrification. Overall, the residual N2O fraction of 45.9 ± 15.0% suggested extensive nitrogen loss as N2 via denitrification. The dominance of nosZ over nirK and nirS further implied substantial conversion of nitrogen to N₂.
In addition, bacterial DNA was more abundant than fungal DNA, and denitrifiers were more abundant than nitrifiers. No clear differences in processes or gene copy numbers were observed between chambers. Seasonally, gene copy numbers of most functional genes were higher during the growing season and lower in winter, consistent with higher N₂O fluxes during the growing season and lower fluxes in winter.
In future analysis, we will show how soil characteristics, isotopic signatures, and functional genes jointly shape the spatial and temporal variation of the measured N2O fluxes.
How to cite: Englert, P., Buchen-Tschiskale, C., Beule, L., Apostolakis, A., Siebert, S., Well, R., and Meijide, A.: Mechanistic insights into in situ N₂O fluxes in a German crop rotation integrating chamber measurements, isotopocule signatures, and functional nitrogen cycling genes , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20191, https://doi.org/10.5194/egusphere-egu26-20191, 2026.
Congo Basin forests are exposed to high nitrogen inputs by atmospheric deposition which so far could not be matched by measured nitrogen losses and plant uptake (Bauters et al., 2019, Makelele et al., 2022, Barthel et al., 2022). High nitrogen deposition in the Congo Basin mainly originates from biomass burning as practiced during shifting cultivation (Bauters et al., 2018). Overall, this N budget imbalance suggests that other gaseous N-losses, such as N2 fluxes, may play a major role in the N-cycle of Afrotropical forests (Barthel et al., 2022) although not yet quantified as such in the Congo Basin. Thus, the main research question we focus upon here: Can the high fire-derived nitrogen deposition be balanced by N2 emissions in the Congo Basin? To answer this key question, we first used permanently installed throughfall and lysimeter networks, sampled weekly, in Gilbertiodendron dewevrei forests to analyse nitrogen deposition and leaching (ammonium, nitrate, nitrite, total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON)) over a full year. Secondly, we measured weekly soil greenhouse gas fluxes (CO2, CH4 and N2O) in the same forest over a full year based on manual static chamber measurements. Thirdly, we used the He/O2 gas-flow-soil-core method to measure for the first time N2 and N2O fluxes and calculated the denitrification product ratio (N2O/(N2O+N2)). Our results confirmed, in line with previous studies, high atmospheric N deposition in the Gilbertiodendron dewevrei forest with substantially low in-situ soil nitrous oxide fluxes. Furthermore, Gilbertiodendron dewevrei forest, with ectomycorrhizal symbiosis, showed highest reduction rates of N2O to N2 (complete denitrification). Therefore, the N budget imbalance in the Congo basin, and especially the Gilbertiodendron dewevrei forests of the Congo Basin, can be explained by high N2 emissions from denitrification processes.
How to cite: Alebadwa, S., Bauters, M., Landuyt, D., Barthel, M., Makelele, I., Ewango, C., Boeckx, P., and Kiese, R.: Nitrogen cycling (deposition, leaching and N trace gas) in lowland tropical forests of the Congo Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3931, https://doi.org/10.5194/egusphere-egu26-3931, 2026.
Nitrous oxide (N2O) emissions contribute notably to the greenhouse effect and are driven mainly by agricultural practices, while nitrogen (N) losses as N2O and dinitrogen (N2) also impair plant N nutrition. The 15N gas-flux method (15NGF) can be used for the direct quantification of N2 and N2O from denitrification, while the natural abundance isotopic composition of N2O provides valuable clues about its microbial sources and its reduction to N2. However, both methods suffer from limited sensitivity, causing field data sets to have gaps when fluxes are below the detection limit. Soil air sampling can, in principle, overcome these limitations. Accounting for diffusive isotopic effects, admixture with atmospheric N2O, and changes in produced N2 and N2O during transport in the soil remains challenging. To evaluate detection limits and to correct raw data, calibration with standard gases that cover the isotopic range of the experimental samples is required. Until recently, suitable gases were not commercially available. Our aim is to develop and test solutions that overcome these limitations.
To obtain continuous field and lab data, we combined results from conventional N2O flux studies with isotopic data. 15NGF was applied in the field under normal atmosphere as well as under artificially N2-depleted atmosphere (15NGF+) to improve detection limits. Additionally, under normal atmosphere, chamber accumulation was extended to 20 hours and soil air was analyzed. Precision and bias were evaluated using custom-made gas standards. In parallel treatments, isotopic N2O fluxes at natural abundance were determined and evaluated using the N2O isotopocule mapping approach to evaluate N2O pathways. To enhance our understanding of N2O processes, a combined approach of 15NGF and N2O isotopocules is also promising. All approaches were compared to evaluate how the data can be combined to obtain continuous field flux data.
How to cite: Buchen-Tschiskale, C., Potthoff, T., Mielenz, H., Rösel, T., Stenfert Kroese, J., and Well, R.: Improving and combining isotopic approaches to optimize sensitivity and accuracy of N2 and N2O fluxes in the field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7571, https://doi.org/10.5194/egusphere-egu26-7571, 2026.
Biogeochemical models are useful tools for modeling nitrous oxide (N2O) emissions from agricultural mineral soils. However, most biogeochemical models assume that conditions favoring N2O producing reactions, nitrification and denitrification, are spatially homogeneous distributed in soil. Recent studies have shown, that conditions favoring N2O producing, like the availability of easy degradable organic carbon, nitrate and ammonium and the establishment of anaerobic conditions are often concentrated in hot spots around particular organic matter originating from crop residues and organic amendments. In contrast to common biogeochemical models, spatially explicit models are required to better describe dynamics of N2O emissions.
To address the role of spatial heterogeneity of conditions responsible for nitrification and denitrification, we introduce a model approach combining biogeochemical process descriptions at hot spots and the bulk soil. The „hot spot” part of the model includes the description of diffusive transport of solutes and gases in a spherical object combined with process descriptions like mineralization, nitrification, nitrifier denitrification and denitrification. Several instances of the hot spot module then interact with the model approach describing processes in the bulk soil.
In this study, a laboratory experiment will be modeled in order to simulate the application of a single type of manure on the surface and the injection of manure into the center of a column in sandy and loess soils with water contents set at 40% and 60% WFPS.
The submodel will be integrated into the DNDCv.Can model, which provides the boundary conditions and input data required for the submodel and also participates in determining the O2 concentration by explicitly calculating the diffusion from the surface to the soil layer where the hot spot is located.
Our hypothesis is that the DNDCv.Can model with the hot spot submodule will be able to describe the daily dynamics of the nitrification and denitrification processes generated by the hot spot induced in the experiment more accurately, which would greatly help the further development of the model for the application of this approach in real agricultural practice.
How to cite: Grosz, B. and Dechow, R.: Integration and testing of the Hot-Spot submodel with the DNDCv.Can model on the results of a manure-soil laboratory experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10754, https://doi.org/10.5194/egusphere-egu26-10754, 2026.
Organic fertilisers including farmyard and liquid manure supply organic matter and nutrients to grassland soils, with potential benefits for organic carbon (C) storage, soil fertility and ultimately, productivity. However, these benefits may be partially offset by changes in nitrogen (N) turnover and associated emissions of nitrous oxide (N₂O), a potent greenhouse gas, contributing to climate change. Here, we investigated legacy effects of long-term fertilisation on N transformations and N₂O production pathways from an alpine grassland soil from Styria, Austria, subjected to organic (ORG), mineral (NPK), or no fertilization (NIL) since 1971. Combining the 15N pool dilution and the 15N gas flux method enabled to quantify gross rates of mineralisation, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA), together with N2O production pathways and the reduction to environmentally benign dinitrogen (N2) in a soil microcosm experiment. Long-term organic fertilisation increased soil organic C and CO2 emissions compared to NPK and NIL, consistent with increased rates of mineralisation, nitrification, and increased N retention via DNRA. Under the conditions of the experiment, long-term fertilisation showed no effect on magnitude of N2O and N2 emissions. Denitrification was the main pathway of N2O production across treatments, with its contribution increasing from 65% under NIL and NPK, to >85% under ORG. The main product of denitrification was N2, accounting for 95% of N2O+N2 under NIL and NPK. Organic fertilisation however shifted the N2O:N2 ratio towards N2O, accounting for more than 15% of N2O+N2 emitted. These results show a clear legacy effect of long-term organic fertilisation on N2O production, which may be explained by higher C availability, fuelling microbial activity and O2 consumption, shifting N2O production towards denitrification. Even though not reflected in overall amounts of N2O and N2 emitted, the shift in the N2O:N2 ratio towards N2O under organic fertilisation denotes an increased risk for N2O emissions, likely amplified by increased N supply via nitrification. Our findings demonstrate a clear increase of N substrate supply via mineralisation and nitrification turnover under long-term organic fertilisation and highlight the need to consider potential environmental offsets for alpine grassland management in the form of N2O emissions, driven by denitrification.
How to cite: Friedl, J., Claramonte Manrique, C., Meeran, K., Bohner, A., Kirkby, R., Wieser, S., Gerzabek, M., and Keiblinger, K.: Long-term organic fertilisation shifts N2O production towards denitrification in an alpine grassland soil, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11221, https://doi.org/10.5194/egusphere-egu26-11221, 2026.
Denitrification, the degradation of nitrate (NO3-) into nitrous oxide (N2O) or dinitrogen (N2), is an important process of the soil nitrogen cycle. Without denitrification, NO3- is leached through the soil and reaches the groundwater. In the groundwater NO3- lowers the water quality and leads to eutrophication of water bodies. Therefore, denitrification in soil and groundwater is a well-observed topic. But what happens with the denitrification in the unsaturated zone between soil and groundwater? The drainage zone, also known as deep vadose zone, has been widely overlooked until now. But this zone may play an important role in reducing NO3- leaching into the groundwater. Indeed, the frequent lack of available organic carbon as an electron donor and mostly oxic conditions in large parts of the drainage zone typically prevent intense denitrification. But due to the possibly large thickness of the drainage zone and the long travel time of NO3-, it is possible that even low denitrification rates could lead to relevant NO3- attenuation.
In the project DeniDrain, we focus on the denitrification process in the drainage zone. Using the direct push drilling method, we collect undisturbed samples at depths between 2 and 10 meters at representative locations throughout Germany. In the laboratory, we measure the N2O and N2 emissions with the 15N gas flow method to obtain the current denitrification rates. Our initial findings suggest that denitrification happens in certain sections of our investigated profiles, where the amount of degraded NO3- depended on the properties of the drainage zone. Therefore, the denitrification in the drainage zone plays an important role in the nitrogen cycle and should be incorporated in future research about the fate of NO3- leached from soil. To minimize the effort of measuring the denitrification in the drainage zone, the application of models is useful. Therefore, using our data we test the existing soil models DENUZ and BODIUM in their ability to predict denitrification in the drainage zone.
We will show the initial results of our measuring and modelling work.
How to cite: Schoner, D., Begill, N., Well, R., Buchen-Tschiskale, C., and Stange, F.: The denitrification in the drainage zone as an important process of the nitrogen cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12996, https://doi.org/10.5194/egusphere-egu26-12996, 2026.
Abstract: Human activities have dramatically increased the deposition of reactive nitrogen (N) globally, leading to enhanced N losses from terrestrial ecosystems through hydrological pathway. This loss of reactive N from soils is controlled by complex transformation and transport processes, which are influenced by a suite of factors, including soil water conditions, vegetation types, soil microorganisms, and soil physicochemical properties. A crucial, yet often overlooked, factor is the availability of phosphorus (P). P is a limiting nutrient for soil microbial activity and vegetation productivity in many regions worldwide, especially in tropical ecosystems. Here we established a P addition experiment (+0, +25, +50, +100 kg P ha−1 yr−1) in an evergreen broadleaf mixed plantation. We found that cumulative dissolved total N (DTN) exhibited a concave-shaped nonlinear response to P addition. During the wet season (July 20 to September 18, 2023), a sharp cumulative increase in the mean values of DTN runoff was observed under P additions. In contrast, the cumulative DTN flux from runoff showed minimal increase during the dry season. Furthermore, the enhanced DTN runoff under P additions were linked to the elevated inorganic N assimilatory reduction genes and SWC, and seasonal precipitation. These findings offer insights into the hydrological loss of N under different P supply conditions in tropical forests, with direct implications for projecting and managing nutrients in tropical forests in the context of global change.
Acknowledgements: This work was supported by the National Natural Science Foundation of China (32301443), the Guangdong Basic and Applied Basic Research Foundation (2022A1515110926, 2023A1515010957 and 2022B1515020014, and NSFC Sino-German Mobility Program (No. M-0749).
How to cite: Ye, H., Luo, X., Chang, Z., Li, Z., Guo, C., Hou, E., and Bol, R.: Phosphorus additions promote soil nitrogen runoff in a tropical forest, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16932, https://doi.org/10.5194/egusphere-egu26-16932, 2026.
Denitrification is the main pathway of nitrogen loss from soils, releasing nitrous oxide (N2O) and dinitrogen (N2). These emissions reduce nitrogen availability for crops and N2O also contributes to climate change. Nitrogen-based fertilizers drive soil nitrogen transformations – including denitrification. The magnitude of resulting emissions varies with management practices, climate and soil properties. Transitioning toward climate-smart agriculture requires understanding the interconnected nature of N-losses and possible trade-offs associated with mitigation strategies. For instance, suppressing N2O emissions through nitrification inhibitors might increase ammonia volatilization (Zhang et al. 2022). Quantifying soil-emitted N2 at the field-scale remains a significant challenge, and consequently, this component is often absent from evaluations of management practices. As a result, agricultural practices that improve nitrogen use efficiency while reducing denitrification losses have yet to be clearly identified.
The overall objective of the joint project MinDen is to assess mitigation measures aimed at reducing both direct and indirect denitrification emissions while enhancing nitrogen use efficiency. To this end, a three-year field experiment was conducted across three sites in Germany, representing a gradient from heavy clayey soils with higher emissions to lighter, sandy soil with lower emissions.
Four liquid-organic fertilizer application techniques - drag hose with incorporation, slit injection, slit injection with nitrification inhibitor, and drag hose with acidified slurry - were tested alongside two mineral-fertilizer treatments (a standard rate according to crop demand and a 20% reduced rate). At one site, an additional comparison was made between organically- and conventionally-managed fields. Ammonia (NH3), N2O and N2 fluxes were determined using passive samplers, static chambers and the 15N gas flux method. Here, we present the temporal dynamics of NH3, N2O and N2 emissions from the first two years at all three sites. Ultimately, these data sets are being used to validate biogeochemical models to regionalize N-losses from agricultural soils across Germany.
Zhang, C., Song, X., Zhang, Y., Wang, D., Rees, R. M., & Ju, X. (2022). Using nitrification inhibitors and deep placement to tackle the trade-offs between NH₃ and N₂O emissions in global croplands. Global Change Biology, 28(14), 4409–4422. https://doi.org/10.1111/gcb.16198
How to cite: Stenfert Kroese, J., Buchen-Tschiskale, C., Cordes, J., Dechow, R., Dittert, K., Dix, B., Fuchs, K., Gattinger, A., Grosz, B., Hauschild, M., Jarrah, M., Kühne, J., Mielenz, H., Potthoff, T., Scheer, C., Schulz, F., Simpson, C., Wolf, B., and Well, R.: Mitigating denitrification N-losses with optimized liquid organic fertilizer application strategies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22998, https://doi.org/10.5194/egusphere-egu26-22998, 2026.
Accounting for denitrification losses from agroecosystems remains challenging, particularly due to methodological challenges regarding measurements and upscaling of highly episodic nitrous oxide (N₂O) and dinitrogen (N₂) emissions to seasonal and system levels. Denitrification losses, and thus nitrogen (N) budgets remain therefore poorly constrained across a wide range of agro-ecosystems, hindering targeted development of N loss mitigation strategies. Here we present an innovative data driven upscaling approach using high-frequency N₂O datasets, and the XGBoost model, establishing seasonal N2O and N2O emissions. Emissions of N2O and N2 were measured in-situ using the 15NGF method. The XGBoost model was trained using in-situ N2O and N2 data across eight site-seasons in different grains systems in southeastern Australia, expressing the ratio of N2 to N2O emitted as a function of water-filled pore space, nitrate content and soil temperature. The trained model was then applied to additional 38 site-season-treatment high-frequency N2O datasets to estimate daily N₂ emissions, followed by aggregation to seasonal scales. Emissions N2O over the cropping season accounted for only 3.2% (on median, 2.1–4.1% at quartiles) of denitrification. Seasonal denitrification losses ranged from 2.6 to 66.1 kg N ha⁻¹ and were dominated by N2, exceeding N2O emissions by a factor of ~30 (on median, 23–46 at quartiles). Our approach delivers for the first time denitrification budgets for a range of different grains systems, providing a blueprint to investigate the effects of environmental drivers and management on denitrification. Extending this approach to other soil types and production systems offers the opportunity to derive more generic relationships between drivers and the N2O:N2 ratio as way forward to improve N budgeting for agronomic and environmental benefits.
How to cite: Takeda, N., Bailey, T., Kirkby, R., O'Hearn, L., Friedl, J., Rowlings, D., Schwenke, G., Armstrong, R., Bell, M., and Grace, P.: Establishing denitrification (N2O+N2) budgets – a data driven scaling approach in grains systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12162, https://doi.org/10.5194/egusphere-egu26-12162, 2026.
Process-based models provide an avenue to assess greenhouse gas emissions on scales where measurements alone are impractical. However, outcomes obtained from models are also subject to sources of error, which include model parameter uncertainty, model input uncertainty and model bias. For process-based models, the high-dimensional parameter spaces lead to large uncertainty contributions arising from model parameterization. Here, we show how time series measurements of 15N intramolecular N2O isotopic composition, i.e., site preference (SP), can be used to constrain parameter uncertainty in the process-based biogeochemical model LandscapeDNDC in connection with the Stable Isotope MOdel for Nutrient cyclEs (SIMONE). We develop a multivariable calibration framework that incorporates isotope tracing simulations from SIMONE into the calibration of LandscapeDNDC parameters, based on measurements of both N2O and SP simultaneously. We perform site-scale calibrations using the SP and N2O flux measurements from Swiss grassland at Chamau, and use a Sampling Importance Resampling scheme to estimate model parameter uncertainties, both with and without using SP as a calibration variable. Our results show that including SP into a calibration-uncertainty estimation framework for N2O emissions significantly reduces model parameter uncertainty.
How to cite: Chojnacki, L., Weber, C., Wolf, B., Kraus, D., Haas, E., Smerald, A., Mohn, J., Scheer, C., and Kiese, R.: Constraining uncertainty of modelled N2O emissions using isotopic composition, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16721, https://doi.org/10.5194/egusphere-egu26-16721, 2026.
Understanding soil nitrogen (N) cycling is essential for predicting nutrient availability and losses in agricultural systems. Although microbial processes such as mineralization and immobilization control the internal N supply to plants, these sub-processes remain poorly constrained in biogeochemical models. This limits our ability to accurately simulate N flows and environmental N losses in agricultural systems.
In this study, we combined data from a 15N tracer experiment with the biogeochemical model LandscapeDNDC and the isotope model SIMONE to test and refine N cycling processes. Initial model–data comparisons revealed a consistent bias: LandscapeDNDC overestimated the uptake of fertilizer N by plants while underestimating the recovery of 15N in soils and N losses. These discrepancies indicated insufficient mineralization of soil organic nitrogen (SON) and an imbalance in the mineralization–immobilization sub-cycle that regulates the internal nitrogen supply.
To address these issues, we reparametrized key soil process rates in LandscapeDNDC using constraints from the 15N data. Specifically, we increased mineralization rates and adjusted immobilization parameters to improve the partitioning between fertilizer-derived N and mineralized SON in plant uptake. The recalibrated model improved the simulations of observed seasonal dynamics of 15N in plant and soil pools, and N loss estimates.
Our results demonstrate that integrating ¹⁵N tracer data with isotope modeling provides a powerful approach for constraining microbial N processes in biogeochemical models. Improving the representation of mineralization–immobilization dynamics resulted in more realistic estimates of the internal N supply, thereby enhancing confidence in modelled fertilizer use efficiency and environmental losses, and improving the prediction of nitrogen dynamics in agricultural ecosystems under future climate and land use change scenarios.
How to cite: Fuchs, K., Scheer, C., Kraus, D., Smerald, A., Kiese, R., and Wolf, B.: Using 15N tracer experiments and the stable isotope model SIMONE to test and refine nitrogen cycling processes in the biogeochemical model LandscapeDNDC, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17816, https://doi.org/10.5194/egusphere-egu26-17816, 2026.
