Fluxes and chemical composition of precipitation is substantially changed after passing through tree canopies, particularly in the case of atmospheric nitrogen compounds, with important implications on forest nitrogen cycling. The causes of these changes, however, have mostly focused on the passive role of foliar surfaces to scavenge pollutants from the atmosphere and to ion exchange processes, while biological processes involving microbes hidden in the phyllosphere have been less investigated. We combined triple oxygen isotopes approach and molecular analyses with the aim of quantifying canopy nitrification and identify microbes responsible for it, respectively. Ten sites included in the European ICP Forests monitoring network, chosen along climate and nitrogen deposition gradients, were selected to include the two most dominant tree species in Europe (Fagus sylvatica L. and Pinus sylvestris L.). Specifically, in this study we: 1) estimated the relative contribution of nitrate derived from biological canopy nitrification vs. atmospheric deposition by using δ¹⁸O and Δ¹⁷O of nitrate collected in water samples, i.e., in the open field (bulk deposition) and underneath tree canopies (throughfall); 2) quantified the functional genes related to nitrification for the two dominant tree species in European forests by using next-generation sequence analyses. Based on the isotope approach, we found that up to 80% of the nitrate reaching the soil via throughfall derived from biological transformations in the phyllosphere, equivalent to a flux of gross canopy nitrification of up to 5.76 kg N ha^-1 y^-1. The fraction of microbiologically derived nitrate increased with raising nitrogen deposition, thus suggesting that the process can be substrate limited. Molecular analyses confirmed the presence on foliar surfaces of bacterial and archaeal autotrophic ammonia oxidisers and bacterial autotrophic nitrite oxidisers across the investigate European forests. Our study demonstrates the potential of integrating stable isotopes with molecular analyses to advance our understanding on key processes underpinning forest nitrogen cycling, which should no longer exclude microbial processes occurring in the phyllosphere.

How to cite: Guerrieri, R., Joan, J., Mattana, S., Casamayor, E., Peñuelas, J., and Mencuccini, M. and the Collaborators at the ICP Forests sites: Quantifying tree canopy nitrification across European forests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17028, https://doi.org/10.5194/egusphere-egu24-17028, 2024.

It has been recognized recently that trees can assimilate NO₂ directly through leaf stomata. Both laboratory and field studies have measured the foliar NO₂ deposition velocity, which could be determined by some environmental factors, e.g. light irradiation intensity, ambient NO₂ concentration, and leaf characteristics. However, the NO₂ uptake capacity and allocation of foliar uptake NO₂ under these environmental factors remain unclear. To clearly understand the foliar NO₂ uptake process and refine the forest NO₂ uptake models, we conducted a dynamic ¹⁵NO₂ fumigation experiment.

We selected Fraxinus mandshurica (F. mandshurica), Pinus koraiensis (P. konraiensis), Quercus mongolica (Q. mongolica), and Larix gmenilii (L. gmenilii) saplings, four dominant tree species in temperate forests of northeastern China, as our experimental materials. Meanwhile, we chose a pair of broad-leaved and coniferous tree species (F. mandshurica and P. konraiensis) to perform fumigation experiment under dark/light irradiation and another pair (Q. mongolica and L. gmenilii) to perform fumigation experiment with soil N addition. All saplings were dynamically fumigated with 50 ppb ¹⁵NO₂ for 8 h and destructively sampled immediately after fumigation. We rinsed the samples surface with purified water, dried and grinded all samples, then measured the ¹⁵N abundance in leaves, twigs, stems and roots with EA-IRMS.

The results showed that tree saplings can absorb NO₂ under both dark and light irradiation treatments. The total ¹⁵N recovery ranged between 30 to 80% under the light condition in all species. Under the dark condition, the total ¹⁵N recovery were (29.8±9.16) % and (1.1±0.47) % for F. mandshurica and P. konraiensis, which were significantly lower than under the light condition, (59.6±5.2) % and (8.8±2.5) %, respectively. With the soil N addition, the total ¹⁵N recovery in Q. mongolica ((56.2±8.8) %) were significantly larger than non-N addition ((27.6± 4.8) %), while L. gmenilii showed the opposite result that the total ¹⁵N recovery ((31.7±7.8) %) significantly decreased, compared to that without N addition ((73.6±4.3) %). These results are likely attributed to different amount of N demand for different tree species, more N needed for Q. mongolica than L. gmenilii. Moreover, coniferous species could assimilate more N through foliar uptake than broad-leaved species, probably due to bigger leaf surface areas of coniferous trees. After 8 h fumigation, the largest proportion of ¹⁵NO₂ was recovered in leaves in all species and treatments, accounting for 60-97%, which indicates that NO₂ stays in leaves in a short-term period after foliar assimilation. However, further studies are needed to explore the transformation of foliar incorporated NO₂ to other organs in a long-term scale.

This study quantified the foliar NO₂ uptake capacity of different tree species and figured out the effects of light irradiation and soil nitrogen availability on foliar NO₂ uptake. Our results would provide references for the model estimation of canopy NO₂ uptake magnitude at a regional scale.

How to cite: Yao, M. and Kang, R.: 2152, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5816, https://doi.org/10.5194/egusphere-egu24-5816, 2024.

Boreal forests play an important role in the global carbon (C) cycle, and their productivity is strongly limited by nitrogen availability.  Thus, understanding whether nitrogen availability in boreal forests is changing has important implications for understanding past, present, and future trends of forest growth. We utilized a unique archive of tree cores collected by the Swedish National Forest Inventory, to evaluate temporal patterns (1950-2017) of wood δ¹⁵N, which is commonly used as an indicator of N limitation. First, we focused on an area of ca. 55,000 sq. km in central Sweden to evaluate how sensitive the wood δ¹⁵N approach is to tree age and two alternative sampling methodologies: a) analysis of single trees sampled in the present, versus b) tree chronologies constructed from multiple trees of the same age sampled during different decades.  By analysing 1038 woods samples, and covering two key boreal tree species (Picea abies and Pinus sylvestris), we found strong trends of declining δ¹⁵N through time, suggestive of progressive N limitation.  We further found that temporal patterns were highly sensitive to method choice, where the multiple tree approach supported by the tree core archive showed much stronger temporal patterns than reliance on more conventional contemporary sampling approaches, where N mobility appeared to obscure temporal patterns.  We further found that temporal trends were relatively insensitive to tree age class. Using the more powerful Multiple Tree Approach, we further evaluated δ¹⁵N values from an additional 1000 P. abies and P. sylvestris wood samples covering the entire forested area of Sweden and spanning the same time period, to investigate how temporal patterns in wood δ¹⁵N varied in areas with historically high N deposition (Southern Sweden) versus low N deposition (Northern Sweden).  These data help address current debates regarding whether temporal patterns in δ¹⁵N are indicative of oligitrophication (i.e. progressive N limitation), or are instead the result of changing δ¹⁵N signatures from nitrogen deposition inputs.  

How to cite: Gundale, M., Bassett, K., Östlund, L., Fridman, J., Perakis, S., and Jämtgård, S.: Evaluating temporal patterns in wood δ15N in Swedish forests as an indicator of changing N limitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17469, https://doi.org/10.5194/egusphere-egu24-17469, 2024.

Climate warming was shown to strongly affect the biogeochemical cycles in global forests, reducing soil carbon storage and accelerating soil nitrogen (N) and phosphorus cycling. In a long-term soil warming experiment in a temperate old-growth forest in Achenkirch, Austria, we recently showed faster root turnover and growth, decreases in microbial biomass, carbon use efficiency and soil carbon storage, increases in ecosystem phosphorus limitation, and varied responses of the soil N cycle in warmed plots (+4 ° C above ambient for 14 years). In this study we therefore employed natural stable isotope techniques to better understand ecosystem-level responses of the N cycle in Achenkirch, studying the abundance of ¹⁵N and ¹⁴N (expressed as δ¹⁵N values) in a wide range of soil nitrogen pools (bulk soil N, root N, microbial biomass N, extractable organic N, ammonium, nitrate) and employed isotope fractionation models to explain the patterns found dependent on soil warming. Specific N cycle processes such as mineralization, nitrification and denitrification cause substantial isotope fractionation (against the heavy stable isotope ¹⁵N), leading to ¹⁵N enrichment of the residual substrates and ¹⁵N depletion of the cumulative products, depending on the fraction on substrates consumed and the isotope fractionation factor of that process. Other processes such as diffusion, (de)sorption and depolymerization exert negligible isotope fractionation. We found a significant warming effect on the isotopic signatures of root N and the soil ammoniumpool, i.e. a ¹⁵N enrichment in these pools. ¹⁵N enrichment of tree fine roots, considered to be isotopic integrators of the plant available N pool, suggest increased soil N cycling and greater soil N losses in warmed plots causing a ¹⁵N enrichment of the soil inorganic N pool (ammonium and nitrate). The increased ¹⁵N enrichment in ammonium of warmed soils highlights an increased activity of nitrifiers, with greater fractions of ammonium oxidized to nitrate causing the observed ¹⁵N enrichment of ammonium. However, soil nitrate did not show the expected ¹⁵N depletion imparted by nitrifiers but matched or even exceeded δ¹⁵N values of soil ammonium. Isotope fractionation calculations indicated that >50% of the soil nitrate produced was lost, particularly through denitrification promoting gaseous N losses in the form of NO, N₂O and/or N₂ and less through nitrate leaching. Natural ¹⁵N abundance studies thereby hold great potential for evaluating the status quo of the complex N cycle in terrestrial ecosystems and to monitor in situ responses to climate change with minimal invasion and improved time integration.

How to cite: Wanek, W., Bachmann, M., Tian, Y., Kwatcho Kengdo, S., Heinzle, J., Inselsbacher, E., Borken, W., and Schindlbacher, A.: Long-term soil warming causes acceleration of soil nitrogen losses in a temperate forest studied by 15N isotope fractionation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15470, https://doi.org/10.5194/egusphere-egu24-15470, 2024.

Current soil biogeochemical models have difficulties matching the observed composition of soil organic matter (i.e., the relative proportions of deadwood, raw litter, organic horizon, particulate organic carbon, and mineral-associated organic carbon). In reality, nitrogen (N) controls microbial decomposition and physiological processes, whereas in most models it is merely considered a plant nutrient. In addition, many N fertilization studies have shown that N exerts different effects on different C pools via changing exoenzyme activities, microbial growth, and necromass production via microbial turnover. These divergent effects control SOM composition and have C-cycle consequences.

We expanded the CENTURY model by incorporating multiple hypothesized microbial responses to nitrogen availability, including 1) decomposition reduction of recalcitrant substrates when N is in excess; 2) decomposition stimulation of high C:N substrates when N limitation is alleviated; 3) microbial adaptation of turnover rate; 4) microbial adaptation of CUE; and 5) secondary feedback to decomposition via changes in microbial biomass in response to N. We systematically tested multiple model variants using two sets of simulations, one along a natural N gradient in Swiss forests, and another one with artificially increased N input (i.e., simulating an N-fertilization experiment). We evaluated the simulated outputs using data on soil organic matter fraction stocks, their relative proportions, and temporal responses under N addition.

From the simulation results, we identified the necessary processes to explain the temporal response pattern of different C pools to N addition, in accordance with findings from meta-analyses. In addition, we identified patterns of SOM composition over a natural gradient of N supply (no artificial N addition), which can again be explained by the N-driven processes we implemented. We conclude that considering the direct effects of nitrogen as a key additional constraint on microbial processes is essential to improve the realism and accuracy of soil biogeochemistry models.

How to cite: Yeung, C. C., Bugmann, H., Hagedorn, F., and Díaz-Yáñez, O.: How does nitrogen control soil organic matter composition? – A theory and model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4706, https://doi.org/10.5194/egusphere-egu24-4706, 2024.

Representing plant-microbe-soil organic matter interactions and their coupling with land surface processes are critical to understanding of ecosystem responses to climate change. More specifically, microbes play an important role in the nitrogen (N) cycle by providing acquisition pathways for plants and overcoming N limitation through mycorrhizal symbiosis and bacterial fixation. Even though biological nitrogen fixation acts as a primary N source for the organisms, ecosystem N availability is still strongly affected by N losses, including atmospheric volatilization.

One of the major challenges to accurately representing N availability in Earth System Models (ESM) is the representation of the atmospheric losses that are not necessarily controlled by the organisms. For instance, the conversion of soil ammonium into gaseous ammonia (i.e., volatilization) is driven by ambient environmental conditions and not directly controlled by the biological demand of plants and soil microbes. Thus, rapid losses of N via volatilization (e.g., after precipitation events) could induce feedback on soil microbial activity and plant growth by impeding biological assimilation.

Even though the representation of ammonia emissions is progressively integrated into ESMs, the focus has been mainly on parameterizing losses from agricultural or managed ecosystems. However, ammonia volatilization from natural soils occurs worldwide and can reach 9 TgN/yr, a non-negligible source, especially in alkaline drylands. Up to now, no proper representation of emissions of ammonia, applicable to unmanaged lands, has been included in ESMs and challenged by observations. In the future, these emissions are likely to follow the rising trends of nitrogen deposition and increasing precipitation due to climate change.

Here we describe a mechanistic parameterization of ammonia emissions in natural ecosystems with explicit treatment of microbes and vegetation dynamics in the fully integrated terrestrial component of the GFDL ESM, LM4.2-GIMICS-N. We apply observational constraints, including measurements of soil 15N isotope and estimates of nitrogen fluxes (BNF, nitrification, mineralization, and ammonia exchange) at different sites to reduce uncertainty in the model simulations. Finally, we examine the main drivers of ammonia volatilization across various ecosystems by considering aridity, soil pH, and nitrogen deposition as well as the key environmental conditions such as precipitation, temperature, and soil moisture.

How to cite: Beaudor, M., Shevliakova, E., Malyshev, S., and Lee, M.: Resolving nitrogen gaseous pathways in the atmosphere-plant-microbial-soil continuum in the NOAA/GFDL Earth System Modeling Framework, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13239, https://doi.org/10.5194/egusphere-egu24-13239, 2024.

The biogeochemical nitrogen (N) cycle in South Africa is influenced by, and in turn influences a number of crucially important global change processes. However, the natural N cycling in South Africa is not well-understood. The “Emissions, deposition, impacts - Interdisciplinary study of N biogeochemical cycling (EDI-SA)” project is working to improve our baseline understanding of the natural biogeochemical cycling of N in non-industrialized ecosystems across South Africa. This includes quantifying N fluxes from emissions through to deposition, identifying linkages between N cycling and related species such as sulphur (S) and ozone, and evaluating ecosystem impacts. Previous work has focused on the impact of atmospheric deposition of N and S species on ecosystems at sites almost exclusively on the industrialized Highveld. This has left large gaps of knowledge in the biogeochemical cycling and ecosystem impacts, particularly within the diverse natural ecosystems found across South Africa. In order to address this gap, EDI-SA is applying a more holistic approach using measurements (from two South African Research Infrastructures; EFTEON and BIOGRIP) and modelling to investigate multiple linkages within the biogeochemical cycling of N with a focus on improving the understanding of the natural cycling. The project is applying a variable resolution sampling approach to investigate processes which occur at multiple spatial scales, and applying multiple measurement techniques including atmospheric measurements, stable isotope analysis of aerosol particles, rainwater and soil, and analysis of soil chemistry and biology. This contribution will detail the approach of this interdisciplinary project, highlight results from the first soil and air sampling campaigns, as well as the atmospheric composition modelling that assesses the relative importance and impacts of N emissions from soil across South Africa. This baseline understanding will allow future research to assess the potential changes to N biogeochemical cycling into the future in a changing climate.  

How to cite: Garland, R. M., Naidoo, M., Altieri, K., Dlamini, P., Feig, G., Jaars, K., Sekhohola, L., van Zyl, P., Muthelo, N., Leroko, J., Sakwe, P., Hamilton, T., van Niekerk, T., Bixirao Neto Marinho, P., and Smart, K.: Examining the natural nitrogen biogeochemical cycling and impacts across South African ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12116, https://doi.org/10.5194/egusphere-egu24-12116, 2024.

Nitrous oxide (N₂O) is one of the most important greenhouse gases with a global warming potential of about 298 times stronger than carbon dioxide (CO₂) over a period of 100 years. From 1800 to 2023, the atmospheric concentration of N₂O has increased from 273 to 336 ppbv, whereby more than half of this rise is due to the addition of fertilisers and manure on agricultural soils. Whilst these managed, nutrient-rich soils have been relatively well studied, little is known about N₂O fluxes in nutrient-poor ecosystems (e.g., the Arctic).

Since many Arctic soils contain very low amounts of available nitrogen, in the past it has been generally assumed that Arctic soils are not a significant source of N₂O. Only recently, several studies have reported significant N₂O emissions from organic-rich Arctic soils; however, due to methodological challenges, extensive investigations on N₂O fluxes in Arctic soils have been limited. As a result, the importance of N₂O fluxes from this region to the global budget remains highly uncertain. 

With the recent advances in portable GHG analyser technology, extensive manual chamber measurements based on in-situ N₂O concentration measurements can provide novel information to close this knowledge gap. However, guidelines on measuring techniques (e.g., chamber closure time) and data quality (e.g., no flux vs. low flux) are still lacking. In this study, we provide new insights on N₂O fluxes in a nutrient-poor ecosystem and give general practical guidelines for measuring low N₂O fluxes with a portable gas analyser and manual chambers. In May, July, and September 2023, we used a portable N₂O/CO₂ analyser to measure N₂O fluxes in a thawing sub-Arctic permafrost peatland in northern Sweden. Recommendations on practical use in the field are given to support future N₂O research with portable gas analysers. 

How to cite: Triches, N. Y., Marushchak, M., Virkkala, A., Vesala, T., Heimann, M., and Göckede, M.: Advances in measuring low N2O fluxes by a portable gas analyser and manual chambers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5728, https://doi.org/10.5194/egusphere-egu24-5728, 2024.

Chinese agriculture has long been characterized by low nitrogen use efficiency (NUE) associated with substantial ammonia (NH₃) loss, which contributes significantly to fine particulate matter (PM_2.5) pollution. However, the knowledge gaps in the spatiotemporal patterns of NH₃ emissions and the states of nitrogen management of agricultural systems render it challenging to evaluate the effectiveness of different mitigation strategies and policies. Here we explored the NH₃ mitigation potential of various strategies and its subsequent effects on PM_2.5 pollution, and their effectiveness in improving NUE of Chinese agricultural systems. We developed and used a nitrogen flow model for evaluating NUE of different crop and livestock types at a provincial scale in China. We then used the bottom-up NH₃ estimates to drive an air quality model (GEOS-Chem High Performance, GCHP) to provide an integrated assessment of four improved nitrogen management scenarios: improving NUE of crop systems (NUE-C), increasing organic fertilizer use (OUR), improving NUE of livestock systems (NUE-L) and combined measures (COMB). The total agricultural NH₃ emission of China was estimated to be 11.2 Tg NH₃ in 2017, of which 46.24% and 53.76% are attributable to fertilizer use and livestock animal waste, respectively, and emission hotspots can be identified in the North China Plain. Our results show that grain crops have higher NUE than fruits and vegetables, while high livestock NUE can be found in pork and poultry, and NUE for the entire crop and livestock systems are both better in Northeast China than the rest of China. We also found that agricultural NH₃ emissions can be reduced from 11.2 Tg to 9.1 Tg, 9.3 Tg, 9.9 Tg and 6.8 Tg, and consequently annual population-weighted PM_2.5 reductions are estimated to be 1.8 µg m^–3, 1.6 µg m^–3, 1.3 µg m^–3 and 4.1 µg m^–3 under NUE-C, OUR, NUE-L and COMB scenarios, respectively. Our results are expected to provide decision support policy making concerning agricultural NH₃ emissions.

How to cite: Luo, B. and Tai, A. P. K.: Improving Agricultural Nitrogen Use Efficiency to Reduce Air Pollution in China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6942, https://doi.org/10.5194/egusphere-egu24-6942, 2024.

For the agri-environmental monitoring of Switzerland, nitrogen balances on farm level for all Swiss farms were calculated and aggregated in order to obtain regionalized nitrogen balances. This monitoring attempts to incorporate as much existing data as possible to minimize multiple data collections from farmers. Data from the agricultural policy information system of Switzerland was used as basis for the calculation. This database contains information on livestock numbers, the crops grown, and the direct payments received for each farm. This information was supported with different data sources from federal offices, cantons, agricultural associations, and research institutions. Balances were calculated as a soil-surface balance according to the OECD method, which includes N input via organic and mineral fertilizers, biological N-fixation, atmospheric N-deposition, and seedlings as well as N outputs via plant yields.

The regional balances showed a high variability, resulting in an average N surplus of around 105 kg N per hectare of utilized agricultural area in cantons with highly intensive livestock farming and around 16 kg N in cantons with more extensive farming practices, i.e. in mountain regions. On national scale, highest N input occurred via organic fertilizers, whereas mineral fertilizers and biological N-fixation account for around 15% of the total input each.

Our approach of calculating N balances on farm level for the whole Swiss farming system has some limitations, which are mainly due to missing or incomplete data sources.  As an example, the use of mineral fertilizers had to be estimated by application data of a rather small sample of farms (~300 farms). Nevertheless, the obtained results show that this methodology is a promising tool to gain a regional overview of the environmental status of Swiss farms. Over the years, this approach will be refined and new data (e.g. additional administrative data, satellite data) can be incorporated in order to better estimate the N balances of Swiss farms.

How to cite: Gilgen, A., Baumgartner, S., Spiess, E., and Liebisch, F.: Regionalized nitrogen balances of Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5701, https://doi.org/10.5194/egusphere-egu24-5701, 2024.

One of the most pressing issues in intensive agriculture is how we can reduce post-harvest losses of nitrogen (N) on agricultural land. In terms of N use efficiency, the focus so far has been on optimizing the amount and timing of N fertilization, including spatially targeted application (precision agriculture). However, we must be aware that this will not be sufficient to solve the problem of N surplus. The mineralization of crop residues and soil organic matter, especially after harvest, can lead to very high mineral N concentrations in the soil, which ultimately result in high N losses, mainly in the form of nitrate leaching, but also as nitrous oxide (N₂O) if the excess N is not immobilized before winter. In crop rotations that do not allow the cultivation of a catch crop, e.g. before winter cereals, the N immobilization potential is by far not high enough to immobilize the available mineral N. In this case, a different approach than plant N immobilization is required to immobilize the excess N before winter.

Here, we present results from laboratory incubations and field trials with different soils under a wide range of conditions based on the stimulation of microbial biomass growth by readily available organic soil amendments. They show that effective immobilization of mineral N in large quantities (almost 100 % reduction of nitrate concentration in the soil) is possible for several months, even under winter conditions. A consistent picture emerges from the results, suggesting that the optimal and longest-lasting effect of N immobilization can be achieved with nitrogen-free organic compounds that are moderately available to microorganisms (i.e., within several weeks rather than a few days). If the microorganisms are offered compounds that are too readily available (extreme case: glucose), a rapid stimulating effect can be triggered, which, however, does not last long enough to immobilize N for several months due to too early remineralization. If too recalcitrant organic compounds are introduced into the soil, the utilization of the additional carbon source takes too long to lead to effective N immobilization. We can therefore say that we have taken a significant step forward in understanding the mechanisms and timing of microbial N immobilization and remobilization, which may prove key to solving the N surplus problem in agriculture. However, the extent to which such management measures can be implemented in agricultural practice also depends on the political framework conditions that make them economically feasible.

How to cite: Brüggemann, N., Zhao, K., and Reichel, R.: How can we reduce post-harvest nitrogen losses on agricultural land? Evaluating the potential of easily degradable, nitrogen-free organic soil additives, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20772, https://doi.org/10.5194/egusphere-egu24-20772, 2024.

High nitrification rates and substantial nitrogen (N) losses through nitrate leaching and N₂O emissions make current agricultural practices unsustainable, contributing to greenhouse gas emissions and environmental pollution. Synthetic nitrification inhibitors (SNIs) can be amended with N-fertilizers to reduce the conversion of ammonia to nitrate by soil nitrifiers. SNIs aim to increase agricultural nitrogen use efficiency (NUE), but they have several disadvantages (e.g., costs, ineffectiveness in the field, possible accumulation in the food chain). The use of biological nitrification inhibitors (BNIs), naturally occurring in plant root exudates, could become an alternative to SNIs. Potential BNIs should be highly specifically targeting nitrification, but for most known BNIs it is unclear if and how they affect other soil microorganisms and biogeochemical processes.

This study aimed to investigate possible off-target effects of BNIs in agricultural soils. We tested the effect of two candidate BNIs (Methyl 3-(4-hydroxyphenyl)propionate and DL-limonene) in slurry assays on soil microbial communities from a typical Austrian agricultural field (Linz, pH 6.89±0.12, fertilized with 120 kg N ha^-1 yr^-1), and compared them to a known SNI (nitrapyrin), and two further nitrification inhibitors (phenylacetylene and octyne). The slurries were incubated for eight days and CO₂ production, pH, as well as nitrate- and N₂O accumulation were measured. At the end of the incubation, we analyzed fluorescence-based enzyme activity, as well as microbial substrate use efficiency using ‘Biolog©’ assays to test the influence on general microbial activity, selected microbial soil processes, and the effectiveness of nitrification inhibition, respectively.

Our results showed that both tested BNIs significantly reduced net nitrification rates, but also affected other biogeochemical processes, even though limonene lost some effectiveness during the incubation. MHPP was heavily respired by heterotrophic microorganisms, leading to a drop in pH and heterotrophic competition for the remaining ammonium, therefore likely acting as an indirect nitrification inhibitor. Extracellular enzymes were also affected: MHPP led to increased potential β-glucosidase activity, while nitrapyrin led to a decrease in potential phosphatase activity. General soil microbial substrate use diversity seemed to be unaffected by the input of either BNIs or SNIs. Whether or not the observed off-target effects are positive and what they mean for the large-scale application of BNIs in the agricultural industry remains to be further investigated.

How to cite: Karbon, I., Madani, K., Prommer, J., Rojas, P., Giguere, A., Sedlacek, C., Sandén, T., Spiegel, H., Pjevac, P., and Fuchslueger, L.: Off-target effects of biological nitrification inhibitors on soil microbial substrate use and enzyme activity in an agricultural soil , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21470, https://doi.org/10.5194/egusphere-egu24-21470, 2024.

Under the framework of Circular Economy, EU Green Deal, and UN Sustainable Development Goals the addition of organic amendments to agricultural soils is highly promoted as a cost-efficient solution to improve soil quality and agrosystem sustainability. Nonetheless, their agronomic use comes with an uncertainty of their potential to release ample plant-available N, and to emit soil greenhouse gases.

This mesocosm study investigated short-term (90 d) soil N dynamics of a loamy soil receiving four organic amendments (50 t ha^-1) (i) cow manure compost (CMC), (ii) food waste compost (FWC), (iii) used digestate substrate (UDS) and (iv) municipal sewage sludge (MSS), without and with N fertilization (160 kg N ha^-1; urea). An unamended soil mesocosm was included as a control (C). During the incubation soil NO₂-, NO₃-, NH₄+, N₂O and CO₂ were regularly monitored.

During the incubation, org. amendments did not affect NH₄⁺ availability (AUC) compared to unamended soil, except MSS treatment which had 5.7x more NH₄⁺ than C. The co-application of urea increased available NH₄⁺ by 2.9x, 4.1x, 4.4x, 4.6x, and 5.9x for MSS, UDS, CMC, FWC, and C, respectively. There was no difference in available NO₂^- among org. amendment treatments and the C, except MSS (2.4x). There was a substantial and temporal accumulation of NO₂^- (2.4x to 3.6x) when urea was co-applied with org. amendments. Co-application of urea with org. amendments increased AUC NO₃- in all treatments ranging to 2.7x from 13.6x, except MSS. Considering cum. CO₂ we did not observe any differences between org. amended treatments without and with urea. However, org. amendments increased cum. N₂O emission by 1.4x, 1.6x, and 3x, for UDS, FWC, and MSS, and reduced by 0.6x for CMC relative to C, respectively. The co-application of urea increased cum. N₂O emissions for MSS, UDS, and CMC by 6%, 65%, and 90%, respectively, and reduced by 58% for FWC, compared to the corresponding org. treatment without urea.

Interestingly, co-application of urea with org. amendments reduced N₂O emission factor (EF) by 4x, 6x, 6x, and 9x, relative to org. amendments without urea, for CMC, MSS, UDS and FWC, respectively. However, the EF N₂O exceeded 1% in most cases. Treatments with urea lost substantial amounts of org. C as CO₂-equivalent emissions, for instance, UDS+U and MSS+U lost 22% and 68%, respectively.  

In conclusion, our preliminary results indicate that the co-application of org. amendments with urea-N could potentially fuel soil N₂O emissions, thus offsetting any favorable aspects of the aforementioned policies. Org. amendment, urea-N, and their interaction were significant factors (p≤0.05) driving CO₂ and N₂O emissions. The quality and composition of the amendments may stimulate soil microbial N transformations, and further investigation will elucidate the intrinsic role of soil microbes and their dynamics in regulating CO₂ and N₂O emissions from soils.

The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers”; Project #01053 awarded to P.I. Dr Georgios Giannopoulos. This project was co-implemented with industrial partner Corteva Agriscience Hellas SA.     

How to cite: Giannopoulos, G., Pasvadoglou, E., Kourtidis, G., Elsgaard, L., Zanakis, G., and Anastopoulos, I.: Co-application of organic amendments and urea-N in a loamy soil reduced the N2O emission factor but substantial amounts of organic C were lost as CO2., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17096, https://doi.org/10.5194/egusphere-egu24-17096, 2024.

Intensively managed pasture systems receive large inputs of nitrogen (N) in the form of fertiliser and through the  deposition of ruminant urine, creating hot-spots for denitrification which results in variable amounts of nitrous oxide (N₂O) and dinitrogen (N₂) emitted. Here we investigated the potential of increased  irrigation frequency to reduce N₂O and N₂ emissions from an intensively managed pasture in the subtropics after ruminant urine deposition. Irrigation volumes were estimated to replace evapotranspiration and were applied either once (Low-Frequency) or split into four applications (High-Frequency). This irrigation schedule was applied 3 times over the 60 day monitoring period, and fluxes of N₂O and N₂ were  measured using the ¹⁵N gas flux method. In line with farming practice, simulated urine patches (equivalent of 80 g N m^-2 applied) were also fertilised three times with 2 g urea N m^-2 to show the combined effects of urinary and fertiliser N on N₂O and N₂ emissions. Highest N₂O emissions of up to 60 mg N₂O-N m^-2 day^-1 were observed briefly after urine deposition, decreasing thereafter, resulting in cumulative N₂O losses of 169.9 mg N₂O-N m^-2 from the Low-Frequency treatment. Denitrification was dominated by N₂, accounting for more than 89% of  N₂O+N₂ emitted. Irrigation treatments had no effect on cumulative N₂ losses of more than 2700 mg N₂-N m^-2. However, High frequency irrigation reduced cumulative N₂O losses by 35%. Our findings suggest that under conditions of high N availability, increased irrigation frequency can reduce the environmental impact (N₂O) of denitrification, but not overall N losses via this pathway. The response of N₂O emissions may further indicate that less frequent, but more intense rainfall events will shift the product ratio of denitrification towards N₂O, increasing environmentally harmful N losses from intensively managed pasture systems.

How to cite: Friedl, J., De Rosa, D., Scheer, C., Fitzgerald, M., Grace, P. R., and Rowlings, D. W.: Increased irrigation frequency reduces N2O, but not overall denitrification losses (N2O+N2) from an intensively managed pasture following ruminant urine deposition and nitrogen fertilisation., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19499, https://doi.org/10.5194/egusphere-egu24-19499, 2024.

In this study, we aimed to constrain and characterize the dynamics of denitrification in three different fields: one conventional arable and two types of pasture (“leys”). During a one-year field campaign, denitrification was measured using our newly developed method combining the application of ¹⁵N tracer and artificial atmosphere for the incubation of soil cores under field conditions (Micucci, 2022), while total N₂O emissions were measured using static flux chambers during parallel incubations. Our objectives were to determine the best way to upscale soil core denitrification measurements and trace the fate of applied synthetic nitrogen fertilizer via denitrification in conventional agriculture in comparison to pastures under regenerative agriculture practices.

We determined that the best way to derive field-scale fluxes of denitrification was to use the core method to calculate the source partitioning coefficient (SPC) and product ratio (PR) and use these metrics in combination with static chamber data. The SPC is defined as the proportion of total N₂O emissions that originates from denitrification while the product ratio measures the proportion of denitrification product emitted as N₂O rather than N₂.

During the field campaign, we estimated that 22 kgN ha^-1 were lost via denitrification in the arable field, amongst which 15.17 were attributed to fertilizer application, representing around 8% of the 200 kgN ha^-1 applied. Furthermore, 9 % of the denitrified fertilizer was emitted as N₂O rather than N₂. On the other hand, the unfertilized ley emitted only 2.6 kgN ha^-1 via denitrification annually. Overall, the total N₂O emissions in the fertilizer arable field were responsible for around 2 t eqCO2 ha^-1 year^-1 compared to 0.15 in the unfertilized ley, highlighting the importance of land management in strategies of greenhouse gas emission reduction.

How to cite: Micucci, G., Sgouridis, F., Krause, S., Lynch, I., McNamara, N. P., Roos, F., Jonathan, L., and Ullah, S.: Constraining the denitrification process in conventional and regenerative agriculture, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4369, https://doi.org/10.5194/egusphere-egu24-4369, 2024.

Nitrogen is a fundamental plant nutrient and the most important fertilizer in modern agriculture. At the same time nitrate based nitrogen loss from agroecosystems becomes an increasing environmental problem in ground- and surface waters. The lysimeter station Brandis in Saxony, Germany, provides detailed observations of water and solute fluxes under representative agricultural landuse since 1981. Despite substantial efforts and success in regulation and assessment of fertilizer needs and the reduction of fertilization excess, the seepage water analysis reveals increasing or stagnating levels of nitrate concentration in groundwater recharge in a broad range of soil types. This apparent decoupling between input and output is evident in all soil types under investigation and raises some important questions concerning the nitrate loss in agricultural soils:

• Which part of the soil N-cycle contributes to the seepage water nitrate export?
• What are the main drivers of nitrate loss in agricultural soils?
• Can residence times of mineral fertilizer nitrogen be estimated?
• Will reduced fertilization excess lead to timely reductions in nitrate loss to the groundwater?

We investigated these questions with long-term solute balances and state-of-the-art isotope methods. Analysis of source δ ¹⁵N ratios in soil, atmospheric deposition and fertilizer in combination with a 5-year campaign of δ¹⁵N and δ¹⁸O analysis of seepage water nitrate allows a source identification with dual-isotope plots and mixing models. The results clearly show that the main source of nitrate loss with the seepage water is the soil organic matter pool in all investigated soils. Analysis of the long-term nitrogen balances and the soil samples show furthermore a substantial accumulation of fertilization excess within the upper meter of agricultural soils and indicate that the residence time of nitrogen in the lysimeters might be substantially longer than water residence times. Isotope analysis in combination with mixing model analysis suggest that the nitrate loss is mainly driven by nitrification of this nitrogen legacy in the post-harvest period. Thus, the results hold an explanation why the current regulation efforts have not yet led to the desired reductions in nitrogen loadings of seepage water fluxes. Furthermore, the apparent decoupling between nitrogen input in agricultural soils and the seepage water output makes a timely reduction of nitrate concentrations, by reductions in fertilization excess alone, in groundwater recharge unlikely.

How to cite: Werisch, S., Alexandra, T., and Diana, B.: Insights into nitrogen dynamics and nitrate loss from agricultural soils based on long-term lysimeter observations and a 5-year isotope measurement campaign, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12647, https://doi.org/10.5194/egusphere-egu24-12647, 2024.

There is limited knowledge on the N (nitrogen) cycling in winter, on the role of organic matter quality on N cycling, and on the microbes involved.

We studied Lake Viinijärvi and Lake Höytiäinen, large boreal lakes in Finland, each lake with clear-water and brown-water sides. Viinijärvi has an additional side affected by mining activities in the catchment showing higher nitrate and sulphate levels. During winter of 2021 we sampled 5 sites at the beginning and at the end of the ice-covered period. Using the Isotope Pairing Technique we incubated sediment cores with ¹⁵NO₃^- and quantified the products of 1) complete denitrification (N₂), 2) truncated denitrification (nitrous oxide, N₂O), and 3) dissimilatory nitrate reduction to ammonium (DNRA, NH₄⁺) to infer the process rates. We characterized the DOM using FT-ICR MS. We explore the genetic potential (DNA) of the sediment microbiome by using several sequencing techniques.

During winter the sediment-water interface is an active compartment. The top sediment microbiome has heterotrophic bacteria with flexible metabolism, breaking-down OM during winter despite most of the DOM is recalcitrant. Impacts of browning and mining with major differences between sites. The genetic potential of the sediment microbiome indicates more DNRA and N2O consumption in clear-waters, while in the mining-impacted site and brown-water sites the dominant pathway depends on the sediment layer with truncated denitrification in top layer, and methanogenesis and N-fixation in sub-top layer. The N₂O production (d14), that fits the genetic potential, is highest in the mining-impacted site (35-43 µmol N/m²/d), followed by the brown-water sediments (6-11 µmol N/m²/d), with the lowest rates in the clear-water sediments (0-1 µmol N/m²/d).

How to cite: Palacin-Lizarbe, C., Bertilsson, S., Siljanen, H. J., Buck, M., Kolh, L., Paul, D., Maréchal, M., Nykänen, H., Liu, T., Kiljunen, M., Aalto, S. L., Rissanen, A. J., Biasi, C., Vainikka, A., and Pumpanen, J.: Browning and mining increase the nitrous oxide production in sediments of large boreal lakes during winter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20301, https://doi.org/10.5194/egusphere-egu24-20301, 2024.

Atmospheric deposition of natural and anthropogenic sourced reactive nitrogen (Nr, mainly including NH₃, NH₄⁺, NO_x, NO₃^- and etc.) has substantial influence on terrestrial and aquatic ecosystems, driving global nutrient imbalances and increasing risks to human health. Although it has been demonstrated that atmospheric Nr deposition has a substantial impact on nitrogen pools in remote and/or sensitive lakes, there is a scarcity of systematic evaluations regarding atmospheric Nr deposition's impact on the nitrogen burden in eutrophic lakes with riverine input as the primary source. Utilizing a regional chemical transport model, combined with observations of riverine nitrogen input, we investigate the contribution of atmospheric Nr deposition to a eutrophic Lake Chaohu in eastern China. The results indicate that riverine total nitrogen (TN) input to the lake was 11553.3 t N yr^-1 and atmospheric TN deposition was 2326.0 t N yr^-1 in the year of 2022. For Nr species which are directly available for the biosphere supporting algae and plant growth, riverine NH₄⁺ input was 1856.1 t N yr^-1 and atmospheric NH_x (NH₃ and NH₄⁺) deposition was 824.5 t N yr^-1. The latter accounts for ~ 1/3 of total NH_x input to the lake. For NO_y (HNO₃ and NO₃^-) species, atmospheric deposition was estimated to also contributes a similar amount to the NH_x species. The results suggest that even in regions with dense human activities with primary riverine N input, atmospheric deposition of Nr could also contribute significantly to the bio-available nitrogen in lake systems, and addressing eutrophication in Lake Chaohu and other eutrophic lakes will also need to consider the reduction of NH₃ and NO_x (i.e., NO + NO₂, the precursor of NO_y) emissions, in addition to the mitigation of riverine N input.

How to cite: Li, W., Wang, X., Zhang, Z., Liu, X., and Geng, L.: On the Contribution of Atmospheric Nitrogen Deposition to Nitrogen Burden in an Eutrophic Lake in Eastern China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5161, https://doi.org/10.5194/egusphere-egu24-5161, 2024.

Direct emission of the greenhouse gases methane and nitrous oxide (N₂O) constitute a significant fraction of the overall carbon footprint of wastewater treatment. Measurement methods to identify emission sources and to quantify emissions are key in mitigating these direct emissions. Nitrous oxide is formed in biological nitrogen removal process units, which are the main source of N₂O emission from wastewater treatment.

Liquid phase sensors (LPS) have recently been developed and installed at various Danish wastewater treatment plants to measure N₂O concentrations in the liquid phase of biological nitrogen removal tanks. These sensors can be used to implement adjustments on the operation of the plant (for example duration of aeration), which affects N₂O emission. In addition, LPS can be utilized to calculate N₂O emission through mass transfer modelling. However, there is a need for validation of liquid-based modelled emission rates against measurement methods, which measure direct N₂O emission rates. In this study, emission rates determined by two remote sensing methods, the tracer gas dispersion method (TDM) and Eddy covariance method (EC) were compared to LPS derived N₂O emission rates.

TDM relies on continuous, controlled release of a gaseous tracer at the source combined with downwind measurements of concentration of target gas (N₂O here) and tracer gas (often acetylene - C₂H₂).  This method is well-established, validated, and has been used to quantify fugitive emissions from various sources such as landfills, composting plants, biogas plants, etc. EC is a stationary method, which relies on high-frequency measurements of N₂O concentration and wind vector on a tower near the source. EC can be set up for continuous monitoring, while TDM as applied here is a discrete measurement method.

In the study, N₂O emission rates were measured over a period of 1.5 years at a relatively large wastewater treatment plant in the greater Copenhagen area. TDM measurements were conducted on 15 measurement days covering both periods of relatively high and low N₂O emission rates. TDM measurements were compared to LPS derived emission rates, where N₂O emission was measured using sensors in four of eight process units for biological nitrogen removal. Overall, daily average emission rates between approximately 0.38 and 13.4 kg N₂O h^-1 were measured. High emission rates of 120 kg N₂O h^-1 were observed on a day, where plant maintenance is believed to be the cause of unusual high emission. Emission rates from simultaneous TDM measurements and LPS derived values (n=43) showed good correlation (R²=0.70). On average, emission rates from TDM were 35% higher than LPS rates. The model implementation to derive LPS determined emission rates was further developed during the study, and the listed results were the final values after some correction. Several factors can explain the difference – including liquid sensor drift, which for the specific sensors tends towards lower N₂O concentration readings than actual concentrations. Continuous EC measurements showed the same emission dynamics as measured by the liquid sensors located inside the footprint of the station.

How to cite: Fredenslund, A., Kissas, K., and Scheutz, C.: Comparison of liquid phase and remote sensing measurements of nitrous oxide emission from biological nitrogen removal at a wastewater treatment facility, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16147, https://doi.org/10.5194/egusphere-egu24-16147, 2024.