Displays

Tree-ring δ¹⁵N provide long-term reliable information of soil available nitrogen, especially under increasing atmospheric N deposition upon forested ecosystems. In this study tree-ring δ¹⁵N values of Larix kaempferi and Cryptomeria japonica were measured for five trees each from a heavy snowfall region in north-eastern Japan, respectively. Larch and cedar tree-ring δ¹⁵N showed a range of -1.2 to 3.8 ‰ and -1.6 to 1.6 ‰, respectively. Larch trees showed a stable increase in δ¹⁵N values, while cedar trees showed a steady decreasing trend for the period 1970-2017. However, the divergence in the two tree series started in the early 1990’s. Both tree stands are exposed to the same atmospheric N deposition, therefore in principle both should have been affected equally. Similarly, an increase or a decrease in the δ¹⁵N value of the soil available N cannot be the reason since tree-rings values showed contrasting trends, unless this difference exists in each forest stand, however this seems unlikely. Another possibility could be the canopy uptake of depleted N from the atmosphere in cedar as the N demand increases could be responsible for cedar tree-ring δ¹⁵N temporal decrease but it does not explain the increase observed in larch. We speculate that low N demand and the increase in root biomass as tree grows could have decreased ¹⁵N discrimination by the EM fungi. The most plausible explanation for the contrasting results is that fractionation by the mycorrhiza fungi (Ecto and Arbuscular, respectively) during N root uptake was lower in larch (3.2 ‰) than in cedar (4.7 ‰) trees, which was related to the lower N demand as tree-ring wood %N was in average 0.06 and 0.10, respectively.

How to cite: Lopez Caceres, M. L., Tanabe, S., Santini, F., Voltas, J., Seidel, F., and Yamanaka, T.: Differential nitrogen isotope variation of tree-rings in two coniferous forests under climate change, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1795, https://doi.org/10.5194/egusphere-egu2020-1795, 2020.

Anthropogenic activities have notably disturbed the natural carbon and nitrogen cycle since the industry revolution. The consequential results include a warmer climate and enhanced nitrogen deposition on forest ecosystems. Covering one-third of the landmass, forests possess vital ecosystem functions such as N retention and the resulting C sequestration. However, the ongoing changes in climate and nitrogen deposition could potentially alter these important processes. Therefore, measures need to be taken to assess the distribution of deposited N in warming forest ecosystems. Here, we use ¹⁵N tracer method to investigate the short-term (2 weeks, 1 month and 3 months) fate of deposited N in a temperate forest, and by taking advantage of the in situ infrared heating, we also attempt to explore the effect of warming ( 2 °C ) on deposited N in forest ecosystems. The results showed that deposited N was largely retained by litter in all treatments (38%-57%) and mineral soil layers contained the least nitrogen. Total ¹⁵N recovery of different ecosystem compartments between warmed and control treatments showed distinct pattern, recovery in warming treatment increased from 72% to 97% after 1 month while the respect recovery of control treatment gradually decreased with time. In the top mineral soil layer (0-10cm), control treatments had higher recovery than warming treatment, suggesting warming may hinder deposited N from forging downwards, however, higher δ¹⁵N value in top mineral soil layer suggesting enhanced microbial activities maybe in action. Little leaching loss of deposited N both in warm and control was observed. Difference in deposited N fate between warming and control could deepen our understanding of how global warming influence forest ecosystem processes, particularly the N cycle.

Key words: Soil warming; ¹⁵N tracer; N deposition; N retention and redistribution; Global warming; Temperate forest

How to cite: Duan, Y. and Fang, Y.: Short-term fate of atmospherically deposited nitrogen in temperate forest under warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19227, https://doi.org/10.5194/egusphere-egu2020-19227, 2020.

Forests are important regulators of carbon dioxide fluxes, whereas overall greenhouse gas (GHG) budgets, in particular, nitrous oxide (N₂O), are still largely unknown. No studies on ecosystem-level N₂O budgets (soil and tree stem fluxes with eddy covariance (EC) measurements above the canopy) are found. Only a few examples are available on N₂O emissions from tree stems. Nevertheless, estimation of the N₂O and the full GHG balance in different forest ecosystems under various environmental conditions is essential to understand their impact on climate.

During the period of August 2017 to December 2019, we measured the N₂O budget of a 40-yr old hemiboreal grey alder (Alnus incana) forest stand on former agricultural land in Estonia considering fluxes from the soil, tree stems and whole ecosystem. Grey alder (Alnus incana) is a fast-growing tree species typically found in riparian zones, with great potential for short-rotation forestry. Their symbiotic dinitrogen (N₂) fixation ability makes alders important for the regulation of nitrogen (N) cycle in forested areas.

We measured the N₂O budget considering fluxes from the soil surface (12 automated chambers; Picarro 2508), tree stems (60 manual sampling campaigns from 12 model trees with chambers at 0.1, 0.8 and 1.7 m; gas chromatographic analysis in lab) and whole ecosystem (EC technique: Aerodyne TILDAS). Simultaneously, soil water level, temperature and moisture were measured automatically, and composite soil samples were taken for physico-chemical analysis. Potential N₂ flux in intact soil cores was measured in the lab using the He-O incubation method.

Average N₂O fluxes from the soil and tree stems varied from 1.2 to 3.0 and 0.01 to 0.03 kg N₂O-N ha^–1 yr^–1, respectively, being the highest during the wet periods, peaking during the freezing-thawing, and being the lowest in dry periods. The average annual potential N₂ flux in the soil was 140 kg N₂ ha^–1 yr^–1 which made the average N₂:N₂O-N ratio in the soil about 60. According to the EC measurements, the forest was a net annual source of N₂O (3.4 kg N₂O ha^–1). Thus, the main gaseous nitrogen flux in this forest was N₂ emission. Our carbon (C) budget showed that the forest was a significant net annual C sink.

Results of our long-term study underline the high N and C buffering capacity of riparian alder forests. For better understanding of C and nutrient budgets of riparian forests, we need long-term, high-frequency measurements of N₂O fluxes from the soil and tree stems in combination with ecosystem-level EC measurements. The identification of microorganisms and biogeochemical pathways associated with N₂O production and consumption is another future challenge.

How to cite: Mander, Ü., Schindler, T., Macháčová, K., Krasnova, A., Escuer-Gatius, J., Maddison, M., Pärn, J., Veber, G., Krasnov, D., and Soosaar, K.: Three-year dynamics of N2O fluxes from soil, stem and canopy in a hemiboreal forest: Impacts of floods and droughts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7760, https://doi.org/10.5194/egusphere-egu2020-7760, 2020.

Moss-associated nitrogen (N) fixation provides a substantial but heterogeneous input of new N to nutrient limited ecosystems at high latitudes. The presence of “hot spots”, defined as a rate of N fixation greater than three standard errors over the mean rate, can further increase the difficulty of scaling N inputs to plant communities or ecosystems. We used ¹⁵N₂ incubations to quantify the fixation rates associated with 34 moss species from 24 sites ranging from 60 to 68 degrees N in Alaska, USA. The total moss-associated fixation rates ranged from 0.08 to 4.4 kg N ha^-1yr^-1, with an average of 1.1 kg N ha^-1yr^-1, based on abundance-weighted averages of all mosses summed for each site. Five of the 24 sampled sites were hot spots of N fixation. We hypothesized that host moss diversity would be correlated with higher N fixation rates, since different mosses often have distinct microbial assemblages and higher microbial diversity has been linked with higher N fixation rates in other ecosystems. However, we found no significant correlation between either moss taxonomic richness or Simpson’s D and N fixation rates (p=0.102, R²=0.01 and p=0.522, R²=0.02, respectively). What we found instead was that certain high-fixing species, most importantly Tomentypnum nitens, were present in almost all hot spots. The relevance of moss taxonomic identity in driving N fixation rates was repeatedly observed in our survey, where both machine learning and mixed model approaches found that moss family was a significant predictor of associated fixation rates across ecosystems in Alaska. Taken together, these results indicate the importance of moss identity in driving hot spots and illustrate that host taxonomy may be a useful tool in generating more accurate large-scale assessments of associated N inputs in these vulnerable and valuable ecosystems.

How to cite: Stuart, J. and Mack, M.: “Hot spots” in high-latitude moss-associated N fixation: What drives locally high fixation rates? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5983, https://doi.org/10.5194/egusphere-egu2020-5983, 2020.

Biological nitrogen fixation, the main input of fixed N into ecosystems, converts inert N₂ gas into bioavailable ammonium in an energetically costly reaction catalyzed by the prokaryotic metalloenzyme nitrogenase.  The high ATP and reductant requirements of N₂ fixation explain why this process is highly regulated in diazotrophs, with the presence of ammonium inhibiting nitrogenase expression and activity. Yet, several reports of N₂ fixation in ammonium- and nitrate-rich (10 to 300 µM) benthic environments challenge our understanding of a key environmental sensitivity of N₂ fixation. Field studies point to heterotrophic sulfate reducers as the likely diazotrophs in these benthic settings, but the fixed N sensitivity of sulfate-reducing diazotrophs is not well understood due to a dearth of culture studies. Additionally, assays of N₂ fixation in incubations rarely involve parallel measurements of dissolved inorganic nitrogen, possibly leading to experimental bias in favor of detecting activity under ammonium-replete initial conditions.

To help reconcile the environmental results, we investigate the ammonium sensitivity of N₂ fixation using the acetylene reduction assay and ¹⁵N₂ tracer methods in i) the model sulfate-reducing diazotroph, Desulfovibrio vulgaris str. Hildenborough (DvH), ii) four enrichment cultures from salt marsh sediments of New Jersey, and iii) slurry incubations of sediments collected from three northeastern salt marshes. In all instances, we found that ammonium strongly inhibits biological nitrogen fixation, with nitrogenase activity only detectable when ammonium concentration is below a threshold of 10 µM (slurry incubation) or 2 µM (pure cultures, enrichments). Amendment of ammonium quickly inhibits nitrogen fixation and nitrogenase activity only resumes  once ammonium is depleted to the threshold level. Ammonium additions to actively fixing samples show complete inhibition of N₂ fixation within several hours post-addition. 

Our measurements of the ammonium sensitivity of benthic N₂ fixation are consistent with the traditional understanding of nitrogen fixer metabolism and with early findings of Postgate et al. (1984) demonstrating that N₂ fixation by the sulfate reducer Desulfovibrio gigas is inhibited by ammonium levels that exceed 10 µM. These results help clarify a long-standing paradox in benthic nitrogen cycling. We suggest that prior observations of N₂ fixation at elevated ammonium levels could reflect methodological artifacts due to very fast depletion of ammonium during activity assays, legacy N₂ fixation activity associated with incomplete inhibition by ammonium, or spatial heterogeneity. Further work to standardize fixed N sensitivity assays could help with cross-study comparisons and with clarifying inconsistencies in our understanding of how environmental fixed nitrogen levels control nitrogen fixation.

How to cite: Darnajoux, R., Zhang, R., Luxem, K., and Zhang, X.: The fixed nitrogen sensitivity of biological nitrogen fixation in salt marshes sediments from the northeastern United States, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11857, https://doi.org/10.5194/egusphere-egu2020-11857, 2020.

Biological nitrogen fixation (BNF) is a key contributor to sustaining the terrestrial carbon cycle, providing nitrogen input that plants require. This is particularly salient for projections of carbon uptake under increased atmospheric carbon dioxide concentrations, which may allow for so-called ‘carbon dioxide fertilisation’ if other plant requirements, such as nitrogen, do not prevent increases in productivity. The amount, processes, and global distribution of BNF is highly disputed and consequently land surface models represent it in varying ways. Looking at the latest generation of CMIP6 earth system models with terrestrial nitrogen cycles, we consider their performance with regard to BNF. We assess models against a new comprehensive meta-analysis of BNF field measurements that gives a global range and site-specific values. We find that compared to the wide range of upscaled observations, the models still have a larger range, with under and overestimates.

How to cite: Davies-Barnard, T., Meyerholt, J., Zaehle, S., Friedlingstein, P., Brovkin, V., Fan, Y., Fisher, R., Jones, C., Lee, H., Peano, D., Smith, B., Wårlind, D., Wiltshire, A., and Ziehn, T.: Terrestrial Biological Nitrogen Fixation in CMIP6 Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1951, https://doi.org/10.5194/egusphere-egu2020-1951, 2020.

Nitrogen deposition into arid ecosystems is increasingly shaping biogeochemical dynamics worldwide. We propose a framework to investigate the role of pulsed soil N emission as a mechanism that relays the influences of atmospheric anthropogenic N deposition to areas that otherwise would be minimally affected. We use nutrient spiraling theory, developed in lotic ecosystems, to quantify how regeneration of N by soils influences N deposition downwind of an urban plume. Our hierarchical framework of landscape functioning thereby connects ecosystem processes occurring from microbial (<1cm and hours) to regional (>100km and inter-annual) scales through reciprocal interactions among soils and microbes, pollution sources, and the atmosphere. We use southern California, USA as a case study for evaluation where we test the terrestrial nitrogen spiraling framework using a combination of field experiments, isotopic measurements, theoretical models, and atmospheric transport and chemistry model outputs. Initial results from field wetting experiments, isotope measurements and contrasting modeling approaches all support a spiraling framework and the increasing importance of soil regeneration of nitrogen to deposition farther from the urban source. Soil microbiome communities associated with nitrogen cycling vary both spatially across the deposition gradient and temporally in response to wetting events. From these results we derive terrestrial spiraling metrics that can identify consequences of both soil and anthropogenic inputs to regional nitrogen cycling. New landscape frameworks for evaluating the role of transport and transformations on N cycling can help understand and predict spatial variation in ecosystems connected across multiple scales.

How to cite: Jenerette, D., Krichels, A., Piper, S., Greene, A., Botthoff, J., Shulman, H., Aronson, E., Homyak, P., Sickman, J., and Wang, J.: Pulses of Biogenic Nitrogen Cycling Lead to Atmospheric-Based Nutrient Spiraling in Southern California, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21311, https://doi.org/10.5194/egusphere-egu2020-21311, 2020.

The abundances of functional genes and transcripts have provided new insights into microbially mediated biogeochemical processes and might improve quantitative predictions of turnover rates.
However, the relationship between reaction rates and the gene and transcript abundances may not be a simple correlation.
Most mechanistic reaction models cannot predict molecular-biological data, and it is unclear how they can be informed by such data.

We developed a mechanistic model that considers transcript abundances of denitrification genes, enzyme concentrations, biomass, and solute concentrations as state variables that are interrelated by ordinary differential equations, and thus mechanistically links molecular-biological data to reaction rates.
Important features of transcript dynamics can be reproduced with the transcript-based model.

We calibrated the model using data from a batch experiment with a denitrifying organism at the onset of anoxia.
We explored the relationship between transcript abundances and reaction rates by analyzing the model results.
The transcript abundances reacted very quickly to substrate concentrations so that we could simplify the model by assuming a quasi steady state of the transcripts.

We compared our model to a classical Monod-type formulation, which was as good at simulating the concentrations of nitrogen species as the transcript-based model, but it cannot make use of any molecular-biological data.
Our results, thus, suggest that enzyme kinetics (substrate limitation, inhibition) control denitrification rates more strongly than the dynamics of gene expression.

How to cite: Störiko, A., Pagel, H., and Cirpka, O.: Does it pay off to link functional gene expression to denitrification rates via modelling?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4954, https://doi.org/10.5194/egusphere-egu2020-4954, 2020.

Restoring degraded peat soils to wetlands can be an attractive and efficient measure with many benefits including carbon sequestration, water quality improvement, food and habitat for wildlife, flood control, and opportunities for recreation. Agricultural lands which are restored to wetlands will start rebuild soils and reverse land subsidence. Using eddy covariance towers in four wetlands that were restored in 1997, 2010, 2013 and 2016 in the Sacramento-San Joaquin Delta in California, we saw high carbon sequestration potentials and peat accumulation. Since soil restoration takes place gradually, it is important to specify the critical turning-points in the process of improving soil microbial community structure and nitrogen cycling. In August 2018, soil samples from four wetlands with different restoration ages in the Delta were collected for chemical and microbial analyses. The bacterial and archaeal 16S rRNA genes and functional genes involved in nitrogen cycling (nirS, nirK, nosZ-I, nosZ-II, bacterial and archaeal amoA, nifH, nrfA, and ANAMMOX-specific genes) in soils were determined using a quantitative PCR method. Soil chemical parameters such as C%, N%, Al, Mn, Fe and two different organic and inorganic P pools were analysed as well. Preliminary results indicate significant dissimilarities in the abundance of soil bacterial and archaeal communities, as well as nirS, nirK, nosZ, nifH, nrfA and archaeal amoA gene-possessing microbial communities in different wetlands. Data analysis showed several statistically significant relationships between target gene parameters and soil chemical parameters that were different when comparing the sites with the restoration age. It is clear, that the complexity of the relationships increases as the wetland gets older. For example, in younger wetlands the availability of C and N plays a crucial role in gene abundances while in the oldest wetland, the most important chemical parameters were different phosphorus forms. This might indicate that more than 20 years of C and N accumulation has led to the availability of phosphorus for N transformation now to be the main limiting factor. Another important finding was that the design criteria can also determine how the wetland acts in terms of nitrogen gas emissions. For example, one of the wetlands was designed with more varied bathymetry that includes many open channels and a fluctuating water table. We saw that the nifH gene-possessing microbes that are responsible for molecular N fixing are highly abundant in open water areas while at the same time this wetland has also the highest abundance of nir genes that control N₂O production by denitrifiers. Our study demonstrates that the design of the wetland can have a significant impact on N-transforming processes, but most importantly at some age, restored wetlands become more similar to natural wetlands.

How to cite: Kasak, K., Anthony, T., Valach, A., Hemes, K., Kill, K., Silver, W., Mander, Ü., Szutu, D., Verfaillie, J., and Baldocchi, D.: Variability of nitrogen-cycle microbial communities determined by the age of restored wetlands , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12965, https://doi.org/10.5194/egusphere-egu2020-12965, 2020.

The anaerobic ammonium oxidation (anammox) process converts ammonium to dinitrogen gas (N2) using nitrite as an electron acceptor under anaerobic conditions, which plays an important role in global nitrogen cycle. Anammox has been extensively investigated at different spatial scales. However, most previous studies have focused on the impacts of environmental factors on anammox bacterial community composition, whereas the influence of spatial factors, such as geographical distance, remains unclear. Here, we took sediment and water samples from two large-scale river in China: the Yangtze River. High-throughput biomolecule analysis was performed to explore the spatial patterns of anammox bacterial community and their response to environmental factors, spatial factors, community interchange and anammox bacterial traits. Additionally, 15N tracer analyses has been performed to estimated anammox activity and its contribution to N2 production (ra), and factors shaping its occurrence. Main conclusions are draw as follows: 
(1) The Three Gorges Dam (TGD) induced sediment coarsening could enhance anammox role as an important N-sink and decrease anammox bacterial alpha diversity. Anammox is ubiquitous in sediment of the Yangtze River, with high bacterial abundance (1.0×105 to 2.90×108 copies g-1 dry sediment), and activity (0.003-6.67 nmol N g-1 h-1), accounting for 3.5-82.8% of total N2 production (ra). Our results showed that the ra at the post-dam site was steeper than that before the dam, whereas the alpha diversity of anammox bacteria showing an opposite trend. Further analysis showed that hydraulic erosion leads to sediment coarsening and loss of organic matter downstream of the dam, which ultimately leads to the enhancement of the ra and the decrease of anammox bacterial alpha diversity. TGD induced sediment coarsening would extend downstream nearly to the river mouth in the coming decades, which would inevitably enhance the importance of anammox in nitrogen loss and alter anammox bacterial community in the Yangtze River for a long time.
(2) A significant distance-decay relationship was observed for anammox bacterial community similarity in the Yangtze River, which was significantly influenced by geographical distance rather than local environmental factors. This implied that niche-independent dispersal limitation plays an important role in shaping anammox community assembly. Furthermore, the slope of the distance-decay curve was much higher than previously reported for whole bacteria, which indicating the species turnover rate of anammox bacteria (z-value = 0.35) was significantly higher than that of the whole bacteria (approximately 0.008-0.05). Anammox bacteria harbor stronger adsorption ability and film-forming ability than other bacteria. As such, anammox bacterial harbor lower dispersal potential, and ultimately exhibited a higher species turnover rate than whole bacteria.
 This study investigated the geographical patterns and the driving mechanisms of anammox bacterial community in large-scale riverine ecosystems, and estimated the key shaping factors of anammox activity and its contribution to total N2 production. The results or conclusions of this study are of scientific significance for further revealing the community assembly and geographical patterns of anammox bacteria on a global scale, as well as the theoretical system of nitrogen cycle.

How to cite: Liu, S. and Chen, L.: Key shaping factors of anammox bacterial geographical distribution and function in riverine ecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6367, https://doi.org/10.5194/egusphere-egu2020-6367, 2020.

Stable isotope measurements of nitrogen and oxygen in nitrogen-containing molecules provide important constraints on the sources, sinks and pools of these molecules in the environment. Anammox is one of two known biological processes for converting fixed nitrogen to N₂, and through its consumption of ammonium and nitrite and production of nitrate, it impacts the supply of a wide variety of fixed N molecules. Nevertheless, the isotope fractionations associated with the various anammox-associated redox reactions remain poorly constrained. We have measured the isotope effects of anammox in microbial communities enriched for the purpose of nitrogen removal from wastewater by anammox. In this system, we can replicate the ecological complexity exhibited in environmental settings, while also performing controlled experiments. We find that under a variety of conditions, the nitrogen isotope effect for the anaerobic oxidation of ammonium in this system (NH₄⁺ to N₂) is between 19‰ and 32‰, that for the reduction of nitrite (NO₂^– to N₂) is between 7‰ and 18‰, and that for the production of nitrate (NO₂^– to NO₃^–) is between -16‰ and -43‰. We propose that these ranges reflect both (1) a mixture of signals from different anammox-performing species and (2) variation of the isotope effect associated with the anammox process within a given microbial community under different conditions. We seek to understand further what factors control this variability to better interpret stable isotope measurements of N-bearing molecules in environmental settings.

How to cite: Magyar, P., Hausherr, D., Niederdorfer, R., Wei, J., Mohn, J., Bürgmann, H., Joss, A., and Lehmann, M.: The imprint of anammox on the stable isotope compositions of nitrogen-bearing molecules, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15588, https://doi.org/10.5194/egusphere-egu2020-15588, 2020.

Nitrite (NO₂^-) is a crucial compound in the complex N soil cycle. As an intermediate of nearly all N transformations its isotopic signature may provide precious information on the active pathways and processes. NO₂^- analyses have been already applied in ¹⁵N tracing studies increasing their interpretation perspectives. Natural abundance NO₂^- isotope studies in soils were so far not applied and this study aims at testing if such analyses are useful in tracing the soil N cycle.  

We conducted laboratory soil incubations with parallel natural abundance and ¹⁵N treatments accompanied by analyses of soil N compounds (NO₃^-, NO₂^-, NH₄⁺) and released N gases (N₂O and N₂). Water content was varied during the experiment from 55 to 86% water-filled pore space. NO₂^- was immediately extracted and analysed with the denitrifier method for selective nitrite reduction with Stenotrophomonas nitritireducens.

NO₂^- content varied in the wide range from 0.6 to 6.6 μmol kg^-1 soil, whereas NO₃^- content was one order higher and quite stable from 1.3 to 1.7 mmol kg^-1 soil. Similarly, the δ¹⁵N(NO₂^-) varied largely from -16.7 to +8.8‰, whereas δ¹⁵N(NO₃^-) was very stable from 3.5 to 5.9‰. The δ¹⁵N(NO₂^-) was correlated with NO₂^- content. Applying Keeling plot the isotopic signature for the NO₂^- input of -11.7‰ was determined. When related to δ¹⁵N(NO₃^-) this gives the ε(NO₂^-/ NO₃^-) of -16.2‰, which is within the literature data for NO₃^- to NO₂^- reduction step of denitrification.

The parallel ¹⁵N treatment was used to provide interpretation for the natural abundance isotope nitrite dynamics. We observed a sudden drop in ¹⁵N abundance in NO₂^- (a¹⁵N(NO₂^-)) after water addition to the soil from 14.7 to 3.2 at%, whereas  ¹⁵N abundance in NO₃^- (a¹⁵N_NO₃^-) showed only slight decrease from 14.2 to 13.1 at%. This indicates an incorporation of another source of unlabelled NO₂^- for the wet part of the experiment. In natural abundance isotopes this change was also reflected in higher Δ¹⁵N(NO₂^-/ NO₃^-); for the wet part of the experiment it was even positive with +1.6‰, whereas for the dry part it was lower with -5.9‰. This additional nitrite source is most probably oxidation of organic N, which will be clarified by further studies, including detailed analysis with the ¹⁵N Ntrace model.

The observed changes in nitrite isotope characteristics were not reflected in N₂O. Whereas a¹⁵N(NO₂^-) droped to 3.2 at%, for a¹⁵N(N₂O) still 13.6 at% were found. The pool derived N₂O fraction calculated with the ¹⁵N gas flux method showed that the entire N₂O originated from NO₃^- in the wet part of the experiment. This shows that NO₂^- pools originating from different pathways must be isolated and not the entire NO₂^- pool undergoes further reduction to N₂O.

Natural abundance nitrite isotope studies may provide a new important tool in constraining the N soil cycling.

How to cite: Lewicka-Szczebak, D. and Well, R.: Nitrite isotope characteristics in 15N-labelled and non-labelled agricultural soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3692, https://doi.org/10.5194/egusphere-egu2020-3692, 2020.

The controlling factors of biotic denitrification in soil as a source of the greenhouse gas nitrous oxide (N₂O) and of dinitrogen (N₂) are still not fully understood due to the challenges in observing processes that co-occur in soil at microscopic scales and the difficulty to measure N₂ fluxes. N₂O production and reduction depend on the extent of anoxic conditions in soil, which in turn are a function of O₂ supply through diffusion and O₂ demand by soil respiration in the presence of an alternative electron acceptor (e.g. nitrate).

This study aimed to explore microscopic drivers that control total denitrification, i.e. N₂O and (N₂O+N₂) fluxes. To provoke different levels of oxygen supply and demand, repacked soils from two locations in Germany were incubated in a full factorial design with soil organic matter (1.2 and 4.5 %), aggregate size (2-4 and 4-8mm) and water saturation (70%, 83% and 95% WHC) as factors. The sieved soils were repacked and incubated at constant temperature and moisture and gas emissions (CO₂ and N₂O) were monitored with gas chromatography. The ¹⁵N tracer application was used to estimate the N₂O reduction to N₂. The internal soil structure and air distribution was measured with X-ray computed tomography (X-ray CT).

The interplay of anaerobic soil volume fraction (ansvf) as an abiotic proxy of oxygen supply and CO₂ emission as a biotic proxy of oxygen demand resulted in 81% and 84% explained variability in N₂O and (N₂O+N₂) emissions, respectively. These high values dropped to 5-30% when only ansvf or CO₂ was considered indicating strong interaction effects. The extent of N₂O reduction in combination with ansvf and CO₂ even increased the explained variability for N₂O fluxes to 83%. Average O₂ concentration measured by microsensors was a very poor predictor due to the extreme variability in O₂ at short scales in combination with the small footprint of the micro sensors probing only 0.2% of the entire soil volume. The substitution of predictors by independent, readily available proxies for O₂ supply (diffusivity based on air content) and O₂ demand (SOM) leads to a reduction in predictive power.

To our knowledge this is the first study analyzing total denitrification in combination with X-ray CT image analysis, which opens up new perspectives to estimate denitrification in soil and also contribute to improving models of N₂O fluxes and fertiliser loss at all scales and can help to develop mitigation strategies for N₂O fluxes and improve N use efficiency.

How to cite: Rohe, L., Schlüter, S., Apelt, B., Vogel, H.-J., and Well, R.: Oxygen supply and demand as controls of denitrification at the microscale in repacked soil , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5257, https://doi.org/10.5194/egusphere-egu2020-5257, 2020.

Eutrophication of aquatic ecosystems provoked by excess nitrogen (N) concentration is still a major concern worldwide with severe consequences such as hypoxia, biodiversity loss, and degradation of drinking water quality. To face these challenges, a novel N mitigation measure has emerged in the last decades consisting of biofilters made of woodchips. Drainage water from agricultural areas infiltrate through a layer of woodchips before it discharges to an aquatic recipient such as a ditch or a stream. The goal with this technique is to provide optimal conditions for denitrification i.e. an easy degradable carbon source (the woodchips) and an anaerobic environment. There is, however, some concerns regarding the emissions of the greenhouse gas nitrous oxide (N₂O) which can be a by-product of denitrification.

Here, we present results on N removal and N₂O emissions from 9 biofilters differing in age (1–8 years) and representing a total of 18 years of monitoring. The biofilters were all located in agricultural catchments in Denmark (temperate climate conditions). Nitrogen removal in the biofilters was estimated using a mass balance approach measuring N species dissolved in the water (total N, nitrate, nitrite, ammonium) using time proportional automated samplers placed at inlet and outlet of the biofilters. Nitrous oxide emissions were measured every third week both as gaseous form at the surface of the biofilters (closed chamber technique and gas chromatography) and in dissolved form in the water phase at inlet and outlet of the biofilters (headspace technique and gas chromatography). We take advantage of this unique dataset to identify the factors enabling to maximize N removal while minimizing N₂O emissions. Furthermore, we make a first assessment of the potential impact of the increasing number of biofilters on N₂O emissions in agricultural landscapes.

How to cite: Audet, J., Zak, D., and Hoffmann, C. C.: Nitrogen removal and nitrous oxide emissions in woodchips biofilters treating agricultural drainage waters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3318, https://doi.org/10.5194/egusphere-egu2020-3318, 2020.

Ammonia (NH3) has received a lot of attention nowadays due to its major implications for the population and the environment. Global sources of ammonia include wild animals, ammonia-containing water areas, traffic, sewage systems, humans, biomass burning (mainly from dung fires and domestic coal combustion), volcanic eruptions and agriculture. In the present study, we used 10 years (2008–2017) of satellite measurements of ammonia retrieved from the Infrared Atmospheric Sounding Interferometer (IASI) to calculate surface emissions. In contrast to other methods, we first used a sophisticated Inverse Distance Weighting (IDW) interpolation algorithm to define a grid of column-integrated ammonia concentrations globally. In a hypothetical box model, emissions are given as a function of the mass of ammonia in each atmospheric box (in molecules cm-3) divided by the lifetime of ammonia in the box (in seconds) based on all the potential removal processes that affect atmospheric ammonia. Instead of considering the lifetime of ammonia as a constant value, such as in the relevant literature, we used calculated gridded lifetimes from a Chemistry Transport Model (CTM). The estimated emissions were then imported in a CTM and were simulated for the same 10–year period. To verify the improvement of the calculated emissions of ammonia, we evaluated the modelled surface concentrations against ground–based measurements from different monitoring stations. The same comparison was performed for the most recent state–of–the–art emission dataset for ammonia.

How to cite: Evangeliou, N., Balkanski, Y., Eckhardt, S., Hauglustaine, D., Cozic, A., and Stohl, A.: Validation of satellite-constrained ammonia using a CTM and ground and satellite measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5396, https://doi.org/10.5194/egusphere-egu2020-5396, 2020.

Gaseous nitrous acid (HONO) and nitric oxide (NO) play a significant role in atmospheric chemistry through the contribution to the hydroxyl radical (OH) and influencing atmospheric oxidization capacity. Soil HONO emissions are considered as a major source of atmospheric HONO. However, soil HONO emissions and their contribution to air quality are rarely quantified. In this study, HONO and NO emissions from cropland, forest, urban green land, and grassland soils in Shanghai were measured by a dynamic chamber system under controlled laboratory conditions. HONO and NO emissions at the optimal water content (10 - 40% of water holding capacity) were highest from forest soil (50.3 ± 30.1 and 70.4 ± 43.9 ng m^-2 s^-1; average ± standard error, respectively), following by cropland soil (48.6 ± 17.4 and 55.8 ± 23.1 ng m^-2 s^-1, respectively), urban green land soil (44.2 ± 9.5 and 39.3 ± 13.3ng m^-2 s^-1, respectively), and grassland soil (27.7 ± 15.6 and 18.4 ± 6.9 ng m^-2 s^-1, respectively). Correlation analysis showed that soil HONO and NO emissions were significantly related with nitrate, total nitrogen, and total carbon (P < 0.01). The total soil emissions of HONO and NO in Shanghai were estimated based on “wetting-drying method”, and then upscaling to China and global. Results showed that global NO emissions from natural and fertilized soils were ~ 4.5 and 2.6 Tg N yr^-1, respectively, which are comparable with the results from IPCC report (2013). The estimated global HONO emissions from natural and fertilized soils were ~ 3.3 and 2.7 Tg N yr^-1, respectively, while those were 0.12 and 0.35 Tg N yr^-1 for China, and 0.01 and 0.33 Gg N yr^-1 for Shanghai, respectively.

The impact of soil HONO emissions on atmospheric oxidation capacity and O₃ concentrations in Shanghai were evaluated using the WRF-Chem model in March of 2016. Daytime HONO concentrations were increased by 0.036 ± 0.015 ppb after considering soil HONO emissions during typical wetting-drying days, and the contribution of HONO photolysis to OH radicals enhanced from 0.095 ppb h^-1 to 0.22 ppb h^-1 and was ~ 2 times the contribution of O₃ photolysis (0.1 ppb h^-1), leading to 0.5 - 1.0 ppb enhancement of 8h-O₃. The sensitivity test showed that O₃ enhancement caused by soil HONO emissions were larger (1.0-1.5 ppb) under low NO_x (cutting down 50%) conditions compared with the current conditions, implies that the importance of soil HONO emissions could be even larger in future considering the on-going NO_x reducing management in China.

How to cite: Wang, M., Zhang, J., An, J., Zhou, F., Zhang, X., Wang, R., Deng, L., Hou, L., Liu, M., and Wu, D.: Emissions of nitrous acid (HONO) and nitric oxide (NO) from soils and its impact on air quality in Shanghai, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-397, https://doi.org/10.5194/egusphere-egu2020-397, 2020.

Climate change has gained extensive international attention due to the impacts on the regional agriculture and water supply. According to IPCC, the global mean temperature will increase by 0.3-0.8 centigrade. Greenhouse gases such as CO₂, CH₄, and N₂O will concentrate and global mean temperature are projected to be increasing. This study separately examines the Greenhouse gases effect arise from different tillage type (dry land and paddy crop) in Wujiang river basin using DeNitrification - DeComposition (DNDC) model. The simulations indicate that, the atmospheric CO₂ and CH₄ concentration increases with the paddy crop plants. Although between two irrigation periods, the field drying event can decrease the CH₄ production effectively. In addition, the paddy soils in this region tend to increase the effect of carbon source resulted from the flooding irrigation. Especially in the first flood irrigation period, the N₂O increases to the maximum value. By contrast, in crop land under rotation of rape and Maize, the effect of carbon sink induced from CO₂ fertilization could generally offset the effect of carbon source. Meanwhile, the effect of carbon sink increased resulted by the plant grows. Thus, the production of CO₂ is always negative. There is no CH₄ production in crop land under rotation of rape and Maize. By contrast, with fertilization input, the N₂O production increases from 0.05 kg C/kg to 0.5kg N/ha/day. The SOC from the top soils (0-10 cm) to bottom (40-50 cm) decreases from 0.021 kg C/kg to 0.014 kg C/kg in either dry land and paddy soils of the Wujiang River region from 1991 to 1994, respectively. These results suggest that SOC storage in paddy and dry land of this region is steady. For the dry land crop (rotation of rape and Maize), the N₂O increased with the fertilization. But for the paddy soils, the irrigation time is the key point period for greenhouse gases production and the variation of carbon and nitrogen in soil. As a representative of paddy crop and dry land crop (rotation of rape and Maize) in western China, the insights gained from the Wujiang River basin may be potentially transferable to other similar agricultural practices in other part of China.

How to cite: wu, X.: The Effect of Cultivation on the Greenhouse Gases Emissions in wujiang river Basin, Yangtze River, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2345, https://doi.org/10.5194/egusphere-egu2020-2345, 2020.

Nitrogen is the primary limiting nutrient in high latitude ecosystems. Biological nitrogen fixation (BNF) by microorganisms associated with cryptogamic covers, such as cyanolichens and bryophytes, is an important source of new reactive nitrogen in pristine, high-latitude ecosystems. BNF is catalyzed by the enzyme nitrogenase, for which three isoforms have been described; the canonical molybdenum (Mo) nitrogenase which requires Mo in its active site and two alternative nitrogenases, the vanadium and iron-only nitrogenases. The low availability of Mo on land has been shown to limit BNF in many ecosystems from the tropical forest to the arctic tundra. Alternative nitrogenases have been suggested as viable alternatives to cope with Mo limitation of BNF, however, field data supporting this long-standing hypothesis have been lacking.

Here, we elucidated the contribution of the vanadium nitrogenase to BNF by cyanolichens across a 600 km latitudinal transect in eastern Canadian boreal forests. We report a widespread activity of the vanadium nitrogenase which contributed between 15 to 50% of total BNF rates on all sites. Vanadium nitrogenase contribution to BNF was more robust in the northern part of the transect. Vanadium nitrogenase contribution to BNF also changed during the growing season, with a three-fold increase between the early (May) and late (September) growing season. By including the contribution of the vanadium nitrogenase to BNF, estimates of new N input by cyanolichens increase by up to 30%, a significant change in these low N input ecosystems. Finally, we found that Mo availability was the primary driver for the contribution of the vanadium nitrogenase to BNF with a Mo threshold of ~ 250 ng.g_lichen^-1 for the onset of vanadium based BNF.

This study on N₂-fixing cyanolichens provides extensive field evidence, at an ecosystem scale, that vanadium-based nitrogenase greatly contributes to BNF when Mo availability is limited. The results showcase the resilience of BNF to micronutrient limitation and reveal a strong link between the biogeochemical cycle of macro- and micronutrients in terrestrial ecosystems. Given widespread findings of Mo limitation of BNF in terrestrial ecosystems, additional consideration of vanadium-based BNF is required in experimental and modeling studies of terrestrial biogeochemistry.

How to cite: Bellenger, J.-P., Darnajoux, R., Magain, N., Renaudin, M., Lutzoni, F., and Zhang, X.: Ecosystem scale evidence for the contribution of vanadium-based nitrogenase to biological nitrogen fixation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10069, https://doi.org/10.5194/egusphere-egu2020-10069, 2020.

Elevated atmospheric carbon dioxide concentrations are stimulating photosynthesis and carbon sequestration. However, the extent of photosynthetic stimulation in forests under future climates is highly uncertain given that nutrient limitation in soils may constrain the CO₂ fertilization effect. The Birmingham Institute of Forest Research (BIFoR), University of Birmingham established the only global mature temperate deciduous forests Free Air Carbon Dioxide Enrichment (FACE) experiment to study the response of forests to future climates. Fumigation of the forest with ~550 ppm CO₂ started in 2017 and will continue until at least 2026. Soil nutrients cycling including nitrogen transformation in response to elevated atmospheric CO₂ (eCO₂) fumigation is currently investigated to determine the role of nutrient availability in carbon capture by forests. In this paper, we show preliminary results of the response of asymbiotic biological nitrogen fixation (BNF) in soils and epiphytic bryophytes at BIFoR-FACE following a year of eCO₂ fumigation. It is hypothesized that the demand for available nitrogen by trees will increase under eCO₂ and that competition of roots and soil microbes for available nitrogen will enhance asymbiotic BNF to at least meet microbial metabolic nitrogen demands in the long run. Surface soils (0-5 cm) and epiphytic feather moss (Hypnum cupressiforme) growing on oak tree stems in the FACE site were  collected during the second year of eCO₂ fumigation for the quantification of BNF activity using the ¹⁵N₂ assimilation methods (Saiz et al. 2019). Samples were incubated in 50 mL serum bottles under in situ conditions, followed by the analysis of soil and tissue samples for ¹⁵N signature on an Isotope Ratio Mass Spectrometer for the quantification of BNF activity.

The BNF activity under eCO₂ were 369% higher than in soils under ambient atmospheric CO₂. BNF rates associated with feather mosses (Hypnum cupressiforme) did not differ between the eCO₂ and control plots; however, rates under eCO₂ on average were 60% lower than in the control plots. Unlike soils, the moisture of feather mosses correlated significantly (R² = 51%) with BNF activity. Among nutrients in soil with implications for BNF activity, the concentrations of Mg, K, Co and Ni were significantly lower in soils under eCO₂ than in the control plots, while in feather moss tissues no differences were observed.  Our preliminary results show that eCO₂ fumigation primed asymbiotic BNF activity in soils. An enhancement of BNF together with the observation of a relatively low nutrient content under eCO₂ points to important changes in nitrogen cycling processes in the early years of CO₂ fumigation. Further detailed studies are underway to fully disentangle controls on nitrogen availability to trees under future climates.

Reference

Saiz, E, Sgouridis, F, Drifjhout, F & Ullah, S. 2019. Biological nitrogen fixation in peatlands: comparison between acetylene reduction assay and ¹⁵N₂ assimilation methods. Soil Biol. Biochem:131:157-165

How to cite: Ullah, S., Saiz Val, E., Sgouridis, F., and Drijfhout, F.: Early response of biological nitrogen fixation in a mature oak dominated forest to elevated atmospheric CO2 fumigation at BIFoR-FACE , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8678, https://doi.org/10.5194/egusphere-egu2020-8678, 2020.

Increasing atmospheric CO₂ concentrations in temperate forests may affect soil nitrogen (N) cycling processes due to the increased demand  for nitrogen availability by trees to support CO₂ uptake through photosynthesis. This in turn can affect the emission of nitrous oxide (N₂O) from the forest soil leading to a potential trade-off between the enhanced canopy CO₂ uptake and soil N₂O emission. The Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO₂ Enrichment (FACE) facility in a mature oak forest in Staffordshire, UK, which became operational in 2017. In April 2018 and again in May 2019, two years after the start of fumigation with 550 ppm CO₂, we collected soil samples (0 – 15 cm depth) from the three elevated CO₂ (eCO₂) and three control plots. Soils were amended in the laboratory with 98 at % ¹⁵N-NH₄⁺ and ¹⁵N-NO₃^- . Gross N mineralisation and nitrification were estimated by the isotope dilution technique, while N₂O emission from nitrification (¹⁵N-NH₄⁺ treatment) and denitrification (¹⁵N-NO₃^- treatment) were estimated by the ¹⁵N Gas-Flux method. Additionally, C/N ratio and δ¹⁵N and δ¹³C were measured in unamended eCO₂ and control samples via EA-IRMS. Whilst gross N mineralisation and N₂O emission were only marginally higher in eCO₂ plots compared to controls after one year of fumigation, there was a significant stimulation of N cycling after the second year that led to more pronounced differences. Gross N mineralisation rates doubled in the eCO₂ plots (mean: 4.09 μg N g^-1 d^-1, P < 0.05) compared to the control plots (mean: 2.02 μg N g^-1 d^-1), while a similar twofold increase was observed for gross nitrification rates (mean eCO₂: 1.63 μg N g^-1 d^-1;  mean control: 0.70 μg N g^-1 d^-1, P < 0.05). N₂O emission from both denitrification (mean: 0.03 ng N g^-1 d^-1) and nitrification (mean: 0.02 ng N g^-1 d^-1) were generally low but of similar magnitude and more than double than in the control plots. C/N ratio was conservative between eCO₂ and control plots as a result of proportional increase of C and N contents in the eCO₂ plots. The observed stimulation in N cycling was further corroborated by the significantly enriched δ¹⁵N signal (-0.66 ‰) in eCO₂ plots compared to the controls (-1.38‰). Moreover, the eCO₂ samples had a more depleted δ¹³C signal (-28.37 ‰) compared to the controls (-27.99 ‰), as a result of the additional carbon supplied through fumigation (CO₂ δ¹³C ~ -28 ‰). Following the first year of CO₂ fumigation, there were indications of soil N limitation despite the high rates of atmospheric N deposition (22 kg N ha^-1 y^-1). However, after only 2 years of the FACE experiment there is strong evidence of a shift in key soil N processes to sustain the enhanced nutrient demands to support enhanced canopy CO₂ uptake. Further research is underway in BiFOR-FACE to elucidate whether carbon quantity and/or quality drives the stimulation of soil N cycling and what are the long-term implications of the trade-off between enhanced CO₂ sequestration and a potential increase in N₂O emissions.

How to cite: Sgouridis, F., Cotchim, S., and Ullah, S.: Stimulation of soil N cycling after two years of Free Air CO2 Enrichment increases nitrous oxide emissions in a temperate forest, UK., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15899, https://doi.org/10.5194/egusphere-egu2020-15899, 2020.

Abstract. Forests ability to store carbon is strongly connected with the amount of nitrogen (N) that forest ecosystems can retain; N is indeed considered the most limiting nutrient for terrestrial ecosystem's net primary productivity. Since the industrial revolution, human activities have more than doubled the rate of N input into the nitrogen cycle and this could alleviate N limitation thus stimulating plant growth. However, it has been suggested that when N availability exceeds biotic demand and abiotic sinks, additional N can trigger a negative cascade effect: nutrient imbalance, reduced productivity, increased losses of N, eutrophication and acidification of soil and water, leading toward forest decline and net greenhouse gases emissions. The consequences of increased N deposition on forest depend in large share on the fate of N in the ecosystem, which can be simulated and quantified by a fertilization at a known isotopic signature. Nevertheless, most of the tracer experiments performed so far added the fertilizer directly to the forest floor, neglecting the potential role of N uptake by the forest canopy. In the Italian Alps, we are conducting an experiment where both types of N additions (above and below the canopy layer) are performed in two different forest stands, to understand if canopy fertilization better simulates ecological consequences of increased atmospheric N deposition. These field-scale manipulation experiments are willing to test two different hypotheses: i) the N uptake by trees in the above-canopy N addition experimental sites is higher than under-canopy N addition ii) forest growth rate varies with the type of treatment. To describe the fate of the applied N, stable isotope techniques have been adopted: the forest sites, fertilized with NH₄NO₃ at a known isotopic signature, are sampled for all the ecosystem components (plant, soil and water) periodically to determine the total N content and its isotopic signature. The δ¹⁵N values permit to calculate the recovery of N-fertilizer in tree tissues, soil and leaching-water, allowing us to understand how N allocation varies under these two fertilization strategies and how this affects C sequestration potential. Results regarding the short-term effects over the first 6 years of data collection will be presented.

How to cite: Da Ros, L., Ventura, M., Rodeghiero, M., Gianelle, D., and Tonon, G.: Fate of atmospheric nitrogen depositions in two Italian temperate mountain forests assessed by isotopic analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13067, https://doi.org/10.5194/egusphere-egu2020-13067, 2020.

In the last decades, the effects of nitrogen (N) deposition on temperate forests have received much interest. Studies recorded several changes in soil carbon (C) and N cycles due to extra reactive N available. For instance, past studies reported that N deposition, may influence CO₂ emission, lower CH₄ consumption by the soil and increase the emission of N₂O. Nevertheless, the mechanistic understanding of these ecological responses is still far to be reached. However, most of the studies neglected to include the canopy interception in the experiments simulating N addition, notwithstanding tree canopy have shown to change both the amount and the chemical composition of the N deposition. Hence, experiments simulating this process by applying fertilization above the canopy are needed.

The aim of this study is to explore how N deposition influences greenhouse gas (GHG) emissions in a temperate oak forest (Quercus petraea Liebl.) located in Monticolo (Bolzano, Italy). In this site, a set of three plots was created and replicated three times. Each set includes a control plot, a plot with below-canopy fertilization (N_BL) and a plot with above-canopy fertilization (N_AB). The fertilization is applied, since 2015, from May to September, for a total annual N addition of 20 kg N ha^-1.

Since April 2018, CO₂ emission has been monthly measured with a portable infrared gas analyzer. Measurements were performed on three points per plot, for a total of 27 measurement points. During measurements, soil moisture and soil temperature at 10 cm depth were measured as well.

The measurements of CH₄ and N₂O started during the growing season in 2019 and are performed on a monthly basis by a static chamber method. Three chambers were installed per plots, for a total of 27 chambers.

We will present the preliminary results of this study. The results showed that the 5-year N fertilization did not lead to significant differences between plots in terms of GHG fluxes. The sensitivity of CO₂ emission to temperature was not influenced by extra N. The differences were not significant between fertilized and unfertilized plots, nor between the two fertilization methods.

How to cite: Bortolazzi, A., Ventura, M., Panzacchi, P., Fornasier, F., Mondini, C., and Tonon, G.: Effects of nitrogen deposition on greenhouse gas fluxes from the soil: results from an innovative experimental design, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17258, https://doi.org/10.5194/egusphere-egu2020-17258, 2020.

N₂O and N₂ Emissions from soil in terrestrial ecosystems is a crucial component of the global nitrogen (N) cycle. The response of these two gases emissions from forest soil to temperature change and its underlying mechanisms are essential for predicting N cycle to global warming. Despite the warming-induced effects on soil N cycle is considered to be positive in general, our understanding of temperature sensitivity (Q₁₀) of N₂O and N₂ emissions is rather limited. We quantified the Q₁₀ of N₂O and N₂ emissions in forest soils and explored their major driving factors by conducting an incubation experiment using ¹⁵N tracer (Na¹⁵NO₃) with soil samples from nineteen forest sites from temperate to tropical zones. The environmental conditions largely varied: mean annual temperature (MAT) ranging from -5.4 to 21.5^oC and mean annual precipitation (MAP) ranging from 300 to 2449 mm. The soil pH varied between 3.62 to 6.38. We incubated soil samples under an anaerobic condition with temperature from 5 to 35^oC with an interval of 5^oC for 12 or 24 hours, respectively. Soil temperature strongly affected the production of N₂O and N₂. N₂O and N₂ production rates showed a positive exponential relation with incubate time and temperature for all forest soils. Our results showed that the Q₁₀ values ranged from 1.31 to 2.98 for N₂O emission and 1.69 to 3.83 for N₂ emission, indicating a generally positive feedback of N₂O and N₂ production to warming. Higher Q₁₀ values for N₂ than N₂O implies that N₂ emission is more sensitive to temperature increase. The N₂O/(N₂O+N₂) decreased with increasing temperature in fifteen of nineteen forest soils, suggesting that warming accelerates N₂ emission. Strong spatial variation in Q₁₀ were also observed, with tropical forest soils exhibiting high Q₁₀ values and relatively low Q₁₀ in temperate forest soils. This variation is attributed to the inherent differences in N biogeochemical cycling behavior between the microbial communities among sites. Despite soil temperature primarily controls the N₂O and N₂ emissions, we  explored the effects of other factors such as pH, C/N, DOC and related functional genes. In addition, we partitioned N₂O and N₂ emissions to different microbial processes (e.g., denitrification, co-denitrification and anammox). The results indicated that denitrification was the main pathway of N₂O and N₂ production under anaerobic environment and the contribution increased as temperature rise.

Key words: Temperature sensitivity, N₂O, N₂, Forest soil, Nitrogen cycle, Global warming, Denitrification

How to cite: Yu, H., Fang, Y., and Kang, R.: Response of N2O and N2 Emissions in Forest Soils to Temperature Change across China , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12706, https://doi.org/10.5194/egusphere-egu2020-12706, 2020.

Utilization of forest bioenergy is increasing; however, the overall environmental impacts of forest bioenergy utilization are not fully understood. Especially effects on N₂O emissions in mineral soils are less studied. With current harvesting practices, either whole-tree-harvest or stem-only-harvest, piles of logging residues are left on the forest floor. As a result, soil nitrogen (N) cycling processes can be accelerated on clear cut area under the piles, especially net nitrification. When N is transformed to more mobile form, the risk for N losses via nitrous oxide (N₂O) emissions from the forest floor may increase.

We studied how logging residue piles of three tree species, Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth.), influence gaseous losses of N after clear-cut. A Norway spruce dominated mixed stand on a mineral soil site was clear-cut and N₂O emissions were monitored. There were four treatments; three tree species treatments consisting of 40 kg m^-2 of fresh logging residues and control plot without residues as an additional treatment. Effects of logging residue piles on N₂O emissions were monitored over 4 growing season with closed chamber technic. Simultaneously soil temperatures were recorded over 2 growing season. Soil denitrification activity and the contribution of nitrification and denitrification to N₂O production were determined in laboratory experiments.

Logging residue piles lowered and balanced fluctuation of soil temperatures. N₂O fluxes peaked under the piles during the second and third growing season after the establishment of the piles; however inconsistent fluxes tended to be low. The production of N₂O was driven by both nitrification and denitrification processes, the proportion depending on the tree species. Our results indicate that logging residue piles accelerate N losses as gaseous form; however studies on the same field experiment shows that most of the N losses occur through soil percolation waters. Spruce residues tend to stimulate N₂O emissions longer compared to other tree species. There was a positive correlation with net nitrification and microbial biomass C (Törmänen et al. 2018, FORECO). These results have implications for sustainable and productive forest management practices and nutrition of re-growing vegetation.

How to cite: Törmänen, T., Lindroos, A.-J., Kitunen, V., and Smolander, A.: Response of N2O emissions to logging residue piles of Norway spruce, Scots pine and silver birch , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20450, https://doi.org/10.5194/egusphere-egu2020-20450, 2020.

The effects of intermittent drying of normally saturated organic systems such as peatlands, swamps, or wetlands has not been reported quite as often as those of wetting and drying cycles of normally dry soils. We report here the effects of weekly drying and rewetting events on saturated woodchips used as denitrification bed. We used denitrification rates and gas effluxes as indicators of the response of normally saturated organic substrate to intermittent aerobic conditions. We used replicated eight upflow columns in the lab fed with nitrated water, and undergoing variable duration of intermittent aerobic conditions (none, 2, 8, and 24 hours) over a 400d experiment.  We used high-frequency sensors to measure in- and outflow nitrate and DOC concentrations on a 2-hour basis, from which we calculated denitrification rates. We also measured the CO2 and N2O effluxes in the headspace on an hourly basis. The results show a burst of respiration activity during drying events and for several days after rewetting. Isotopic data suggest that respiration was bacterial denitrification. Intermittent aerobic conditions seem to provide the conditions conducive to the generation of more and better quality DOC, which microbes use during subsequent saturated conditions. Our results suggest that intermittent aerobic conditions may have lasting impacts on microbial respiration in wetlands.

How to cite: Birgand, F., Maxwell, B., Thomas, A., Schipper, L., Williams, D., Christianson, L., Tian, S., Helmers, M., Chescheir, C., and Youssef, M.: Effects of drying and rewetting cycles on denitrification and greenhouse gas emissions in normally saturated organic substrate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22205, https://doi.org/10.5194/egusphere-egu2020-22205, 2020.

Biochar additions may mitigate N₂O emissions from soil. The mechanisms underpinning the mitigation of emissions remain to be elucidated. A series of incubation experiments were performed to investigate the effects of biochar on N₂O production and reduction in columns with a low-fertility or high-fertility soil, with or without the injection of N₂O in the subsoil and with and without glucose (to stimulate denitrification). Biochar was added to the calcareous soils in 0 and 1% (w/w) amounts and moisture was maintained at 70% water-filled pore space (WFPS) over the incubation period. The results revealed that biochar reduced the emissions of soil-produced N₂O by 37−47% and those of injected N₂O by 23−44%. The addition of glucose solution strongly increased N₂O emissions, while biochar reduced total N₂O emissions by as much as 64−81% and those of injected N₂O alone by 29−51%. Differences between the low-fertility and high-fertility soils in the apparent N₂O emission mitigation by biochar were relatively small, but tended to be larger for the low-fertility soil. The results suggest that biochar addition can suppress the production of N₂O in soil and simultaneously stimulate the reduction of N₂O to N₂. Further studies are needed to elucidate the regulatory effects of biochar in soil.

How to cite: Dong, W., Walkiewicz, A., He, C., Bieganowski, A., and Hu, C.: The effect of boichar on the emission of N2O from a calcareous soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6628, https://doi.org/10.5194/egusphere-egu2020-6628, 2020.

Liming to increase pH of acidic soils is a common agricultural practice to optimize crop yields, which also modulates greenhouse gas emissions from soils. In particular, soil pH has been identified as a primary regulator of denitrification pathways with enhanced ratio of nitrous oxide (N₂O) to dinitrogen (N₂) emissions (i.e., enhanced N₂O/N₂ ratio) at lower soil pH. Therefore liming could represent a potential management option to mitigate soil N₂O emissions. However, changes in soil pH have pervasive effects on general microbial activity and on soil properties, including transformations of carbon (C) and bioavailability of phosphorus (P), with a feedback on microbial processes. Thus, the eventual net effects of liming on microbially derived N₂O emissions may be complex. The aim of this study was to discern the interaction between liming (soil pH), and availability of C and P in regulating N₂O emissions from acidic fertilized agroecosystems. Using coarse sandy soils from a long-term liming field experiment, N₂O/N₂ ratios from denitrifying enzyme activity was shown to be strongly affected by liming, i.e., with gradually decreasing ratios at increasing soil pH. Although liming acidic soil (pH, 3.6) to almost neutral (pH, 6.4) favored the reduction of N₂O to N₂, it also enhanced the overall denitrification rate, which eventually resulted in the highest N₂O emission from moderately limed treatments (pH, 4.7). Interactions between P availability and denitrification (and N₂O emission) occurred, where P addition generally increased cumulative N₂O emissions with strongest effect at the moderately limed soil. Mechanistic hypotheses for this effect are discussed. Overall, our results suggest that a critical liming rate should be pursued which may lead to substantial mitigation of N₂O emissions from acidic arable soil.

How to cite: Liang, Z., Abalos, D., and Elsgaard, L.: Interactions between liming and availability of C and P regulate nitrogen transformations and denitrifying potential in an acidic arable soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3890, https://doi.org/10.5194/egusphere-egu2020-3890, 2020.

Denitrification usually takes place under anoxic conditions and over short periods of time and depends on readily available nitrate and carbon sources. Variations in CO₂ and N₂O emissions from soils amended with plant residues have mainly been explained by differences in their decomposability. Another factor rarely considered so far is water-extractable organic matter (WEOM) released into soil during residue decomposition. Here, we examined the potential effect of plant residues on denitrification with special emphasis on WEOM. A range of fresh and leached plant residues was characterized by elemental analyses, ¹³C-NMR spectroscopy, and extraction with ultrapure water. The obtained solutions were analyzed for the concentration of organic carbon (OC), organic nitrogen (ON), and by UV-VIS spectroscopy. To test the potential denitrification induced by plant residues or three different OM solutions, these carbon sources were added to soil suspensions and incubated for 24 hours at 20 °C in the dark under anoxic conditions; KNO₃ was added to ensure unlimited nitrate supply. Evolving N₂O and CO₂ were analyzed by gas chromatography and acetylene inhibition was used to determine denitrification and its product ratio. The production of all gases as well as the molar N₂O+N₂-N/CO₂-C ratio was directly related to the water-extractable OC (WEOC) content of the plant residues and the WEOC increased with carboxylic/carbonyl C and decreasing OC/ON ratios of the plant residues. Incubation of OM solutions revealed that the molar N₂O+N₂-N/CO₂-C ratio and share of N₂O are influenced by the WEOM’s chemical composition. In conclusion, the effect of plant residues on potential denitrification is governed by their composition and the related production of WEOM.

How to cite: Surey, R., Schimpf, C. M., Sauheitl, L., Mueller, C. W., Rummel, P. S., Dittert, K., Kaiser, K., Böttcher, J., and Mikutta, R.: Denitrification induced by plant residues is driven by water-soluble organic carbon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2760, https://doi.org/10.5194/egusphere-egu2020-2760, 2020.

Controlling soil N cycling to mitigate N-oxide emissions and optimize N use efficiency is an important aspect of agricultural soil management. Numerous denitrification models exist that can inform management decisions, but these are limited by the lack of soil N2 flux measurements to validate the model estimates. Measurements of soil denitrification - including both N₂O and N₂ fluxes - are challenging, however, due to methodological limitations for the measurement of N₂ and the spatial/temporal heterogeneity of denitrification in soils.

We used laboratory incubations of re-packed soil cores, combined with both soil flushing and stable isotope techniques, to measure denitrification in two agricultural soils, as part of the DFG-research unit “Denitrification in Agricultural Soils: Integrated Control and Modelling at Various Scales (DASIM)”. The laboratory incubations used an automated mesocosm system, with regular measurements of both N₂O and N₂, to assess the response of soil denitrification to a variety of control factors. Control factors simulated typical scenarios that might occur in the field, including different amounts/types of plant residue, and changes in moisture, temperature, NO₃^- and oxygen concentration. Both natural abundance and ¹⁵N labeling of the soil mineral N pool were used to assess denitrification pathways.

Here we contrast the results of the incubation data from a sandy Podzol and silt-loam Luvisol. These data will be used to calibrate newly developed DASIM models as well as denitrification sub-modules of existing biogeochemical models. They will also inform the next steps of this work, which will extend the laboratory incubation technique to measure denitrification in undisturbed field soils.

How to cite: Matson, A., Burkart, S., Grosz, B., Köster, J. R., Merl, S., and Well, R.: Response of soil N2 and N2O fluxes to denitrification control factors in two agricultural soils , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15158, https://doi.org/10.5194/egusphere-egu2020-15158, 2020.

Living roots and arbuscular mycorrhiza fungi (AMF) are widespread in most terrestrial ecosystems and play an important role in ecosystem nitrogen (N) cycling. However, the influence of living roots and AMF on soil N₂O emissions remains poorly understood. In this study, we conducted a pot experiment with ryegrass (Lolium perenne) growing in a greenhouse for three months with three factors: root and AMF presence (None or unplanted, Root or with roots, and Root+AMF or with roots colonized by AMF), two N addition levels (N0 and N1 with 0 and 50 mg N kg^-1 soil) and two P addition levels (P0 and P1, with 0 and 20 mg P kg^-1 soil).

Our results showed that N addition didn’t have significant effect on N₂O emission, however, we detected significant effects of Root and Root+AMF, particularly under P addition. Though the colonization of AMF didn’t significantly influence N₂O emission, the presence of roots (Root and AMF+Root treatments) deceased N₂O emission by 58%-67% compared with the None treatment. P addition increased (+134%) N₂O emission from unplanted soil but decreased (74%-98%) N₂O emission under planted soil regardless of AMF colonization. Moreover, there were no significant relationship between N₂O emission and soil pH, NH₄⁺-N and net N mineralization. The lower N₂O emission from rooted treatments were mainly due to the lower soil NO₃^--N (and MBN) content which might be immobilized by plant biomass, while the higher N₂O emission from unplanted soil under P addition was attributed to increased soil available (r=0.760, P<0.01) and total (r=0.654, P<0.01) phosphorus content. We conclude that root presence and P addition played an important role in regulating N₂O emission from P-limited soils.

How to cite: Shen, Y., Xu, T., and Zhu, B.: The effects of living roots and arbuscular mycorrhiza fungi (AMF) on soil N2O emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6822, https://doi.org/10.5194/egusphere-egu2020-6822, 2020.

Nitrous oxide (N₂O) is a potent greenhouse gas associated with nitrogen fertiliser inputs to agricultural production systems. Minimising N₂O emissions is important to improving the efficiency and sustainability of grassland agriculture. Multispecies grassland swards composed of plants from different functional groups (grasses, legumes, herbs) have been considered as a management strategy to achieve this goal. Numerous soil nitrogen transformation pathways can lead to the production of N₂O emissions. These transformation pathways are regulated by soil microbial communities and the environmental conditions and management practices that impact on them. Much research has been carried out on N cycling and N₂O emissions from predominantly grass monoculture systems. However, there is a lot yet to understand about how agricultural grasslands with diverse plant communities influence soil N cycling and N₂O emissions. A lysimeter experiment was set up as a completely randomised block design and carried out over a full year to investigate N₂O production, and nitrogen cycling associated with four sward types. The swards four swards were: perennial ryegrass (PRG, Lolium perenne); PRG and low white clover (PRG + LWC, Trifolium repens); PRG and high white clover (PRG + HWC); PRG, WC and ribwort plantain (PRG + WC + PLAN, Plantago lanceolata) managed at 250, 90, 0, and 45 kg N ha^-1yr^-1, respectively. Fertiliser N was applied by syringe as urea in splits at suitable timings to meet grass growth demands. N₂O fluxes were measured using a static chamber technique and additional samples were taken after the final flux sample to measure the associated N₂O isotopomers using a novel Cavity Ring Down Spectroscopy technique. Leachate volumes were measured on a weekly basis and composite monthly samples were used to determine the total amount of N leached from each treatment over the full year. Herbage was harvested on a monthly basis to measure DM yield (kg DM ha^-1), total N (%) and N yield (kg N ha^-1).This work reports on the N₂O emissions and N leaching associated with the four sward treatments and related these N losses to the treatments DM yields and N uptake as an estimation of the efficiency of these differing grassland management strategies. N₂O isotopomer measurements were used to indicate N transformation pathways driving N loss over the growing season particularly around periods of peak N₂O emissions.

How to cite: Bracken, C., Lanigan, G., Richards, K., Tracy, S., Müller, C., and Murphy, P.: Analysis of N2O emissions and isotopomers to understand nitrogen cycling associated with multispecies grassland swards at a lysimeter scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9218, https://doi.org/10.5194/egusphere-egu2020-9218, 2020.

Growing plants affect soil moisture, mineral N and organic C (C_org) availability in soil and may thus play an important role in regulating denitrification. The availability of the main substrates for denitrification (C_org and NO₃^-) is controlled by root activity and higher denitrification activity in rhizosphere soils has been reported. We hypothesized that (I) plant N uptake governs NO₃^- availability for denitrification leading to increased N₂O and N₂ emissions, when plant N uptake is low due to smaller root system or root senescence. (II) Denitrification is stimulated by higher C_org availability from root exudation or decaying roots increasing total gaseous N emissions while decreasing their N₂O/(N₂O+N₂) ratios.

We tested these assumptions in a double labeling pot experiment with maize (Zea mays L.) grown under three N fertilization levels S / M / L (no / moderate / high N fertilization) and with cup plant (Silphium perfoliatum L., moderate N fertilization). After 6 weeks, all plants were labeled with 0.1 g N kg^-1 (Ca(¹⁵NO₃)₂, 60 at%), and the ¹⁵N tracer method was applied to estimate plant N uptake, N₂O and N₂ emissions. To link denitrification with available C in the rhizosphere, ¹³CO₂ pulse labeling (5 g Na₂¹³CO₃, 99 at%) was used to trace C translocation from shoots to roots and its release by roots into the soil. CO₂ evolving from soil was trapped in NaOH for δ¹³C analyses, and gas samples were taken for analysis of N₂O and N₂ from the headspace above the soil surface every 12 h.

Although pots were irrigated, changing soil moisture through differences in plant water uptake was the main factor controlling daily N₂O+N₂ fluxes, cumulative N emissions, and N₂O production pathways. In addition, total N₂O+N₂ emissions were negatively correlated with plant N uptake and positively with soil N concentrations. Recently assimilated C released by roots (¹³C) was positively correlated with root dry matter, but we could not detect any relationship with cumulative N emissions. We anticipate that higher C_org availability in pots with large root systems did not lead to higher denitrification rates as NO₃^- was limited due to plant uptake. In conclusion, plant growth controlled water and NO₃^- uptake and, subsequently, formation of anaerobic hotspots for denitrification.

How to cite: Rummel, P. S., Well, R., Pfeiffer, B., Dittert, K., Floßmann, S., and Pausch, J.: NO3- uptake and C exudation – do plant roots stimulate or inhibit denitrification?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19445, https://doi.org/10.5194/egusphere-egu2020-19445, 2020.

Sufficient nitrogen fertilisation is essential for obtaining the crop yields required to feed the growing population. Moreover, nitrogen applied to fields is often lost to a number of processes including denitrification, surface run-off and leaching. These processes can damage the local ecology and contaminate water supplies. Additionally, nitrogen lost as ammonia gas and the large energy input required to synthesize ammonia are both large contributors to global greenhouse gas emissions. Choosing fertilisation strategies to optimise the proportion of nitrogen taken up by crops (nitrogen use efficiency) can reduce the production of ammonia and the pollution of water supplies.

We developed a mathematical model that describes the movement of water and multiple nitrogen species in soil at the field scale over a growing season. The model was then used to assess the nitrogen use efficiency of varying fertilisation strategies. We consider the effects of a number of biological, chemical, and physical processes including: root growth, root uptake, the transformation of nitrogen between different nitrogen species, and the effect of soil water movement on nitrogen transport. The resulting model is comprised of a coupled system of partial and ordinary differential equations that describe the mathematical interplay between nitrogen transport, water movement, and root uptake, which were solved numerically using a finite element approach. Numerical experiments were conducted to determine how nitrogen uptake efficiency was affected by different fertilisation strategies. We examine numerous cases by varying the quantity of fertiliser applied to the soil and the fertiliser application times.

The numerical experiments suggest that, under uniform rainfall rates, the optimal fertilisation times (within the bounds of typical times found in agriculture) can result in 25% more nitrogen uptake than the worst strategies. However, there were large time periods, 28 days for the first application and 10 days for the second, which resulted in close-to-optimal nitrogen use efficiency. The results of this study, in addition to crop health and past and predicted rainfall, could be taken into consideration by farmers while choosing fertilisation times to optimise nitrogen uptake efficiency.

How to cite: McKay Fletcher, D., Ruiz, S., Duncan, S., Chadwick, D., Jone, D., and Roose, T.: Optimising Fertilisation Strategy for Nitrogen Uptake Efficiency, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8609, https://doi.org/10.5194/egusphere-egu2020-8609, 2020.

There is growing evidence that liming reduces the magnitude of N2O emissions in acidic soils. Here we report N2O emissions from a liming experiment with olivine, dolomite and calcite and of maintenance liming with the same materials in clay loam soil at Norwegian University of Life Sciences research farm. The field was bulk limed in 2014 and monitored for N2O fluxes by an autonomous filed flux robot (FFR). Over the course of four years, the fluxes varied but showed a potential of lime as a mitigation tool, with calcareous treatments (dolomite and calcite) displaying a clear decline in N2O emissions compared to unamended plots. To explore the effect of maintenance liming, subplots were maintenance limed and compared with bulk limed controls after sowing the field to winter wheat (Triticum aestivum L.) in summer and fertilizing with 50 kg NPK-N.

Growing-season N2O emissions (June-September) in maintenance limed dolomite plots were on average 26% lower than bulk limed plots and the corresponding reduction in calcite plots was 16%. There was no effect of maintenance liming in the olivine treatment. N2O emissions decreased in the order unlimed control > olivine > dolomite > calcite, covering a pH_CaCl2 range of 4.9 to 6.5.

Our results suggest that maintenance liming, as a component of good agricultural practice, is important to maintain the N2O reducing effect of liming over time. However, the amount of CO2 released by the dissolution of lime should be investigated in order to fully explore the mitigation potential of soil pH management in crop production.

How to cite: Todorcic Vekic, T., Bakken, L., and Dörsch, P.: Effect of maintenance liming with olivine, dolomite and calcite on growing-season N2O emissions in an arable soil of SE Norway , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11023, https://doi.org/10.5194/egusphere-egu2020-11023, 2020.

Changes in climate will bring along changes in precipitation patterns, and as such, it will determine the availability of water in agricultural systems. We aimed to investigate the impact of climate-induced altered precipitation regimes on crop performance and soil processes such as organic matter mineralisation and nutrient release. The experiment took place at the lysimeter station located in Hirschstetten, Vienna, Austria (48° 15' 22" N, 16° 289 3" E, 160 m a.s.l.) where a future precipitation scenario was compared with current precipitation patterns on two different soil types – a sandy calcaric Phaeozem and a calcic Chernozem, both being representative for the Marchfeld region in Lower Austria. The future precipitation regime was calculated from four regionalised scenarios from Euro-Cordex out of the ÖKS 15 ensemble following the GHG emission scenarios RCP 4.5 and RCP 8.5.

Stable isotope analysis has become a useful tool for sensitively tracing biogeochemical processes in soils. In this study, plant residues of white mustard (Sinapis alba), isotopically labelled with carbon ¹³C and nitrogen ¹⁵N in a controlled laboratory environment were applied as organic fertiliser (green manure) on the lysimeter soils in April 2018. Soil, plant, gas and groundwater samples were collected from the lysimeters throughout the growing season of 2018 and 2019 and analysed using cavity ring-down spectrometry (CRDS) for ¹⁵N-N₂O in the field and by isotope ratio mass spectrometry.

Crop results showed an increase in the shoot ¹³C signatures, indicative of drought stress, which resulted in diminished plant production by -20 to -50% under the decreased precipitation. Isotope analysis showed lower decomposition and mineralisation rates of labelled green manure only during the first few days under the future precipitation treatment, followed by an increase in ¹⁵N enrichment of soil solution NO₃^- during summer, emphasising the importance of plant biomass production on root NO₃^- uptake from the soil. N₂O emissions were higher after the application of synthetic fertiliser during the first year, highlighting the importance of available NO₃^- in agricultural systems for nitrification and denitrification processes. However, lower N₂O emissions were observed during the second year, indicating possible N stress. Overall we found that N losses through NO₃^- leaching and N₂O emissions were most sensitive to reduced precipitation when NO₃^- is available, which can cause aggravating environmental problems in the future.

The stable isotope labelling technique proved to be successful for tracing and identifying drought stress effects on plant and soil processes in agricultural systems, allowing for a better understanding of soil-plant processes under changing climate conditions.

How to cite: Spiridon, A., Kisielinska, W., Hood-Nowotny, R., Leitner, S., Heiling, M., Wawra, A., Hösch, J., Murer, E., Formayer, H., Wanek, W., Prommer, J., and Watzinger, A.: Interaction of decreased crop growth and retarded mineralisation of 15N 13C labelled green manure under decreased precipitation patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8818, https://doi.org/10.5194/egusphere-egu2020-8818, 2020.

The amount of bioavailable nitrogen is directly linked to anthropogenic activity (Kuypers, Marchant, & Kartal, 2018), particularly with the intensive application of synthetic nitrogen fertilisers. Although high nitrogen inputs are required to support the ever-increasing need for food production, nitrogen use efficiency is in many cases low, to the extent that even with extra nitrogen inputs over time, increases of food production are small and slow (Battye, Aneja, & Schlesinger, 2017).

It has been suggested that roughly 40% of reactive nitrogen is denitrified in the soil (Seitzinger, et al., 2006), and most of the reactive nitrogen that results from human activities is removed by denitrification, with consequent production of N₂ and N₂O. However, even if most reactive N forms are removed by denitrification, this is an indicator that N use efficiency is not at optimum levels.

A study is being conducted in field and controlled conditions, that aims to understand denitrification and nitrogen use efficiency in a long-term experiment (running continuously since 2013) at Rothamsted Research. The experiment was designed to provide a clearer look at the effect of applications of organic amendments and/or inorganic fertilisers on nitrogen dynamics and crop yields in a conventional cereal-based cropping system.

Simultaneously, using yield data from the same trial, we aim to understand a) if the application of organic amendments leads to a reduction of the nitrogen threshold for optimum yields and, by using a modelling approach, b) if the eventual higher yields obtained with organic amendment application are due to the effect of the extra nutrients contained in the amendment or to some other effect caused by the amendments.

Soil and gas samples are being collected from a) different treatments of the field experiment (four different organic amendments: anaerobic digestate, compost, farmyard manure, straw and unamended control; and different nitrogen application rates; area of each plot: 54 m²) to assess nitrogen dynamics, and b) from soil columns (height 35 cm; width 25.5 cm)  placed in a controlled environment using soil collected from the same trial. Different measurements are being taken including leachate (measurements of mineralised nitrogen), microbiology and gas emissions (using a Picarro device that measures NH₃, N₂O, CO₂, CH₄, O₂, H₂O). Simultaneously, underground sensors are being used to understand moisture and temperature evolution in the soil column, while electrochemical nitrate sensors are being used to understand nitrate dynamics before and after application of organic amendments and inorganic fertilisers.

With this, we aim at having a better understanding on denitrification processes and nitrogen use efficiency issues that may occur when using a joint regime of organic amendments and inorganic fertilisers. The main objectives of the project are the validation of the effect of organic amendments in the Fosters long-term experiment and the quantification of nitrogen gas emissions with the application of organic amendments and nitrogen fertilisers.

How to cite: Albano, X., Sakrabani, R., and Haefele, S.: Effect of organic amendments and inorganic fertiliser application on nitrogen use efficiency and denitrification in controlled and field conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15032, https://doi.org/10.5194/egusphere-egu2020-15032, 2020.

Quantifying soil nitrogen processes – especially denitrification – are critical for the adequate prediction of the produced and emitted N₂O and N₂ gasses and the production and consumption of NO₃^- and NH₃. Biogeochemical models are useful tools for the description of these N processes, but recent research progress is not considered on the denitrification sub-modules of these models. Denitrification typically occurs in hot-spots of the soils but the models describe the soils as a homogenized system. Another critical problem is the calibration of the decomposition sub-modules. Suitable soil N₂ flux data were not available during the development of the extensively used models but new measurement techniques provide appropriate N₂ gas flux data.

In this study we investigate the N₂ and N₂O fluxes from mesocosm experiments of different complexity and use the measured data and experimental settings for testing the denitrification sub-module of existing biogeochemical models.

Two arable soils – a silty loam and a sandy soil – were used for the experiments and varied with N fertilization and organic matter amendment. The soils were incubated in laboratory incubation systems over 42 and 58 days, respectively. N₂, N₂O and CO₂ fluxes were quantified by gas chromatography and isotope-ratio mass spectrometry. Seven moisture and three NO₃^- contents were set up to the loamy soil and only the temperature was manipulated during the experiment, while other factors were kept constant. In the experiment with the sandy soil, incubations were conducted with or without incorporation of organic litter (ryegrass) and initial water content was adjusted equivalent to 80% water-filled pore space. Temperature, water content and NO₃^- content were manipulated during that experiment.

Three commonly used biogeochemical models – namely CoupModel, DNDC and DeNi (a self programmed early stage version of the nitrification and denitrification sub-model of the DailyDayCent) – were tested on the experimental data.

The average N₂+N₂O fluxes of the loamy soil as given by measurements, DNDC, DeNi and CoupModel calculations was 287.5±202.3, 1.8±0.5, 779.1±282.2, 67.9±8.4 gN ha^-1 day^-1, respectively. For the sandy soil, these fluxes were 166.6±377.7, 23.7±34.7, 491.2±819.9 and 13.3±7.8 gN ha^-1 day^-1, respectively. The results show that the models did not calculate the same magnitude of the measured values. The DeNi model overestimated and the DNDC and CoupModel underestimated the measured fluxes. However, in some cases the temporal patterns of the measured and the modeled emission were similar. Most cases of over- or underestimations by the models could be explained by certain deficiencies of the models or of the experimental data.

How to cite: Grosz, B., Well, R., Dechow, R., Köster, J. R., Khalil, M. I., He, H., Merl, S., Rode, A., and Ziehmer, B.: Evaluation and adjustment of description of denitrification in the CoupModel, DNDC and DeNi model based on N2 and N2O laboratory mesocosm incubation system measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2543, https://doi.org/10.5194/egusphere-egu2020-2543, 2020.

Denitrification in unsaturated soils is widely assumed to be a result a result of the formation of so-called hot spots. However, this is a hypothesis, which is hard to test experimentally. Furthermore a better understanding of the microscale dynamics might be very helpful to derive better models at the macroscale.

Experiments have been conducted, where artificial aggregates from sintered glas have been inocculated with microorganisms and been placed in environments with different oxygen availabilities. Very high-resolution simulations are conducted to reproduce the dynamic of the generation of nitric and nitrous oxide based on a model of microbial growth parametrised with experimental data from batch experiments. The simulations allow a detailed analysis of the local and temporal dynamics of denitrification inside the aggregates.

How to cite: Ippisch, O., Zawallich, J., Dörsch, P., Schlüter, S., Horn, M. A., and Vogel, H.-J.: Understanding the Dynamics of Denitrification with high-resolution Simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19467, https://doi.org/10.5194/egusphere-egu2020-19467, 2020.

Nitrogen (N) fertilization is an essential agronomic practice, which increases crop yields and improves soil fertility. Globally, more than 110 x 10⁹ kg of chemical N fertilizers are applied each year with urea-N being one of the most affordable options. Upon urea hydrolysis, any portion not assimilated by crops is either volatilized as NH₃ or microbially nitrified (i.e., NH₄⁺ oxidized) to leachable NO₃^- and NO₂^-. Nitrification inhibitors (NI) are increasingly co-applied as a sustainable agricultural practice and block the process of nitrification, resulting in a temporal increase of NH₄⁺ in the soils. Several studies have documented the effectiveness of NIs in retaining soil NH₄⁺ and increasing crop yields, but less is known about the effects of NIs on the fate of urea–N and the overall impact of NIs on the soil microbial community.

In a 60 day soil mesocosm experiment, we investigated the effects of Nitrapyrin (NI; 2-chloro-6-(trichloromethyl)pyridine) co-applied with a selection of urea-based fertilizers: urea (U); U with urease inhibitors (U+UI); methylene-urea (MU); and zeolite-coated urea (ZU), on a typical Mediterranean soil under ambient summer conditions. We showed that NI applied with urea fertilizers resulted in a slower decay of extractable NH₄⁺ with a concurrent increase in NH₃ volatilization. Integrated measures of soil NH₄⁺ were 1.5 to 3.3-fold greater when NI was applied. At the same time, there was a 10 to 60% reduction in integrated measures of NO₃^- and NO₂^- when NI was applied with the tested fertilizer types, except MU fertilizer where the integrated measures of NO₃^- and NO₂^- doubled. Upon urea hydrolysis, the released NH₄⁺ was transformed to NO₃^- and NO₂^-, which subsequently decreased in concentration following a typical nitrification - denitrification pathway in the absence of plants. Soil N₂O emissions from urea fertilizers were reduced by 40% with UI, 50% with NI, and 66% with NI + UI.

Interestingly, 15 days after the application of NI, there was a decrease in bacterial abundance (eub genes; qPCR) in all fertilized treatments. NI dramatically reduced the abundance of ammonia-oxidizing microbes (amoA genes) and there were fewer bacteria associated with denitrification genes (nirK, nirS, nosZ) when NI was applied. 

At the end of the experiment, there was no significant difference in total N among all fertilized soils. Total N was in excess when compared to the control, and it was a considerable N pool potentially immobilized in microbial biomass in the absence of crops.

In conclusion, the use of NI doubled NH₄⁺ retention in the soil and decreased soil N₂O emission by 50%, through negatively affecting ammonia oxidizing and denitrifying microbes and subsequently reducing soil available NO₃^- and NO₂^-. The application of NIs should be carefully planned and synchronized (timing) with crop growth to reduce subsequent N transformations and N loss to the environment.

Keywords: urea, zeolite, methylene-urea, nitrification inhibitor, nitrapyrin, calcareous soil, soil nitrogen

How to cite: Giannopoulos, G., Elsgaard, L., Zanakis, G., B. Franklin, R., L. Brown, B., and Barbayannis, N.: Effects of the nitrification inhibitor nitrapyrin on urea-based fertilizers in a Mediterranean calcareous soil: N dynamics and microbial functional genes., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1763, https://doi.org/10.5194/egusphere-egu2020-1763, 2020.

Sources of N₂O (nitrous oxide) are multiple in the biosphere, but the only known process consuming N₂O is the microbial reduction of N₂O to dinitrogen (N₂), which has traditionally been attributed to denitrifying bacteria and archaea. Recently, N₂O reductase genes (nosZ) clearly phylogenetically differentiated from “typical” NosZ (Clade I) were shown to be more abundant in many soil ecosystems than “typical” nosZ genes, suggesting that our understanding of the role of nosZ in controlling soil N₂O emissions was incomplete. This more abundant group of nosZ genes was designated as “atypical NosZ” or Clade II.  Here, by synthesizing a meta-data of the 631 peer-reviewed papers published on NosZ in the six years since NosZ Clade II was first reported in the literature, we found that only 10% of studies evaluated Clade II NosZ and an additional 7% of papers merely mentioned Clade II NosZ showing little awareness of this novel gene. In addition, disciplinary silos also contribute to the slow spread of awareness about Clade II nosZ. A lack of consensus on the terminology used to refer to Clade I versus Clade II nosZ (more than 17 terminologies) may contribute to confusion about the two clades. Finally, we proposed several recommendations to accelerate progress in understanding the roles of Clade I versus Clade II N₂O reducers in controlling soil N₂O emissions.

How to cite: Shan, J., Ooi, S., Sanford, R. A., Chee-Sanford, J., Löffler, F., Konstantinidis, K., and Yang, W. H.: Advancing our understanding of novel nitrous oxide reducers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20866, https://doi.org/10.5194/egusphere-egu2020-20866, 2020.

Using reclaimed water as a resource for landscape water replenishment may alleviate the major problems of water resource shortages and water environment pollution. However, the safety of the water and the risk of eutrophication remain doubted by the public. Our study aimed to reveal the difference between natural water and reclaimed water and to discuss the rationality of reclaimed water replenishment from the perspective of microorganisms. We analyzed the microbial community structures in natural water, reclaimed water and natural biofilms and the community succession was clarified along the ecological niches, water resources, liquidity and time using 16S rRNA gene amplicon sequencing. Primary biofilms without the original community were added to study the formation of microbial community structures under reclaimed water acclimation. The results showed that the difference caused by ecological niches was more than those caused by the liquidity of water and different water resources. No significant difference was found in the microbial diversity and community structure caused by the addition of reclaimed water. Based on the microbial analysis, reclaimed water replenishment is a feasible solution that can be used for supplying river water. Innovatively, we introduced the study of biofilms and determined that the monitoring of biofilms or sediments closely related to water was also important for the early warning of water bloom, providing a unique perspective for the management of eutrophication.

How to cite: Li, J., Sun, Y., Wang, X., Yin, M., and Xu, S.: Formation and succession of microbial community structure in different ecological niches under reclaimed water acclimation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12613, https://doi.org/10.5194/egusphere-egu2020-12613, 2020.

Freshwater microalgae isolates from a UK headwater catchment (collected in 2017) were tested for their growth and media nitrogen speciation changes when presented with low molecular weight dissolved organic nitrogen compounds. The location has input from livestock run off increasing organic matter in stream. Experimental treatments and initial isolation took place in controlled culture cabinets kept at 15°C, with a 16:8 light:dark cycle and light c.a. 50 µmol m^-2 s^-1. Treatments included separately presented urea and glutamate, alongside negative (no N or P sources) and positive controls (nitrate or ammonium). Nitrogen addition treatments were provided with the same phosphorus source, trace minerals, trace metals and took place for two weeks. Different species isolated from the location showed optimal growth on different organic nitrogen sources. Organic nitrogen compounds caused growth at least comparable to inorganic sources. Cell growth was best on dissolved organic nitrogen compounds for some species. This relatively quick cycling of organic nitrogen compounds in river systems to photosynthetic growth has implications for ecosystem heath and capacity to mitigate organic nitrogen inputs. Anthropogenic activity that increases organic nitrogen may favour certain species compositions, altering downstream ecosystem functions such as algal bloom formation and dominant microalgae species. Further work will use stable isotope investigation of potential uptake mechanisms and wider work is required on understanding how the ecosystem may respond to organic nitrogen changes. 

How to cite: Bayliss, C. E., Johnes, P., Evershed, R. P., Sanchez-Baracaldo, P., and Maberly, S. C.: Responses to dissolved organic nitrogen compounds by recently isolated freshwater microalgae species, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5713, https://doi.org/10.5194/egusphere-egu2020-5713, 2020.

Magnetic-nanoparticle mediated isolation (MMI) is a new method for isolating active functional microbes from complex microorganisms without substrate labeling. In this study, the composition and properties of the magnetic nanoparticles （MNPs）were characterized by a number of techniques. And then the MNPs were added to activated sludge rich in ammonia nitrogen-degrading bacteria after long-term stable treatment,  another set of experiments plus urea was set as the only carbon source in the system. Compared with the group without MNPs, degradation experiment results showed that the ammonia nitrogen degradation ability of a group of MNPs was slightly improved. The high-throughput sequencing results showed that the addition of MNPs did not change the microbial community structure of activated sludge under long-term stable conditions, and that the addition of urea as a nitrogen source significantly changed the microbial community structure. RDA analysis results also showed that Comamonadaceae_unclassified and Thiobacillus absolutely dominated in situ ammonia degradation, and the change in dominant genera showed the same trend as the degradation rate of ammonia nitrogen. It has also proved that the complex flora after adding magnetic nanoparticles is more adaptable to newly introduced pollutants, using MMI to study pollutant-degrading microorganisms under in-situ conditions has a broad application prospect.

How to cite: Yin, M., Sun, Y., Zheng, D., Wang, L., Zhao, X., and Li, J.: Separating and characterizing functional nitrogen degraders via magnetic-nanoparticle mediated isolation technology in high concentration of ammonia nitrogen wastewater treatment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12833, https://doi.org/10.5194/egusphere-egu2020-12833, 2020.

The membrane rotary energy-yielding ATPases represent the cornerstone of cellular bioenergetics for all three domains of life. The archaeal ATPases (A-type ATPases) are functionally similar to the eukaryotic and bacterial F-type ATPases that catalyze ATP synthesis using a PMF. However, they are structurally more similar to the vacuolar-type (V-type) ATPases of eukaryotes and some bacteria that function as proton pumps driven by ATP hydrolysis. Significant variation in subunit composition, structure, and mechanism of the archaeal ATPases is thought to confer adaptive advantage in the variety of extreme environments that archaea inhabit.

The ammonia-oxidizing archaea are recognized to exert primary control of nitrification in the marine environment, are major contributors to soil nitrification, and have a habitat range extending from geothermal systems, to acidic soils and the oceanic abyss. The basis for their remarkable adaptive radiation is obscured by a relatively simple metabolism – autotrophic growth using ammonia for energy and nitrogen. In this study, we find that their adaptation to acidic habitats and the extreme pressures of the hadal zone of the ocean at depths below 6000 meters is correlated with horizontal transfer of a variant of the energy-yielding ATPase (atp) operon. Whereas the ATPase genealogy of neutrophilic soil and upper ocean pelagic AOA is congruent with their organismal genealogy inferred from concatenated conserved proteins, a common clade of V-type ATPases unites phylogenetically disparate clades of acidophilic and piezophilic species.

A function of the so-acquired V-ATPases in pumping excessive cytoplasmic protons at low pH is consistent with its increased expression by acid-tolerant AOA with decreasing pH. Consistently, heterologous expression of the thaumarchaeotal V-ATPase significantly increased the growth rate of E.coli at low pH. Additional support for adaptive significance derives from our observation that horizontal transfer is also associated with the adaptive radiation of Micrarchaeota, Parvarchaeota and Marsarchaeota into acidic environments. Their ATPases are affiliated with the acidophilic lineage ATPases of Thermoplasmatales and phylogenetically divergent from the corresponding species tree.

Another notable finding is that single hadopelagic AOA species contain both A- and V-type ATPases, suggesting that extensive horizontal transfer of atp operons is a highly active and ongoing process within AOA. The presence of an additional V-type ATPase in hadopelagic AOA may provide fitness advantages in the deep ocean with elevated hydrostatic pressure, as the proposed function of V-ATPase in pumping excessive cytoplasmic protons at high pressure may serve to maintain the cytosolic pH homeostasis in marine AOA.

Taken together, our study provides the first clear evidence of a significant role of horizontal transfer of atp operon in the adaptive radiation of AOA, one of the most successful organisms on Earth, and other archaeal species, spanning the TACK and DPANN superphyla as well as Euryarchaeota phylum.

How to cite: Wang, B. and Qin, W.: Archaea as Global Explorers: Let`s Exchange ATPase and Occupy More Extreme Habitats!, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20953, https://doi.org/10.5194/egusphere-egu2020-20953, 2020.

Bulk sedimentary nitrogen isotopes (δ¹⁵N) have been used as an accurate redox proxy in well-preserved sedimentary systems, however, fewer studies of N-isotope have been performed in lacustrine shales.  In this paper, we report the first δ¹⁵N data from the Chang 7 Shale from a core drilled in the Ordos Basin. Bulk δ¹⁵N values are significantly higher in Zone A (the Chang 7₃ and the lower part of the Chang 7₂ submembers, average = 9.4 ± 1.3‰) than in Zone B (the upper part of the Chang 7₂ and the Chang 7₁ submembers, average = 5.4 ± 1.5‰). Given the lithological characteristics and previous geochemical measurements, we suggest that sediments within Zone A of the Chang 7 Shale were mainly deposited under suboxic bottom water conditions, whereas Zone B sediments show evidence of deposition under oxic deep water regimes. Additionally, organic carbon isotopes (δ¹³C_org) and total nitrogen (TN) values were measured to characterize any processes that might control alteration of the bulk δ¹⁵N signal, including changes in organic matter source and post-depositional processes. Our results show that there is no significant difference in the organic carbon isotopes (δ¹³C_org) and total nitrogen (TN) values between the two zones. In conclusion, we suggest that the difference in δ¹⁵N values through the Chang 7 Shale primarily reflects differences in the depositional redox conditions and δ¹⁵N values of shale can provide important details regarding the depositional history of unconventional resource plays.

How to cite: Chen, R. and Liu, G.: Evaluating nitrogen isotopes as proxies for depositional redox conditions in shales: A case study from the Chang 7 Shale in the Ordos basin, North China., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1814, https://doi.org/10.5194/egusphere-egu2020-1814, 2020.