BG1.7

In order to assess peatland carbon sink potential under multiple global change perturbations, we examined the individual and combined effects of long-term warming and enhanced nitrogen (N) and sulfur (S) deposition on ecosystem CO₂ exchange at one of the longest-running experiments on peatlands, Degerö Stormyr poor fen, Sweden. The site has been treated with NH₄NO₃ (15 times ambient annual wet deposition), Na₂SO₄ (6 times ambient annual wet deposition) and elevated temperature (air +3.6 C) for 23 years. Gross photosynthesis, ecosystem respiration and net CO₂ exchange were measured weekly during June-August using chambers. After 23 years, two of the experimental perturbations: N addition and warming individually reduced net CO₂ uptake potential down to 0.3-0.4 fold compared to the control mainly due to lower gross photosynthesis. Under S only treatment ecosystem CO₂ fluxes were largely unaltered. In contrast, the combination of S and N deposition and warming led to a more pronounced effect and close to zero net CO₂ uptake potential or net C source. Our study emphasizes the value of the long-term multifactor experiments in examining the ecosystem responses: simultaneous perturbations can have nonadditive interactions that cannot be predicted based on individual responses and thus, must be studied in combination when evaluating feedback mechanisms to ecosystem C sink potential under global change.

How to cite: Larmola, T., Maanavilja, L., Kiheri, H., Nilsson, M., and Peichl, M.: Impacts of long-term warming and enhanced nitrogen and sulfur deposition on carbon sink potential in a boreal peatland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1210, https://doi.org/10.5194/egusphere-egu21-1210, 2021.

Dissimilatory nitrate (NO₃⁻) reduction to ammonium (DNRA) and denitrification (DNF) are major dissimilatory NO₃⁻ reduction processes, competing for the available NO₃⁻ under anoxic conditions. The competition among these processes leads to different fates of NO₃⁻ in soil, i.e., loss of nitrogen (N) as benign N₂ or potent greenhouse gas (nitrous oxide, N₂O), or retaining of N by converting NO₃⁻ to ammonium. Unfortunately, little is known about the soil-environmental factors controlling the NO₃⁻ partition. Here we report DNF and DNRA in soils from the top and bottom of the hillslope.

We sampled soils from a hillslope of forest to generate a soil-environmental gradient. The soil-environmental factors including soil pH, available carbon (potassium chloride-extractable organic carbon: EOC), NO₃⁻, and microbial C and N (MBC and MBN) were determined. We incubated the soils under anoxic condition (i.e., helium atmosphere) and applied a ¹⁵N isotope pairing technique to quantify the potential rates of DNRA and DNF. Briefly, we incubated the soil under anoxic condition (i.e., helium atmosphere) to remove any N oxides and oxygen, then we added ¹⁵NO₃⁻ (99.9%) and measured the production rates of ¹⁵NH₄⁺, ³⁰N₂, and ⁴⁶N₂O.

The results showed that (1) a good gradient of the soil-environmental variables was observed along the hillslope from top to bottom, including pH (top–bottom; 3.95–4.78), EOC:NO₃⁻ (184–18.7), and MBC: MBN (8.2–6.3); (2) DNRA rate tended to be higher at the top of the hillslope with DNF being nearly inactive, resulting in a dominance of DNRA (59–97%), while the trend was reversed at the bottom, with DNF rates being much higher and dominantly contributing to NO₃⁻ reduction (89–97%); and (3) during DNF process, the magnitude of N₂O production rates was comparable or even higher than that of N₂ in the soils from the bottom of the hillslope. The ratio of the N₂O to N₂ production (N₂O:N₂) was much higher in the soils from the top despite the low DNF rates.

The remarkably different patterns of DNRA and DNF rates and relative contributions between the top and bottom of the hillslope are controlled by the EOC:NO₃⁻: DNRA was preferred over DNF when NO₃⁻ was limited (i.e., high EOC:NO₃⁻) because more free energy is liberated per unit of NO₃⁻ reduced for DNRA as compared to DNF. The substantial production of N₂O at the bottom of the hillslope indicates that previous studies that considered only ³⁰N₂ production rate could have highly underestimated the DNF rate. The high N₂O:N₂ at the top is likely caused by the low pH as well as the dominance of fungi, of which the N₂O reductase is generally lacking, pointing to the key roles of soil pH and microbial community structure in regulating the product stoichiometry of N₂O and N₂ in DNF.

How to cite: Du, J., Zheng, J., Shibata, M., Wang, Z., Watanabe, T., and Funakawa, S.: Dissimilatory nitrate reduction processes along a forest hillslope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10536, https://doi.org/10.5194/egusphere-egu21-10536, 2021.

Some studies show that photolysis of nitrate and deposited nitrate and gas nitric acid (HNO₃) on the ground surface is much faster than that of HNO₃. The former mechanism has been considered as a possible daytime HONO source and discussed in many laboratory and field studies. Although this mechanism is also coupled into some three-dimensional chemical transport models, the effect of large changes in the ratio of photolysis rate of nitrate to that of HNO₃ (RAT) on HONO concentrations has not been assessed and will be discussed here by using the updated WRF-Chem model. Simulations indicate that in the morning, this mechanism only resulted in a HONO increase of a few ppt, while the heterogeneous reaction of NO₂ enhanced HONO by about 150 ppt; in the afternoon, however, this mechanism led to a significant HONO increase, with its contribution to HONO concentrations being close to the contribution of the heterogeneous reaction of NO₂. In some heavily nitrate-polluted areas, this mechanism contributed more than 80% of HONO concentrations during the afternoon. Large changes in RAT produced a substantial impact on HONO concentrations. When RAT was altered from 15 to 100, increase of HONO concentrations was enhanced by about 6 times. Our results suggest that more laboratory and field studies on the photolysis rates of nitrate and deposited nitrate and HNO₃ on the ground surface are still needed.

How to cite: Guo, Y., An, J., and Zhang, J.: Assessment of the impact of large changes in the ratio of photolysis rate of nitrate to that of gas nitric acid on HONO concentrations simulated by a 3-D chemical transport model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2471, https://doi.org/10.5194/egusphere-egu21-2471, 2021.

Plastic-shed vegetable production system is becoming the main type of vegetable production in China, while excessive irrigation and fertilization input lead to significant N loss by leaching, runoff, and gaseous N. The current study established a field experiment to investigate the effects of drip irrigation and optimized fertilization on vegetable yield, water and fertilizer efficiencies and N₂O emission in a typical intensive plastic-shed tomato production region of China. The treatments include CK (no fertilization, flood irrigation), FFP (farmers’ conventional fertilization, flood irrigation), OPT1 (80% of FFP fertilization, flood irrigation), OPT2 (80% of FFP fertilization, drip irrigation). N₂O isotopocule deltas, including δ¹⁵N^bulk, δ¹⁸O and SP (the ¹⁵N site preference in N₂O), have been used to investigate microbial pathways of N₂O production under different treatments. Our results showed: i) optimized fertilization and drip irrigation significantly improved the fertilizer and water use efficiency without reducing tomato yield, ii) compared with flood irrigation, drip irrigation decreased soil WFPS and soil ammonium content, but increased soil nitrate content. When soil moisture was higher than 60%WFPS, drip irrigation led to a decrease of N₂O emission with lower N₂O SP signature observed than that of food irrigation, suggesting a reduction of denitrification derived N₂O. In contrast, drip irrigation significantly increased N₂O emission and N₂O SP value when soil moisture status was lower than 55% WFPS, which may be due to the enhanced nitrification or fungal denitrification derived N₂O.

How to cite: Xu, X., Wu, D., Zhang, W., Ni, B., Yang, X., and Meng, F.: Drip irrigation affects N2O emission differently depending on soil moisture status in an intensive vegetable production system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15035, https://doi.org/10.5194/egusphere-egu21-15035, 2021.

Pyrite is the most common sulfide mineral occurring in sedimentary and igneous rocks and globally contributes a greater flux of sulfate. Large quantity of reactive nitrogen as fertilizers for agricultural production has been released into the environment in China over recent decades. Sulfuric acid formed by oxidative weathering of pyrite (OWP) and nitric acid formed by oxidation of reducing nitrogen fertilizer (ONF) through neutralization with carbonate minerals can counteract CO₂ drawdown from chemical weathering. Here, we use the multiple isotopes (¹³C-DIC, ³⁴S and ¹⁸O-SO₄^2–, ¹⁵N and ¹⁸O-NO₃^–, and ¹⁸O and D-H₂O) and water chemistry, as well as historical hydrochemical data to assess the roles of strong acids in chemical weathering and the carbon cycle in a karst river system (Chishui River, southwestern China). The variations in alkalinity and the δ¹³C-DIC along with theoretical mixing models demonstrate the involvement of strong acids in carbonate weathering. However, the strong acid weathering flux determined by δ¹³C-DIC and mixing models is considered to be overestimated due to the effects of photosynthesis and degassing of CO₂ on δ¹³C-DIC signal. The protons liberated from OWP and ONF can be constrained by water chemistry and isotope techniques with the use of a Bayesian isotope mixing model. The strong acid weathering flux determined using proton information is higher that determined by δ¹³C-DIC and mixing models. This suggests that the additional protons derived from OWP and ONF might be consumed in other ways without affecting the δ¹³C-DIC signals, such as the neutralization of acidic waters. These results indicate that OWP and ONF coupled with carbonate dissolution significantly enhanced the coupling cycles of carbon, nitrogen and sulfur in this river system.

How to cite: Xu, S., Li, S.-L., Su, J., Yue, F.-J., Zhong, J., and Chen, S.: Assessing the effects of oxidation of pyrite and reducing nitrogen fertilizer on chemical weathering and the carbon cycles in a karst river system by using multiple isotopes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3682, https://doi.org/10.5194/egusphere-egu21-3682, 2021.

The recent discovery of comammox Nitrospira performing complete ammonia oxidation to nitrate has fundamentally renewed the 120-year-held perspective of “two-step” nitrification. Rivers are known as the “Arteries” of the Earth, coupling the biogeochemical cycling of continents and oceans. Frequent human activities usually increase nitrogen load, and nitrifying microorganisms are crucial for the management of nitrogen load in rivers. The ecological roles of truncated nitrifiers, including canonical ammonia-oxidizing bacteria, ammonia-oxidizing archaea and nitrite-oxidizing bacteria in rivers have been fully understood, however, investigations of the newly discovered comammox Nitrospira are very scarce. To fill this gap, we used the metagenomic shotgun sequencing to provide the first biogeographic patterns of comammox Nitrospira in the Yangtze River over a 6030 km continuum.

First, ten novel comammox genomes (71~96% completeness) were reconstructed with the metagenome assemblies from fluvial water in the upper reach and surface sediments from the middle reach to the estuary. Gene arrangements in ammonia oxidation-related gene clusters of these novel genomes were more complex and diverse than those of the previously discovered ones. For instance, multi-copy amoA or amoB genes, peptidases, cupredoxin and fkpA-cytochrome c-nirK gene sets were first found within the ammonia oxidation-related gene clusters of comammox Nitrospira, which might confer them advantages in adapting to the relatively oligotrophic environments and stabilizing the ammonia-oxidation process in rivers. Taxonomic analysis demonstrated that all riverine comammox genomes (constituting four new species) belonged to clade A. Based on the phylogenies of their 37 “elite” conserved marker genes, we further separated all reported comammox clade A into five sublineages, named clade A-Ia, A-Ib, A-Ic and A-IIa, A-IIb. The reclassified sublineages were sufficiently divergent to be meaningful in expanding the taxonomic/functional diversity and improving the phylogenetic resolution.

Second, based on the improved phylogenetic resolution, we explored the biogeographic patterns of planktonic and benthic comammox Nitrospira subjected to natural and anthropogenic factors along the Yangtze River. Our study revealed the wide existence of comammox Nitrospira and their significant contributions to nitrifier abundances, constituting 30% and 46% of ammonia-oxidizing prokaryotes (AOPs) and displaying 30.4- and 17.9-fold greater abundances than canonical Nitrospira representatives in water and sediments, respectively. Comammox Nitrospira were found to contribute more to nitrifier abundances (34~87% of AOPs) in typical oligotrophic environments with a higher pH and lower temperature, particularly in the plateau (clade B), mountain and foothill (clade A-Ic) of the upper reach. Environmental selection determined the niche replacement of planktonic comammox Nitrospira by canonical ammonia-oxidizing bacteria and Nitrospira sublineages I/II from upstream to downstream, leading to a higher spatial turnover rate than observed for the benthic counterpart, while the dissimilarity of benthic comammox Nitrospira was moderately driven by geographic distance. A considerable decrease (83%) in benthic comammox Nitrospira abundance occurred immediately downstream of the Three Gorges Dam, which was consistent with a substantial decrease in the overall bacterial taxa in sediments.

Together, this study highlights the previously unrecognized dominance of comammox Nitrospira in major river systems and underlines the importance of revisiting the distributions of and controls on nitrification processes within global freshwater environments.

How to cite: Liu, S., Wang, H., and Ni, J.: Biogeographic patterns of complete ammonia oxidizers (comammox) within the Yangtze River continuum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7121, https://doi.org/10.5194/egusphere-egu21-7121, 2021.

Terrestrial ecosystems worldwide are experiencing increasing atmospheric nitrogen (N) deposition because of fossil-fuel combustion and fertilizer applications. As a C₄ feed crop, sweet sorghum (Sorghum bicolor L.) is widely used in the arid region of China since its high sugar content, good palatability and high yield. However, impacts of atmospheric N deposition on production of sweet sorghum are poorly understood in arid land ecosystems where soils are typically low in plant available N. At Hui Autonomous Region, Ningxia, China, a complete random block design was used to study the effects of four levels of N additions (45, 169, 197, and 224 kg ‍‌‌‍ha^-1 year^-1) on sorghum, node number, stem diameter, leaf number, plant height, yield per plant, dry matter, and sugar Brix of stem. Nitrogen application significantly affected the above parameters. When the amount of N applied was 224 kg ‍‌‌‍ha^-1 year^-1, the plant height (mean ± standard deviation, 256.9 cm ± 10.7, n=9), stem diameter (16.9 mm ± 1.1 ,n=9), number of leaf (10.8 ± 1.3, n=6) and node (4.9 ± 0.4, n=9), and dry matter per unit area (1.48 t ha^-1 ± 0.3, n=9) was highest. While N application did not affect sugar Brix of stem. Therefore, N deposition plays a linearly positive role in enhancing the productivity of sweet sorghum in the arid region of China.

Keywords: Agronomic traits, C₄ plant, Feed crop, Nitrogen addition

How to cite: Liu, W., Sun, L., Lan, J., and Li, Y.: Nitrogen deposition enhanced productivity of sweet sorghum (Sorghum bicolor L.) in the arid region of China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10759, https://doi.org/10.5194/egusphere-egu21-10759, 2021.

Increasing evidence suggests that alkaline mineral amendments from industrial wastes (e.g., phosphorus tailing, PT) have potential to ameliorate soil acidification and improve agriculture sustainability. However, the effects of such amendment on nitrous oxide (N₂O) production remain elusive. To fill this knowledge gap, an incubation experiments were conducted with an acidic soil of pH 4.80 treated with i) control (CK, no amendments), ii) urea at 60 mg N kg^-1 (U), iii) 10 g kg^-1 amendments (S) and iv) 10 g kg^-1 amendments plus urea at 60 mg N kg^-1 (S+U). A 184-h experiment was conducted with a robotized incubation system for monitoring real-time gases (O₂, N₂O, N₂, CO₂) dynamics. Results from this batch experiment showed that the soil pH was significantly increased with the amendment addition (S and S+U) from 4.80 to above 6.00. Meanwhile, the mineralization, nitrification and denitrification processes were stimulated with the amendment addition. The N₂O production was reduced by an average of 65.7% with the amendment addition compared to that without the amendment application. After incubation, higher N₂ productions were observed from the soil with amendment addition (S and S+U) than untreated soil (P<0.05). The findings suggest that the N₂O emissions from acidic soils can be considerably controlled by valorization of PT.

How to cite: Yin, J., Liu, R., and Chen, Q.: Mitigation of nitrous oxide emissions from an acidic soil by valorization of industrial waste, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14027, https://doi.org/10.5194/egusphere-egu21-14027, 2021.

A common approach to reduce N loss is inclusion of inhibitors with the fertilizer formulation. However, the performance of commercially available inhibitors is often highly unpredictable, with efficacies lasting for few weeks or even only days, for reasons that are still poorly understood. Furthermore, how repeated use of these inhibitors affect soil microbes and nitrogen (N) transformation remains elusive. In the current study, we investigated the response of the community compositions of critical N-cycling biomarkers involved in nitrification and denitrification (including amoA, nxrA, nxrB, narG, nosZ), potential nitrification rates (PNR) and N₂O emissions to repeated addition of urease and nitrification inhibitors (nBPT, DMPP) based on a 4-year field experiment. The results demonstrated that 4 years of fertilization significantly affected the community structure of nitrifiers, except AOA. Repeated addition of inhibitors significantly reduced N₂O emissions (P ˂ 0.05) and changed the community structure of bacterial-amoA, nxrB, narG and nosZ compared to urea application alone (P ˂ 0.05), but the effects of DMPP and nBPT on N₂O emissions and soil microbes were different. Inhibitors had no significant effect on the soil PNR and archaeral-amoA and nxrA. Based on structural equation modeling, amendment with the inhibitors reduced N₂O emissions through directly affecting NH₄⁺-N substrate availability for soil microbes and subsequently changing community composition of AOB. These findings help to disentangle the interplay of nBPT and DMPP with soil microbes and N₂O emissions in North China Plain.

How to cite: Zhang, W., Liu, R., and Meng, F.: How repeated use of urease and nitrification inhibitors affect soil microbes and N2O emissions in North China Plain?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15030, https://doi.org/10.5194/egusphere-egu21-15030, 2021.

One of the most important measures to reduce ammonia (NH₃) and nitrous oxide (N₂O) fluxes from crop production is the adoption of low-emission application techniques for slurry. Application techniques may also impact dinitrogen (N₂) emission, as they can influence denitrification activity by changing slurry and soil aeration (e.g. by injection techniques), nitrate formation (e.g. by adding nitrification inhibitors) and the pH value (e.g. by slurry acidification). However, measuring N₂ emissions and following pathways of slurry nitrogen (N) transformation under field conditions is still challenging.

Thus, we set up a field experiment using undisturbed soil cores with growing winter wheat as small lysimeters. Cattle slurry treatments include the following application techniques: trailing hose with and without acidification (H₂SO₄), slot injection with and without nitrification inhibitor (DMPP). Soil cores without slurry application were used as control. In a first step, soil nitrate was ¹⁵N labelled by homogeneous injection of a K¹⁵NO₃ solution (98 at% ¹⁵N, equal to 4 kg N ha^-1). One week later, we applied 72 kg N ha^-115N-labelled slurry (NH₄⁺ labelled at 65 at% ¹⁵N). NH₃ emissions were measured by Dräger-Tube method (Pacholski, 2016). N₂O and N₂ emission were measured using the ¹⁵N gas flux method with N₂-depleted atmosphere (Well et al., 2018). To close the N balance and follow the different N transformation pathways, ¹⁵N losses by leaching, ¹⁵N uptake by plant and residual ¹⁵N in roots, plant residues, microbial biomass and soil were analysed by IRMS.

N₂O emission were very low (up to 0.1 kg N₂O-N ha^-1) and not significantly different between treatments during the experimental period of 60 days. Since the N₂O/(N₂+N₂O) ratio of denitrification (N₂Oi) was also very low, most labelled N was lost via N₂ (up to 3 kg N ha^-1). Nevertheless, the major gaseous loss pathway was NH₃ with up to 8 kg N ha^-1 in the trailing hose treatment. Slot injection significantly reduced NH₃ emission, while N leaching losses were up 5 kg N ha^-1. Recovery of ¹⁵N was higher in the soil N pool (32-48 %) than in plants (19-37 %) and roots (5-7 %). Total ¹⁵N recovery was almost complete, indicating that the experiment was able to catch the relevant N pathways.

References:

Pacholski, A., 2016. Calibrated passive sampling-multi-plot field measurements of NH₃ emissions with a combination of dynamic tube method and passive samplers. Journal of visualized experiments: JoVE 109, e53273.

Well, R., Burkart, S., Giesemann, A., Grosz, B., Köster, J., Lewicka-Szczebak, D., 2018. Improvement of the ¹⁵N gas flux method for in situ measurement of soil denitrification and its product stoichiometry. Rapid Communications in Mass Spectrometry 33, 437–448.

How to cite: Buchen-Tschiskale, C., Flessa, H., and Well, R.: Applying slurry with different techniques in spring – which pathway does the nitrogen take?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-674, https://doi.org/10.5194/egusphere-egu21-674, 2021.

Fertilizing arable soils with liquid manures affects gaseous N losses to the atmosphere including NO, N₂O and N₂ as well as nitrate leaching. These emissions impair nitrogen use efficiency of crops and contribute to the greenhouse effect and stratospheric ozone destruction and pollution of aquatic resources. Their extent depends on the complex interaction between manure application techniques and properties of manures and soil. Whereas the types of manure effects on N transformations and associated gaseous fluxes are known, their prediction is still poor because previous investigations mostly excluded N₂ flux.

Our mesocosm experiment addresses the questions, (1) how liquid manure fertilization and its application mode impact N₂, N₂O and CO₂ fluxes from agricultural soil, and (2) how the water and dissolve organic carbon content and the pH of the manure amended soil change between the soil layers. We use these data to set up a dataset to test and develop new biogeochemical model approach to describe the manure-soil interactions.

A sandy arable soil (Fuhrberg, Germany) was used for the experiments and amended with artificial slurry (artificial liquid and cow digestate mixture) in various treatments. The soil was incubated in laboratory incubation systems over 10 days. N₂, N₂O and CO₂ fluxes were quantified by gas chromatography and isotope-ratio mass spectrometry. Incubations were conducted with (surface or injected application) or without (control) of slurry treatment and initial water content was adjusted equivalent to 40% and 60% water-filled pore space. The environmental conditions were kept constant during that experiment.

The average N₂+N₂O flux decreased at the 40% WFPS surface and injected treatments with 70% and 60%, respectively, compared to the non-fertilized control. For the 60% WFPS surface and injected treatments, the average N₂+N₂O flux increased with more than 610% and 1690%, respectively. The results show that the initial water content and the application method can influence drastically the N₂+N₂O flux of the manure amended soil.

How to cite: Grosz, B., Well, R., and Burkart, S.: The impact of liquid organic fertilization and associated application techniques on N2 and N2O fluxes from agricultural soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8305, https://doi.org/10.5194/egusphere-egu21-8305, 2021.

How to cite: Stoll, E., Harris, E., Diaz-Pines, E., Reinthaler, D., Radolinski, J., Glatzel, S., Zechmeister-Boltenstern, S., Pötsch, E., and Bahn, M.: Using online N2O isotopic measurements to understand grassland N2O emission processes in a changing climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5125, https://doi.org/10.5194/egusphere-egu21-5125, 2021.

Diurnal variations in soil nitrous oxide (N_­2O) emissions have been documented for nearly four decades yet consensus on their significance is still lacking. Resolving this question is important as soil N₂O emissions have some of the highest uncertainties in national greenhouse gas inventories. A major challenge for understanding diurnal variation is that conventional measurements rarely operate at temporal frequencies that can observe and report this phenomenon. Some higher frequency studies have observed daytime peaking of soil N_­2O emissions and often ascribe it to the diurnal oscillation of soil temperature. However, night-time peaking and irregular diurnal N_­2O patterns have also been reported in a number of studies.

To investigate the prevalence and characteristics of diurnal N_­2O variability, we systematically reviewed published studies that measured N₂O at high temporal frequencies (≥ 5 times/day). We identified 46 published studies covering cropland, grassland and forest soils; and extracted sub-daily N_­2O flux data and other soil parameters, yielding 286 individual days of data. Diurnal variability of N_­2O emissions were found in ~80% of the data, with ~60% peaking during the day and ~20% at night. Diurnal N_­2O patterns were associated with non-diurnal factors including soil texture and land use but the relationship between soil temperature and N_­2O flux was inconsistent, with strong positive correlations (R > 0.7) only found in one-third of the datasets.

This talk explores the implications of the review results on the time of sampling using conventional approaches (single time-point flux measurements), and the potential drivers of diurnal N_­2O variations for future research. In addition, this talk will also introduce a novel automated measurement technique allowing flux measurements at high temporal resolutions and how its application could enable experimental investigations of potential drivers of diurnal N_­2O variability.

How to cite: Wu, Y. F., McNamara, N., Whitaker, J., and Toet, S.: Investigating the diurnal variability of nitrous oxide emissions from soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12807, https://doi.org/10.5194/egusphere-egu21-12807, 2021.

The main prerequisites for denitrification are availability of nitrate (NO₃^-) and easily decomposable organic substances, and oxygen deficiency. Growing plants modify all these parameters and may thus play an important role in regulating denitrification. Previous studies investigating plant root effects on denitrification have found contradictive results. Both increased and decreased denitrification in the presence of plants have been reported and were associated with higher C_org or lower NO₃^- availability, respectively. Accordingly, it is still unclear whether growing plants stimulate denitrification through root exudation or restrict it through NO₃^- uptake. Furthermore, reliable measurements of N₂ fluxes and N₂O/(N₂O+N₂) ratios in the presence of plants are scarce.

Therefore, we conducted a double labeling pot experiment with either maize (Zea mays L.) or cup plant (Silphium perfoliatum L.) of the same age but differing in size of their shoot and root systems. The ¹⁵N gas flux method was applied to directly quantify N₂O and N₂ fluxes in situ. To link denitrification with available C in the rhizosphere, ¹³CO₂ pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil.

Plant water uptake was a main factor controlling soil moisture and, thus, daily N₂O+N₂ fluxes, cumulative N emissions, and N₂O production pathways. However, N fluxes remained on a low level when NO₃^- availability was low due to rapid plant N uptake. Only when both N and water uptake were low, high NO₃^- availability and high soil moisture led to strongly increased denitrification-derived N losses.

Total CO₂ efflux was positively correlated with root dry matter, but there was no indication of any relationship between recovered ¹³C from root exudation and 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 limiting denitrification due to plant N uptake. Overall, we conclude that root-derived C stimulates denitrification only when soil NO₃^- is not limited and low O₂ concentrations enable denitrification. Thus, root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.

How to cite: Rummel, P. S., Well, R., Pfeiffer, B., Dittert, K., Floßmann, S., and Pausch, J.: Nitrate and water uptake, rather than rhizodeposition, control denitrification in the presence of growing plants, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1223, https://doi.org/10.5194/egusphere-egu21-1223, 2021.

Accurate models of soil N cycling are an important tool for optimizing N use efficiency within agricultural systems and predicting N emissions to the environment. However, within such models, denitrification remains a challenge to describe and predict. One issue in achieving accurate denitrification estimates is the limited number of soil N₂ flux datasets that can be used to validate model estimates. Measurements of soil denitrification, which include both N₂O and N₂ fluxes, are challenging, however, due to methodological limitations for the measurement of N₂ and the heterogeneity of denitrification in soils.

As part of the DFG-research unit “Denitrification in Agricultural Soils: Integrated Control and Modelling at Various Scales (DASIM)”, we are building on previous data collected from laboratory incubations to take in situ field measurements. We use soil flushing and stable isotope techniques, combined with real-time monitoring of soil conditions, to assess the response of soil denitrification to a variety of control factors. These include: soil texture, previous crop, irrigation, and fertilizer application, in addition to the ambient changes in field conditions over one growing season of winter wheat. Both natural abundance and ¹⁵N labeling of the soil mineral N pool will be used to assess denitrification pathways.

Here we introduce the experimental field setup, summarize key elements of method testing and present early results of N₂ and N₂O. Once complete, this data set will provide valuable insight into the temporal and spatial heterogeneity of denitrification in agricultural soils. Data will be used to calibrate newly developed DASIM models as well as denitrification sub-modules of existing biogeochemical models.

How to cite: Matson, A., Lempio, D., Höppner, F., and Well, R.: Field measurements of soil denitrification using 15N gas flux, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12482, https://doi.org/10.5194/egusphere-egu21-12482, 2021.

Nitrous oxide (N2O) is a major greenhouse gas whose presence in atmosphere is continuously increasing. Hence it’s important to understand its production and consumption mechanisms. During the summer of 2020, we conducted lab experiments using heavy nitrogen tracers of Potassium Nitrate 15N 98% atom (Sigma Aldrich) and Ammonium Chloride 15N 98% atom (Sigma Aldrich) under different moisture conditions to get an insight into N2O production mechanisms and on their dependence on soil moisture. We applied the tracer to peat samples (Kärevere, Estonia) placed in 36 (12 control, 12 nitrate treatment & 12 ammonia treatment) plastic buckets (radius-10cm, height-20cm) with soil height of 10 cm and a 10 cm head space left for gas collection. We installed oxygen sensors, water table indicators and temperature sensors on all buckets. We focused on studying physical conditions (soil oxygen, temperature, water table and soil moisture), gas (N2O) emission data, soil chemistry, gas isotope 15N, soil isotope and soil microbiology to get a complete picture of the processes involved in production of N2O gas. Under the ammonia treatment, emissions increased more than ten-fold which could be due to multiple processes of the nitrogen cycle in play. N2O emissions increased as the oxygen conditions shifted from anoxic (Omg/L=0) to sub-oxic (Omg/L=0.5–6) and then decreased as oxygen conditions reached the oxic (Omg/L>6) state. Furthermore, we witnessed negative site preference and 18O values during the nitrate treatment indicating nitrifier-denitrification. Under the ammonia treatment, we recorded both negative as well as high positive site preference values indicating presence of multiple production mechanisms. This was expected as ammonia triggers multiple processes in the nitrogen cycle. In some samples, we observed N2O consumption with little change in site preference as compared to the N2O producing samples. This indicates some bacterial-denitrification along with the prevailing nitrifier-denitrification. We also observed that under both treatments, heavy oxygen increased with increasing site preference. This indicates reduction of N2O (Ostrom et al, 2007) as redox supports 15N and 18O enrichments. After these lab experiments, we conducted the same experiment at a large scale in a drained peatland forest in Agali, Estonia. In this experiment, we established 1m2 triangle-shape mesocosms using experimental draining and flooding to achieve varying oxygen conditions. Preliminary results of qPCR analysis of N-cycle control genes support the domination of ammonia oxidation and denitrification as sources of N2O.

How to cite: Masta, M., Gadegaonkar, S., Sepp, H., Espenberg, M., Pärn, J. P., Kirsimäe, K., and Mander, Ü.: Isotope and microbiome analysis indicates variety of N-cycle processes controlling N2O fluxes in a drained peatland forest soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10809, https://doi.org/10.5194/egusphere-egu21-10809, 2021.

Coastal wetlands, including mangrove and melaleuca forests, are globally important due to their capacity in sequestering carbon and nitrogen, and intercepting nutrient pollution from vast, nutrient-rich, tropical riverine networks. Despite this, the environmental drivers controlling soil biogeochemistry in these ecosystems remain poorly understood. Here we conducted a study across gradients of restoration and land-use in the mangrove forest of Xuan Thuy National Park in the Red River Delta, northern Vietnam and the melaleuca forest of U Minh Thuong National Park in the Mekong River Delta, southern Vietnam. We investigated nitrogen transformation processes and greenhouse gas production in mangrove and melaleuca forest soils using a ¹⁵N-Gas flux method to determine rates of denitrification, and its relative contribution to soil N₂O emissions. We found that denitrification was a more dominant source of N₂O in the melaleuca soils, despite higher rates of denitrification in the mangrove soils resulting from more complete denitrification in the mangroves. N₂O and CO₂ emissions were significantly higher from the melaleuca soils. Disturbance and subsequent recovery or restoration of these forests did not have a significant effect on soil biogeochemistry. The mangrove system, therefore, may remove excess nitrogen and improve water quality while maintaining low emissions of greenhouse gases whereas melaleucas process nutrients at a cost of N₂O and CO₂ emissions. Melaleucas, however, may act as a significant CH₄ sink at least partially balancing these emissions.

How to cite: Comer-Warner, S., Nguyen, A., Nguyen, M., Wang, M., Turner, A., Sgouridis, F., Krause, S., Le, H., Kettridge, N., Nguyen, N., and Ullah, S.: The role of land-use change and restoration on nitrogen processing in tropical coastal wetlands of Vietnam, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1619, https://doi.org/10.5194/egusphere-egu21-1619, 2021.

Treatment wetlands are widespread measures to reduce agricultural diffuse pollution. Systems that are often planted with emergent macrophytes such as Typha spp. and Phragmites spp. are efficient to reduce nutrients, particularly nitrogen and phosphorus compounds. While many experiments have been conducted to study the emission of carbon dioxide (CO₂) and methane (CH₄), little attention has been paid for the emission of nitrous oxide (N₂O). Few studies have been shown that usually N₂O emission from water saturated ecosystems such as wetlands is low to negligible. In Vända in-stream treatment wetland that was built in 2015 and located in southern Estonia, we carried out first long term N₂O measurements using floating chambers. The total area of the wetland is roughly .5 ha; 12 boardwalks, each equipped with two sampling spots, were created. Samples were collected biweekly from March 2019 through January 2021. In each sampling campaign water table depth, water and air temperature, O₂ concentration, oxygen reduction potential, pH and electrical conductivity were registered. Water samples for TN, NO₃-N, NO₂-N, TOC, TIC and TC were collected from inflow and outflow of the system in each sampling session and the average concentrations were 5.1 mg/L, 3.68 mg/L, <0.1 mg/L, 41.2 mg/L and 28.7, respectively. Our results showed a very high variability of N₂O emission: the fluxes ranged from -4.5 ug m^-2 h^-1 to 2674.2 ug m^-2 h^-1 with mean emission of 97.3 ug m^-2 h^-1. Based on gas samples (n=687) we saw a strong correlation (R² = -0.38, p<0.0001) between N₂O emission and water depth. The average N₂O emission from sections with the water table depth >15 cm was 45.9 ug m^-2 h^-1 while sections with water table depth <15 cm showed average emission of 648.3 ug m^-2 h^-1. The difference between these areas was more than 10 times. Water temperature that is often considered as the main driver had less effect to the N₂O emission. For instance, at lower temperatures, when the emissions from deeper zones decreased, there was no temperature effect on emissions from shallow zones. We also saw that over the years the overall N₂O emission followed clear seasonal dynamics and has a slight trend towards lower emissions. This can be related to the more intensive vegetation growth that has been increased from ~40% in 2019 to approximately 90% in 2020. Our study demonstrates that the design of the wetland is not only important for the water treatment, but it can also determine the magnitude of greenhouse gas emissions. We saw that even slight changes in water table depth can have a significant effect on the annual N₂O emission. Thus, in-stream treatment wetlands that have water table depth at least 15 cm likely have remarkably lower N₂O emissions without losing water treatment efficiency.

How to cite: Kasak, K., Kill, K., Uuemaa, E., and Mander, Ü.: Design of the treatment wetland determines nitrous oxide emission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7340, https://doi.org/10.5194/egusphere-egu21-7340, 2021.

Stable isotope fingerprinting is widely applied to plant-soil-groundwater systems in an aim to identify and even quantify the sources of nitrates found in groundwater. Frequently, in such studies, the δ¹⁵N and δ¹⁸O values of nitrogen sources, such as inorganic fertilizers and manure, are directly compared to the isotope signatures of nitrate encountered in groundwater bodies below agricultural watersheds. We submit that the underlying assumptions (conservative behavior of isotope composition, rapid transfer from surface to groundwater) may only be realistic under very specific conditions whereas, in most cases, significant isotope effects exerted by the soil-microbial-plant system on the δ¹⁵N and δ¹⁸O values of nitrate need to be taken into account when attempting a quantitative apportionment of sources of groundwater nitrate.

We hypothesise that the isotopic signature of nitrate exported from below the root zone and migrating towards the groundwater will reflect the nitrogen isotope composition of the soil organic N pool, rather than the isotope composition of source fertilizer or organic amendments, due to processes that reset source isotope compositions within soil N pools. We test this hypothesis using empirical observations from a diversity of settings, in France, Spain and Canada with a relatively constant historic anthropogenic N source or a simple and well constrained landuse history. Furthermore, through the use of a process-based model (SIMSONIC, Billy et al., 2010) we estimate to what extent the isotopic composition of the predominant N input to the soil-microbial-plant system and the soil N pool has been modified in an attempt to consider these changes in source apportionment studies elucidating the sources of groundwater nitrate.

This research was supported through the Consortium award MUTUAL, by the LE STUDIUM® Loire Valley Institute for Advanced Studies via its SMART LOIRE VALLEY (SLV) fellowship programme, co-funded by the H2020 Marie Sklodowska-Curie programme, Contract No. 665790.

Billy C., Billen G., Sebilo M., Birgand F., Tournebize J. (2010) Nitrogen isotopic composition of leached nitrate and soil organic matter as an indicator of denitrification in a sloping drained agricultural plot and adjacent uncultivated riparian buffer strips. Soil Biology and Biochemistry, 42, 108-117.

How to cite: Otero, N., Sebilo, M., Mayer, B., Gooddy, D., Lapworth, D., Surridge, B., Petelet-Giraud, E., and Kloppmann, W.: Modelling the impact of soil processes on the N and O isotope signatures of nitrate in groundwater, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12432, https://doi.org/10.5194/egusphere-egu21-12432, 2021.