BG1.3

Nitrate contamination in groundwater is affected by both anthropogenic activities and natural conditions, becoming one of the most prevalent problems worldwide. In this study, several machine learning methods including decision tree (DT), k nearest neighbors (KNN), logistic regression (LR), support vector machine (SVM), and extreme-gradient-boosted trees (Xgboost) were applied to predict the risk of groundwater nitrate contamination (NO3- > 50 mg L^-1) in the riverside areas of lower reaches of Yangtze River, east China. The developed model included 13 hydrochemical parameters (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-, NH₄⁺, NO₂^-, Fe, Mn, As, Sr, pH) and well depth as explanatory variables, and a total of 1089 groundwater samples. The results showed the hydrochemical dataset could effectively predict the risk of nitrate contamination, with a minimum accuracy of 82.7% in LR and maximal accuracy of 91.7% in SVM and Xgboost. However, only the Xgboost model under a cutoff probability of 0.3 had the best performance with the highest sensitivity of 80.3% and AUC 0.95, whereas other models had sensitivity lower than 60% with insufficient capability of identifying contaminated groundwater samples. The results showed that the ensemble learning method had a strong, robust prediction capability. In addition, the relative importance of K⁺, SO₄^2-, and Cl^- exceeded 0.65, indicating the dominant influence of domestic or industrial sewage in the study area due to widespread urbanization. Finally, we examined the relationship among nitrate contamination risk, land use type, the intensity of anthropogenic activities, and redox conditions and obtained the risk map of nitrate contamination in the study area. This study successfully proved the validity of predicting the risk of groundwater nitrate contamination using machine learning tools, which favors regional groundwater management and protection.

Keywords: groundwater; nitrate contamination; risk prediction; machine learning

How to cite: Huang, X., Jin, M., Liang, X., Su, J., and Ma, B.: Predicting the risk of groundwater nitrate contamination using machine learning tools, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1945, https://doi.org/10.5194/egusphere-egu22-1945, 2022.

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) is a crucial link between carbon and nitrogen cycles in estuarine and coastal ecosystems. However, the factors that affect the heterogeneous variability in n-DAMO microbial abundance and activity across estuarine and intertidal wetlands remain unclear. This study examined the spatiotemporal variations in n-DAMO microbial abundance and associated activity in different estuarine and intertidal habitats via quantitative PCR and ¹³C stable isotope experiments. The results showed that Candidatus 'Methylomirabilis oxyfera' (M. oxyfera)-like DAMO bacteria and Candidatus 'Methanoperedens nitroreducens' (M. nitroreducens)-like DAMO archaea cooccurred in estuarine and intertidal wetlands, with a relatively higher abundance of the M. oxyfera-like bacterial pmoA gene (4.0×10⁶-7.6×10⁷ copies g^-1 dry sediment) than the M. nitroreducens-like archaeal mcrA gene (4.5×10⁵-9.4×10⁷ copies g^-1 dry sediment). The abundance of the M. oxyfera-like bacterial pmoA gene was closely associated with sediment pH and ammonium (P<0.05), while no significant relationship was detected between M. nitroreducens-like archaeal mcrA gene abundance and the measured environmental parameters (P>0.05). High n-DAMO microbial activity was observed, which varied between 0.2 and 84.3 nmol ¹³CO₂ g^-1 dry sediment day^-1 for nitrite-DAMO bacteria and between 0.4 and 32.6 nmol ¹³CO₂ g^-1 dry sediment day^-1 for nitrate-DAMO archaea. The total n-DAMO potential tended to be higher in the warm season and in the upstream freshwater and low-salinity estuarine habitats and was significantly related to sediment pH, total organic carbon, Fe(II), and Fe(III) contents (P<0.05). In addition to acting as an important methane (CH₄) sink, n-DAMO microbes had the potential to consume a substantial amount of reactive N in estuarine and intertidal environments, with estimated nitrogen elimination rates of 0.5-224.7 nmol N g^-1 dry sediment day^-1. Overall, our investigation reveals the distribution pattern and controlling factors of n-DAMO bioprocesses in estuarine and intertidal marshes and gains a better understanding of the coupling mechanisms between carbon and nitrogen cycles.

How to cite: Chen, F., Zheng, Y., and Hou, L.: Microbial abundance and activity of nitrite/nitrate-dependent anaerobic methane oxidizers in estuarine and intertidal wetlands: Heterogeneity and driving factors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2143, https://doi.org/10.5194/egusphere-egu22-2143, 2022.

Even though the ability of fungi to produce the greenhouse gas nitrous oxide (N₂O) during denitrification has been demonstrated, the proportion N₂O emissions from fungal denitrification in soils cannot yet be determined or predicted. In order to develop methods for estimating the fungal proportion, N₂O must be partitioned to bacterial and fungal denitrification. The denitrification regulatory phenotype (DRP) is well described for a number of bacterial strains (Bergaust et al. 2010, Bergaust et al. 2011), but to our knowledge there are only few data relating to the fungal DRP in terms of oxygen (O₂) tension in fully stirred cultures at which they start producing N₂O. The aim of this study was to analyse the kinetics of fungal denitrification combined with analysis of the isotopic composition of N₂O. In particular, the ¹⁵N site preference of N₂O (SP-N₂O) is known to be a promising tool to differentiate between N₂O produced during bacterial and fungal denitrification.

Four fungal species (Fusarium oxysporum, Fusarium decemcellulare, Fusarium solani fsp. pisi and Chaetomium funicola) were incubated as batch cultures in a robotized incubation system (Molstad et al. 2007) for 165h. Batch cultures were incubated in 120 ml flasks containing 50 ml of growth medium amended with ample amounts of carbon and nitrate in a He atmosphere with 2 vol%O₂. To test for pH effects, a complex medium (Shoun et al. 1992) with pH values adjusted to 6.9 and 7.4 as well a minimal medium (Dox 1910) with a pH value of about 7.9 were used. O₂ consumption and production of nitric oxide (NO), N₂O, dinitrogen (N₂) and carbon dioxide (CO₂) were monitored at high temporal resolution while isotopic composition of N₂O was analysed in samples taken manually at selected time points.

All four fungal cultures quickly consumed O₂. NO production increased strongly before O₂ was completely consumed and was followed by immediate N₂O production. The kinetics of N₂O production differed to published kinetics of denitrifying prokaryotes by showing a lower sensitivity to O₂. This could result in a larger share of fungal denitrification under microaerobic conditions in soil.

Isotopic analysis of N₂O confirmed previous results of specifically high SP-N₂O values of fungal produced N₂O. We further showed that SP-N₂O values of fungal N₂O are quite stable and do not depend on denitrification kinetics. Likewise, incubation conditions such as pH of the medium had little impact on SP-N₂O values. These findings support the usage of SP-N₂O values for partitioning N₂O soil fluxes and provide a tool to study the biology of fungal denitrification under field conditions, which is needed to develop mitigation strategies of N₂O from fungal denitrification.

References:

• Bergaust, Y. Mao, L. R. Bakken, Å. Frostegård, Appl Environ Microbiol 2010, 76.
• Bergaust, L. R. Bakken, Å. Frostegård, Biochem Soc Trans 2011, 39.
• Molstad, P. Dörsch, L. R. Bakken, J Microbiol Methods 2007, 71, 202.
• Shoun, D.-H. Kim, H. Uchiyama, J. Sugiyama, FEMS Microbiol. Lett. 1992, 94, 277.
• W. Dox, U.S. Dept. of Agriculture, Bureau of Animal Industry, Washington, D.C., 1910, 70 p.

How to cite: Rohe, L., Well, R., Nadeem, S., and Dörsch, P.: Gas kinetics and stoichiometry from four fungi incubated under conditions favouring denitrification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5064, https://doi.org/10.5194/egusphere-egu22-5064, 2022.

The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (C) and total N concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought.

Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously.

How to cite: Dannenmann, M., Ramm, E., Liu, C., Ambus, P., Butterbach-Bahl, K., Hu, B., Martikainen, P. J., Marushchak, M. E., Mueller, C. W., Rennenberg, H., Schloter, M., Siljanen, H. M. P., Voigt, C., Werner, C., and Biasi, C.: A review of the importance of mineral nitrogen cycling in the plant-soil-microbe system of permafrost-affected soils – changing the paradigm, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11621, https://doi.org/10.5194/egusphere-egu22-11621, 2022.

The availability of nutrients is one of the main factors in soils that affect plant growth. This is something that has always been worrying humans; initially using natural fertilizers such as animal and human waste to enhance crop productivity. However, the industrial revolution brought the Haber-Bosch process (artificial nitrogen fixation), which posed a milestone on artificial fertilizers. The production of fertilizers increased exponentially during the twentieth century and is still increasing although at a slower pace. It has had not only positive results, e.g. food production, but is also causing major environmental, health and economic problems.

Because of these problems, it is critical to improve soil management strategies at the precise spatial scales in order to protect human health and the environment, while food production is also guaranteed. To do so, what is needed is a low-cost way to measure nutrients in the soil, in real-time, at different spatial scales.

During recent years, researchers have been working on the adaptation and modification of Ion Selective Electrodes for the analysis of nutrients directly in the soil. However, low precision and accuracy, intense instrument handling (pre and post-calibration), and complex data processing is preventing its general use.

We will present here two modifications of our first prototype of a low-cost ISE-based sensor probe. The probe allows measurements in situ and/or continuous monitoring of up to 16 chemical species. We will here showcase preliminary data obtained by measuring four replicates of four analytes. We also apply the Bayesian calibration methodology previously developed by us in order to improve the precision and accuracy of measurements.

How to cite: Saiz, E., Radu, A., Ullah, S., Del-Rio-Ruiz, R., Alsaedi, M., and Sonkusale, S.: Real-time soil nutrients monitoring, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8543, https://doi.org/10.5194/egusphere-egu22-8543, 2022.

In recent years, irrigation with nanobubbles aerated water (NB-water) [i.e., air or oxygen-NB (ONB)] has emerged as a new method to alleviate transient hypoxic conditions in the rhizosphere. We aimed to study the effect of surface and subsurface drip irrigation with ONB aerated waters [i.e., fresh (0.4 dS/m), secondary urban treated wastewater (TWW; 1.3 dS/m), saline (3dS/m)] on soil nitrogen transformations. Greenhouse lysimeter experiments were conducted in vertisol (58% clay), sand (98% sand), compost, and sand:compost (1:1) mixture, under well aerated and poorly aerated conditions. Ammonium-N to nitrate-N ratios in the irrigation waters ranged from 15% to 50%.  In all the experiments, irrigation with ONB water, with dissolved oxygen (DO) concentrations of 11 to 35 mg/L, increased the transient buildup of nitrite in the porewater, even under well-aerated conditions (soil air O₂> 19%). The most significant effects were observed in the sand, sand:compost, and compost media, where nitrite concentrations were 2 – 8 times greater than the controls and reached over 65 mg/L. Despite the increased nitrite concentrations, irrigation with ONB waters reduced the nitrous oxide fluxes by 4 – 85%. Both phenomena suggested higher oxygen availability in the soil. Nitrite buildup implies that ammonia (NH₃) oxidation may not be the rate-limiting step of nitrification under irrigation with ONB aerated water. 

How to cite: Baram, S., Weinstien, M., Kaplan, G., and Friedman, S.: The effects of drip irrigation with nanobubbles aerated water on soil N transformation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6937, https://doi.org/10.5194/egusphere-egu22-6937, 2022.

Non-native coniferous plantations in the UK have long been associated with potentially negative impacts on surface water and groundwater quality due to high levels of nitrogen accumulation in their soils. Recent changes in UK forestry policy and targets and in attitudes towards biodiversity triggered a shift towards restocking conifer forests with broadleaved species. Broadleaved species are typically associated with lower rates of nitrogen deposition, scavenging and nitrate leaching, so it is often assumed that this change in management will enhance water quality. However, the conversion of coniferous woodland to broadleaved woodland typically stimulates the breakdown of organic matter, leading to a pulse release of nutrients which cannot be taken up rapidly enough by the nascent broadleaved forest.

To assess the significance of this process we conducted a study at Thetford Forest, Norfolk, a forest exposed to elevated levels of nitrogen deposition.  We measured throughfall and soil solution chemistry, soil C/N ratios, pH and net nitrification in a chronosequence of stands (0-72 years old) in the conversion process. Observed changes in organic soil C/N ratios indicate the potential for elevated nitrate leaching fluxes within the first decade post-conversion. Results also show an increase in net nitrification in the summer five to eight years post-conversion, followed by an accumulation of nitrogen in the deep mineral soils (30-90 cm depth) ten years post-conversion. Our ongoing analysis of deep soil solution and throughfall chemistry will confirm whether these observations are linked to elevated leaching fluxes in the first decade after conversion. Mature broadleaf stands were unexpectedly associated with greater concentrations of throughfall nitrate from August-October, and lower rates of soil nitrification in the summer than coniferous stands. Further analyses from winter-spring 2022 will explore seasonal variations in throughfall chemistry between broadleaf and coniferous stands in the context of elevated nitrogen deposition.

Our observations highlight the need to consider interactions between the effect of land management, seasonality and elevated deposition on nitrogen cycling processes to understand the impact of intensive nitrogen use on terrestrial nitrogen fluxes.    

How to cite: Lewis, C., Lukac, M., Vanguelova, E., and Ascott, M.: Risks of converting coniferous forests to broadleaved species, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11430, https://doi.org/10.5194/egusphere-egu22-11430, 2022.

Anthropogenic disturbances of the nitrogen cycle are one of the most important issues for world ecology. Increased fluxes of atmospheric nitrogen deposition, including ammonium, nitrates, nitrites and other nitrogen oxides characterize the current state of nitrogen cycle. Scientists have found that this process might provide unproductive lakes with nitrogen enough for phytoplankton to turn from nitrogen to phosphorus limitation of growth. Nevertheless, atmospheric nitrogen deposition and its effect on phytoplankton primary production have not been uniformly studied around the world and significant areas remain understudied.

In this study, we measured the winter atmospheric deposition of nitrogen and phosphorus in the snow cover of an understudied region: the Ergaki Natural Park in the south of central Siberia. The concentrations in winter precipitation (40±16 mg of NO3-N m^-2 0.58±0.13 mg of total P m^-2) were used to estimate yearly yields (119±71 mg of NO3-N m^-2 year^-1 and 1.71±0.91 mg of total P m^-2 year^-1). These values approximately corresponded to the forecasts of worldwide mathematical models in the literature and were notably low for terrestrial sites, especially in the case of phosphorus. Measurements of d¹⁵N, total N and P in lake sediment cores confirmed the minor role of eventual atmospheric N deposition in the studied lakes, as compared to terrestrial inputs.

The atmospheric nitrogen deposition on the Ergaki mountain ridge was slightly lower than in northern Sweden, where the low atmospheric nitrogen deposition had been found to trigger nitrogen (instead of phosphorus) limitation of phytoplankton growth in unproductive lakes. Nevertheless, atmospheric phosphorus deposition in the study site was among the lowest ones on the mainland, if not the lowest. Due to this extremely low content of atmospheric nutrient deposition, the stoichiometry of N:P in snow and lake water did not correlate, so our lakes did not belong to the group of lakes in the world that are influenced by atmospheric deposition of nutrients. According to our observations, both nitrogen and phosphorus can periodically be limiting factors of phytoplankton in Ergaki lakes.

In conclusion, firstly, the Ergaki Natural Park is an ideal place to study the effects of global warming with a minimal interference of atmospheric nitrogen deposition. Secondly, even at low levels of atmospheric nitrogen deposition in places where atmospheric phosphorus deposition is very low, nitrogen is not necessarily the limiting factor of phytoplankton growth, which may contradict the general character of the currently accepted paradigm. Further studies should check the year-round deposition of nutrients and expand the number of lakes and regions in Siberia, where a significant part of the lakes is not subject to severe anthropogenic pollution.

This study was funded by the Russian Foundation of Basic Research grant number 20-04-00960.

How to cite: Diaz-de-Quijano, D., Ageev, A. V., Moshkin, N. V., Ivanova, E. A., Anishchenko, Y. D., Anishchenko, O. V., and Sushchik, N. N.: Low atmospheric nitrogen deposition in southern central Siberia does not trigger any nitrogen limitation in the growth of mountain lake phytoplankton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13309, https://doi.org/10.5194/egusphere-egu22-13309, 2022.

The current climate trajectory in conjunction with agricultural intensification and the reliance on synthetic fertilisers, present further threat to the resilience of future food production through their contributions to soil degradation and consequent climatic feedback. Innovative sustainable agricultural technologies are needed to produce nutritious and equitable food products in line with the UN’s goal for Zero Hunger and sustainable development. Glacial Rock Flour (GRF) is a fine mineral rock dust, made available through the glacial abrasion of bedrock, and is often enriched in nutrients (e.g. Potassium, Phosphorous, Silicon, trace elements) but low in Nitrogen. It would therefore be a suitable soil fertility amendment for legume crops grown in acidic, nutrient poor soils often found in many mountainous regions (e.g. Hindu Kush Himalaya), where GRF is considered an alluvial ‘waste’ silting up dams and reservoirs. We have investigated the effect of GRF soil amendments in soil-plant mesocosms using a typical UK silt loam arable soil (pH~7) for cultivating red clover (Trifolium pratense) inoculated with Rhizobium. GRF from the Chhota Shigri (India) and Sólheimajökull (Iceland) glaciers were applied at 2 and 20 T/ha, while no GRF treatments included synthetic fertilizer applications of phosphorus (P), potassium (K) and P+K, and they were all compared against control red clover plants grown with no soil amendments. The nitrogen fixation capacity of red clover was estimated via ¹⁵N natural abundance against a rye grass control (Lolium perenne) in two harvests on weeks 14 and 19. Both 20 T/ha GRF treatments appeared to stimulate fixed nitrogen yield compared to synthetic fertilizer treatments and control red clover plants, while the stimulation was more pronounced in the 2^nd harvest as the soil nutrients were progressively depleted. Soil greenhouse gas fluxes over the growth period (weeks 4-14) were monitored by enclosing pots in sealed chambers. While no difference was observed in carbon dioxide fluxes between treatments, nitrous oxide (N₂O) flux was negative for all red clover mesocosms with the N₂O reduction being more prominent in both 20 T/ha GRF treatments towards the end of the first growth period (week 14). Gross N mineralization and nitrification were estimated in post-harvest soils from all the mesocosms using the isotope dilution method, while ¹⁵N-N₂O and ¹⁵N-N₂ production were also measured after amending the soils with 98 at% ¹⁵N-NH₄⁺ and ¹⁵N-NO₃^-_.  Gross N mineralization was not different between treatments, while nitrification was non-detectable, indicating a very tightly coupled N cycle between Rhizobium and red clover. However, when excess nitrate was applied, bacterial denitrification was active but the amendment of the soils with GRF appeared to reduce the production of N₂O and promote complete denitrification to N₂. Our novel study on the properties and application of GRF as a sustainable soil fertility amendment under a low nitrogen cropping system, holds promise that it can promote leguminous nitrogen fixation and a tightly-coupled N cycle that maximises N-use efficiency while mitigating N₂O emissions by promoting complete denitrification.

How to cite: Sgouridis, F., Forrester, H., Tingey, S., and Wadham, J.: Glacial Rock Flour as soil fertility amendment increases N fixation activity in red clover and enhances soil N2O reduction., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5489, https://doi.org/10.5194/egusphere-egu22-5489, 2022.