Distribution and molecular chemodiversity of dissolved organic nitrogen in the vadose zone-groundwater system of a fluvial plain, northern China: Implications for understanding its loss pathway to groundwater

Nitrate and nitrite reduction by adsorbed Fe(II) generated from ligand-promoted dissolution of biogenic iron minerals in groundwater

Chemical denitrification by redox-active Fe(II) species is pivotal in the coupled iron and nitrogen cycles. The reductive dissolution of ferric minerals by ligand can generate Fe(II)-ligand complexes, but their reducing capability for electrophilic pollutants like nitrate and nitrite remains uncertain. Here, biogenic secondary iron minerals (SIM) after dissimilatory iron reduction were reductively dissolved by oxalate and the siderophore desferrioxamine B, and subsequently the partially-dissolved SIM (SIMD) effectively removed NO2- from groundwater via reduction, while exhibiting much lower reactivity towards NO3-. The dissolution and removal processes were well-fitted with the Kabai model and the pseudo-second-order adsorption model, respectively. The equilibrium NO2- removal capacity (qe) of SIMD reached 0.146-0.223 mmol/g, accompanied with the rate constants as 0.433-0.810 g/(mmol·h). The emission of N2O and NO verified the occurrence of chemical denitrification during NO2- removal by SIMD. From the perspective of Fe(II) reactivity, SIMD exhibited higher densities of surface Fe(II) and more negative Eh values than SIM, and these two indicators showed linear correlations with the removal rates. Combined with microscopic, electrochemical and spectral analysis, our results indicated the redox reaction of adsorbed Fe(II)-complexes with NO2- on SIMD surface. The concurrent substance biochar was also considered, as it indirectly influenced dissolution and pollutant removal by shifting the iron mineral phase in SIM from magnetite to goethite. These findings highlight the significant role of reductive dissolution of iron mineral in N transformation, expand the electron pool available to support chemical denitrification, and have implications for Fe and N cycling coupling with pollutant reduction.

New insights into the controlling factors of nitrate spatiotemporal characteristics in groundwater of Dagu aquifer in Qingdao, China

Identifying spatiotemporal variation of groundwater NO3-N and its primary controlling factors are vital for groundwater protection. This study, under the data scarce conditions and based on time series monitoring data in Dagu aquifer, applied methods including hydrochemical ion ratio, multiple linear regression, support vector regression and grey relational analysis and dedicated to revealing primary controlling factors of temporal variation patterns of groundwater NO3-N. The results showed that agricultural and manure fertilizer are the main sources of NO3-N in north and central area (vegetable farming area), and that domestic sewage discharge and manure fertilizer are the main sources of NO3-N in south area (residential and grain planting area). In addition, results identified the dominant influencing factors of variation of NO3-N in different regions, with human wastewater discharge, nitrogen load amount and water-table depth being the dominant factors of variations of NO3-N in north area, human wastewater discharge being the main factor of variations of NO3-N in central area, and irrigation water and human wastewater being the leading factors of variations of NO3-N in south area. Moreover, types of controlling factors can influence the seasonal variations of NO3-N. NO3-N in vegetable farming area that dominantly affected by fertilization generally shows higher concentration and larger variation range of concentration during summer and autumn than that during spring. NO3-N which mainly affected by human wastewater discharge and manure inputs shows minimal seasonal variation of mean concentration. NO3-N in grain area influenced by irrigation could show more significant variations during spring and autumn than that during summer. The conclusions can enhance understandings of major influencing factors on NO3-N variation in local aquifer. Importantly, the dominant roles of water-table depth and irrigation in NO3-N variation of N2 site (vegetable planting area) and S5 site (grain planting area), respectively, were highlighted.

Shallow groundwater table fluctuations weaken nitrogen accumulation in the thin layer vadose zone of cropland around plateau lakes, Southwest China

Excessive accumulation of nitrogen (N) in the soil profile in the intensive agricultural region will seriously threaten groundwater quality and safety. However, the impact of shallow groundwater table (SGWT) fluctuations driven by seasonal variations on the N accumulation characterizations in the soil profiles has not been well quantified, particularly in the regions with thin layer vadose zone. Through in-situ monitoring and simulation experiments, the changes in the SGWT and N accumulation of soil profile in intensive cropland around 7 plateau lakes in Yunnan were studied during the rainy season (RS) and dry season (DS), and the N loss in soil profile of cropland driven by SGWT fluctuations was estimated. The results showed that the SGWT and N accumulation in soil profile of cropland around the plateau lakes had obvious seasonal variation characteristics. The proportion of N storage in different forms in 60-100 cm soil layer in the RS was greater than that in the DS, particularly the proportion of NH4+-N storage was as high as 55 %, while N accumulation in surface soil was obvious in the DS. Compared with the DS, due to the rising SGWT in the RS, the maximum storages of TN and NO3--N in the 0-100 cm soil layer decreased by17% and 36 %, respectively. The TN loss intensities from the 0-100 cm soil profiles of cropland around Fuxian Lake, Yilong Lake, Qilu Lake, Dianchi Lake, Yangzong Lake, Erhai Lake, and Xingyun Lake were 74, 54, 127, 105, 93, 72 and 207 kg/ha, respectively. Moreover, if the SGWT was <30 cm, the average TN loss intensity and amount could reach 177 kg/ha and 1250 t, respectively. Therefore, the SGWT regulation was one of the key measures to reducing soil N loss from the thin layer vadose zone of cropland around plateau lakes and improving groundwater quality.

The impact of dissolved organic nitrogen (DON) retention in the vadose zone on nitrogen leaching losses

Leaching of dissolved organic nitrogen (DON) is a significant pathway for nitrogen (N) loss in agricultural ecosystems. The excessive application of N for enhanceing agricultural productivity often results in the leaching of N into groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. This study aims to advance simulation techniques for modelling the transport process of reactive DON within a heterogeneous vadose zone. Through a combination of laboratory experiments and numerical simulations, the study firstly examines the extent of DON retention in the vadose zone and quantitatively analyse groundwater contamination risk from this kind of accumulation. Our findings indicate that heavy N fertilizer application and high-intensity rainfall events led to elevated contents of DON in the vadose zone and increased DON leaching fluxes into groundwater. Besides, intensifier rainfall reduced the N storage more quickly in scenarios devoid of DON application with higher mineralization rate, while DON slowly mineralized to other forms, largely accumulated in the top layer and migrated deeper with intensifier rainfall after input of urea. In our scenarios, DON accounted for a substantial portion (33-68%) of the total dissolved nitrogen (TDN) leaching fluxes, with exogenous DON content contributing significantly (25-85%) to the overall DON leaching into the aquifer. These results underscore the need for effective strategies to mitigate groundwater contamination risks associated with agricultural N use.

Spectral and molecular insights into the characteristics of dissolved organic matter in nitrate-contaminated groundwater

The characteristics of dissolved organic matter (DOM) serve as indicators of nitrate pollution in groundwater. However, the specific DOM components associated with nitrate in groundwater systems remain unclear. In this study, dual isotopes of nitrate, three-dimensional Excitation emission matrices (EEMs) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were utilized to uncover the sources of nitrate and their associations with DOM characteristics. The predominant nitrate in the targeted aquifer was derived from soil organic nitrogen (mean 46.0%) and manure &sewage (mean 34.3%). The DOM in nitrate-contaminated groundwater (nitrate-nitrogen >20 mg/L) exhibited evident exogenous characteristics, with a bioavailable content 2.58 times greater than that of uncontaminated groundwater. Regarding the molecular characteristics, DOM molecules characterized by CHO + 3N, featuring lower molecular weights and H/C ratios, indicated potential for mineralization, while CHONS formulas indicated the exogenous features, providing the potential for accurate traceability. These findings provided insights at the molecular level into the characterization of DOM in nitrate-contaminated groundwater and offer scientific guidance for decision-making regarding the remediation of groundwater nitrate pollution.

Review of soil dissolved organic nitrogen cycling: Implication for groundwater nitrogen contamination

Dissolved organic nitrogen (DON) in groundwater is derived from soil DON transformation and migration processes, which has been considered an emerging nitrogen (N) pollutant. However, due to the limitations of the analytical methods and the complexity of the involved transformation process, the role of DON in soil N cycling remains unclear. Therefore, this review aims to critically examine previous related studies on DON and highlight the knowledge gaps related to DON transformations and molecular characteristics in soils. In addition, the DON distributions and key transformation processes, as well as their influencing factors, were summarized. About 60% of DON components have not been determined due to the limited analytical techniques and methodologies. The depolymerization process of polymers into DON is the rate-limiting step of N mineralization. Furthermore, DON leaching amounts accounted for 7-1500% of soil nitrate (NO3--N) amounts, becoming the dominate pathway of N loss. Further studies are required to provide accurate information on DON compositions and transformation mechanisms, as well as their influencing factors, in soils. The suggested studies can provide further insights into the role of DON in soil N cycling, thereby controlling effectively groundwater N contamination.

Unveiling the biogeochemical mechanism of nitrate in the vadose zone-groundwater system: Insights from integrated microbiology, isotope techniques, and hydrogeochemistry

Clarifying the biogeochemical mechanism of nitrate (NO3-) in the vadose zone-groundwater system, particularly in agricultural contexts, is crucial for mitigating groundwater NO3- pollution. However, comprehensive studies on the impacts of changes in chemical indicators and microbial communities on NO3- are still lacking. This paper aims to address this gap by employing hydrogeochemistry, stable isotopes, and microbial techniques to assess the NO3- biogeochemical processes in the vadose zone-groundwater system. The findings suggested that NO3- in upper soil layers was primarily influenced by plant root absorption, assimilation, and nitrification processes. The oxygen contents gradually decreased with the nitrification process, resulting in the occurrence of the denitrification. However, denitrification predominantly occurred in the 60-80 cm soil layer in the study area. The limited thickness of the denitrification layer results in less NO3- consumption, leading to increased NO3- leaching into groundwater. Hydrochemical and isotopic characteristics further indicated that groundwater NO3- concentrations were mainly controlled by nitrification, followed by denitrification and mixing processes. The 16S rRNA sequencing analysis revealed great influences of soil sampling depths and groundwater NO3- concentrations on the microbial community structure. Additionally, the PICRUSt2-based prediction results demonstrated a stronger potential for dissimilatory reduction of NO3- to ammonium (DNRA) in both soil and groundwater compared to the other processes, potentially due to the widespread presence of the nrfH functional genes. However, the chemical indicators and isotopes used in this study did not support the occurrence of DNRA process in the vadose zone-groundwater system. This finding highlights the importance of an integrated approach combining microbiological, isotopic, and hydrogeochemical data to comprehensive understanding biogeochemical processes. The study developed a conceptual model elucidating the NO3- biogeochemical processes in the vadose zone-groundwater system within an agricultural area, contributing to enhanced comprehension and advancement of sustainable management practices for groundwater nitrogen.

Distribution, sources and main controlling factors of nitrate in a typical intensive agricultural region, northwestern China: Vertical profile perspectives

Nitrate (NO3-) pollution of groundwater is a global concern in agricultural areas. To gain a comprehensive understanding of the sources and destiny of nitrate in soil and groundwater within intensive agricultural areas, this study employed a combination of chemical indicators, dual isotopes of nitrate (δ15N-NO3- and δ18O-NO3-), random forest model, and Bayesian stable isotope mixing model (MixSIAR). These approaches were utilized to examine the spatial distribution of NO3- in soil profiles and groundwater, identify key variables influencing groundwater nitrate concentration, and quantify the sources contribution at various depths of the vadose zone and groundwater with different nitrate concentrations. The results showed that the nitrate accumulation in the cropland and kiwifruit orchard at depths of 0-400 cm increased, leading to subsequent leaching of nitrate into deeper vadose zones and ultimately groundwater. The mean concentration of nitrate in groundwater was 91.89 mg/L, and 52.94% of the samples exceeded the recommended grade III value (88.57 mg/L) according to national standards. The results of the random forest model suggested that the main variables affecting the nitrate concentration in groundwater were well depth (16.6%), dissolved oxygen (11.6%), and soil nitrate (10.4%). The MixSIAR results revealed that nitrate sources vary at different soil depths, which was caused by the biogeochemical process of nitrate. In addition, the highest contribution of nitrate in groundwater, both with high and low concentrations, was found to be soil nitrogen (SN), accounting for 56.0% and 63.0%, respectively, followed by chemical fertilizer (CF) and manure and sewage (M&S). Through the identification of NO3- pollution sources, this study can take targeted measures to ensure the safety of groundwater in intensive agricultural areas.

Nitrogen fixation and diazotroph diversity in groundwater systems

Biological nitrogen fixation (BNF), the conversion of N2 into bioavailable nitrogen (N), is the main process for replenishing N loss in the biosphere. However, BNF in groundwater systems remains poorly understood. In this study, we examined the activity, abundance, and community composition of diazotrophs in groundwater in the Hetao Plain of Inner Mongolia using 15N tracing methods, reverse transcription qPCR (RT-qPCR), and metagenomic/metatranscriptomic analyses. 15N2 tracing incubation of near in situ groundwater (9.5-585.4 nmol N L-1 h-1) and N2-fixer enrichment and isolates (13.2-1728.4 nmol N g-1 h-1, as directly verified by single-cell resonance Raman spectroscopy), suggested that BNF is a non-negligible source of N in groundwater in this region. The expression of nifH genes ranged from 3.4 × 103 to 1.2 × 106 copies L-1 and was tightly correlated with dissolved oxygen (DO), Fe(II), and NH4+. Diazotrophs in groundwater were chiefly aerobes or facultative anaerobes, dominated by Stutzerimonas, Pseudomonas, Paraburkholderia, Klebsiella, Rhodopseudomonas, Azoarcus, and additional uncultured populations. Active diazotrophs, which prefer reducing conditions, were more metabolically diverse and potentially associated with nitrification, sulfur/arsenic mobilization, Fe(II) transport, and CH4 oxidation. Our results highlight the importance of diazotrophs in subsurface geochemical cycles.

Effect of mixed physical barrier on seawater intrusion and nitrate accumulation in coastal unconfined aquifers

Physical barrier has been proven to be one of the most effective measures to prevent and control seawater intrusion (SWI) in coastal areas. Mixed physical barrier (MPB), a new type of physical barrier, has been shown to have higher efficiency in SWI control. As with conventional subsurface dam and cutoff wall, the construction of MPB may lead to the accumulation of nitrate contaminants in coastal aquifers. We investigated the SWI control capacity and nitrate accumulation in the MPB using a numerical model of variable density flow coupling with reactive transport, and performed sensitivity analysis on the subsurface dam height, cutoff wall depth and opening spacing in the MPB. The differences in SWI control and nitrate accumulation between MPB and conventional subsurface dam and cutoff wall were compared to assess the applicability of different physical barrier. The numerical results show that the construction of MPB will increase the nitrate concentration and contaminated area in the aquifer. The prevention and control efficiency of MPB against SWI is positively correlated with the depth of the cutoff wall, reaching the highest efficiency at the minimum effective dam height, and the retreat distance of the saltwater wedge is positively correlated with the opening spacing. We found a non-monotonic relationship between the change in subsurface dam height and the extent of nitrate accumulation, with total nitrate mass and contaminated area increasing and then decreasing as the height of the subsurface dam increased. The degree of nitrate accumulation increased linearly with increasing the height of the cutoff wall and the opening spacing. Under certain conditions, MPB is 46-53% and 16-57% more efficient in preventing and controlling SWI than conventional subsurface dam and cutoff wall, respectively. However, MPB caused 14-27% and 2-12% more nitrate accumulation than subsurface dam and cutoff wall, respectively. The findings of this study are of great value for the protection of coastal groundwater resources and will help decision makers to select appropriate engineering measures and designs to reduce the accumulation of nitrate pollutants while improving the efficiency of SWI control.

Enhancing nitrate attenuation in groundwater via selectively applying surface agricultural practices: A novel and sustainable strategy for non-point source pollution mitigation

Non-point nitrate pollution in groundwater has been accelerated by agricultural development, but sustainable nitrogen removal is a challenge because of its wide distribution and negative side effects. Surface agricultural practices (SAPs), which are demonstrably effective in driving the downward infiltration of dissolved organic carbon (DOC), have not been well explored for their potential to enhance nitrate attenuation in groundwater. Therefore, a combination of soil column and groundwater incubation experiments was performed to investigate the carbon and nitrogen responses to different SAPs (manure fertilization, lucerne planting, and straw return). The soil column experiment showed that SAPs promoted DOC and reduced nitrate leaching into groundwater, and straw treatment witnessed the highest DOC leaching flux (252.71 g m^-2 yr^-1) and lowest nitrate leaching flux (9.51 g m^-2 yr^-1). The groundwater incubation experiment showed that leachates from the straw treatment displayed the best denitrification-enhancement performance, with the highest NO₃^--N reduction efficiency (92.93%) and rate (1.627 mg/day), N₂ selectivity (99.78%), and net nitrogen removal (0.09 mg). Furthermore, Fourier transform ion cyclotron resonance mass spectrometry confirmed that CHOS molecules with lower double bond equivalents (0-5) and larger carbon numbers (10-15) were more accessible to denitrifiers. This study provides a new path for the sustainable control of non-point source nitrate pollution.

A critical review of the central role of microbial regulation in the nitrogen biogeochemical process: New insights for controlling groundwater nitrogen contamination

With the increase of nitrogen (N) input in vadose zones-groundwater systems, N contamination in groundwater has become a global environmental and geological issue that has a profound impact on the ecological environment and human health. N migration in the vadose zone is the most significant means of contaminating the groundwater aquifer. However, the current research on the control of groundwater N contamination focuses solely on the content change of certain indicators and is unable to comprehend the cause and subsequent development of groundwater N contamination. These factors pose significant environmental management challenges in areas where groundwater is contaminated with nitrate. In recent years, research on the migration and transformation behavior of various N forms in vadose zones-groundwater systems has yielded some breakthroughs but also encountered some roadblocks. The biogeochemical behavior of nitrogen consists of a series of intricate chain reaction cycles (called N-cycle). The crucial role of microorganisms in the N biogeochemical process has attracted the interest of soil carbon- and N-cycle researchers and become a hot topic of study. Nonetheless, the role of microbial regulation in groundwater systems has been largely neglected and needs to be summarized immediately. Consequently, this review summarizes recent advancements, mechanisms, and challenges, and proposes a dynamic perspective on microbial regulation. On the basis of these findings, we propose a dynamic and comprehensive groundwater N system centered on microbial regulation. In addition, we critically summarized the migration and transformation behavior of the most recent N indicators, the impact of global environmental change on each N component, and the non-negligible effects of these factors on the control of groundwater N contamination. Future research must focus on the migration and transformation behavior of nitrogen in the deep vadose zone, based on the dynamic regulation of microorganisms, and complete the missing pieces of the developed N-cycle index system. These are essential for providing scientific guidance for global N management and effectively mitigating N contamination in groundwater.

Unveiling microbial community and function involved in anammox in paddy vadose under groundwater irrigation

The extensive application of nitrogen fertilizer in intensive irrigation areas poses a potential threat to groundwater. Given that the vadose zone acts as a buffer zone for the underground entry of surface pollutants, an in-depth understanding of its microbial community structure and function was crucial for controlling groundwater nitrogen pollution. In this study, soil samples from paddy vadose under groundwater irrigation with different depths (G1: 6.8 m, G2: 13.7 m, G3: 15.6 m, and G4: 17.8 m) were collected to unravel the differences in microbial community structure and function at different vadose depths (0-250 cm), as well as their relationship with soil properties. Results showed some differences among soil physicochemical factors under groundwater irrigation with different depths and that some electron acceptors were more abundant than others under deep groundwater irrigation (G2-G4). Remarkable differences in microbial communities under shallow- and deep-groundwater irrigation were found. The high abundances of anammox bacteria Candidatus_Brocadia in G2 and G3 indicated that deep groundwater irrigation was beneficial to its enrichment. Iron-reducing bacteria Anaeromyxobacter and sulfate-reducing bacteria Desulfovibrio were widely distributed in vadose zone and possessed the potential for anammox coupled with Fe(III)/sulfate reduction. Norank_f_Gemmatimonadaceae had nitrate- and vanadium-reducing abilities and could participate in anammox in vadose zone. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Geobacter facilitated Fe(II)-driven DNRA and thus provided electron donors and acceptors to anammox bacteria. Soil nutrients and electron donors/acceptors played important roles in shaping microbial community structure at phylum and genus levels. Microorganisms in vadose zone under groundwater irrigation showed good material/energy metabolism levels. Deep groundwater irrigation was conducive to the occurrence of anammox coupled with multi-electron acceptors. Our findings highlight the importance of understanding the structure and function of microbial communities in paddy vadose under groundwater irrigation and reveal the potential role of indigenous microorganisms in in-situ nitrogen removal.

Nitrate transport behavior behind subsurface dams under varying hydrological conditions

The construction of subsurface dams for controlling seawater intrusion triggers the accumulation of nitrate upstream of a dam. This is raising the concerns about nitrate contamination in those regions of coastal aquifers that are supposed to be used as a fresh groundwater source behind a subsurface dam. Research on this subject has been mostly restricted to the use of a simplified sea boundary (e.g., static and no slope), ignoring sea level fluctuations driven by tides. In this study, the combined effect of tides and subsurface dams on nitrate pollution in upstream groundwater was examined through laboratory experiments and numerical simulations. The results revealed that the difference in the extent of nitrate contamination under various conditions (i.e., static, tidal, static with a dam, and tidal with a dam) was related to the temporal pollution behavior. In the early stage, nitrate contamination in upstream groundwater was essentially identical for different scenarios. Both tides and subsurface dams were found to increase nitrate contamination in upstream aquifers. The extent of nitrate contamination increased with higher tidal amplitudes, whereas the increment was more evident for a large tidal amplitude. The effects of tides and subsurface dams on nitrate contamination were also regulated by the locations and infiltration rates of the pollution source. Interestingly, under the joint action of tides and subsurface dams, the increment in the extent of nitrate pollution was greater than the sum of their individual effects. The increased pollutions caused by subsurface dams and tides were quantified as 9.47% and 37.22%, respectively, whereas the increased value caused by their joint action was measured as 51.10%. These findings suggest that tidal activity should not be overlooked when assessing nitrate contamination in upstream groundwater.

APCS-MLR model: A convenient and fast method for quantitative identification of nitrate pollution sources in groundwater

Nitrate (NO₃^-) contamination in groundwater has diverse sources and complicated transformation processes. To effectively control NO₃^- pollution in groundwater systems, quantitative and accurate identification of NO₃^- sources is critical. In this work, we applied hydrochemical characteristics and isotope analysis to determine NO₃^- source apportionment. For the first time, the NO₃^- source contributions were calculated using hydrochemical indicators combined with multivariate statistical model (PCA-APCS-MLR). The results interpret that chemical fertilizers (58.11%) and natural sources (22.69%) were the primary NO₃^- sources in the vegetable cultivation area (VCA) which were rather close to the estimation by Bayesian isotope mixing model (SIAR). In particular, the contributions of chemical fertilizers in the VCA differed by only 3.79% between the two methods. Compared with previous approaches e.g. SIAR, the key advantage of the proposed PCA-APCS-MLR model is that it only requires the hydrochemical indicators which can be easily measured. A series of complicated experiments including measurement of isotope data of NO₃^- in groundwater, monitoring of in-situ pollution source information and calculation of isotopic enrichment factor can be simply avoided. The PCA-APCS-MLR model offers a much more convenient and faster method to determine the contribution rates of NO₃^- pollution sources in groundwater.

Molecular characterization of dissolved organic nitrogen and phosphorus in agricultural runoff and surface waters

Agricultural runoff is a significant contributor to nitrogen (N) and phosphorus (P) pollution in water bodies. Limited information is available about the molecular characteristics of the dissolved organic N (DON) and P (DOP) species in the agricultural runoff and surface waters. We employed Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometry (FT-ICR-MS) to investigate the changes in the molecular characteristics of DON and DOP at three watershed positions (upstream water, runoff from agricultural fields, and downstream waters). Across three watershed locations, more-bioavailable compounds (such as amino sugars, carbohydrates, lipids, and proteins) accounted for <5% of DON and 4-31% of DOP molecules, whereas less-bioavailable compounds (such as lignin, tannins, condensed hydrocarbons, and unsaturated hydrocarbons) were >95% of DON and 69-96% of DOP. Of the dissolved organic matter, runoff waters from agricultural fields contained the greatest proportion of DON formulas (20-25%) than upstream (18%) and downstream (13-14%) waters, indicating the presence of a greater diversity of DON species in the runoff. Various nutrient sources present in agricultural fields such as crop residues, soil organic matter, and transformed fertilizers likely contributed to the diverse composition of DON and DOP in the runoff, which were likely altered as the surface water traversed along the flow pathways in the watershed. The presence of more-bioavailable molecules detected in upstream compared to agricultural runoff and downstream waters suggests that photochemical and/or microbial processes likely altered the characteristics of DON and DOP compounds. The findings of this study increase our understanding of DON and DOP compounds lability and transformations in runoff and surface waters , which may be useful in quantifying the contribution of organic N and P sources to water quality impairment in aquatic ecosystems.

Dual roles of dissolved organic nitrogen in groundwater nitrogen cycling: Nitrate precursor and denitrification promoter

Dissolved organic nitrogen (DON) has been reported to be prevalent in groundwater worldwide. Owing to the diversity of physicochemical properties, DON plays complex roles in nitrogen cycling processes, which has further implications for nitrate (NO₃^--N) pollution control in groundwater. To characterize these crucial roles, we investigated the effects of three types of DON (amino acid, urea, and protein) on NO₃^--N accumulation in groundwater with a 60-day incubation experiment and established quantitative correlations between microbial indicators (bacterial communities and nitrogen functional genes) and nitrogen content. The results showed that NO₃^--N content increased by 30.3% and 38.8% and was strongly correlated with the presence of amino acid and urea; however, the addition of protein did not lead to an additional increase in NO₃^--N, possibly due to different extents of mineralization and denitrification caused by different types of DON. Molecular biological experiments demonstrated that Nitrospira (1.8-3.2%) contributed to nitrification in the urea treatment, whereas Arthrobacter (2.0-6.9%) and Thermomonas (11.9-13.1%) were key communities controlling denitrification in amino acid and protein treatments. amoA and nxrA were continuously enriched in the presence of urea; however, amino acid and protein were strongly correlated with napA-dominated and narG-dominated denitrification processes, with the path coefficient - 2.912 and - 2.450 respectively. Combined analyses showed that DON with distinct physicochemical properties played dual roles (NO₃^--N precursor and denitrification promoter) to varying degrees, which could have significant impacts on NO₃^--N accumulation in groundwater. This study may provide guidance for environmental risk evaluation and control strategies for NO₃^--N pollution in groundwater.

Spatial variation of dissolved organic nitrogen in Wuhan surface waters: Correlation with the occurrence of disinfection byproducts during the COVID-19 pandemic

Intensified sanitization practices during the recent coronavirus disease-2019 (COVID-19) led to the release of chlorine-based disinfectants in surface water, potentially triggering the formation of disinfection byproducts (DBPs) in the presence of dissolved organic nitrogen (DON). Thus, a comprehensive investigation of DON's spatial distribution and its association with DBP occurrence in the surface water is urgently needed. In this study, a total of 51 water samples were collected from two rivers and four lakes in May 2020 in Wuhan to explore the regional variation of nitrogen (N) species, DON's compositional characteristics, and the three classes of DBP occurrence. In lakes, 53.0% to 86.3% of N existed as DON, with its concentration varying between 0.3-4.0 mg N/L. In contrast, NO₃^--N was the dominant N species in rivers. Spectral analysis revealed that DON in the lakes contained higher humic and fulvic materials with higher A₂₅₄, A₂₅₃/A_203, SUVA₂₅₄, and P_III+IV/P_I+II+V ratios, while rivers had higher levels of hydrophilic compounds. Trihalomethanes (THMs) were the most prevalent DBPs in the surface waters, followed by N-nitrosamines and haloacetonitriles (HANs). The levels of N-nitrosamines (23.1-97.4 ng/L) increased significantly after the outbreak of the COVID-19 pandemic. Excessive DON in the surface waters was responsible for the formation of N-nitrosamines. This study confirmed that the presence of DON in surface water could result in DBP formation, especially N-nitrosamines, when disinfectants were discharged into surface water during the COVID-19 pandemic.