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In the present study, 3-year (1997, 2008 and 2017) satellite images as well as different hydro-chemical parameters, nitrogen and oxygen isotopic composition of nitrate were used to examine the impacts of land use and land cover change on surface and groundwater quality. Through isotopic composition, sources of surface and groundwater pollutants were also elucidated. The results showed significant land use transition whereby land use changed from forest and bare land to agricultural land and built-up areas. A slight reduction in the size of areas covered by water bodies was also observed. Results indicate differences in nitrate concentration that mirror land use changes. Samples with elevated levels of nitrate above 10 mg/L were located near agricultural fields and areas with intensive livestock keeping activities. In groundwater, ẟ15N-nitrate and ẟ18O-nitrate ranged from 3.2‰ to 20.1‰ with a mean value of 11.7 ± 1.8‰ and from 2.1‰ to 12.0‰ with mean value of 5.4 ± 1.8‰, respectively. In surface water, ẟ15N-nitrate and ẟ18O-nitrate ranged from 2.4‰ to 19.3‰ with mean value of 4.9 ± 1.4‰ and from 1.5‰ to 21.9‰ with a mean value of 13.5 ± 2.8‰, respectively. Isotopic composition data suggest sources of nitrate in both ground and surface water dominated by synthetic and organic fertilizer application and to a lesser extent a natural soil nitrate source.

The microbiota of the human lower intestinal tract helps maintain healthy host physiology, for example through nutrient acquisition and bile acid recycling, but specific positive contributions of the oral microbiota to host health are not well established. Nitric oxide (NO) homeostasis is crucial to mammalian physiology. The recently described entero-salivary nitrate-nitrite-nitric oxide pathway has been shown to provide bioactive NO from dietary nitrate sources. Interestingly, this pathway is dependent upon oral nitrate-reducing bacteria, since humans lack this enzyme activity. This pathway appears to represent a newly recognized symbiosis between oral nitrate-reducing bacteria and their human hosts in which the bacteria provide nitrite and nitric oxide from nitrate reduction. Here we measure the nitrate-reducing capacity of tongue-scraping samples from six healthy human volunteers, and analyze metagenomes of the bacterial communities to identify bacteria contributing to nitrate reduction. We identified 14 candidate species, seven of which were not previously believed to contribute to nitrate reduction. We cultivated isolates of four candidate species in single- and mixed-species biofilms, revealing that they have substantial nitrate- and nitrite-reduction capabilities. Colonization by specific oral bacteria may thus contribute to host NO homeostasis by providing nitrite and nitric oxide. Conversely, the lack of specific nitrate-reducing communities may disrupt the nitrate-nitrite-nitric oxide pathway and lead to a state of NO insufficiency. These findings may also provide mechanistic evidence for the oral systemic link. Our results provide a possible new therapeutic target and paradigm for NO restoration in humans by specific oral bacteria.

In temperate karst aquifers under intensive anthropogenic impacts and high heterogeneity, groundwater contamination tracking has predominantly focused on nitrate, but inadequate evidence for co‐occurring ammonium sources undermines accurate nitrogen pollution assessments. This study pioneered the application of a δ15NNH4 ${\\delta }^{15}{\\mathrm{N}}_{{\\text{NH}}_{4}}$ isotope approach within the groundwater flow framework of the Jinan Spring Catchment, constructed a novel ammonium‐nitrate isotope tracing system (δ15NNH4 ${\\delta }^{15}{\\mathrm{N}}_{{\\text{NH}}_{4}}$, δ15NNO3 ${\\delta }^{15}{\\mathrm{N}}_{{\\text{NO}}_{3}}$, and δ18ONO3 ${\\delta }^{18}{\\mathrm{O}}_{{\\text{NO}}_{3}}$) for full‐form source tracking, and implemented the Bayesian mixing model (Simmr) to quantitatively apportion nitrate sources in karst groundwater. This integrated system enabled simultaneous NH4+‐N and NO3−‐N pollution source identification, enhancing the resolution of key nitrogen cycling pathways, particularly ammonium‐dominated nitrification. The NO3−‐N was the dominant form of inorganic nitrogen in karst groundwater (0.4−42.2 mg/L), and demonstrated an overall upward trend from 1958 to 2019. δ15NNH4 ${\\delta }^{15}{\\mathrm{N}}_{{\\text{NH}}_{4}}$ and nitrate isotope both confirmed that NH4+‐N and NO3−‐N primarily originated from ammonium fertilizer and soil nitrogen. The Simmr model quantified ammonium fertilizer (28.4%−58.3%) and soil nitrogen (23.1%−62.7%) as primary contributors to groundwater NO3−‐N. Hydrochemical and dual nitrogen isotope evidences revealed that mineralization and re‐nitrification of soil nitrogen, nitrification of ammonium fertilizers, and mineralization‐immobilization‐turnover of nitrate fertilizers all promoted nitrate accumulation in karst groundwater. Groundwater flow analysis identified mixing between shallow and deep karst groundwater as the primary mechanism for nitrate attenuation of spring waters in the urban discharge area, with denitrification playing a negligible role. These findings provide new insights into nitrogen behavior in temperate karst groundwater, offering valuable guidance for water resource management and protection in similar karst systems.

On page 625, Cheng et al.3 report that sources of river nitrogen pollution in the United States are often spatially separated from existing wetlands (Fig. 1), which can remove nitrate from water, and show that wetland restoration targeted to nitrate sources would yield substantial benefits for downstream water quality. [...]wetlands provide other important ecosystem services such as sequestering atmospheric carbon, supporting biodiversity, and reducing flooding and stream-bank erosion6. [...]the benefits of wetland restoration would extend to other ecosystem services. [...]substantial policy and legal uncertainties regarding US federal rules governing water management on private land11 must be resolved to overcome barriers to conservation efforts.

Studying the biogeochemical cycle of biogenic nitrogen and its influence on hydrological processes and anthropogenic nitrogen input is of great significance for water resource management and the maintenance of aquatic ecosystems in ecologically sensitive areas. Currently, there is a limited understanding of the sources contributing to nitrate levels during thermal stratification in deep and large reservoirs, as well as the transformation processes of nitrate under varying hydrological conditions. This study collected water samples from the Longyangxia Reservoir, located in the upper reaches of the Yellow River, during January and April of 2024. Utilizing hydrogeochemical analysis, multivariate stable isotope technology, the Bayesian isotope mixing model, and multivariate statistical analysis, we analyzed the vertical distribution characteristics of nitrogen in the reservoir across different periods. The transformations and sources of nitrogen were identified, and the contribution rates of each nitrogen source were estimated. The results indicate that January serves as the mixing period for the Longyangxia Reservoir, during which the differences in nitrogen concentration among the vertical water layers are relatively minimal. The concentration ranges for nitrate (NO₃⁻), dissolved organic nitrogen (DON), and ammonium (NH₄⁺) were observed to be 0.598–0.647 mg/L, 0.124–0.397 mg/L, and 0.015–0.157 mg/L, respectively. Beginning in April, the reservoir enters the thermal stratification period, characterized by higher concentrations of various nitrogen forms compared to the mixing period. During the stratification period, the concentration of various nitrogen forms within the vertical profile of the reservoir demonstrates a characteristic distribution of being low in the upper section, maximum values of total nitrogen (TN) and dissolved DON in the middle section, and maximum concentrations of NO₃⁻ and NH₄⁺ in the bottom section. Nitrate nitrogen and dissolved organic nitrogen are the primary forms of nitrogen present in the Longyangxia Reservoir, constituting 66.71% and 25.83% of the total dissolved nitrogen in January, and 62.39% and 21.59% in April, respectively. During the sampling period at Longyangxia Reservoir, the δ 15 N-NO 3 - values in the water ranged from 5.58 ‰ to 7.38 ‰, while the δ 18 O-NO 3 - values varied from −5.87 ‰ to 2.58 ‰. Nitrification is identified as the primary nitrogen conversion process occurring in the reservoir water. Under aerobic conditions, denitrification does not occur in aquatic environments. The dynamics of nitrate in the bottom layer are influenced by nitrification processes and the release of nitrogen from sediment. Soil organic nitrogen is the primary source of nitrate in Longyangxia water, contributing 42.1% and 51.8% during the sampling period, respectively. This study introduced sediment as an additional end member, highlighting that the contribution of sediment to nitrate in water is significant, accounting for 24% and 14.1%, respectively. This study offers valuable insights for precise nitrogen management and control in deep reservoirs by tracking nitrate sources and quantifying their contributions.

Nitrate (NO3-) concentrations and their dual isotopic compositions (δ15N-NO3- and δ18O-NO3-) were measured to constrain N sources and their cyclic processes in summer using samples from the water column of the northern South China Sea (NSCS). Our data revealed that higher NO3- concentrations and δ15N-NO3- values were observed in the upper waters of the coastal areas near the Pearl River Estuary (PRE). The Bayesian stable isotope mixing model was used to calculated the proportion of nitrate sources, the results indicated that the nitrate in the upper waters of the coastal areas near PRE were mainly influenced by manure and sewage (63%), atmospheric deposition (19%), soil organic nitrogen (12%) and reduced N fertilizer (6%). For the upper waters of the outer areas, low NO3- concentrations and δ15N-NO3- values, but high δ18O-NO3- values, reflected that NO3- was mainly influenced by Kuroshio water intrusion (60%), atmospheric deposition (32%) and nitrogen fixation/nitrification (8%). Complex processes were found in bottom waters. Nitrification and phytoplankton assimilation may be responsible for the higher nitrate concentrations and δ15N-NO3- values. Our study, therefore, utilizes the nitrate dual isotope to help illustrate the spatial variations in nitrate sources and complex nitrogen cycles in the NSCS.

Nitrate (NO 3 − ) pollution in karst areas has been widely discussed, which affects human health and the ecological environment. To investigate nitrate sources and their perturbations on cave hydrogeochemistry in karst cave systems, this study was conducted in Mahuang Cave, a karst cave in Southwest China, to assess the impact of human activities on the karst carbon cycle and the environment. The results show that (1) the variations of the water-soluble ions in Mahuang Cave are mainly controlled by carbonate weathering, and the cave water chemistry is characterized as the HCO 3 –Ca–Mg and HCO 3 –SO 4 –Ca–Mg types. (2) The dual isotopes and stable isotope Bayesian mixing model (SIAR) show that chemical fertilizers (41.5%) and soil nitrogen (33.75%) are the main nitrate sources in the cave water bodies, followed by manure and sewage (17.25%) and atmospheric precipitation (7.5%). (3) The significant enrichment of dissolved inorganic carbon isotope (δ 13 C DIC ) in Mahuang Cave reveals that nitric acid produced by nitrification accelerates carbonate weathering in Mahuang Cave, and the carbon source effect of the carbon cycle in the cave is enhanced. Consequently, the response of cave drips and sediments to external environments is disturbed.

Coastal aquifer is a fragile environment due to the interaction of groundwater with seawater, especially in arid environments. Groundwater along Kuwait’s Bay is polluted due to discharge of waste from desalination plants, power plants, and other anthropogenic activities. Earlier studies on submarine groundwater discharge in Kuwait’s Bay region have reported the transfer of nutrient flux from the groundwater to Kuwait's Bay. The current study focused on nitrate sources and processes governing their distribution in groundwater samples collected from the southern part of Kuwait’s Bay. The concentration of nitrate in the samples ranged from 22.7 to 803.9 mg/L. Higher values were noted in the samples collected inland and a few samples adjacent to the Bay. Spearman’s correlation analysis of the data indicated that NO 3 − has a strong positive correlation with SO 4 2− and moderate positive correlation with Na  +  , TDS/EC. The PCA analysis and factor scores revealed the different sources for groundwater nitrate contamination as follows: leakage of sewer lines in the urban region has led to the infiltration of contaminated sewage, high saline environment due to seawater intrusion, chemical weathering, and influence of denitrifying bacteria. The health risk has resulted due to the NO 3− concentration being above the standard limit for adults. Furthermore, the nitrate concentration was higher in the region adjoining the landfills. In addition, the discharge of groundwater with higher nitrate to the adjacent open water in the Bay may lead to eutrophication. Hence, proper management strategies are to be adopted to control the nitrate pollution in groundwater.

Efforts to reduce nitrogen and carbon loading from developed watersheds typically target specific flows or sources, but across gradients in development intensity there is no consensus on the contribution of different flows to total loading or sources of nitrogen export. This information is vital to optimize management strategies leveraging source reductions, stormwater controls, and restorations. We investigate how solute loading and sources vary across flows and land‐use using high frequency monitoring and stable nitrate isotope analysis from five catchments with different sanitary infrastructure, along a gradient in development intensity. High frequency monitoring allowed estimation of annual loading and attribution to storm versus baseflows. Nitrate loads were 16 kg/km2/yr. from the forested catchment and ranged from 68 to 119 kg/km2/yr., across developed catchments, highest for the septic served site. Across developed catchments, baseflow contributions ranged from 40% of N loading to 75% from the septic served catchment, and the contribution from high stormflows increased with development intensity. Stormflows mobilized and mixed many surface and subsurface nitrate sources while baseflow nitrate was dominated by fewer sources which varied by catchment (soil, wastewater, or fertilizer). To help inform future sampling designs, we demonstrate that grab sampling and targeted storm sampling would likely fail to accurately predict annual loadings within the study period. The dominant baseflow loads and subsurface stormflows are not treated by surface water management practices primarily targeted to surface stormflows. Using a balance of green and gray infrastructure and stream/riparian restoration may target specific flow paths and improve management. Key Points Baseflows contributed nearly as much or more of nitrogen export as stormflows across low to moderate development intensity catchments Stormflow NO3− sources were diverse and highly variable in time, while baseflows mobilized few sources with little temporal variability Grab sampling methods did not reliably estimate the loading observed from in situ monitoring for small catchments

The traceability of groundwater nitrate pollution is crucial for controlling and managing polluted groundwater. This study integrates hydrochemistry, nitrate isotope (δ 15 N-NO 3 − and δ 18 O-NO 3 − ), and self-organizing map (SOM) and end-member mixing (EMMTE) models to identify the sources and quantify the contributions of nitrate pollution to groundwater in an intensive agricultural region in the Sha River Basin in southwestern Henan Province. The results indicate that the NO 3 − -N concentration in 74% (n = 39) of the groundwater samples exceeded the WHO standard of 10 mg/L. According to the results of EMMTE modeling, soil nitrogen (68.4%) was the main source of nitrate in Cluster-1, followed by manure and sewage (16.5%), chemical fertilizer (11.9%) and atmospheric deposition (3.3%). In Cluster-2, soil nitrogen (60.1%) was the main source of nitrate, with a significant increase in the contribution of manure and sewage (35.5%). The considerable contributions of soil nitrogen may be attributed to the high nitrogen fertilizer usage that accumulated in the soil in this traditional agricultural area. Moreover, it is apparent that most Cluster-2 sampling sites with high contributions of manure and sewage are located around residential land. Therefore, the arbitrary discharge and leaching of domestic sewage may be responsible for these results. Therefore, this study provides useful assistance for the continuous management and pollution control of groundwater in the Sha River Basin.