δ15N Of NO3 Research Articles

Variation of nitrate sources affected by precipitation with different intensities in groundwater in the piedmont plain area of alluvial-pluvial fan

A substantial reservoir of nitrogen (N) in soil poses a threat to the quality and safety of shallow groundwater, especially under extreme precipitation that hastens nitrogen leaching into groundwater. However, the specific impact of varying precipitation intensities on the concentration and sources of nitrate (NO3−) in groundwater across diverse hydrogeological zones and land uses remains unclear. This study aims to elucidate the fluctuations in NO3− concentration, sources, and controlling factors in shallow groundwater under different intensities of precipitation (extreme heavy precipitation and continuous heavy precipitation) in a typical alluvial-pluvial fan of the North China Plain by using stable isotopes (δ2H–H2O, δ18O–H2O, δ15N–NO3-, δ18O–NO3-), hydrochemical analyses and the SIAR model. Affected by extreme heavy precipitation the depleted isotopes of δ2H–H2O and δ18O–H2O in groundwater of the entire area suggested the rapid recharge of fast flow by precipitation. The enriched isotopes of δ2H–H2O and δ18O–H2O of north part in alluvial fan after continuous heavy precipitation showed the recharge of translatory flow of soil water. NO3−concentrations increased to 78.9 mg/L after extreme heavy precipitation and increased to 105.3 mg/L after continuous heavy precipitation when compared to those in normal year (56.8 mg/L) of north part of the alluvial fan. However, NO3− concentrations had slight variation after continuous heavy precipitation of south part of the fan due to the deep vadose zone. The contribution ratio of sources of NO3− in groundwater by using SIAR analysis revealed manure & sewage (MS) as the primary NO3− source (accounting for 59.7–78.1%) before extreme heavy precipitation, chemical fertilizer (CF) making a minor contribution (6.9–17.3%). Different precipitation events and land use types lead to changes in NO3− sources. Affected by extreme heavy precipitation, the contribution of MS decreased while CF increased, particularly in vegetables (26.2–28.1%) and farmland (29.2–34.7%). After continuous heavy precipitation, MS increased again, particularly in vegetables (50.0%) and farmlands (20.4–66.4%), with CF either increasing or remaining steady. This indicated that continuous heavy precipitation accelerated the leaching of nitrogen (organic manure application) stored in deep soil to groundwater and it has a larger influence on the increasing of NO3− concentrations of groundwater than extreme heavy precipitation which carried nitrogen (chemical fertilizer application) in shallow soil to groundwater by fast flow. These findings underscore the importance of considering soil chemical N stores and their implications for groundwater contamination mitigation under future extreme climate scenarios, particularly in agricultural management practices.

Nitrate sources and transformations in a river-reservoir system: Response to extreme flooding and various land use

Nitrate (NO3−) pollution poses a global aquatic threat, promoting eutrophication and endangering human health. Large floods can swiftly drive significant nitrogen load into rivers and downstream water bodies, nevertheless, precisely quantifying NO3− sources in watersheds with complex land use patterns remains challenging. In June 2020, heavy and persistent rainfall triggered severe floods across southeastern China, causing Lake Qiandao, the largest reservoir in this region, to reach its highest recorded level. This study investigated NO3− sources and transformations within the Xin'an River-Lake Qiandao system, particularly focusing on the impact of the catastrophic summer flooding event. Through extensive sampling, we identified a distinct nitrogen concentration gradient: from the pristine Xin'an River source, characterized by 95 % forest cover and consistently low nitrogen levels for the initial 125 km, to a gradual rise as the river traversed human-impacted regions, culminating in peak concentrations within residential and agricultural areas. Isotopic analysis identified a general upward trend in nitrate stable isotope (δ15N–NO3–) associated with the expansion of agricultural and urban lands. Mean values of δ15N–NO3– gradually increased from headwaters and forest-dominant catchments (+2.88 ‰) to agricultural regions (+6.64 ‰) and residential areas (+7.10 ‰). A Bayesian modeling identified that during the flood season, soil erosion and chemical fertilizers were the primary contributors, collectively accounting for 74 % of NO3− sources in the Xin'an River. However, accumulated sewage in tributaries and the overflow of sewage from treatment plants, exacerbated by the flood, significantly impacted Lake Qiandao, particularly in densely populated urban areas, contributing 53 % of the total NO3− input. Additionally, NO3− levels and isotopic values in Lake Qiandao were influenced by a mixture of sources and nitrogen cycling processes, including nitrification and algae assimilation following the flood event. In contrast, during baseflow conditions, the contribution of domestic sewage and livestock wastewaters increased to 41 % in the upper river, while Lake Qiandao remained affected by non-point source pollution, with soil erosion and chemical fertilizers contributing 58 % to the total nitrogen pollution. This study sheds light on the complex dynamics of NO3− dynamics within river–reservoir systems, particularly in the context of extreme flooding, emphasizing the critical need for comprehensive water quality management strategies along the entire watershed.

Coupled processes involving ammonium inputs, microbial nitrification, and calcite dissolution control riverine nitrate pollution in the piedmont zone (Qingshui River, China)

Rivers in agricultural countries widely suffer from diffuse nitrate (NO3−) pollution. Although pollution sources and fates of riverine NO3− have been reported worldwide, the driving mechanisms of riverine NO3− pollution associated with mineral dissolution in piedmont zones remain unclear. This study combined hydrogeochemical compositions, stable isotopes (δ18O–NO3−, δ15N–NO3−, δ18O–H2O, and δ2H–H2O), and molecular bioinformation to determine the pollution sources, biogeochemical evolution, and natural attenuation of riverine NO3− in a typical piedmont zone (Qingshui River). High NO3− concentration (37.5 ± 9.44 mg/L) was mainly observed in the agricultural reaches of the river, with ~15.38 % of the samples exceeding the acceptable limit for drinking purpose (44 mg/L as NO3−) set by the World Health Organization. Ammonium inputs, microbial nitrification, and HNO3-induced calcite dissolution were the dominant driving factors that control riverine NO3− contamination in the piedmont zone. Approximately 99.4 % of riverine NO3− contents were derived from NH4+-containing pollutants, consisted of manure & domestic sewage (74.0 % ± 13.0 %), NH4+-synthetic fertilizer (16.1 % ± 8.99 %), and soil organic nitrogen (9.35 % ± 4.49 %). These NH4+-containing pollutants were converted to HNO3 (37.2 ± 9.38 mg/L) by nitrifying bacteria, and then the produced HNO3 preferentially participated in the carbonate (mainly calcite) dissolution, which accounted for 40.0 % ± 12.1 % of the total riverine Ca2+ + Mg2+, also resulting in the rapid release of NO3− into the river water. Thus, microbial nitrification could be a new and non-negligible contributor of riverine NO3− pollution, whereas the involvement of HNO3 in calcite dissolution acted as an accelerator of riverine NO3− pollution. However, denitrification had lesser contribution to natural attenuation for high NO3− pollution. The obtained results indicated that the mitigation of riverine NO3− pollution should focus on the management of ammonium discharges, and the HNO3-induced carbonate dissolution needs to be considered in comprehensively understanding riverine NO3− pollution in piedmont zones.

Apportionment of NOx and NH3 emission sources in an urban coastal airshed: Insights from stable isotopes and a novel approach to intermittent sources

Inorganic nitrogen (IN) wet deposition flux and emission sources have been intensively investigated over the past two decades with a primary focus in rural areas and large metropolitan cities. Smaller coastal cities are often overlooked in these studies despite a substantial portion of the earth's population inhabiting these regions. Ammonia and NOx emission source apportionment estimations in these studies can be misleading without proper inspection of intermittent source (e.g., biomass burning and lightning) impacts on the airmass of rain events. This study measured the chemical composition (NH4+, NO3−, Cl−, SO42− and pH) and isotopic composition of ammonium and nitrate (δ15N–NH4+, δ15N–NO3-, and δ18O–NO3-) in rainwaters collected in a small coastal city (Corpus Christi, TX, USA) to aid in determining the emission source apportionments of NH3 and NOx. A novel approach coupling lightning and fire/smoke mapping products along with airmass back trajectories was developed to help determine the influence of biomass burning and lightning on individual rain events before applying to isotope source apportionment models. The annual IN wet deposition was 3.9 kg N·ha−1·yr−1, in which NH4+ and NO3− constituted 65% and 35%, respectively. Isotope mixing model results suggest vehicle emission contribution to NH3 can rival agriculture sources (i.e., fertilizer and livestock waste) in urban areas. Vehicles, biomass burning and lightning were significant NOx sources, while soil “biogenic” emissions increased substantially in certain rain events occurring in warmer months. This work qualified IN wet deposition flux and estimated IN emission source apportionments in a coastal small-sized city, which will inform regional N emission regulations and watershed restoration practices.

Assessment major NOx sources to nitrate of TSP around the Danjiangkou reservoir using isotopes and a Bayesian isotope mixing model

Nitrate (NO3−) is a key component of atmospheric particulate matter. However, studies quantifying NO3− formation and sources in water source areas remain scarce. This study employs NO3− concentration, dual isotopic analyses (δ15N, δ18O), and SIAR and PSCF models to elucidate NO3− formation mechanisms and apportion sources in the Danjiangkou Reservoir area. The result showed that the concentration of NO3− (0.52 μg m−3-2.39 μg m−3), δ15N–NO3- (−2.3‰-4.2‰) and δ18O–NO3- (64.2‰–96.6‰) values varied seasonally in total suspended particulate matter (TSP). The contribution of the NO2 + ·OH pathway was the highest in summer (84%), while the main contributions came from the N2O5 + H2O and NO3 + VOCs pathways in winter (75%). The SIAR model revealed that the contributions of coal combustion and traffic emissions were 34% and 23%, respectively, indicating that fossil fuel release contributed most. Moreover, the contribution of biomass combustion sources was 31%, and soil emission sources had the least (12%). Considering liquefied petroleum gas, gasoline, and diesel vehicles as traffic emission sources, their contribution to NO3− in TSP increased to 50%. The PSCF analysis indicated that external input areas were predominantly from central Henan Province, northern Hubei Province, and central Shaanxi Province. This study quantitatively analyzed the sources of NO3− in atmospheric particulate matter in water sources for the first time, indicating that controlling the emissions of nitrogen oxides from gasoline cars, heavy-duty diesel trucks, and ships could help alleviate haze pollution and enhance air quality in the Danjiangkou reservoir area.

Vadose zone flushing of fertilizer tracked by isotopes of water and nitrate

AbstractA substantial fraction of nitrogen (N) fertilizer applied in agricultural systems is not incorporated into crops and moves below the rooting zone as nitrate (NO3−). Understanding mechanisms for soil N retention below the rooting zone and leaching to groundwater is essential for our ability to track the fate of added N. We used dual stable isotopes of nitrate (δ15N–NO3− and δ18O–NO3−) and water (δ18O–H2O and δ2H–H2O) to understand the mechanisms driving nitrate leaching at three depths (0.8, 1.5, and 3.0 m) of an irrigated corn field sampled every 2 weeks from 2016 to 2020 in the southern Willamette Valley, Oregon, USA. Distinct periods of high nitrate concentrations with lower δ15N–NO3− values indicated that a portion of that nitrate was from recent fertilizer applications. We used a mixing model to quantify nitrate fluxes associated with recently added fertilizer N versus older, legacy soil N during these “fertilizer signal periods.” Nitrate leached below 3.0 m in these periods made up a larger proportion of the total N leached at that depth (∼52%) versus the two shallower depths (∼13%–16%), indicating preferential movement of recently applied fertilizer N through the deep soil into groundwater. Further, N associated with recent fertilizer additions leached more easily when compared to remobilized legacy N. A high volume of fall and winter precipitation may push residual fertilizer N to depth, potentially posing a larger threat to groundwater than legacy N. Optimizing fertilizer N additions could minimize fertilizer losses and reduce nitrate leaching to groundwater.

Water-soluble ions and nitrogen and oxygen stable isotope ratios in nitrate in sake in Akita, Japan

Sake is a traditional alcoholic beverage in Japan, and demand for it has increased overseas in recent years. In this study, we analyzed oxygen and nitrogen of stable isotope ratios in nitrate (δ18O–NO3– and δ15N–NO3–) in beverages by using the denitrifying bacteria for the first time. We measured the concentrations of five water-soluble ions and the δ18O–NO3– and δ15N–NO3– values of 49 commercial sakes (11 sake breweries) made in Akita Prefecture, Japan. The similarity of the concentrations of NO3− in sake and groundwater suggested that these ions were derived from brewing water. The δ18O–NO3– value of the 49 sake samples ranged from 2.2 to 12.3‰ (mean, 4.7 ± 2.3‰). The δ15N–NO3– value in the sake ranged from 1.8 to 8.7‰ (mean, 6.1 ± 1.8‰). The δ15N–NO3– value of sake could be used to identify breweries because the δ15N–NO3– value was similar for different sake types in the same brewery. Principal component analysis revealed that the first four principal components could account for 85.2% of the variance of the experimental data. The factor loading plots was found to be similar for different sake types in the same brewery.

Continental Emissions Influence the Sources and Formation Mechanisms of Marine Nitrate Aerosols in Spring Over the Bohai Sea and Yellow Sea Inferred From Stable Isotopes

AbstractThe influence of continental emissions on the origin and formation mechanisms of atmospheric particulate nitrate (ρ‐NO3−) aerosols in the marine boundary layer remains unclear. Here, synchronous observations of nitrogen isotope ratios (δ15N–NO3−) and oxygen isotope anomaly (Δ17O–NO3−) in ρ‐NO3− were conducted across the Yellow Sea and Bohai Sea in Eastern China. Nitrate concentrations, δ15N–NO3− and Δ17O–NO3− exhibited a pronounced north‐to‐south latitudinal gradient. Combined with backward air mass trajectory analysis, the high nitrate concentration and isotopic characteristics in the northern sea area were found to be affected by the continental outflow near China while the low values in the southern sea area were more related to the oceanic inflow. Stable isotope analysis in R (SIAR) indicated that near the northern sea area, the nitrate radicals (NO3) reacted with hydrocarbons (HC) or dimethyl sulfides (DMS) pathway (NO3 + HC/DMS) played a leading role in nitrate production, whereas the NO2 + OH pathway dominated near the southern sea area. Nitrate in the northern seas originated mainly from nitrogen oxides (NOx = NO + NO2, the gaseous precursor of nitrate) emitted from continental sources, especially coal combustion and biomass burning. While closer to the southern seas, the proportion of NOx generated in the marine environment (from ship and biogenic emissions) increased. Overall, the differential relative contributions of continental and marine atmospheric chemistry and NOx sources lead to the spatial distribution characteristics of atmospheric nitrate concentrations and isotopic values over the Yellow and Bohai Seas.

Sources and transformation of nitrate in an agricultural riverine reservoir watershed of the Sichuan Basin

Elucidating the origins and transformations of nitrate in groundwater and surface water within agricultural regions is imperative for effectively managing watershed contamination and environmental improvement. This study conducted a comprehensive geochemical investigation using multiple stable isotopes (δD-H2O and δ18O–H2O, δ15N–NO3- and δ18O–NO3-, and δ15N–NH4+) and hydrogeochemical characteristics to explore the potential sources and transformation of nitrate in groundwater and reservoir water within an agricultural riverine reservoir watershed. The results indicated that nitrate pollution in groundwater exhibited a higher severity compared to the reservoir water within this watershed, thereby posing potential risks to the quality of the reservoir water through hydraulic connection. The integration of δ15N–NH4+ and dual-isotopes of nitrate, combined with hydrogeochemical analyses, revealed that the presence of NO3− in groundwater and reservoir water primarily originated from manure/sewage sources. Moreover, intricate biogeochemical processes containing denitrification, nitrification, and DNRA were identified within this watershed. The findings of the Bayesian stable isotope mixing model (SIAR) further corroborated the conclusion that manure/sewage was the primary source of nitrate. It revealed that manure/sewage accounted for an average of 46 % of groundwater NO3− concentration, followed by atmospheric precipitation (25 %), soil nitrogen (18 %), and chemical fertilizer (11 %). There were no distinct differences in nitrate source contributions between groundwater and reservoir water. For reservoir water, manure or sewage (45 % on average) > atmospheric precipitation (28 %) > soil nitrogen (16 %) > chemical fertilizer (11 %). This study demonstrates the effectiveness of incorporating δ15N–NH4+ with dual isotopes of nitrate and hydrogeochemical analyses, serving as a reliable methodology that yields valuable insights into the origins and transformations of NO3− in an agricultural reservoir watershed, with potential applicability to other regions.

Nitrate leaching characteristics of red soils from different parent materials in subtropical China

Globally, nitrate (NO3−) leaching from agroecosystems has been of major concern. There is evidence that NO3− leaching exhibits intense seasonal variation in subtropical regions. However, influencing factors to the seasonal dynamics remain unclear. In this study, a two-year field lysimeters experiment was conducted with three red soils derived from different parent materials (Quaternary red clay (QR), red sandstone (RS), and basalt (BA)). An N fertilizer (15N-enriched urea, 10 atom% excess) of 200 kg N ha−1 yr−1 was applied for maize. The effect of parent material on NO3− leaching characteristics was examined in surface (0–20 cm) and subsoil (20–100 cm) layers. The results showed due to the weakening of abundant drainage, there was no significant effect of parent materials on NO3− leaching characteristics in surface layers. Environmental factors (precipitation and temperature) and fertilization together led to obvious seasonal characteristics, i.e. abundant NO3− leaching during both crop growth and fallow periods. In subsoil layers, NO3− leaching characteristics were completely different among three soils. The concentrations and δ15N of NO3− in QR and RS soils showed a continuous increase after first year's fertilization, while those in BA soil remained relatively stable after reaching peak levels around harvest in first year. Meanwhile, the NO3− leaching amount in BA soil was significantly lower than in the other two soils. These might be explained by different NO3− adsorption capacities caused by the differences in mineral composition and free iron and aluminium contents. These elucidated in subsoil layers, NO3− leaching characteristics highly depended on parent materials. Meanwhile, adsorption capacity was limited and cannot slow NO3− leaching in the long run. Our results suggest that seasonal variation of NO3− leaching in surface layers and temporary retardant effect from NO3− adsorption capacity in subsoil layers should receive much attention when calculating and predicting NO3− leaching in subtropical regions.

A Quantitative Approach for Identifying Nitrogen Sources in Complex Yeongsan River Watershed, Republic of Korea, Based on Dual Nitrogen Isotope Ratios and Hydrological Model

Effective management of nitrate loading in complex river systems requires quantitative estimation to trace different nitrogen sources. This study aims to validate an integrated framework using soluble nitrogen isotope ratios (δ15N–NH4 and δ15N–NO3) and hydrological modeling (hydrological simulation program SPARROW) of the main stream and tributaries in the Yeongsan River to determine anthropogenic nitrogen fluxes among different land-use types in the complex river watershed. The δ15N–NH4 and δ15N–NO3 isotopic compositions varied across different land-use types (4.9 to 15.5‰ for δ15N–NH4 and −4.9 to 12.1‰ for δ15N–NO3), reflecting the different sources of nitrogen in the watershed (soil N including synthetic fertilizer N, manure N, and sewage treatment plant effluent N). We compared the soluble nitrogen isotopic compositions (δ15N–NH4 and δ15N–NO3) of the river water with various nitrogen sources (soil N, manure N, and sewage N) to assess their contribution, revealing that N from sewage treatment plant effluent as a point source was dominant during the dry season and N from forest- and soil-derived non-point sources was dominant due to intensive rainfall during the wet season. The coefficient of determination (R2) between the measured pollution load and the predicted pollution load calculated by the SPARROW model was 0.95, indicating a high correlation. In addition, the EMMA-based nitrogen contributions compared to the SPARROW-based nitrogen fluxes were similar to each other, indicating that large amounts of forest- and soil-derived N may be transported to the Yeongsan River watershed as non-point sources, along with the effect of sewage treatment plant effluent N as a point source. This study provides valuable insights for the formulation of management policies to control nitrogen inputs from point and non-point sources across different land-use types for the restoration of water quality and aquatic ecosystems in complex river systems. Given the recent escalation in human activity near aquatic environments, this framework is effective in estimating the quantitative contribution of individual anthropogenic nitrogen sources transported along riverine systems.

Using PMF model and Bayesian model to quantify NOx sources in non-typhoons and Super Typhoon Lekima (2019) in Hangzhou

Quantifying and comparing nitrate (NO3−) sources in typhoons and non-typhoons is important for controlling and reducing their emissions. In this study, the chemical compositions and stable isotopes (δ15N–NO3- and δ18O–NO3-) of non-typhoon and Typhoon Lekima precipitation in Hangzhou (from May–August during 2019 and 2020) were determined. The results revealed that severe acid rain occurred in Hangzhou, except during Typhoon Lekima. According to the positive matrix factorization (PMF) model results, the ion contributions of sources in non-typhoons were as follows: secondary inorganic aerosols + biomass burning > sea salt aerosols > agricultural sources > terrestrial sources, whereas sea salt aerosols + biomass burning were the primary contributors of ions in Typhoon Lekima. The δ15N–NO3- values of non-typhoon and Typhoon Lekima ranged from −4.0 to −1.3‰ and −2.7 to 5.8‰, respectively. The δ18O–NO3- values of non-typhoon and Typhoon Lekima ranged from 56.3‰ to 74.7‰ and 32.2–55.3‰, respectively. The calculated contribution of N2O5 pathway in NOx oxidation to NO3− during non-typhoon precipitation was 64.7 ± 12.2%. If the contribution of OH· pathway during Typhoon Lekima precipitation was 87%, the Monte Carlo simulation results demonstrated that the estimated oxidation proportions of NO via O3 and RO2 (or HO2) ranged from 48.8 to 100% and 0–51.2%, respectively. Based on the improved Bayesian model combined with nitrogen isotope fractionation, the NOx contributions from four sources (biomass burning > coal combustion > mobile sources > microbial N cycle) in non-typhoon precipitation ranged from 21 to 30% (Ԑ = 3.6). In Typhoon Lekima (Ԑ = 5.2), lightning (23.6 ± 11.2%) was one of the primary NOx contributors and a lower contribution from microbial N cycle (17.9 ± 5.4%) compared with the PMF model (22.8%). Moreover, for Typhoon Lekima, the sum of biomass burning, coal combustion, and mobile sources in the improved Bayesian model (58.5%) was slightly lower than that of secondary inorganic aerosols in the PMF model (62.4%). Therefore, the coupled PMF–improved Bayesian model is a feasible and reliable method for source identification and apportionment of NO3− in precipitation.

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.

Using dual stable isotopes method for nitrate sources identification in Cao-E River Basin, Eastern China

Excess nitrate (NO3−) of water is a worldwide environmental problem. Therefore, identifying the sources and analyzing respective contribution rates are of great importance for improving water quality. The current study was carried out to identify the potential sources of NO3− pollution in Cao-E River basin, in Eastern China. Surface water samples were collected during the dry season and wet season. Multiple hydrochemical indices, dual NO3− isotopes (δ15N–NO3− and δ18O–NO3−) and a Bayesian model (stable isotope analysis in R, MixSIAR) were applied to identify NO3− sources and estimate the proportional contributions of multiple NO3− sources. During the sampling period, nitrification was a dominant nitrogen transformation process in the study area. The results of the NO3− isotopes suggested that manure and sewage (M&S), soil nitrogen (SN) and nitrogen fertilizer (NF) were the major contributors to NO3−. Moreover, the results obtained from the MixSIAR model showed that the proportional contributions of atmospheric deposition (AD), NF, M&S and SN to NO3− were 2.82, 15.45, 44.25, 37.47% and 3.14, 23.39, 31.78, 41.69% in the dry and wet season, respectively. This study provided evidence to further understand the sources, transport, and transformation of N in Cao-E River basin, which deepens the understanding of the management of N contaminant.

Nitrogen sources and conversion processes in shallow groundwater around a plain lake (Northwest China): Evidenced by multiple isotopes and water chemistry

The groundwater quality is severely impacted by Nitrate (NO3−-N) pollution worldwide. Effective lake quality management depends on understanding the origin and fate of nitrogen (N) in the groundwater around lakes. This study combined data for multiple stable isotopes (δ2H–H2O and δ18O–H2O, δ15N–NO3 and δ18O–NO3) and hydrochemistry with the hydrodynamic monitoring profile and a Bayesian isotope mixing (MixSIAR) model to clarify the sources and transformation of N within shallow groundwater around Shahu Lake in the arid area plain of Northwest China. In May 2022, multiple water samples were collected from aquifers (n = 33), drainage water (n = 1), channel water (n = 1), and lake water (n = 7). The results showed that 57% of groundwater samples had high NO3−-N concentrations exceeding the World Health Organisation threshold for drinking water (10 mg/L). The high variation in δ15N–NO3 (from −9.21‰ to +27.57‰) and δ18O–NO3 (from −8.32‰ to +19.04‰) revealed multiple N sources and conversion processes. According to nitrate isotopes and the MixSIAR model, N fertilizer, soil organic N and manure, and sewage are the main sources of nitrogen in groundwater and lake water, which account for 40.61%, 35.86%, and 21.55% of groundwater NO3−-N, respectively, and 35.07%, 34.43%, and 27.49% of lake water NO3−-N. Hydrodynamic monitoring combined with water isotopes showed that upper groundwater (5–10 m) within 1.22 km of the adjacent lake shore strongly interacted with the lake. In groundwater, nitrification predominated, while local denitrification remained a possibility. In conclusion, this research offers a comprehensive approach to determining the sources and conversion of N in contaminated groundwater.

Formation pathways and source apportionments of inorganic nitrogen-containing aerosols in urban environment: Insights from nitrogen and oxygen isotopic compositions in Guangzhou, China

Air quality issues caused by PM2.5-induced persistent extreme haze episodes have become increasingly serious in urban environments in recent years. Secondary water-soluble inorganic sulfate (SO42−), nitrate (NO3−) and ammonium (NH4+) ions are the major components of PM2.5. However, the contributions of inorganic nitrogen species, especially nitrate, to PM2.5 have greatly increased during haze episodes in many cities in China. Therefore, better understanding of their emission sources and formation pathways holds the key to controlling urban PM2.5 pollution more efficiently and effectively. In this study, water-soluble ionic characteristics and isotopic compositions and sources of NH4+ and NO3−, as well as NO3− formation pathways were determined in PM2.5 aerosol samples collected in Guangzhou, China, during 2015–2018. The PM2.5 concentrations varied from 30.5 to 189.8 μg⋅m−3 and their mean values were highest in spring (111.1 μg⋅m−3) and lowest in summer (63.5 μg⋅m−3). The δ15N–NH4+ and δ15N–NO3- values ranged from +4.5 to +20.2‰ and from +4.8 to +14.8‰, respectively, with their mean values being highest in winter (14.0‰ and 9.5‰, respectively) and lowest in summer (10.4‰ and 7.1‰, respectively). The seasonal δ15N variability was mainly attributed to isotopic equilibrium fractionation, and partly due to the changes in NH3 and NOx sources. The average δ18O–NO3- value of 63.0‰, ranging seasonally from +58.6‰ in summer to +68.0‰ in spring, suggests that NO2 +⋅OH pathway played a vital role (70.3–85.9%) in NO3− formation. The Bayesian isotope mixing model results revealed fossil fuel combustion sources as dominant sources of atmospheric NH3 and NOx. This study suggests that more effort should be devoted to reduce NH3 and NOx from combustion-related processes and highlights the importance of δ18O–NO3- analysis for exploring variations of nitrate formation pathways in urban atmospheres.

A seasonal analysis of aerosol NO3− sources and NOx oxidation pathways in the Southern Ocean marine boundary layer

Abstract. Nitrogen oxides, collectively referred to as NOx (NO + NO2), are an important component of atmospheric chemistry involved in the production and destruction of various oxidants that contribute to the oxidative capacity of the troposphere. The primary sink for NOx is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NOx sources and NOx-to-NO3- formation pathways remain poorly constrained in the remote marine boundary layer of the Southern Ocean, particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ15N, δ18O, and Δ17O) of atmospheric nitrate in coarse-mode (> 1 µm) aerosols, collected between South Africa and the sea ice edge in summer, winter, and spring. Similar latitudinal trends in δ15N–NO3- were observed in summer and spring, suggesting similar NOx sources. Based on δ15N–NO3-, the main NOx sources were likely a combination of lightning, biomass burning, and/or soil emissions at the low latitudes, as well as oceanic alkyl nitrates and snowpack emissions from continental Antarctica or the sea ice at the mid-latitudes and high latitudes, respectively. Snowpack emissions associated with photolysis were derived from both the Antarctic snowpack and snow on sea ice. A combination of natural NOx sources, likely transported from the lower-latitude Atlantic, contribute to the background-level NO3- observed in winter, with the potential for a stratospheric NO3- source evidenced by one sample of Antarctic origin. Greater values of δ18O–NO3- in spring and winter compared to summer suggest an increased influence of oxidation pathways that incorporate oxygen atoms from O3 into the end product NO3- (i.e. N2O5, DMS, and halogen oxides (XO)). Significant linear relationships between δ18O and Δ17O suggest isotopic mixing between H2O(v) and O3 in winter and isotopic mixing between H2O(v) and O3/XO in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate chlorine chemistry with the potential to increase peroxy radical concentrations, contributing to oxidant chemistry in the marine boundary layer. As a result, isotopic mixing with an additional third end-member (atmospheric O2) occurs in spring.

Treated wastewater and weak removal mechanisms enhance nitrate pollution in metropolitan rivers

The focus of urban water environment renovation has shifted to high nitrate (NO3−) load. Nitrate input and nitrogen conversion are responsible for the continuous increase in nitrate levels in urban rivers. This study utilized nitrate stable isotopes (δ15N–NO3− and δ18O–NO3−) to investigate NO3− sources and transformation processes in Suzhou Creek, located in Shanghai. The results demonstrated that NO3− was the most common form of dissolved inorganic nitrogen (DIN), accounting for 66 ± 14% of total DIN with a mean value of 1.86 ± 0.85 mg L−1. The δ15N–NO3− and δ18O–NO3− values ranged from 5.72 to 12.42‰ (mean value: 8.38 ± 1.54‰) and −5.01 to 10.39‰ (mean value: 0.58 ± 1.76‰), respectively. Based on isotopic evidence, the river received a significant amount of nitrate through direct exogenous input and sewage ammonium nitrification, while nitrate removal (denitrification) was insignificant, resulting in nitrate accumulation. Analysis using the MixSIAR model revealed that treated wastewater (68.3 ± 9.7%), soil nitrogen (15.7 ± 4.8%) and nitrogen fertilizer (15.5 ± 4.9%) were the main sources of NO3− in rivers. Despite the fact that Shanghai's urban domestic sewage recovery rate has reached 92%, reducing nitrate concentrations in treated wastewater is crucial for addressing nitrogen pollution in urban rivers. Additional efforts are needed to upgrade urban sewage treatment during low flow periods and/or in the main stream, and to control non-point sources of nitrate, such as soil nitrogen and nitrogen fertilizer, during high flow periods and/or tributaries. This research provides insights into NO3− sources and transformations, and serves as a scientific basis for controlling NO3− in urban rivers.

Identifying nitrate sources and transformations in an agricultural watershed in Northeast China: Insights from multiple isotopes

Accurate identification of riverine nitrate sources is required for preventing and controlling nitrogen contamination in agricultural watersheds. The water chemistry and multiple stable isotopes (δ15N–NO3, δ18O–NO3, δ2H–H2O, and δ18O–H2O) of the river water and groundwater in an agricultural watershed in China's northeast black soil region were analyzed to better understand the sources and transformations of riverine nitrogen. Results showed that nitrate is an important pollutant that affects water quality in this watershed. Affected by factors such as seasonal rainfall changes and spatial differences in land use, the nitrate concentrations in the river water showed obvious temporal and spatial variations. The riverine nitrate concentration was higher in the wet season than in the dry season, and higher downstream than upstream. The water chemistry and dual nitrate isotopes revealed that riverine nitrate came primarily from manure and sewage (M&S). Results from the SIAR model showed that it accounted for more than 40% of riverine nitrate in the dry season. The proportional contribution of M&S decreased during the wet season due to the increased contribution of chemical fertilizers and soil nitrogen induced by large amounts of rainfall. The δ2H–H2O and δ18O–H2O signatures implied that interactions occurred between the river water and groundwater. Considering the large accumulation of nitrates in the groundwater, restoring groundwater nitrate levels is essential for controlling riverine nitrate pollution. As a systematic study on the sources, migration, and transformations of nitrate/nitrogen in agricultural watersheds in black soil regions, this research can provide a scientific support for nitrate pollution management in the Xinlicheng Reservoir watershed and provide a reference for other watersheds in black soil regions in the world with similar conditions.

National-scale investigation of dual nitrate isotopes and chloride ion in South Korea: Nitrate source apportionment for stream water

Nitrate sources in surface water have been identified using dual-isotope compositions of nitrate with various tools to efficiently manage the water quality at the local scale. Correlation between Cl and NO3 has also been used to identify NO3. In this study, we assess the reliability of the dual-isotope approach and Cl in terms of nitrate source apportionment. To this end, we collected stream water samples throughout South Korea to estimate nitrate sources in streams and determine whether the land-use pattern was closely related to nitrate sources. The δ15N–NO3 ranging from −1.3 to 14.8‰ showed a spatial distribution that was lower in mountain ranges (<7‰) than plain areas (>8‰). The Cl concentration in this national-scale distribution was also assessed. The relationship between the proportion of Cl and δ15N–NO3 classifies nitrate sources into areas characterized by three land-use patterns: (1) agricultural and business areas, (2) forests in highlands, and (3) lowland forests, of which (1) had proportions of Cl >50%, while (2) and (3) were <50%. The samples in (3) showed δ15N–NO3 values > 6‰, similar to those of (1). Deuterium excess of samples was negatively correlated (R2 = 0.53) with δ15N–NO3, accounting for the fact that δ15N–NO3 reflected land-use patterns. Samples were dominantly affected by agriculture-derived sources and domestic sewage showed NO3/Cl of <0.4 and δ15N–NO3 of >6‰. These results suggest that nitrate source apportionment should be comprehensively evaluated considering the dual-isotope approach, land-use patterns, and Cl proportions.