Isotopic Composition Of Nitrate Research Articles

Experimental Study on the Impact of Sorption-Desorption on Nitrate Isotopes

Nitrate pollution is a global environmental problem, and mean nitrate levels have risen by an estimated 36% in global waterways since 1990. Tracing nitrate sources is important for water quality management, and nitrate isotopes (δ15N-NO3− and δ18O-NO3−) are commonly used for this purpose because of the different isotopic compositions of different sources. However, the impact of nitrate sorption on matrix and desorption from matrix on N and O isotopic composition of nitrate in liquid phase has not been well clarified. To explore the mechanism for the changes in nitrate concentration and isotopes in liquid phase during sorption and desorption, this study took a shale sample (enriched in clay minerals and commonly exposed in the Earth), conducted a series of laboratory experiments for nitrate sorption and desorption, and studied the impact of sorption and desorption on nitrate N and O isotopic composition in liquid phase. The results showed that the shale sample exhibited a rapid sorption and desorption rate for nitrate in the surface water samples, with the nitrate concentration in the solution decreasing from 14.3 mg/L to 4.1 mg/L within 5 min. The sorption data fit the Langmuir model better than that of the Freundlich model. The maximum possible sorption (Qmax) for the shale sample was estimated to be 46 μg/g. Preliminary laboratory experiments showed that changes in δ15N-NO3− values were not obvious, and changes in δ18O-NO3− values in liquid phase were minor during sorption and desorption of the shale sample, suggesting that nitrogen isotopic fractionation can be neglected, and the sorption of nitrate by the shale sample has a very limited impact on the distribution of nitrate isotopes in liquid phase. However, the impact of nitrate desorption on the nitrate isotopes in liquid phase depends on the isotopic composition of exchangeable nitrate in the solid phase, which may be related to antecedent water–rock interactions. This study provides important information for elucidating the evolution mechanism of nitrate and its isotopic compositions following sorption-desorption, and is conducive to revealing the nitrogen cycle law in the environment.

Hydrochemical and dual-isotope approach to the identification of denitrification in arable field drainage in the Wensum catchment, eastern England

The global pool of reactive nitrogen has doubled in the last century in response to the need to increase food production with the consequent increase in fertiliser-derived reactive nitrogen detrimentally affecting aquatic ecosystems. This study investigates the spatial distribution and significance of denitrification in the lowland, agriculturally-impacted River Wensum catchment in eastern England as a natural attenuation process. To investigate the evidence for denitrification, the hydrochemical characteristics and dual stable isotope composition of nitrate (15N and 18O) were measured over a 15-month period, 2015–2017, in 63 samples of field drainage in predominantly clay loam and sandy clay loam soils under mainly arable cultivation. Microbially-mediated denitrification in field drainage was indicated by the gradient of the linear regression of 15NNO3 and 18ONO3 compositions with a value of 0.58. Dual fractionation of the nitrate isotopes yielded enrichment factors for δ15NNO3 (−4.52‰) and δ18ONO3, (−4.51‰) within the reported ranges for denitrification in aquatic studies. Soil type influenced denitrification, with a positive relationship between percentage clay and δ15NNO3 and δ18ONO3 values. The same relationship was observed for denitrification rates calculated via a simple mass balance approach, which ranged from 11.0 to 26.3 kg N ha−1 and accounted for 30–73% of the leached soil nitrogen. Higher denitrification rates were recorded in drainage areas with a greater soil clay content (>20% by weight). Comparing calculated dentification rates for individual drain areas with median δ15NNO3 values of drain samples demonstrated that an isotopic enrichment of +1‰ is associated with a denitrification rate of 2.6 kg N ha−1. In conclusion, sustainable agricultural practices that maintain natural attenuation processes such as denitrification, for example by preserving and increasing the soil organic carbon content, are desirable to improve overall soil health to support ecosystem services that reduce nitrate pollution.

Modeling the Complete Nitrogen and Oxygen Isotopic Imprint of Nitrate Photolysis in Snow

AbstractSnow nitrate is vulnerable to photolytic loss that causes isotopic alteration, and thus its isotopes can potentially track the extent of snow nitrate photolysis and its impacts in environments where loss is significant. Large increases in δ15N‐NO3− below the snow surface have been attributed to photolysis and this behavior is generally consistent amongst theoretical as well as lab and field studies. Oxygen isotope ratios are thought to be influenced by photolysis as well as secondary condensed‐phase chemistry, but the competing effects have yet to be reconciled. Here we use a model that simulates nitrate burial, photolytic fractionation, and re‐oxidation in snow to quantitatively assess these processes with the aim of developing a consistent framework for interpreting the photolytic effects of the complete nitrate isotopic composition (δ15N, δ18O, and Δ17O). This study reveals that isotopic effects of nitrate photolysis and aqueous‐phase re‐oxidation chemistry are important sources of uncertainties in modeling δ18O‐NO3−.

Nitrogen species and microbial community coevolution along groundwater flowpath in the southwest of Poyang Lake area, China

Nitrate and ammonia overload in groundwater can lead to eutrophication of surface water in areas where surface water is recharged by groundwater. However, this process remained elusive due to the complicated groundwater N cycling, which is governed by the co-evolution of hydrogeochemical conditions and N-cycling microbial communities. Herein, this process was studied along a generalized groundwater flowpath in Ganjing Delta, Poyang Lake area, China. From groundwater recharge to the discharge area near the lake, oxidation-reduction potential (ORP), NO3–N, and NO2–N decreased progressively, while NH3–N, total organic carbon (TOC), Fe2+, sulfide, and TOC/NO3− ratio accumulated in the lakeside samples. The anthropogenic influences such as sewage and agricultural activities drove the nitrate distribution, as observed by Cl− vs. NO3−/Cl− ratio and isotopic composition of nitrate. The hydrogeochemical evolution was intimately coupled with the changes in microbial communities. Variations in microbial community structures was significantly explained by Fe2+, NH3–N, and sulfide, while TOC/NO3− controlled the distribution of predicted N cycling gene. The absence of NH3–N in groundwater upstream was accompanied by the enrichment in Acinetobacter capable of nitrification and aerobic denitrification. These two processes were also supported by Ca2+ + Mg2+ vs. HCO3− ratio and isotopic composition of NO3−. The DNRA process downstream was revealed by both the presence of DNRA-capable microbes such as Arthrobacter and the isotopic composition of NH4+ in environments with high concentrations of NH3–N, TOC/NO3−, Fe2+, and sulfide. This coupled evolution of N cycling and microbial community sheds new light on the N biogeochemical cycle in areas where surface water is recharged by groundwater.

Systematic tracing of nitrate sources in a complex river catchment: An integrated approach using stable isotopes and hydrological models

Quantitative estimation for tracking the transport of various nitrate sources is required to effectively manage nitrate loading in complex river systems. In this study, we validated an integrated framework using field isotopic data (δ15NNO3 and δ18ONO3) of nitrates and hydrological modeling (hydrological simulation program FORTRAN; HSPF) to determine anthropogenic nitrate flux among different land-use types within a watershed. Nitrate isotopic compositions showed different ranges among four land-use types (4.9 to 15.5‰ for δ15NNO3, -4.9 to 12.1‰ for δ18ONO3), reflecting the different nitrate sources (sewage, synthetic fertilizer, effluent and soil) within watersheds. Based on the integration of HSPF modeling, we also found that total nitrate loads might be partially controlled by hydrological conditions such as water discharge (12,040.3–22,793.2 L/s) from upstream to downstream. Among the nitrate sources, the sewage transport showed unique enhancement near urban boundaries, along with an increase in total nitrate load (>193.5 NO3-N g/s km2) in downstream areas. In addition, the isotopic- and model-based nitrate fluxes showed good correlation for urban sources (R2=0.73, p < 0.05) but poor correlations for agriculture-dominated land use (R2=0.13, p > 0.05), reflecting the potential influence of surface runoff and ground infiltration into the watershed. Consequently, this research provided useful information to establish nitrogen management policy controlling point and non-point nitrate source loads in various land-use types for the restoration of water quality and aquatic ecosystem in the complex river system. Considering the recent increase in human activities near aquatic environments, this framework would be effective for individually estimating the quantitative contributions of anthropogenic nitrate sources transported along river-coastal systems.

Nitrate sources and transformations along the Yangtze river and its changes after strict environmental regulation

The exploration of the sources and fate of nitrate in surface waters along with the evaluation of the implementation of environmental regulations is essential for preventing and remediating the serious problem of nitrate pollution. However, the nitrate sources and transformations in the Yangtze River and its changes after strict environmental regulation remain unclear. In this pursuit, the present study investigated the spatiotemporal variations of the nitrate source and transformations in the Yangtze River by determining the nitrate isotopic compositions (δ15N-NO3− and δ18O-NO3−) and water chemistry during the wet and dry seasons. Furthermore, the current measurements of the nitrate sources and transformations after the environmental regulations were compared to the historical records in 2006. The results showed that the NO3−-N concentration gradually increased along the river, similar to that in 2006. However, the spatial pattern of δ15N and δ18O of NO3− values obviously changed after the environmental regulations. It was found that the δ15N-NO3− values increased from the headwater to the lower stream, which can be due to assimilation of phytoplankton. The δ18O-NO3− values showed a gradual decrease in the headwater area, an increase at the upper stream, and stabilized in the middle and lower stream. In addition, the δ18O-NO3− value was significantly higher during the wet season than in the dry season (P < 0.05). However, there was no significant seasonal difference in NO3−-N concentrations, δ15N-NO3− value, and nitrate source and transformations. The results of source apportionment indicated that the dominated nitrate source changed from the manure and sewage to soil N after the implementation of the environmental regulations. However, the major nitrate transformations were not affected by the environmental regulations and nitrification was still prevalent in the Yangtze River watershed. The change of the dominated nitrate source validated the effectiveness of the environmental regulation; however, the increase in the nitrate concentration reflected the heavy inputs of residual nitrate from the watershed. This study identified the nitrate sources and transformation processes and its changes after strict environmental regulations, and provided significant information for adjusting and formulating new policies to further control the nitrate pollution.

Nitrogen cycling in sediments on the NW African margin inferred from N and O isotopes in benthic chambers

Benthic nitrogen cycling in the Mauritanian upwelling region (NW Africa) was studied in June 2014 from the shelf to the upper slope where minimum bottom water O2 concentrations of 25 µM were recorded. Benthic incubation chambers were deployed at 9 stations to measure fluxes of O2, dissolved inorganic carbon (DIC) and nutrients (NO3-, NO2-, NH4+, PO43-, H4SiO4) along with the N and O isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) and ammonium (δ15N-NH4+). O2 and DIC fluxes were similar to those measured during a previous campaign in 2011 whereas NH4+ and PO43- fluxes on the shelf were 2 – 3 times higher and possibly linked to a long-term decline in bottom water O2 concentrations. The mean isotopic fractionation of NO3- uptake on the margin, inferred from the loss of NO3- inside the chambers, was 1.5 ± 0.4 ‰ for 15/14N (15ϵapp) and 2.0 ± 0.5 ‰ for 18/16O (18ϵapp). The mean 18ϵapp:15ϵapp ratio on the shelf (&amp;lt; 100 m) was 2.1 ± 0.3, and higher than the value of 1 expected for microbial NO3-reduction. The 15ϵapp are similar to previously reported isotope effects for NO3- respiration in marine sediments but lower than determined in 2011 at a same site on the shelf. The sediments were also a source of 15N-enriched NH4+ (9.0 ± 0.7 ‰). A numerical model tuned to the benthic flux data and that specifically accounts for the efflux of 15N-enriched NH4+ from the seafloor, predicted a net benthic isotope effect of N loss (15ϵsed) of 3.6 ‰; far above the more widely considered value of ~0‰. This result is further evidence that the assumption of a universally low or negligible benthic N isotope effect is not applicable to oxygen-deficient settings. The model further suggests that 18ϵapp:15ϵapp trajectories &amp;gt; 1 in the benthic chambers are most likely due to aerobic ammonium oxidation and nitrite oxidation in surface sediments rather than anammox, in agreement with published observations in the water column of oxygen deficient regions.

Origins of groundwater nitrate in a typical alluvial-pluvial plain of North China plain: New insights from groundwater age-dating and isotopic fingerprinting

Identifying nitrate sources and their temporal evolution in different land use is important for the sustainable management of groundwater resources. In this study, groundwater dating (3H–3He and time series of 3H) was combined with chemical and stable isotope analyses to resolve the evolution of nitrate sources and the driving mechanism of nitrate contamination. Approximately 75% of the groundwater samples (collected in 2014 and 2018) had nitrate concentrations exceeding World Health Organization's guideline for drinking water (50 mg/L), and 44% exceeded the groundwater quality standard of China (88.6 mg/L), indicating severe nitrate pollution. The shift of nitrate sources in different land use was identified using stable isotope composition of nitrate and groundwater age. The decreasing median value of δ15N from 10.6‰ to 7.5‰ of dated groundwater in farmland irrigated by clean water indicated the shift of nitrate sources from manure toward the mixing of fertilizer and manure due to the increased application of chemical fertilizers from intensive plant farming since 1980s. Comparably, the trend of increasing δ15N (the median value from 7‰ to 12‰) in farmland irrigated by wastewater might be attributed to the decreasing proportion of industrial wastewater since 2000s. The prevailing sources of nitrate in residential area were manure and sewage, and showed no obvious change along the recharge time. Driven by rapid urbanization, the nitrate sources of land use change area exhibited a marked shift from inorganic fertilizers toward manure and sewage. Principal component analysis (PCA) on nitrate concentrations with multiple parameters indicated nitrogen input in agricultural development and urbanization were the main controlling factors of nitrate contamination in the study area. The study results are a good reference for groundwater management in regions with nitrate source change during the process of rapid urbanization and agricultural intensification. The coupling of chemical, isotopic analyses and groundwater dating proved to be invaluable and should be applied in similar studies of nitrate contamination.

Multi-tracer approach to understand nitrate contamination and groundwater-surface water interactions in the Mediterranean coastal area of Guerbes-Senhadja, Algeria

Implementing sustainable groundwater resources management in coastal areas is challenging due to the negative impacts of anthropogenic stressors and various interactions between groundwater and surface water. This study focuses on nitrate contamination and transport via groundwater-surface water exchange in a Mediterranean coastal area (Guerbes-Senhadja region, Algeria) that is heavily affected by anthropogenic activities. A multi-tracer approach, integrating hydrogeochemical and isotopic tracers (δ2HH2O, δ18OH2O, 3H, δ15NNO3 and δ18ONO3), is combined with a Bayesian isotope mixing model (MixSIAR) to (i) elucidate the nitrate sources and their apportionments in water systems, and (ii) describe potential interactions between groundwater and surface water. Results from nitrate isotopic composition and the MixSIAR model show that nitrate concentrations mainly originate from sewage and manure sources. Nitrate derived from the sewage is attributed to urban and rural wastewater discharge, whereas nitrate derived from the manure is related to animal manure used to fertilise agricultural areas. High apportionments of nitrate-based atmospheric precipitation are identified in groundwater and surface water; a finding that is specific to this study. The multi-origin stresses combined with evidence of interactions between surface water and groundwater contribute to negatively impacting large parts of the study coastal area. The outcomes of this study are expected to contribute to sustainable management of coastal ecosystems by drawing more attention towards groundwater use and protection. Furthermore, this study may improve scientists' ability to predict the behavior of anthropogenically impacted coastal ecosystems and help decision-makers elsewhere to prepare suitable environmental strategies for other coastal ecosystems currently undergoing an early stage of groundwater resources deterioration.

Determining nitrate sources in storm runoff in complex urban environments based on nitrogen and oxygen isotopes.

Urban storm runoff, as the primary transport medium for nutrients entering urban rivers, contributes to urban water contamination. Accurate source identification is critical for controlling water pollution. Although some studies have used nitrate isotopic composition (δ15N-NO3- and δ18O-NO3-) to identify nitrate (NO3--N) in urban storm runoff, the relatively low frequency of collecting samples in surface runoff within a single functional area hinders the understanding of spatial variations and dynamic process of NO3--N sources over the runoff process. This study investigated the nitrogen (N) concentrations and analyzed dynamic changes of NO3--N sources in surface runoff in different urban functional areas, drainage pipeline runoff, and channels during the complete runoff process in Wuxi, east China. The results showed that N concentrations in pipeline runoff and channels were higher than those in surface runoff, indicating that high concentration of N pollutants were accumulated in drainage pipelines. Information of δ15N-NO3- and δ18O-NO3- suggested that the main NO3--N source varied between runoff stages. NO3--N contribution from atmospheric deposition decreased in the order: surface runoff (57%) > residential pipeline runoff (25%) > channels (14%), while the opposite trend was observed for the contributions from sewage, increasing from 10%, 26% to 39%. In urban storm runoff, more sewage, fertilizers, and soil N were carried into the surface runoff after 30% of cumulative runoff ratio and carried into pipeline runoff in the initial 25% of cumulative runoff ratio in the residential area. As the first attempt to identify nitrate sources over the cumulative runoff in different urban functional areas, this work expands our understanding of the primary nitrate source in urban storm runoff. The findings provide important insights for developing strategies to mitigate non-point source water pollution.

Combined effects of seawater intrusion and nitrate contamination on groundwater in coastal agricultural areas: A case from the Plain of the El-Nil River (North-Eastern Algeria)

This study focuses on coastal aquifers subject to uncontrolled land use development by investigating the combined effects of seawater intrusion and nitrate contamination. The research is undertaken in a Mediterranean coastal agricultural area (Plain of the El-Nil River, Algeria), where water resources are heavily impacted by anthropogenic activities. A multi-tracer approach, integrating hydrogeochemical and isotopic tracers (δ2HH2O, δ18OH2O, δ15NNO3 and δ18ONO3), is combined with a hydrochemical facies evolution diagram, and a Bayesian isotope mixing model (MixSIAR) to assess seawater contamination with its inland intrusion, and distinguish the nitrate sources and their apportionment. Results show that seawater intrusion is circumscribed to the sector neighboring the Mediterranean Sea, with two influencing functions including classic inland intrusion through the aquifer, and upstream seawater impact through the river mouth connected to the Mediterranean Sea. Groundwater and surface water samples reveal nitrate concentrations above the natural baseline threshold, suggesting anthropogenic influence. Results from nitrate isotopic composition, NO3 and Cl concentrations, and the MixSIAR model show that nitrate concentrations chiefly originate from sewage and manure sources. Nitrate derived from the sewage is related to wastewater discharge, whereas nitrate derived from the manure is attributed to an excessive use of animal manure to fertilise agricultural areas. The dual negative impact of seawater intrusion and nitrate contamination degrades water quality over a large proportion of the study area. The outcomes of this study are expected to contribute to effective and sustainable water resources management in the Mediterranean coastal area. Furthermore, this study may improve scientists' ability to predict the combined effect of various anthropogenic stressors on coastal environments and help decision-makers elsewhere to prepare suitable environmental strategies for other regions currently undergoing an early stage of water resources deterioration.

Tracing sources and fate of nitrate in multilayered karstic hydrogeological catchments using natural stable isotopic composition (δ15N-NO3− and δ18O-NO3−). Application to the Toulon karst system (Dordogne, France)

• Nitrate contents and isotopes are measured in a large multilayered karst aquifer. • Primary and secondary sources of nitrate are determined, at the local scale, using nitrate isotopes. • Origins of nitrate, over a karstic hydrogeological catchment, are assessed by nitrate isotopes. • The fate of nitrate in a karst aquifer is specified/ elucidated/ quantified/ combining isotopes and hydrogeological approaches. In groundwater management, the determination of the origins of nitrate (NO 3 – ) endmember sources is the first step in improving the management over a hydrogeological catchment. The generally multiple source contamination by nitrate over the karst systems makes it difficult to identify the exact nitrate origins in surface water and groundwater. This work aims to assess nitrate vulnerability of a karst system fed by several karst aquifers. The Toulon karst system offers the opportunity to study the relationships between two multilayered karst aquifers. It shows an increase in mean NO 3 – concentrations over decades. However, the origins of the NO 3 – remain unclear. Natural stable isotopic composition of NO 3 – (δ 15 N -NO3 – and δ 18 O -NO3 – ) measurements combined with concentrations were used to identify the origins of nitrogen causing this increase. This information was also integrated into the local hydrogeological situation to improve knowledge of the contamination of both confined and unconfined karst aquifers. δ 15 N -NO3 − and δ 18 O -NO3 − in potential nitrate sources over the hydrogeological catchment were analysed in order to identify the range of values that can be measured in both rivers and groundwater. Over the Toulon Springs catchment, rivers are contaminated with nitrate coming from N synthetic fertilizer mineralized in agriculturalsoils. Most of the springs are also contaminated by this nitrate source, but some record an isotope composition of nitrate coming from sewage effluents. Toulon Springs are only contaminated with nitrate from N synthetic fertilizer mineralized in agricultural soils. Finally, from a hydrogeological point of view, the analysis of δ 15 N -NO3 − and δ 18 O -NO3 − values improves knowledge on the functioning of the study area and shows that the upper Jurassic karst confined aquifer has nitrate of the same origins (sources) as the upper Cretaceous unconfined aquifer. The results here highlight that the lower aquifer can be fed by the Upper Cretaceous unconfined aquifer, contrary to previous belief that the lower aquifer was confined. From a more fundamental standpoint, this work provides a set of parameters adapted for karstic hydrogeological catchment. It lays the foundation for the use of isotope biogeochemistry as a powerful tool for understanding the recharge of drinking water catchments and beyond the efficient management of water quality at the scale of karstic hydrogeological catchment.

A case study in the wintertime Vaal Triangle Air-Shed Priority Area on the utility of the nitrogen stable isotopic composition of aerosol nitrate to identify NOx sources

In South Africa, the Highveld region and the Johannesburg-Pretoria megacity are known as global NOx (NOx = NO + NO2) “hotspots” identified by satellite-based instruments. The ultimate sink for atmospheric NOx is conversion to aerosol nitrate. However, measurements of aerosol nitrate concentrations do not provide information on which NOx sources served as nitrate precursors at that location. This complicates efforts to reduce concentrations of particulate matter (PM) in these air quality priority areas. Here, we measured the nitrogen stable isotopic composition of nitrate from daily wintertime collections of coarse mode PM2.5-10 (PM ≤ 10 and &gt;2.5 µm in diameter) at three air quality monitoring stations located in the Vaal Triangle Air-Shed Priority Area (VTAPA). The overall aim of this case study was to evaluate the use of the distinct stable isotopic signatures of various NOx sources to identify their relative contribution to aerosol nitrate across the Highveld. The nitrogen isotopic ratios of aerosol nitrate were similar across the three sites, with greater day-to-day variability than site to site variability. Air mass history was the main driver of the variability in the nitrogen isotopic ratios of aerosol nitrate, with significantly higher isotopic ratios observed for air masses originating from the southwest. Using an isotope mixing model we determined that NOx from coal-burning is the dominant contributor to aerosol nitrate (66%), followed by biomass burning (16%), vehicles (12%), and soil emissions (6%).

Comparison of Nitrate Isotopes Between the South China Sea and Western North Pacific Ocean: Insights Into Biogeochemical Signals and Water Exchange

AbstractReconstructing past changes in the oceanic nitrate inventory with sedimentary N records in the South China Sea (SCS), which is the terminal of North Pacific Intermediate Water (NPIW), has regional and global implications. However, water‐column nitrate cycling that affects N isotope preservation remains poorly understood in the SCS. We present a new data set of nitrate isotopes ( and ) to elucidate nitrate dynamics in the SCS and the adjoining western North Pacific Ocean (wNPO). Greater increases in than in are observed in the SCS euphotic zone, suggesting a combined effect of partial nitrate assimilation and nitrification. In the subsurface and thermocline waters of both regions, upward disproportional decreases in and accompanied by elevated nitrate anomalies (N*) indicate an accumulation of external N. Such changes are less significant in the SCS due to higher nitrate concentrations therein, although external N influxes are comparable in both regions. High and values in the wNPO intermediate water result from the lateral transport of NPIW with isotopically more enriched nitrate from the remote denitrification zones followed by mixing with overlying water containing isotopically depleted nitrate. As NPIW flows into the SCS, its isotopically enriched signal is further diluted by strong vertical mixing with overlying and underlying waters in the interior. Compared to its source water from the wNPO, the SCS deep water has consistent nitrate isotopic compositions but significantly lower N*, indicating increased benthic denitrification at the wNPO margins with an estimated rate of 0.26–0.41 mmol N m−2 day−1.

Identifying the source and transformation of riverine nitrates in a karst watershed, North China: Comprehensive use of major ions, multiple isotopes and a Bayesian model

Nitrate (NO3−) contamination of surface water is a globally concern, especially in karstic regions affected by intensive agricultural activities. This study combines hydrochemistry, and environmental isotopes (δ2HH2O, δ18OH2O, δ15NNO3, and δ18ONO3) with a Bayesian isotope mixing model (Simmr) to reduce the uncertainty in estimating the contributions of different pollution sources. Samples were collected from 32 surface water sites in the Yufu River (YFR) watershed, North China, in September and December 2019. The results revealed that NO3−-N was the predominant form of inorganic nitrogen that caused the deterioration of water quality in the watershed, accounting for approximately 58% of the total nitrogen (TN). The hydrochemical compositions and nitrate isotopes indicated that NO3− mainly originated from soil nitrogen (SN), ammonium fertilizer (AF), but nitrate fertilizer (NF), manure and sewage (M&S) and atmospheric precipitation (AP) were limited. The isotopic composition of nitrate in the upper reaches of the watershed was mainly affected by microbial nitrification, while the mixture of multiple sources was the dominant nitrogen transformation process in the mid-lower reaches of the watershed. Simmr model outputs revealed that SN (56.5%) and AF (29.5%) were the primary contributor to riverine NO3− pollution, followed by NF (7.1%), MS (3.6%), and AP (3.4%) sources. Moreover, an uncertainty index (UI90) of the isotope mixing showed that SN (0.73) and AF (0.67) had the highest values, followed by NF (0.22), M&S (0.22) and AP (0.10). Chemical fertilizer and SN collectively contributed >50% of nitrate during the two sampling campaigns. These results indicated that reducing the application of nitrogen fertilizers and rational irrigation are the keys to alleviate of NO3− pollution. The study is helpful in understanding the source and transformation of riverine NO3− and effectively reducing NO3− pollution in karst agricultural rivers or watersheds.

Nitrogen Cycling and Biosignatures in a Hyperarid Mars Analog Environment.

The hyperarid Atacama Desert is a unique Mars-analog environment with a large near-surface soil nitrate reservoir due to the lack of rainfall leaching for millennia. We investigated nitrogen (N) cycling and organic matter dynamics in this nitrate-rich terrestrial environment by analyzing the concentrations and isotopic compositions of nitrate, organic C, and organic N, coupled with microbial pathway-enzyme inferences, across a naturally occurring rainfall gradient. Nitrate deposits in sites with an annual precipitation of <10 mm carry atmospheric δ15N, δ18O, and Δ17O signatures, while these values are overprinted by biological cycling in sites with >15 mm annual precipitation. Metagenomic analyses suggest that the Atacama Desert harbors a unique biological nitrogen cycle driven by nitrifier denitrification, nitric oxide dioxygenase-driven alternative nitrification, and organic N loss pathways. Nitrate assimilation is the only nitrate consumption pathway available in the driest sites, although some hyperarid sites also support organisms with ammonia lyase- and nitric oxide synthase-driven organic N loss. Nitrifier denitrification is enhanced in the “transition zone” desert environments, which are generally hyperarid but see occasional large rainfall events, and shifts to nitric oxide dioxygenase-driven alternative nitrifications in wetter arid sites. Since extremophilic microorganisms tend to exploit all reachable nutrients, both N and O isotope fractionations during N transformations are reduced. These results suggest that N cycling on the more recent dry Mars might be dominated by nitrate assimilation that cycles atmospheric nitrate and exchanges water O during intermittent wetting, resulting stable isotope biosignatures could shift away from martian atmospheric nitrate endmember. Early wetter Mars could nurture putative life that metabolized nitrate with traceable paleoenvironmental isotopic markers similar to microbial denitrification and nitrification stored in deep subsurface.

Measurement report: Nitrogen isotopes (&amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N) and first quantification of oxygen isotope anomalies (&amp;lt;i&amp;gt;Δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O, &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O) in atmospheric nitrogen dioxide

Abstract. The isotopic composition of nitrogen and oxygen in nitrogen dioxide (NO2) potentially carries a wealth of information about the dynamics of the nitrogen oxides (NOx = nitric oxide (NO) + NO2) chemistry in the atmosphere. While nitrogen isotopes of NO2 are subtle indicators of NOx emissions and chemistry, oxygen isotopes are believed to reflect only the O3 / NOx / VOC chemical regime in different atmospheric environments. In order to access this potential tracer of the tropospheric chemistry, we have developed an efficient active method to trap atmospheric NO2 on denuder tubes and measured, for the first time, its multi-isotopic composition (δ15N, δ18O, and Δ17O). The Δ17O values of NO2 trapped at our site in Grenoble, France, show a large diurnal cycle peaking in late morning at (39.2 ± 0.3) ‰ and decreasing at night until (20.5 ± 0.3) ‰. On top of this diurnal cycle, Δ17O also exhibits substantial daytime variability (from 29.7 ‰ to 39.2 ‰), certainly driven by changes in the O3 to peroxyl radicals (RO2) ratio. The nighttime decay of Δ17O(NO2) appears to be driven by NO2 slow removal, mostly from conversion into N2O5, and its formation from the reaction between O3 and freshly emitted NO. As expected from a nighttime Δ17O(NO2) expression, our Δ17O(NO2) measured towards the end of the night is quantitatively consistent with typical values of Δ17O(O3). Daytime N isotope fractionation is estimated using a general expression linking it to Δ17O(NO2). An expression is also derived for the nighttime N isotope fractionation. In contrast to Δ17O(NO2), δ15N(NO2) measurements exhibit little diurnal variability (−11.8 ‰ to −4.9 ‰) with negligible isotope fractionations between NO and NO2, mainly due to high NO2 / NOx ratios, excepted during the morning rush hours. The main NOx emission sources are estimated using a Bayesian isotope mixing model, indicating the predominance of traffic emissions in this area. These preliminary results are very promising for using the combination of Δ17O and δ15N of NO2 as a probe of the NOx sources and fate and for interpreting nitrate isotopic composition records.

Nitrate Supply Routes and Impact of Internal Cycling in the North Atlantic Ocean Inferred From Nitrate Isotopic Composition

AbstractIn this study we report full‐depth water column profiles for nitrogen and oxygen isotopic composition (δ15N and δ18O) of nitrate (NO3−) during the GEOTRACES GA01 cruise (2014). This transect intersects the double gyre system of the subtropical and subpolar regions of the North Atlantic separated by a strong transition zone, the North Atlantic Current. The distribution of NO3− δ15N and δ18O shows that assimilation by phytoplankton is the main process controlling the NO3− isotopic composition in the upper 150 m, with values increasing in a NO3− δ18O versus δ15N space along a line with a slope of one toward the surface. In the subpolar gyre, a single relationship between the degree of NO3− consumption and residual NO3− δ15N supports the view that NO3− is supplied via Ekman upwelling and deep winter convection, and progressively consumed during the Ekman transport of surface water southward. The co‐occurrence of partial NO3− assimilation and nitrification in the deep mixed layer of the subpolar gyre elevates subsurface NO3− δ18O in comparison to deep oceanic values. This signal propagates through isopycnal exchanges to greater depths at lower latitudes. With recirculation in the subtropical gyre, cycles of quantitative consumption‐nitrification progressively decrease subsurface NO3− δ18O toward the δ18O of regenerated NO3−. The low NO3− δ15N observed south of the Subarctic Front is mostly explained by N2 fixation, although a contribution from the Mediterranean outflow is required to explain the lower NO3− δ15N signal observed between 600 and 1500 m depth close to the Iberian margin.

Global patterns of nitrate isotope composition in rivers and adjacent aquifers reveal reactive nitrogen cascading

Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Isotopes (δ15N, δ18O) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO3−). We synthesized nitrate isotope data (n = ~5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO3− (0.3 ± 0.2 mg L−1, n = 2902) compared to aquifers (5.5 ± 5.1 mg L−1, n = 2291) and slightly lower δ15N values (+7.1 ± 3.8‰, n = 2902 vs +7.7 ± 4.5‰, n = 2291), but were indistinguishable in δ18O (+2.3 ± 6.2‰, n = 2790 vs +2.3 ± 5.4‰, n = 2235). The isotope composition of NO3− was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher δ15N and δ18O values in wintertime, suggesting waste-related N-source signals are better preserved in the cold seasons. Isotopic assays of nitrate biogeochemical transformations are key to understanding nitrate pollution and to inform beneficial agricultural and land management strategies.

Source identification of nitrate in the upper aquifer system of the Wadi Shueib catchment area in Jordan based on stable isotope composition

Groundwater forms the main freshwater supply in arid and semi-arid areas, and contamination of this precious resource is complicated by the slow rate of recharge in these areas. Nitrate contamination of groundwater is a global water quality problem, as it entails threat to human health as well as aquatic ecosystems. Source identification of contamination is the cornerstone and a prerequisite for any effective management program of water quality. Stable isotope composition of the dissolved nitrate ( $${{\rm{\delta}}^{15}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ and $${{\rm{\delta}}^{18}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ ) has been applied to identify $${\rm{N}}{{\rm{O}}_{{3^ - }}}$$ sources and the main transformation processes in the upper aquifer system (A1/2, A4, and B2/A7 aquifers) in the Wadi Shueib catchment area, Jordan. Moreover, the stable isotope compositions of the groundwater (δ2H-H2O and δ18O-H2O) in conjunction with the groundwater hydrochemistry were integrated to investigate the origin and evolution of the groundwater. Results revealed that groundwater in the study area is fresh and hard-very hard water, and mainly a Ca-Mg-Cl type. $${\rm{N}}{{\rm{O}}_{{3^ - }}}$$ concentration was in the range of 7.0–74.0 mg/L with an average of 37.0 mg/L. Most of the samples showed concentration higher than the natural background concentration of $${\rm{N}}{{\rm{O}}_{{3^ - }}}$$ (5.0–10.0 mg/L). The δ2H-H2O and δ18O-H2O values indicated that the groundwater is meteoric, and of Mediterranean origin, with a strong evaporation effect. The $${{\rm{\delta}}^{15}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ values ranged between 6.0‰ and 11.3‰ with an average of 8.7‰, and the $${{\rm{\delta}}^{18}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ values ranged between 1.6‰ and 5.9‰ with an average of 3.4‰. These values are in conformity with the stable isotope composition of nitrate derived the nitrification of wastewater/manure, and soil NH4. Nitrification and denitrification are the main transformation processes affecting nitrogen species. Statistical analysis revealed no significant differences in the δ2H-H2O and δ18O-H2O values, and $${{\rm{\delta}}^{15}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ and $${{\rm{\delta}}^{18}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ values for the three aquifers (A1/2, A4, and B2/A7), indicating that the groundwater of these aquifers has the same origin, and a common source of pollution.