Dual Isotopic Composition Research Articles

Comprehensive evaluation of nitrogen contamination in water ecosystems of the Miyun reservoir watershed, northern China: distribution, source apportionment and risk assessment.

Miyun Reservoir plays a vital role as a source of drinking water for Beijing, however it grapples with nitrogen contamination issues that have been poorly understood in terms of their distribution, source, and associated health risks. This study addresses this knowledge gap by employing data on nitrate nitrogen (NO3--N), chloride (Cl-), dual isotopic compositions of NO3- (δ15N-NO3- and δ18O-NO3-) data in water ecosystems, systematically exploring the distribution, source and health risk of nitrogen contaminants in Miyun reservoir watersheds. The results showed that over the past 30years, surface water runoff has exhibited a notable decrease and periodic fluctuations due to the combined influence of climate and anthropogenic activities, while the total nitrogen (TN) concentration in aquatic ecosystems presented an annual fluctuating upward trend. The TN concentration in the wet season was predominantly elevated because a large amount of nitrogen contaminants migrated into water ecosystems through heavy rainfall or river erosion. The concentration of NO3--N, the main contaminant of the water ecosystems, showed distinct variations across different watersheds, followed as rivers over the Miyun reservoir. Moreover, NO3--N levels gradually increased from upstream to downstream in different basins. NO3--N in surface water was mainly derived from the mixture of agricultural ammonia fertilizer and sewage and manure, with a minority of samples potentially undergoing denitrification. Comparatively, the main sources of NO3--N in groundwater were soil N and sewage and manure, while the denitrification process was inactive. The carcinogenic risks caused by NO3--N in groundwater were deemed either nonexistent or minimal, while the focus should predominantly be on potential non-carcinogenic risks, particularly for infants and children. Therefore, it is crucial to perform proactive measures aimed at safeguarding water ecosystems, guided by an understanding of the distribution, sources, and associated risks of nitrogen contamination.

Changes in fronts regulate nitrate cycling in Zhanjiang Bay: A comparative study during the normal wet season, rainstorm, and typhoon periods

Typhoons and rainstorms (>250 mm/day) are extreme weather events changing hydrological characteristics and thus nitrogen (N) cycle in coastal waters. However, responses of N cycle to rainstorms and typhoons and their underlying mechanisms need to be elucidated. In this study, we conducted an analysis of a comparative dataset encompassing concentrations of nitrate (NO3−), ammonium (NH4+), dissolved oxygen (DO), chlorophyll a (Chl a), hydrological parameters, dual isotopic composition of NO3− (δ15N-NO3− and δ18O-NO3−) in Zhanjiang Bay during three distinct periods: the normal wet season, rainstorm, and typhoon periods. After the rainstorm, the salinity front in Zhanjiang Bay was more weakened and steadier than that during the normal wet season, mainly because onshore wind and a large amount of freshwater was inputted into the ocean surface. This weakened and steady salinity front strengthened water stratification and provided a favorable condition for phytoplankton blooms. Correspondingly, evident NO3− deficits coincided with elevated δ15N-NO3− and δ18O-NO3− values indicated that sufficient NO3− sustained phytoplankton blooms, leading to NO3− assimilation during the rainstorm period. By contrast, due to the onshore wind induced by the typhoon, the salinity front in Zhanjiang Bay was more intensified and unsteady after the typhoon than the normal wet season. The salinity front after the typhoon was unsteady enough to enhance vertical mixing in the water column. Relatively high DO concentrations suggested that enhanced vertical mixing after the typhoon support freshly organic matter decomposition and nitrification via oxygen injection from the air into the water column. In addition, NO3− deficits coincided with elevated δ15N-NO3− values and δ18O-NO3− values demonstrated the coexistence of NO3− assimilation during the typhoon period. This study suggests that the changing processes involved in NO3− cycling after typhoons and rainstorms are associated with the stability and intensity of the salinity front altered by these weather events.

Impact of typhoons on anthropogenic nitrogen sources in Lake Sihwa, South Korea

This study investigated the nitrate dual isotopic compositions (δ15NNO3 and δ18ONO3) of water samples to trace nitrate sources in Lake Sihwa, which encompasses various land-use types (e.g., urban, industry, wetland, and agriculture). The biogeochemical interactions of anthropogenic nitrogen sources (e.g., soil, road dust, and septic water) were also evaluated through multiple pathways from terrestrial boundaries to the water column. Based on increased concentrations of dissolved total nitrogen (DTN; 3.1 ± 1.6 mg/L) after typhoon, the variation of element stoichiometry (N:P:Si) in this system shifted to the relatively N-rich conditions (DIN/DIP; 14.1 ± 8.1, DIN/DSi; 1.4 ± 1.8), potentially triggering the occurrence of harmful algal blooms. Furthermore, discriminative isotopic compositions (δ15NNO3; 4.0 ± 2.1 ‰, δ18ONO3; 6.1 ± 4.3 ‰) after the typhoon suggested the increased DTN input of anthropogenic origins within Lake Sihwa would be mainly transported from urban sources (76 ± 9 %). Consequently, the isotopic-based approach may be useful for effective water quality management under increased anthropogenic activities near aquatic systems.

Tracing the contributions of different nitrate sources associated with submarine groundwater discharge in coastal seawaters off Jeju Island, Korea

We measured the concentrations of dissolved inorganic nutrients and the dual isotopic composition of nitrate (δ15N-NO3− and δ18O-NO3−) in coastal waters off Jeju, a volcanic island in Korea, to trace its main sources. Sampling of seawater and fresh groundwater was conducted in four different coastal areas of Jeju Island: Haengwon (HW), Pyoseon (PS), Ilgwa (IG), and Sagye (SG) in May 2020 and 2021. The significant negative correlations between NO3− and salinity in the four areas indicate that the main source of NO3− is fresh submarine groundwater discharge (FSGD), with the extrapolated fresh groundwater endmember values ranging from 170 to 300 μM (δ15N-NO3−: 4.1–10.8 and δ18O-NO3−: 1.7–6.4). The actual sources of SGD-driven NO3− in these coastal waters were determined using a bi-plot diagram (δ15N-NO3− vs. δ18O-NO3−) and a Bayesian stable isotope mixing model (MixSIAR). The results showed that, besides the background contribution from open-ocean waters, the main sources of NO3− in HW were fertilizer (69 ± 5%) and manure and sewage (24 ± 7%) and those in PS, IG, and SG were manure and sewage (PS: 53 ± 11%, IG: 57 ± 12%, SG: 63 ± 13%) and fertilizer (PS: 27 ± 8%, IG: 24 ± 5%, SG: 22 ± 5%). Our extrapolation approach for NO3− dual isotopes provides a better way to evaluate the main sources of NO3− in coastal waters off volcanic islands where SGD-driven NO3− is significant, but its actual source groundwater cannot be located.

The stable carbon and hydrogen isotopic composition of microbial fatty acids traces microbial metabolism in soils and peats

The compound-specific stable carbon (δ13C) and hydrogen (δ2H) isotopic compositions of microbial fatty acids have been widely used to trace microbial metabolism across a range of mesophilic environments. However, few studies have combined the δ13C and δ2H values of microbial fatty acids. So far none have determined the δ2H of microbial phospholipid fatty acids (PLFAs) in soils or peats, even though these stable isotope combinations could provide new insights into soil microbial metabolism. Here, we measured the δ13C and δ2H values of microbial PLFAs in top soils from peatlands, meadows, and woodlands in the Dajiuhu basin, central China. We observed a significant (p < 0.05) positive correlation between the δ13C and δ2H of microbial PLFAs across the three habitats studied here, indicating that central metabolic pathways affect both carbon and hydrogen isotopic compositions of microbial PLFAs. Moreover, our stable isotope data consistently indicate a relatively conservative metabolic state, which is dominated by expected heterotrophic metabolism. The exception to these observations is PLFAs associated with methanotrophs, as these appear to have decoupled carbon and hydrogen isotopes, providing an additional tool for tracing methanotrophic signals. Our results suggest that the carbon and hydrogen dual isotopic composition of microbial PLFAs can not only provide cross-validation for microbial metabolism in natural environments, but as a combined tool can also provide new insights into the perturbation of soil microbial community metabolism.

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.

A stable isotopic approach for quantifying aquaculture-derived organic matter deposition dynamics in the sediment of a coastal fish farm

The stable carbon and nitrogen isotopic compositions (δ13C and δ15N) of sedimentary organic matter (OM) in fish farms (FFs) were investigated to quantify the aquaculture-derived OM deposition dynamics in the sediment. The dual isotopic compositions of mixed OMs in surface sediments at FFs differed significantly (p < 0.05) from those at reference sites, indicating an increased deposition of fish feces or uneaten feed in the sediments. Furthermore, OM source apportionments revealed that the quantitative contribution of fish feces (23.3 mg g−1 dw) during farming activities was significantly higher than that of other natural OM sources (C3 plants and phytoplankton). After the disassembly of fish cages, the deposited fish feces may be preferentially degraded through processes that require a large amount of oxygen consumption (<0.1 kg C m−2 yr−1). Our isotopic approach may be helpful for assessing the impact of FF wastes and for taking measures to minimize environmental deterioration.

Evaluating the sources and fate of nitrate in riparian aquifers under agricultural land using in situ-measured noble gases, stable isotopes, and metabolic genes

Riparian zones with their buffering ability and abundant water supply are often subjected to intensive agricultural activities. We investigated a riparian aquifer located near a stream in South Korea that recently experienced sharply decreasing groundwater levels and elevated nitrate (NO3–) concentrations, which were attributed to local agricultural activities. Our goal was to identify the predominant nitrogen sources and NO3– removal processes. Multiple approaches including geochemical and isotopic tracers, land-use analysis, metabolic gene quantification, and inert gas tracers were used to elucidate groundwater and nutrient dynamics in stream-side granitic aquifers. The dual isotopic composition of NO3– identified manure and sewage as the major sources of NO3– contamination. Denitrification was the dominant NO3– removal process in the aquifer, as demonstrated by the negative relationship between δ15N and δ18O values in NO3–and NO3–/Cl–. Denitrification and anammox genes were also observed in microbial communities of the aquifer throughout the study site, suggesting that these processes support effective natural NO3– attenuation in groundwater. A mixing model constructed using a catchment-scale dataset including SiO2 concentrations and δ18O-H2O suggested that mixing with paddy soil water was the major driver of denitrification in the aquifer at the study site, where impervious layers provided anaerobic conditions for natural NO3– attenuation. Denitrification reduced the NO3– flux into the nearby stream by up to 114.4 NO3– kg/ha/y (26 kg N/ha/y). The N2 generated by denitrification did not accumulate in the groundwater, but mostly escaped from groundwater to the atmosphere, as demonstrated by the degassed signature of dissolved inert gases below the air saturated water level. This study identified the predominant NO3– sources and conceptualized N cycling in the heavily developed agricultural riparian aquifer using multiple tracers, demonstrating that NO3– is partially removed through denitrification and possibly anammox while N2 mostly escapes into the atmosphere.

Enhanced aerosols over the southeastern Tibetan Plateau induced by open biomass burning in spring 2020

The Tibetan Plateau is the third pole of the world, with an essential role in regulating Northern Hemisphere climate. Previous studies showed that atmospheric aerosols over the Tibetan Plateau are influenced by biomass burning (BB) products from South and Southeast Asia. In fact, open biomass burning (OBB) is also an important form of BB in Southeast Asian countries, causing serious springtime air pollution yearly. However, there are still scientific gaps in the contribution of OBB to surrounding regional aerosols, especially on the Tibetan Plateau. In order to quantify this contribution, we collected samples of fine particulate matter and derived the concentrations of major water soluble ion, water soluble organic carbon (WSOC), and total carbon (TC) and total nitrogen (TN) as well as the dual isotopic compositions of carbon and nitrogen (δ13C and δ15N) during March–June on the southeastern Tibetan Plateau. δ13C and δ15N showed no significant difference (p > 0.05) between the OBB and non-OBB periods. Furthermore, both δ13C and δ15N (−25.7 ± 0.7 ‰ and 8.0 ± 3.6 ‰) values calculated during the whole sampling period were similar to the BB value, indicating that the primary source of TC and TN in aerosols was BB, whether OBB or non-OBB burning periods. TC and TN concentrations during the OBB period (6.5 ± 2.9 μg m−3 and 1.2 ± 0.4 μg m−3, respectively) were significantly higher than during the non-OBB period (4.1 ± 1.7 μg m−3, with p = 0.014, and 0.7 ± 0.3 μg m−3, with p = 0.013, respectively). Active fire data and surface smoke concentrations further indicated that BB emissions from Southeast Asia were higher during the OBB period. This suggests that OBB-related high BB emissions significantly enhanced atmospheric aerosols concentrations on the southeastern Tibetan Plateau.

Provenancing Nineteenth Century Saltpetre From British India Using Nitrogen, Oxygen, and Strontium Isotope Ratios

Highly purified specimen of saltpetre, kept in two glass jars are stored in the collections of the Natural History Museum, London, United Kingdom. Historic catalogs of the specimen collection record their acquisition date as 17th March, 1881. The specimen represent heritage of the saltpetre industry during the times of the colonial British India. Chemical analysis shows that they are highly refined, containing only traces of impurities. Dual isotopic composition (δ15N and δ18O) of nitrate implies their intrinsic origin as British India. In addition, relative to highly radiogenic signature of Sr widespread throughout India, less radiogenic strontium (87Sr/86Sr = 0.707859–0.714168) in the specimens suggests that final stages of the refining took place in western India where relevant waters containing pristine Sr were used. Stable isotope geochemistry in combination with history and socio-economy provide new insights into this research area which is not yet fully understood.

Museum-archived and recent acquisition nitrates from the Atacama Desert, Chile, South America: refinement of the dual isotopic compositions (δ 15N vs. δ 18O)

ABSTRACT Sodium nitrate ores from the Atacama Desert in South America were economically important as they represented huge natural resources for the fertilizer and explosives industries during the early nineteenth to early twentieth centuries. Nitrogen and oxygen isotope ratios (δ 15N and δ 18O) of these desert nitrates generally show unique compositions (from close to 0 and up to ca. +50 ‰, respectively). The nitrates indicate the provenance as atmospheric in origin due to the mass-independent photochemical reaction of nitric oxide (NO) with ozone (O3) in the atmosphere to produce nitrate (NO3 –). This paper examines the previously existing isotope data for specimens acquired from the Atacama Desert. It then reports new data from dual isotope analysis of historic nitrate specimens archived in museums in the UK. In the stable isotope signatures for nitrates from two areas of the Atacama Desert, Tarapacá in the north and Antofagasta in the south, were examined, and this analysis enabled a more detailed definition of their isotopic compositional ranges. This improved database is useful for tracing the provenance of the historic nitrates used in gunpowder and saltpetre, and also the cause of nitrate pollution in natural environments for which routine chemistry alone cannot provide the definite evidence for the origin.

Extreme precipitation accelerates the contribution of nitrate sources from anthropogenetic activities to groundwater in a typical headwater area of the North China Plain

Identification of diffused nitrate (NO3–) sources in groundwater is challenging in headwater areas with diverse land-uses, multiple potential inputs and intensification of precipitation. Although NO3– sources in headwater areas with different land-uses have been observed, the mechanisms of the impact of extreme precipitation on NO3– transport to groundwater and NO3– sources in these areas are not well understood. In this study, dual isotopic compositions of NO3– (δ15N-NO3– and δ18O-NO3–) and hydrochemical data were measured in groundwater samples during August 2014-January 2017 under four land-uses: natural vegetation (NV), farmland (FL), economic forestland (EF), and residential areas (RA). An isotope mixing model (SIAR) was applied to determine the main NO3– sources and the governing factors. Affected by extreme precipitation, the rates of exceeding the WHO standard of NO3– (45 mg/L) increased to 93.3%, 65.6%, 100%, and 46.7% for groundwater samples from RA, FL, EF, and NV, respectively, when comparing with 88.7%, 56.6%, 92.9%, and 6.1% of samples collected before extreme precipitation from RA, FL, EF and NV, respectively. SIAR results showed that the contribution of anthropogenetic activities to NO3– in groundwater was accelerated after extreme precipitation. Before extreme precipitation, soil N (43.5%) and organic matter of soil (32.3%) were the dominant NO3– source in NV, and manure/sewage made substantial contributions to NO3– concentrations in groundwater, accounting for 55.8%, 56.2%, and 52.7% in RA, FL, and EF, respectively. In NV, the contributions of organic matter of soil increased to 46.8% and the contribution of soil N decreased to 28.9%. The contributions of chemical fertilizers in soil increased from 15.2% to 31.3% and there was a corresponding decrease in the contribution from manure/sewage (41.1%) in FL, although the contribution of NO3– from manure/sewage increased in EF (66.3%) and RA (57.6%). Assessing temporal variation of NO3– sources in the headwater areas of the North China Plain can help to protect water resource and avoid further deterioration of water quality during land-use management and extreme precipitation in the future.

Formation pathways and sources of size-segregated nitrate aerosols in a megacity identified by dual isotopes

Nitrate becomes a vital secondary component in atmospheric aerosols, especially in the urban areas with effective control of sulfur emissions. The elucidation of NO3− formation pathways and sources would be very helpful for accurately modeling and controlling the particle pollution. In this study, the dual isotopic compositions (δ15N and δ18O) of size-segregated NO3− aerosols were measured at an urban site of Shanghai in the winter of 2018 and the late summer of 2019. The formation mechanisms and sources of NO3− in five size-modes and different seasons were explored by combining analyses of dual isotopes, ion concentrations, air mass back trajectories and the Bayesian isotopic mixing model. Our results showed that the δ18O-NO3− values in summer (62.1 ± 12.8‰) were generally lower than those in winter (80.1 ± 9.9‰) with the lowest values in Aitken (sub–0.1 μm) and condensation modes (0.1–0.56 μm, 48.6 ± 9.8‰). This suggested that NO2 + ∙OH pathway probably played a significant role in the NO3− formation in summer (56.1 ± 4.8%) especially in Aitken and condensation modes, while N2O5 heterogeneous and NO3 + RH/RSC (hydrocarbons/reduced sulfur compounds) pathways could be critical in winter (55.0 ± 12.6% and 28.3 ± 15.2%, respectively). The isotopic fractionation may play a critical role in the size distribution of δ15N-NO3−. Based on the formation pathways of NO3− and calculated fractionation factors εN, the contribution of major NOx sources to NO3− aerosols in Shanghai was estimated. It was found that diesel engine exhaust contributed to the highest percentages (39.3% and 32.1%) of NO3− aerosols followed by gasoline engine exhaust (30.9%) and coal combustion (29.4%) in summer and winter, respectively. The contribution of coal combustion was almost doubled in winter compared to summer whereas soil emission decreased. Our results suggest that the impact of diesel engine exhaust on particle pollution in eastern Chinese megacities should be emphasized.

A preliminary evaluation of ecohydrological separation in a semi-arid riparian area

In recent years the ecohydrological separation paradigm has stimulated significant scientific debate, as it challenges the basis of existing hydrological process understanding. Through the use of stable isotope analysis several studies have demonstrated that two isotopically distinct water stores may exist within the subsurface, a matric-bound store which sustains transpiration and a mobile store which contributes to the recharge of groundwater and streams. Despite growing evidence supporting this hypothesis, further testing across a range of climatic regimes and environmental settings has been advocated to develop an improved hydrological understand of this paradigm. In this study, the dual isotopic composition of meteoric and environmental waters was analysed in a semi-arid riparian environment during a significant El Nino induced drought, to ascertain if plants were using water that would have otherwise contributed to stream and/or groundwater recharge. Overall, the results of these investigations showed no clear evidence supporting ecohydrological separation. Environmental waters were found to be significantly different from rainfall, whereas no significant differences were established between xylem, bulk soil water, and streamflow. While, these findings represent a unique scenario with regards to the testing of ecohydrological separation, these results are by no means ubiquitous as sampling was conducted during extreme climatic conditions with limited spatio-temporal representation of the system. Therefore, longer-term sampling and monitoring is recommended to obtain an improved understanding of the system under a range of climatic conditions.

Identifying the sources of nitrate contamination using a combined dual isotope, chemical and Bayesian model approach in a tropical agricultural river: Case study in the Mun River, Thailand

Environmental issues triggered by increasing nitrate in agricultural river has become global concern. Identifying nitrate sources and transformation are crucial for water sources protection and eliminating nitrate contamination in an agricultural watershed. In this study, chemical and dual isotopic compositions of nitrate were employed to trace the nitrate sources and transformation processes, and proportional contribution of NO3− source were estimated by SIAR based on Bayesian model. NH4+ concentrations in middle Mun and lower Mun in wet season were significantly higher than NO3−, suggesting enhanced runoff processes by flood promote agricultural fertilized NH4+ leaching into the river. Higher Cl− concentration and NO3−/Cl− indicated that manure and sewage was the dominate nitrate source in the Lam Takhong River and the upper Mun. The overall values of δ15N-NO3− and δ18O-NO3− ranged from −3.9‰ to +16.6‰ and from −5.2‰ to +40.0‰, respectively. The results of nitrate isotopes indicated that NO3− mainly originated from soil N nitrogen, chemical fertilizer, and manure and sewage wastes. Spatially, soil N and chemical fertilizer contributed the most nitrate in the mainstream of lower Mun, middle Mun, and the Lam Takhong River; whereas over 60% of nitrate was derived from manure and sewage in the upper Mun. The spatial variation of water discharge and rainfall, together with the nitrate concentration and isotopes inferred that the nitrate sources and transformations in rain-fed river in tropical zone were distinguished from other rivers. High water discharge driven by rainfall events accelerated the nitrate export and the contribution of atmospheric deposition in wet season, and enlarged the contribution of manure and sewage in dry season. This study provided an example for further researches and approaches to assess the effects of tropical climate and agriculture on nitrate accumulation in watershed.

Tracing nitrate source and transformation in a semiarid loess aquifer with the thick unsaturated zone

The unsaturated zone (UZ) plays an important link between groundwater and the land surface. The surface pollutants would be retained firstly in the UZ when the UZ is thick and the recharge rate is limited before they percolate into groundwater. This study is dedicated to revealing the groundwater nitrate source and transformation by combining evaluations of the thick UZ (40–75 m) with the saturated zone in a loess aquifer using dual isotopic composition of nitrate from precipitation, soil water, and groundwater. According to the percolation velocity of soil water (0.12–0.14 m/yr), the depth above and below 10 m is taken as the shallow and deep part of the UZ, respectively. Results show that recent human activities have already caused nitrate pollution in the shallow part, which originated from chemical fertilizers, manure, and soil nitrogen. However, the nitrate in the deep part mainly originates from soil nitrogen. Denitrification in the shallow and deep part of the UZ is not evident. The groundwaters are all tritium-free (i.e., recharged before the 1950s), and therefore the water quality has not been affected by recent human activities. The groundwater nitrate concentration ranges from 6.5 mg/L to 19.2 mg/L with an average of 12.6 mg/L and mainly originates from the nitrification of soil nitrogen. Denitrification process is also not evident in groundwater, suggesting that the hydrogeological condition (especially the buffering effect of the thick UZ) controls the migration of nitrate. However, nitrate produced by human activities on the land surface would enter the aquifer eventually, posing a potential risk on water quality in the future.

Nitrate processing traced by nitrate dual isotopic composition in the early spring in the Changjiang (Yangtze River) Estuary and adjacent shelf areas

Nitrate is the major chemical form of N-nutrient to sustain primary production in Changjiang Estuary and adjacent seawaters. We employed δ15N-NO3− and δ18O-NO3− to constrain the source, cycling, and sink of nitrate in early spring. Both δ15N-NO3− and δ18O-NO3− differentiate significantly among Changjiang Diluted Water (CDW), Yellow Sea Coastal Current (YSCC), and Taiwan Warm Current (TWC). In coastal areas, nitrate distribution and its isotopes are mainly affected by Changjiang inputs. Chemical fertilizers and sewage & manure originated nitrate jointly contribute the most nitrate in CDW. In offshore areas, nitrification contributes 44 ± 21% of the nitrate in YSCC and 17 ± 16% in TWC; assimilation is the dominant process to remove nitrate in TWC (35 ± 16%). Overall, nitrification and assimilation are the key nitrate cycling processes in early spring and co-shape the offshore distribution pattern of nitrate and its dual isotopes.

The Dual Isotopic Composition of Nitrate in the Gulf of Mexico and Florida Straits

AbstractThe Gulf of Mexico receives considerable riverine nutrient inputs that contribute to eutrophication of northern shelf waters. However, it is unclear whether a significant fraction of these riverine nutrients delivered to the Gulf of Mexico are subsequently exported from the Gulf, potentially influencing the chemistry of the North Atlantic. Here, we report the first dual isotopic composition (“δ15N” and “δ18O”) measurements of nitrate (NO3−) from three profiles collected in different years in the Gulf of Mexico, as well as a new profile from the Florida Straits. We compare these new measurements with prior measurements from the North and South Atlantic, as well as with the magnitude of riverine and other nitrogen fluxes to the Gulf of Mexico. We find that in the upper thermocline, NO3− δ15N measurements from the Gulf of Mexico are intermediate between prior measurements in the Sargasso Sea (lower in NO3− δ15N), and those from the South Atlantic (higher in NO3− δ15N), but are indistinguishable from Florida Straits and Sargasso Sea profiles on deeper isopycnal surfaces. There is little evidence for riverine NO3− influencing the isotopic composition of NO3− exported from the Gulf of Mexico, which, based on its δ15N, would be expected to raise the δ15N of shallow NO3−. Lesser nitrogen fluxes are too small to significantly influence the isotopic composition of water column NO3− in the Gulf of Mexico. These results are consistent with recent modeling work suggesting that the majority of riverine nutrients are retained in the nearshore region.

Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate

Nitrate (NO3−) has become recognized as the most important water-soluble ion in fine particulate (PM2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO3−concentration, δ18O-NO3− and δ15N-NO3− values in PM2.5 were analyzed. NO3− concentrations increased as PM2.5 levels increased during both polluted and non-polluted days over the entire year. δ18O-NO3− values during cold months (63.5–103‰) were higher than those during warm months (50.3–85.4‰), these results suggested that the nitrate formation pathways shifted from the NO2 + OH (POH) in warm months to the N2O5 + H2O (PN2O5) and NO3 + VOCs (PNO3) pathways in cold months. Especially during cold months, δ18O-NO3− values increased from 65.2–79.9‰ to 80.7–96.2‰ when PM2.5 increased from ∼25 to >100 μg/m3, but when PM2.5 > 100 μg/m3, there were relatively small variations in δ18O-NO3−. These results suggested that nitrate formation pathways changed from POH to PN2O5 and PNO3 pathways when PM2.5 < 100 μg/m3, but that PN2O5 and PNO3 dominated nitrate production when PM2.5 > 100 μg/m3. Higher δ15N-NO3− values in warm months (−11.8–13.8‰) than in cold months (−0.7–22.6‰) may be attributed to differences in NOx emission sources and nitrogen isotopic fractionation among NOx and NO3−. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM2.5 levels.

Application of the dual-isotope approach and Bayesian isotope mixing model to identify nitrate in groundwater of a multiple land-use area in Chengdu Plain, China

Nitrate (NO3−) contamination in groundwater is an environmental problem worldwide. Partitioning the pollution into its sources is the key for effective controls. In this study, NO3− dual isotopic compositions (δ15N-NO3− and δ18O-NO3−) were measured in groundwater samples from 28 wells in an area with multiple land-uses, followed by the application of an isotope mixing model (SIAR) to identify the main NO3− sources and their biogeochemical processes. The results showed that denitrification was unlikely occur at significant rates, while nitrification was an important nitrogen transformation processes. Spatial variation of groundwater NO3− and its isotopic compositions was associated with the land-use types. Agricultural areas were characterized by relatively high NO3− concentrations and low δ15N-NO3− values. In contrast, industrial areas were characterized by enriched δ15N-NO3− and δ18O-NO3− values. In crop field, vegetable field and poultry and livestock breading farm, the proportional contribution represented a similar pattern with highest contribution from chemical fertilizer followed by soil organic nitrogen, manure, atmospheric precipitation and sewage in order. Nitrate in groundwater in industrial areas has different pattern of the proportional contribution, in which groundwater NO3− is largely influenced by sewage discharge and atmospheric precipitation. We concluded that the combination of isotopic analysis together with land-use information and chemical analysis was an effective approach for assessing the source apportionment and the fate of nitrate in the aquifer in multiple land-use areas.