Nitrate δ18O Research Articles

Revealing the mechanisms of soil gypsum formation in the Atacama Desert through triple oxygen and hydrogen isotopes of gypsum hydration water

The soils in the hyper-arid core of the Atacama Desert (Chile) harbor substantial quantities of soluble salts, including sulfates and nitrates. Gypsum (CaSO₄∙2H₂O) is a prevalent mineral in the Atacama region. It manifests as ∼10-cm-thick surface crusts exhibiting a spatial pattern resembling polygons, in mixtures with other minerals beneath the surface (<3 m deep) or growing in brine ponds of salars. We examine the triple oxygen and hydrogen stable isotopes (δ17O, δ18O and δD) of structurally-bonded gypsum hydration water (GHW) from Atacama gypsum deposits to investigate their formation mechanisms. We observed considerably positive Δ17O anomalies (Δ17O > 1 ‰) in GHW of samples containing substantial amounts of nitrate (generally soil horizons 0.5–2 m deep). This is interpreted as resulting from oxygen isotope exchange between soil nitrate (Δ17O > 15 ‰) and GHW (Δ17O ∼ 0 ‰), either occurring in-situ within the soils due to bacterial activity, or more likely, during cryogenic extraction of GHW at high temperature and low pH in the laboratory. However, when analyzing pedogenic gypsum in soils with negligible amounts of nitrates, the isotopic composition of its parent solution is preserved. These samples showed that gypsum formed from solutions that did not undergo significant evaporation. These waters are isotopically similar to contemporary unevaporated freshwater sources in the region. This indicates that gypsum formation likely occurred through the hydration of anhydrous or hemihydrate calcium sulfate, either in the atmosphere or in the soil. Subsequently, the gypsum was subject to recrystallization in the soil due to occasional rainfall events. This stands in contrast to subaqueous gypsum crystals growing in brine ponds of salars in the Atacama Desert, which reflect the isotopic composition of highly-evaporated parent fluids.

A Study of Chemical Processes of Nitrate in Atmospheric Aerosol and Snow Based on Stable Isotopes

Nitrate (NO3−) is a prominent atmospheric pollutant and a key chemical constituent of snow and ice, which plays a crucial role in the atmosphere and significantly impacts regional climate and environment conditions through a series of complex chemical processes. By summarizing the recent research progress on the nitrate chemical process (particularly on the isotopic measurements of NO3− (δ15N, Δ17O and δ18O)) in atmosphere and glacier snow, this study mainly investigated the chemical compositions and chemical processes, formation pathways, and photochemical reactions of nitrate in snow and atmosphere. Our results identified that the main ways of atmospheric nitrate formation are the hydrolysis of N2O5 and the reaction of ·OH with NO2; the spatial distribution of Δ17O and δ18O values of atmospheric nitrate have a significant latitudinal trend between 30° N–60° N; the study of stable isotopes (δ15N and δ18O) and the oxygen isotope anomaly (Δ17O) of nitrate have mainly been carried out over the densely populated and coastal mega cities; there exist significant gaps in the study of chemistry processes of nitrate in snow and ice and the air–snow interfaces across glaciated regions. This study provides a basic reference for more robust observations and research of nitrate in glacier areas in the future.

Stable isotopes of nitrate (δ15N and δ18O) as functional indicators of nitrogen processing in constructed wetlands

The effectiveness of nitrogen removal in wetlands relies heavily on the biological processes that control its removal. Here, we used δ15N and δ18O of nitrate (NO3−) to assess the presence and the dominance of transformation processes of nitrogen in two urban water treatment wetlands in Victoria, Australia over two rainfall events. Laboratory incubation experiments were undertaken in both light and dark to measure the isotopic fractionation factor of nitrogen assimilation (by periphyton and algae) and benthic denitrification (using bare sediment). Highest isotopic fractionations were observed for nitrogen assimilation by algae and periphyton in the light, 15ε = −14.6 to −25 ‰ while the 15ε = −1.5 ‰ in bare sediment, consistent with that of benthic denitrification. Transect water samplings of the wetlands showed different rainfall patterns (discrete versus continuous) affect the removal capability of the wetlands. During the discrete event sampling, the observed 15ε of NO3− (an average of 3.0 to 4.3 ‰) within the wetland falls between the experimental 15ε of benthic denitrification and assimilation; coinciding with the decrease in NO3− concentrations, suggesting that both denitrification and assimilation were important removal pathways. Depletion of δ15N-NO3− throughout the whole wetland system also suggested the influence of water column nitrification during this time. In contrast, during continuous rain events, no fractionation effect was observed within the wetland and was consistent with limited NO3− removal. The difference in fractionation factors within the wetland during different sampling conditions suggested that nitrate removal was highly likely limited by changes in overall nutrient inputs, residence time and water temperature which impeded biological uptake or removal. These highlight that consideration of sampling condition is crucial when assessing the efficacy of a wetland in removing nitrogen.

Hyporheic exchange in an urban beaver pond mediates high nutrient groundwater inflow and pond productivity

The rebound of beaver populations across North America has resulted in new understanding of how their alteration of stream channels affects hydrology and biogeochemical cycling, but their increased impact on urban areas has not been well investigated. Urban beaver ponds have the potential to slow and retain runoff, reconnect surrounding floodplains, and reduce nutrients. Where management allows, beaver ponds may partially ameliorate water quality problems in urban streams and enhance the ecological function of the broader riparian zone. In this study we measured water chemistry in surface (over ten months) and hyporheic (over three months) waters, including nitrate, ammonium, and dissolved organic carbon (DOC), to assess the impact of urban beaver dams on water quality. Pond water is sourced from groundwater, with anoxic conditions in the surface water at the top of the pond that become increasingly oxic moving longitudinally through the pond. Large diel dissolved oxygen swings in the pond indicate high sestonic productivity. There are distinct hyporheic geochemical signatures above and below the dam as well as across the dam indicating the dam is driving downwelling above the dam and upwelling below. Hyporheic ammonium concentrations and surface nitrate concentrations are high above the dam and decrease below the dam, showing that shifting oxic conditions drive nitrogen speciation. The δ15N and δ18O values of nitrate point to diffuse sewage leaks as the main source of nutrients to the pond instead of overland runoff. In addition, surface waters have lower δ15N and δ18O of nitrate and DOC concentrations than groundwaters, indicating denitrification may drive the decrease in nitrate concentration. With increasing presence of beaver in urban streams, surface water-groundwater interactions through urban beaver dams have the potential to remediate urban sources of nutrients from ponds.

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 (&gt; 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.

Vertical Differences of Nitrate Sources in Urban Boundary Layer Based on Tower Measurements

Reducing nitrogen oxide (NOx) emissions is key to limiting fine particulate pollution globally. Nitrogen isotopes are powerful tools for exploring sources of atmospheric nitrate and its precursor NOx. This work performed simultaneous vertical measurements of nitrogen isotopes of nitrate (δ15N(NO3–)) at 8 and 260 m heights on a meteorological tower in Beijing. Although δ15N(NO3–) of the two heights were close, a significant difference (P < 0.01) in the δ15N of nitrogen oxides (NOx) was found in winter after considering the nitrogen isotope fractionation between NOx and the nitrate constrained by the oxygen anomaly (Δ17O) of nitrate. Combining the estimated δ15N of NOx and a Bayesian model, it was found that the proportion of coal combustion to nitrate aerosols was higher at 260 m (54% ± 9%) than that at the ground surface (31% ± 10%) in winter, suggesting that the air masses carrying coal-derived aerosols reached the upper air of Beijing through regional transport. In summer, the vertical variations of nitrate sources were not obvious. Our findings suggest that conventional ground-based observations may underestimate the contribution of coal combustion to atmospheric aerosols in the urban boundary layer as air pollutants are influenced by a combination of local emissions, meteorology, and regional transport.

Tracing the source of nitrate in a forested stream showing elevated concentrations during storm events

Abstract. To clarify the source of nitrate increased during storm events in a temperate forested stream, we monitored temporal variation in the concentrations and stable isotopic compositions including Δ17O of stream nitrate in a forested catchment (KJ catchment, Japan) during three storm events: I, II, and III (summer). The stream showed a significant increase in nitrate concentration, from 24.7 to 122.6 µM, from 28.7 to 134.1 µM, and from 46.6 to 114.5 µM, during the storm events I, II, and III, respectively. On the other hand, the isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate showed a decrease in accordance with the increase in the stream nitrate concentration, from +2.5 ‰ to −0.1 ‰, from +3.0 ‰ to −0.5 ‰, and from +3.5 ‰ to −0.1 ‰, for δ15N; from +3.1 ‰ to −3.4 ‰, from +2.9 ‰ to −2.5 ‰, and from +2.1 ‰ to −2.3 ‰ for δ18O; and from +1.6 ‰ to +0.3 ‰, from +1.4 ‰ to +0.3 ‰, and from +1.2 ‰ to +0.5 ‰, for Δ17O during the storm events I, II, and III, respectively. Besides, we found strong linear relationships between the isotopic compositions of stream nitrate and the reciprocal of stream nitrate concentrations during each storm event, implying that the temporal variation in the stream nitrate can be explained by simple mixing between two distinctive endmembers of nitrate having different isotopic compositions. Furthermore, we found that both concentrations and the isotopic compositions of soil nitrate obtained in the riparian zone of the stream were plotted on the nitrate-enriched extension of the linear relationship. We concluded that the soil nitrate in the riparian zone was primarily responsible for the increase in stream nitrate during the storm events. In addition, we found that the concentration of unprocessed atmospheric nitrate in the stream was stable at 1.6 ± 0.4, 1.8 ± 0.4, and 2.1 ± 0.4 µM during the storm events I, II, and III, respectively, irrespective of the significant variations in the total nitrate concentration. We concluded that the storm events have fews impacts on the concentration of unprocessed atmospheric nitrate in the stream, and thus the annual export flux of unprocessed atmospheric nitrate relative to the annual deposition flux can be a robust index to evaluate nitrogen saturation in forested catchments, irrespective to the variation in the number of storm events and/or the variation in the elapsed time from storm events to sampling.

A nitrate budget of the Bohai Sea based on an isotope mass balance model

Abstract. The Bohai Sea (BHS) is a semi-closed marginal sea impacted by one of the most populated areas of China. The supply of nutrients, markedly that of reactive nitrogen, via fluvial and atmospheric transport has strongly increased in parallel with the growing population. Therefore, it is crucial to quantify the reactive nitrogen input to the BHS and understand the processes and determine the quantities of nitrogen eliminated in and exported from the BHS. The nitrogen budget and in particular the internal sources and sinks of nitrate were constrained by using a mass-based and dual stable isotope approach based on δ15N and δ18O of nitrate (NO3-). Samples of water, suspended matter, and sediments were taken in the BHS in spring (March and April) and summer (July and August) 2018. The Yellow River (YR) was sampled in May and July to November, and Daliao River, Hai River, Luan River, and Xiaoqing River were sampled in November of 2018. In addition to nutrient, particulate organic carbon, and nitrogen concentrations, the dual isotopes of nitrate (δ15N and δ18O), δ15N of suspended matters, and sediments were determined. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main nitrate sources are rivers contributing 19.2 %–25.6 % and the combined terrestrial runoff (including submarine fresh groundwater discharge of nitrate) accounting for 27.8 %–37.1 % of the nitrate input to the BHS, while atmospheric input contributes 6.9 %–22.2 % to total nitrate. An unusually active interior nitrogen cycling contributes 40.7 %–65.3 % to total nitrate via nitrification. Nitrogen is mainly trapped in the BHS and mainly removed by sedimentation (70.4 %–77.8 %), and only very little is exported to the Yellow Sea (YS) (only 1.8 %–2.4 %). At present denitrification is active in the sediments and removes 20.4 %–27.2 % of nitrate from the pool. However, a further eutrophication of the BHS could induce water column hypoxia and denitrification, as is increasingly observed in other marginal seas and seasonally off river mouths.

Persistent Nitrate in Alpine Waters with Changing Atmospheric Deposition and Warming Trends.

Nitrate concentrations in high-elevation lakes of the Colorado Front Range remain elevated despite declining trends in atmospherically deposited nitrate since 2000. The current source of this elevated nitrate in surface waters remains elusive, given shifts in additional nitrogen sources via glacial inputs and atmospheric ammonium deposition. We present the complete isotopic composition of nitrate (δ15N, δ18O, and Δ17O) from a suite of nitrate-bearing source waters collected during the summers of 2017-2018 from two alpine ecosystems to constrain the provenance of elevated nitrate in surface waters during the summer open-water season. The results indicate a consistent contribution of uncycled atmospheric nitrate throughout the summer (13-23%) to alpine lakes, despite seasonal changes in source water inputs. The balance of nitrate (as high as 87% in late summer) is likely from nitrate production within the catchment via nitrification of reduced nitrogen sources (e.g., thawed soil organic matter and ammonium deposition) and released with rock glacier meltwater. The role of microbially produced nitrate has become increasingly important over time based on historical surface water samples from the mid-90s to present, a trend coincident with increasing ammonium deposition to alpine systems.

Nitrate Δ17O tracer method for determining gross nitrification rates

Gross nitrification rate (GNR) can reflect the actual status of nitrification process but is difficult to constrain. To develop a practical method for assessing GNR, Δ17O-NO3− tracers were applied to container systems under greenhouse conditions to test their feasibility. Two treatments with double- (15NΔ17O3−, negative Δ17O value) or single-labeled (NΔ17O3−, positive Δ17O value) fertilizer nitrate salts were implemented to constrain nitrification fractions and GNRs in soilless media container systems. The nitrification fractions calculated from the Δ17O-NO3− tracer method concurred with those from the 15NO3− tracer method, with small deviations and a linear regression slope close to 1. This suggested the Δ17O-NO3− tracer inherently works in the same way as the well-established 15NO3− tracer. GNRs in the soilless media container systems ranged from 250.5(±39.1) to 861.5(±275.6) mg N/(m2‧d), generally higher than in natural ecosystems. The discrepancies in GNRs between the two treatments were generally smaller than discrepancies in GNRs between replicate container systems, indicating the applicability of either negatively-labeled or positively-labeled Δ17O-NO3− tracers. Commercial nitrate salts mined from the Atacama and Kumtag Deserts are proposed for use as Δ17O-NO3− tracers for GNR estimation due to their abundant availability, low cost and distinct discrimination from nitrified NO3−.

A Complete Isotope (δ15N, δ18O, Δ17O) Investigation of Atmospherically Deposited Nitrate in Glacial‐Hydrologic Systems Across the Third Pole Region

AbstractThe Himalayas and Tibetan Plateau, identified as the Third Pole (TP), is a unique region because of its insertion into global environmental and climatic changes. Deposition of atmospheric nitrate in this region is one of the most important sources of reactive nitrogen to glacial‐hydrologic system and ecosystems. The isotopic composition of atmospherically deposited nitrate preserved in ice bodies plays a central role in delineating environmental and climatic changes, present, and past. Here, we provide an overview of the complete isotopic compositions (δ15N, δ18O, and Δ17O) of nitrate in aerosol, snow, ice, and water samples (n = 46) collected across the Southern, Southeastern, Central, and Northern TP to constrain complex nitrogen cycles of the atmosphere, cryosphere, hydrosphere, and, potentially, the biosphere. A large variability of snow nitrate isotopic compositions is observed across the TP at different spatial scales (from a single glacier to the entire plateau), likely due to the complex landscape and relevant physical and chemical processes across the TP. Large nitrate Δ17O values are observed in water samples collected from the Mt. Everest region, highlighting the considerable fraction (up to 45%) of atmospheric nitrate to the nitrate load in this Himalayan hydrologic system. Our work reveals the complex chemical, depositional, and postdepositional processes over the TP that are greater than previously thought and identifies further comprehensive investigations, which entail using nitrate isotopic compositions as an identifier for nitrate source apportionment in various ecosystems and understanding past atmospheric and climatic conditions in this region.

Utilization of Δ17O for nitrate dynamics in a subtropical freshwater reservoir

Feitsui Reservoir, a freshwater body in Taiwan with minimal anthropogenic stress, meets the water demand for the population of more than five million living in Taipei city. In view of the biogeochemical processes controlling the long-term trophic status of this socio-economically and ecologically important aquatic system, probing the nitrogen cycle and its dynamics is essential. Here, we monitored the concentration and stable isotopic compositions (δ15N, δ18O, and Δ17O) of nitrate in the Feitsui Reservoir and in the atmospheric wet deposition at intervals of 1–2 weeks for a year, along with measurements of environmental data such as chlorophyll a, dissolved oxygen, and community respiration. Emphasis was laid on Δ17O (= δ17O – 0.52 × δ18O) because of the mass-conservative behavior of Δ17O during partial assimilation and denitrification. The present approach offered an effective method to quantify the gross nitrification and removal/uptake rates of nitrate in the reservoir. The atmospheric nitrate exhibited elevated Δ17O values ranging from 12.6‰ to 30.1‰ (23.3 ± 5.0‰), compared to the lower Δ17O values of ~0 to 4.6‰ (1.1 ± 0.7‰) recorded in the reservoir nitrate. Utilizing Δ17O for dissolved nitrates, we observed a seasonal trend of higher nitrification and removal rates during the summer than in the winter. Our estimates showed annually-averaged nitrification rate of 55 ± 11 mmol m−2 d−1 and removal/uptake rate of 57 ± 11 mmol m−2 d−1 (or a nitrate turnover time of ~2.5 months), representing the active nature of nitrogen cycling in this preserved subtropical reservoir.

Isotopic constraints on the formation pathways and sources of atmospheric nitrate in the Mt. Everest region

Inorganic particulate nitrate (p-NO3−), gaseous nitric acid (HNO3(g)) and nitrogen oxides (NOx = NO + NO2), as main atmospheric pollutants, have detrimental effects on human health and aquatic/terrestrial ecosystems. Referred to as the ‘Third Pole’ and the ‘Water Tower of Asia’, the Tibetan Plateau (TP) has attracted wide attention on its environmental changes. Here, we evaluated the oxidation processes of atmospheric nitrate as well as traced its potential sources by analyzing the isotopic compositions of nitrate (δ15N, δ18O, and Δ17O) in the aerosols collected from the Mt. Everest region during April to September 2018. Over the entire sampling campaigns, the average of δ15N(NO3−), δ18O(NO3−), and Δ17O(NO3−) was −5.1 ± 2.3‰, 66.7 ± 10.2‰, and 24.1 ± 3.9‰, respectively. The seasonal variation in Δ17O(NO3−) indicates the relative importance of O3 and HO2/RO2/OH in NOx oxidation processes among different seasons. A significant correlation between NO3− and Ca2+ and frequent dust storms in the Mt. Everest region indicate that initially, the atmospheric nitrate in this region might have undergone a process of settling; subsequently, it got re-suspended in the dust. Compared with the Δ17O(NO3−) values in the northern TP, our observed significantly higher values suggest that spatial variations in atmospheric Δ17O(NO3−) exist within the TP, and this might result from the spatial variations of the atmospheric O3 levels, especially the stratospheric O3, over the TP. The observed δ15N(NO3−) values predicted remarkably low δ15N values in the NOx of the sources and the N isotopic fractionation plays a crucial role in the seasonal changes of δ15N(NO3−). Combined with the results from the backward trajectory analysis of air mass, we suggest that the vehicle exhausts and agricultural activities in South Asia play a dominant role in determining the nitrate levels in the Mt. Everest region.

Novel GC/Py/GC/IRMS-Based Method for Isotope Measurements of Nitrate and Nitrite. I: Converting Nitrate to Benzyl Nitrate for δ18O Analysis.

The 18O/16O (and 15N/14N) ratio of natural nitrate (NO3-) and nitrite (NO2-) can be used to extract valuable information about their source and fate as environmental contaminants, their metabolism as macronutrients in plants and animals, and their behavior in the N biogeochemical cycle. We developed an accurate, precise, sensitive (minimum sample size: 0.2 μg NO3--equivalent), and reliable (minimal oxygen exchange, loss, or gain) method to selectively isolate and purify nitrate and nitrite from natural water, soil, air, and plant materials by strong anion exchange (SAX) for low- to normal-salinity samples or strong cation exchange (SCX) for high-salinity samples, followed by quantitative conversion to their respective benzyl esters, which can be separated and individually analyzed for δ18O (and potentially δ15N) by gas chromatography (GC)/pyrolysis/GC/isotope-ratio mass spectrometry (IRMS). The method compares favorably with the currently popular bacterial denitrification and chemical reduction methods, in terms of sensitivity and reliability, and has the potential to simultaneously measure δ15N and δ18O of nitrate and nitrite from natural samples of various origins.

Multiyear Measurements on Δ17O of Stream Nitrate Indicate High Nitrate Production in a Temperate Forest.

Nitrification is a crucial step in ecosystem nitrogen (N) cycling, but scaling up from plot-based measurements of gross nitrification to catchments is difficult. Here, we employed a newly developed method in which the oxygen isotope anomaly (Δ17O) of nitrate (NO3-) is used as a natural tracer to quantify in situ catchment-scale gross nitrification rate (GNR) for a temperate forest from 2014 to 2017 in northeastern China. The annual GNR ranged from 71 to 120 kg N ha-1 yr-1 (average 94 ± 10 kg N ha-1 yr-1) over the 4 years in this forest. This result and high stream NO3- loss (4.2-8.9 kg N ha-1 yr-1) suggest that the forested catchment may have been N-saturated. At the catchment scale, the total N output of 10.7 kg N ha-1 yr-1, via leaching and gaseous losses, accounts for 56% of the N input from bulk precipitation (19.2 kg N ha-1 yr-1). This result indicates that the forested catchment is still retaining a large fraction of N from atmospheric deposition. Our study suggests that estimating in situ catchment-scale GNR over several years when combined with other conventional flux estimates can facilitate the understanding of N biogeochemical cycling and changes in the ecosystem N status.

The observation of isotopic compositions of atmospheric nitrate in Shanghai China and its implication for reactive nitrogen chemistry

The occurrence of PM2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NOX = NO + NO2). The oxygen-17 excess of nitrate (Δ17O(NO3−)) can be used to reveal the relative importance of nitrate formation pathways and get more insight into reactive nitrogen chemistry. Here we present the observation of isotopic composition of atmospheric nitrate (Δ17O and δ15N) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m−3 with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m−3 in winter, spring and summer respectively. Δ17O(NO3−) varied from 20.5‰ to 31.9‰ with the mean value being (26.9 ± 2.8) ‰ in winter, followed by (26.6 ± 1.7) ‰ in spring and the lowest (23.2 ± 1.6) ‰ in summer. Δ17O(NO3−)-constrained estimates suggest that the conversion of NOX to nitrate is dominated by NO2 + OH and/or NO2 + H2O, with the mean possible contribution of 55–77% in total and even higher (84–92%) in summer. A diurnal variation of Δ17O(NO3−) featured by high values at daytime (28.6 ± 1.2‰) and low values (25.4 ± 2.8‰) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (τ) and calculations indicate τ is around 15 h during the diurnal sampling period. In terms of δ15N(NO3−), it changed largely in our observation, from −2.9‰ to 18.1‰ with a mean of (6.4 ± 4.4) ‰. Correlation analysis implies that the combined effect of NOX emission sources and isotopic fractionation processes are responsible for δ15N(NO3−) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions.

Nitrate sources and the effect of land cover on the isotopic composition of nitrate in the catchment of the Rhône River

ABSTRACTThe Rhône River originates in the high Alps and drains an intensely cultivated and industrialised catchment before it discharges to the Gulf of Lion. We investigated the interaction of catchment geomorphology with nitrate sources (atmosphere, agriculture, and nitrification of soil organic matter) and removal processes in large and diverse watersheds on the basis of dual nitrate isotope signatures in river water.In March 2015, we took surface water samples along the Rhône River, including its main tributaries, and measured nutrient concentrations and the stable isotopic composition of nitrate (δ15N, δ18O and Δ17O), and water (δ18O-H2O).Results show that high altitude regions are dominated by nitrate from nitrification in pristine soils and atmospheric deposition, while nitrate in the downstream Rhône River originates mainly from nitrification of agricultural/urban sources. Parallel increases in δ15N and δ18O reflect the influence of primary production. Previous studies suggested robust correlations between land use and . Based on our observation that nitrate δ15N values at higher altitudes are lower than expected, we assume that lower nitrate δ15N values likely reflect limited nitrate consumption and lower soil nitrogen turnover rates. We propose that correlation between land use and nitrate δ15N is sensitive to slope and geomorphology.

Nitrogen and carbon cycling over the western continental shelf of India during seasonal anoxia: A stable isotope approach

The western continental shelf of India (eastern Arabian Sea) houses the world's largest seasonally-occurring coastal low-oxygen zone. The natural abundance of nitrogen and oxygen isotopes in dissolved nitrate were measured along four transects of the shelf (off Mumbai, Goa, Mangalore and Calicut) to gain insights into nitrogen cycling in this system. Carbon and nitrogen isotope abundances in particulate organic matter were also measured to understand sources and cycling of organic matter. Three of the four transects (with the exception of Mumbai transect) experienced fully anoxic (sulphidic) conditions over the inner shelf. Denitrification signals, as indicated by a linear increase of δ15N and δ18O in nitrate in oxygen-deficient waters, were observed off Mumbai, Goa and Mangalore. The occurrence of nitrite oxidation was indicated by a decoupling of stable N and O isotopes of nitrate. The fractionation factor computed was found to be quite low (6.1‰), as compared to open ocean oxygen minimum zones, presumably due to a large contribution (>49%) of benthic denitrification to N-loss. Autochthonous production and phytoplankton composition are proposed to exert major controls on the C isotopic composition of particulate organic matter while nitrate and ammonium seem to control the N-isotope variability. Overall, this study attempts to fill the knowledge gap in terms of N and C stable isotope studies in a system that is otherwise well documented from the biogeochemical point of view.

Seasonal variations of nitrate dual isotopes in wet deposition in a tropical city in China

Monthly analytical results for more than two years long of δ15N and δ18O of nitrate in wet deposition are reported for a tropical city of Zhanjiang in the southernmost mainland China, in an effort to elucidate NOx sources and its oxidation pathways to nitrate. The results showed that monthly variations of δ18ONO3- responded well to changes in sunshine hours, with lower δ18ONO3- values corresponding to longer sunshine hours. This pattern suggests that NOx oxidation via the OH radical was the predominant pathway, which, by estimate, accounted for 87% in winter and 94% in summer, for nitrate formation. Remarkably, we found that the δ18ONO3- here are prominently low relative to previous studies, likely due to that annual sunshine hours in this tropical city is relatively long. Moreover, available data of reported δ18ONO3- in wet deposition showed an increasing trend with latitude, reflecting a shift of the predominant NOx oxidation pathways from those via the OH radical in low latitudes to those via O3 in high latitudes, corresponding to the decrease of sunshine hours. A temporarily co-variation of δ15NNO3- and δ18ONO3- was observed, exhibiting higher values in dry winter and lower values in wet summer, which was attributed to the washout effect on the dual isotopes. During wet seasons, complete nitrate washout suggests that δ15NNO3- in precipitation should be similar to the nitrate formed in the atmosphere, and thereby can be used for source apportionment. A Bayesian model showed that the source of atmospheric nitrate deposition is mainly natural (61%) in Zhanjiang, with less anthropogenic contribution.

Probing soil nitrification and nitrate consumption using Δ17O of soil nitrate

Recent analytical and conceptual advances related to the nitrate (NO3−) 17O anomaly (Δ17O) have opened the door to a new method that probes soil nitrification and NO3− consumption using Δ17O of soil NO3−. Because biological NO3− production and consumption processes in soil obey the mass-dependent fractionation law, Δ17O of soil NO3−, an index of excess 17O over that expected from 18O, can be used to trace gross nitrification and NO3− consumption in a way analogous to the 15NO3− tracer typically employed in studies of soil NO3− cycling. Moreover, coupling Δ17O with the dual NO3− isotopes (δ15N and δ18O) at natural abundances offers additional valuable insights into mechanisms that underlie soil NO3− dynamics. In this study, we conducted both laboratory and field experiments to assess the use of Δ17O-NO3- for tracing soil nitrification and NO3− consumption. Soil samples spanning a wide range of physical and chemical properties were sampled from four sites for batch incubations and amendments with a Δ17O-enriched NO3− fertilizer. After amendments, the triple isotopes (δ15N, δ18O, and Δ17O) of soil NO3− were measured periodically and used in a developed Δ17O-based numerical model to simultaneously derive gross rates and isotope effects of soil nitrification and NO3− consumption. The measured Δ17O-NO3- was also used in the classical isotope dilution model to estimate gross NO3− turnover rates. In situ field soil sampling was conducted in a temperate upland meadow following snowmelt input of Δ17O-enriched atmospheric NO3− to assess the robustness of Δ17O-NO3- as a natural tracer. The results show that the temporal dynamics of Δ17O-NO3- can provide quantitative information on soil nitrification and NO3− consumption. In the laboratory incubations, a wide range of gross nitrification and NO3− consumption rates were estimated for the four soils using the Δ17O-based models. The estimated rates are well within the range reported in previous 15N tracer-based studies and not sensitive to oxygen isotopic fractionations during nitrification and NO3− consumption. Coupling Δ17O-NO3- with the dual NO3− isotopes using the numerical model placed strong constraints on the δ15N and δ18O endmembers of nitrification-produced NO3− and revealed soil-specific N isotope effects for nitrification and NO3− consumption, consistent with the inferred differences in soil microbial community structure among these soils. Non-zero Δ17O-NO3- values, up to 4.7‰, were measured in the meadow soil following the snowmelt event. Although soil heterogeneity in the field prevents quantitative rate estimation using Δ17O-NO3-, active NO3− cycling via co-occurring nitrification and denitrification was revealed by the covariations in the triple NO3− isotopes. Integrating the field observations with the incubation results uncovered isotopic overprinting of nitrification on denitrification in the surface soil following the snowmelt, which has important implications for explaining the discrepancies between field- and laboratory-derived isotope systematics of denitrification. We conclude that Δ17O-NO3- is a conservative and powerful tracer of soil nitrification and NO3− consumption and future applications are expected to help disentangle soil NO3− cycling complexity at various scales.