δ15N Of Nitrate Research Articles

Vadose Zone Transport of Tritium and Nitrate under Ponded Water Conditions

Vadose zone transport of tritium and nitrate can be important considerations at radioactive waste sites, landfills, or areas with industrial impacts. These contaminants are of particular concern because they typically have a relatively higher mobility in the subsurface compared to other compounds. Here, we describe a semiarid site with tritium and nitrate contamination involving a manmade ponded water source above a thick unsaturated zone at Los Alamos National Laboratory in New Mexico, USA. This study demonstrates the value of vadose zone flow and transport modeling for the development of field investigation plans (i.e., identifying optimal borehole locations and depths for contaminant characterization), and how a combination of modeling with isotope and geochemical measurements can provide insight into how tritium and nitrate transport in the vadose zone in semiarid environments. Modeling results suggest that at this location, tritium transport is well predicted by classical multiphase theory. Our work expands the demonstrated usefulness of a standard tritium conceptual model to sites with ponded surface conditions and agrees with previous results where a standard model was able to explain the evolution of a tritium plume at an arid waste disposal site. In addition, depth-based analyses of δ18O and δ2H of pore waters helped confirm the extent of pond infiltration into the vadose zone, and the δ15N of nitrate showed that the contaminant release history of the site was different than originally assumed.

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.

Identifying NOx Sources in Arequipa, Peru Using Nitrogen Isotopes in Particulate Nitrate

We reported on the first time series of δ15N in aerosol nitrate from South America. Particulate matter less than 2.5 microns in diameter (PM2.5) was collected at four sites located in Arequipa, a major city in southern Peru. The δ 15N values for nitrate in PM2.5 ranged from -1.7–15.9‰ and averaged 5.3 ± 3.0‰, with no significant difference between the four study sites and no discernable seasonal trend. These values are significantly higher than those in aerosol nitrate from southern hemisphere marine environments and those from the northern hemisphere. We explain the elevated values using an isotope mass balance mixing model that estimates a source NOx δ15N of -8 ± 3‰, derived mainly from anthropogenic sources (vehicles, industry). An isotope enabled 0-D photochemical box model was used to estimate the isotope enrichment of nitrate relative to NOx due to kinetic, equilibrium, and photolysis isotope effects occurring during NOx oxidation. This “source plus photochemistry” approach resulted in general agreement with the observations. This suggests that if the photochemistry effect can be accounted for, nitrate δ15N can be used to assess the relative importance of NOx sources and could be a new tool to validate NOx emission inventories.

Characterization of nitrogen isotope fractionation during nitrification based on a coastal time series

AbstractIsotopic enrichment factors are key to using stable isotope signatures in biogeochemical studies. However, these are typically determined in laboratory experiments and their applicability to environmental conditions is difficult to test. Here, we analyzed nitrogen stable isotope changes associated with nitrification in a coastal basin using weekly time‐series measurements of δ15N in particulate nitrogen, ammonium, nitrite, and nitrate. Two year‐long time series were selected as contrasting natural experiments in the ammonium‐rich, aphotic bottom water of Bedford Basin, Nova Scotia, Canada. In 2014, ammonia oxidation (AO) was associated with Thaumarchaeota and nitrite concentrations remained low (< 0.5 μmol kg−1). In contrast, transient nitrite accumulation (~ 8 μmol kg−1) and a more rapid δ15NNH4 increase in the fall of 2017 were likely caused by ammonia‐oxidizing bacteria, associated with higher AO rates and, possibly, stronger nitrogen‐isotope enrichment (15εAO). Estimates of 15εAO (21.8 ± 2.2‰, 24.1 ± 1.1‰) were derived empirically using Rayleigh models applied to field data from restricted periods during which the bottom waters approximated a closed system and influence on 15εAO from other processes was demonstrably insignificant. Using a numerical reactive‐transport model, we found that the best fit for the δ15N data was obtained with 15εAO values (18.9‰, 25.1‰) close to those determined by the Rayleigh models. The time series also revealed substantial (~ 7‰) 15N‐enrichment of the particulate nitrogen due to light‐independent assimilation of partially nitrified ammonium. Consistent with previous studies, these field‐based nitrogen isotope fractionation experiments suggest that the range of 15εAO values relevant for marine systems may be narrower than determined in laboratory studies.

Seasonal dynamics of anaerobic oxidation of ammonium and denitrification in a dimictic lake during the stratified spring–summer period

AbstractIn aquatic ecosystems, nitrogen (N) loading is mitigated in redox transition zones principally through the processes of denitrification and anaerobic oxidation of ammonium (anammox). Here, we investigate the N cycling processes in the water column of a seasonal stratified lake influenced by benthic processes in Southern Germany (Fohnsee) during the development of the vertical redox stratification between April and September. Concentration profiles and stable isotope compositions of NO3− and NH4+ together with numerical modeling and quantification of the hydrazine synthase gene (hzsB) and nitrite reductase (nirK and nirS) genes were used to identify the predominant nitrogen‐transformation processes at lake Fohnsee throughout the spring and summer periods. Water chemistry data, quantitative polymerase chain reaction analysis and increases of δ15N and δ18O values of nitrate from 7.0‰ to 41.0‰ and 2.0‰ to 28.0‰, respectively, showed that nitrate reduction to nitrite and NO occurs in an upward moving zone of the water column from June to September following the displacement of the oxycline caused by thermal stratification. We also observed an increase in δ15N of ammonium from 15‰ to 28‰ in the anoxic water column. Modeling results suggest that this shift in δ15N‐NH4+ is predominantly controlled by mixing between ammonium stemming from the oxic water column with δ15N values of 25‰ and ammonium that is likely formed in the lake sediments by oxidation of organic matter with δ15N values of 11‰. Observed gene abundances (hzsB, nirK, and nirS) in lake water samples collected in June and July indicated the co‐occurrence of nitrate reduction and low rates of anammox, while the presence of sulfide in August and September may have inhibited the activity of anammox bacteria near the sulfate‐reduction zone at the lake bottom. This study revealed temporal and spatial (e.g., depth dependent) variations in the dominant N‐transformation processes in the investigated lake.

Analysing seasonal variations of methane oxidation processes coupled with denitrification in a stratified lake using stable isotopes and numerical modeling

Since the discovery of nitrite (n-damo) and nitrate dependent anaerobic oxidation of methane in 2006, evidence has arisen about the occurrence of this process in freshwater ecosystems. Anaerobic oxidation of methane (AOM) linked with denitrification has been proposed to be a previously overlooked sink of methane in freshwater environments. However, the occurrence and relevance of AOM coupled with denitrification in methane removal and, therefore, mitigating methane emissions in the environment is so far largely unquantified.We investigated methane oxidation processes in the water column at the seasonally stratified lake Fohnsee located in Southern Germany to determine the seasonal dynamics and to quantitatively describe the contribution of anaerobic and aerobic oxidation of methane removing CH₄ from the water column throughout nearly one year. Vertical concentration profiles and corresponding stable isotope ratios of methane (δ13C) and nitrate (δ15N and δ18O), together with dissolved oxygen concentrations were measured, and a numerical model was developed to evaluate whether anaerobic oxidation of methane coupled with denitrification is a key biogeochemical process at lake Fohnsee. Our data set together with the results of the numerical model revealed a redox zone within the water column where both denitrification and anaerobic oxidation of methane are linked, contributing to approximately 70% of methane removal between June and September. Therefore, AOM linked with denitrification represents the dominant methane sink in the water column of lake Fohnsee during summer stratification.The data patterns also showed that the anaerobic oxidation of methane zone was located near the lake bottom in May, and moved upwards following the seasonal vertical displacement of the oxycline and the availability of nitrate. This redox dynamic within the water column of lake Fohnsee had also an effect on the relevance of aerobic and anaerobic oxidation of methane removing methane from the water column during the year and the formation of a bacterial sulfate reduction zone close to the lake sediments.

The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub><i>x</i></sub> sources and cycling in the summertime Southern Ocean marine boundary layer

Abstract. Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx=NO+NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean, where pristine conditions serve as a useful proxy for the pre-industrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (>1 µm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70∘ S and across the northern edge of the Weddell Sea. The δ15N–NO3- decreased with latitude from −2.7 ‰ to −42.9 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid-latitudes and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of -21.8±7.6 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O–NO3- was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O–NO3- values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea-ice-covered ocean, which is known to enhance peroxy radical production. The observed low δ18O–NO3- is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

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.

Seasonal changes in nitrate assimilation of boreal woody species: importance of the leaf-expansion period

Key messageNitrate served as an important nitrogen source for dominant deciduous tree species, especially during their leaf expansion period, even in boreal forests, where nitrate availability was assumed to be low.Temporal changes in leaf nitrate assimilation with leaf growth were intensively investigated in boreal tree species to demonstrate the contribution of nitrate as a N source and to determine temporal changes in the contribution of nitrate during leaf ontogeny. Leaf area, mass, nitrate reductase activity (NRA), N concentration, and δ15N were repeatedly measured in developing leaves of naturally grown Alnus crispa, Betula neoalaskana, and Populus tremuloides during their leaf expansion period. Alnus crispa and B. neoalaskana showed distinct peaks in NRA during leaf expansion, whereas P. tremuloides did not. The highest peak in NRA occurred for A. crispa, whereas it had low NRA during the summer. Peak NRA in B. neoalaskana was lower than that of A. crispa (p < 0.01, ANOVA), although it showed higher NRA during summer (p < 0.01, ANOVA). All species showed clear decrease in N concentration through the leaf expansion period, but total N content per leaf increased. Only the N-fixing species A. crispa showed a rapid change in δ15N during the leaf expansion, and the decline indicated the changes in N source during the leaf development. The results indicate leaves of target species assimilated nitrate during the leaf expansion period, consuming immense energy, although leaves were considered a carbon sink during the early leaf expansion period. We suggest the early onset of leaf growth due to climate warming could influence plant nutrition via asynchrony between supply and demand for energy during spring.

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.

Quantifying Nitrate Formation Pathways in the Equatorial Pacific Atmosphere from the GEOTRACES Peru-Tahiti Transect

This study aims to better our collective understanding of the oxidative capacity and atmospheric chemistry over the equatorial Pacific. Bulk and size-segregated filter samples were collected during the GEOTRACES Eastern Tropical Pacific transect (4.1° S, 81.9° W to 10.5° S, 152.0° W; October–December 2013) and measured for aerosol concentration and complete isotopic composition of nitrate (δ15N, δ18O, Δ17O where Δ17O = δ17O – 0.52 × δ18O). Combined size-segregated filters produced data similar to that found in bulk filter samples, and notably neither δ15N nor δ18O showed any trends based on aerosol size. Similar to other studies, NO3– is concentrated (>80%) in the coarse size fractions (>1.5 μm). Bulk aerosol concentrations ranged from 6.6 to 89.8 nmol/m3. The bulk δ15N-, δ18O-, and Δ17O-nitrate ranged from −13.1 to −3.2‰, 68.5 to 79.3‰, and 23.5 to 28.4‰, respectively. Higher δ15N values near the coast are best explained by the influence of continental sources; lower δ15N values far from the coast may be associated with chemical fractionation during long-range transport or an oceanic source. Both Δ17O and δ18O are interpreted using kinetic analysis and gas concentrations from a global atmospheric chemical transport model (GEOS-Chem), which showed that nitrate production in this environment is dominated by OH oxidation (60%) and RONO2 hydrolysis (15%). To best match the δ18O and Δ17O observations in this study, the terminal oxygen isotopic values for ozone must be higher than those suggested by available observations and/or halogen-mediated chemistry must be more important than the models currently suggest.

Nitrate sources and transformations in surface water of a mining area due to intensive mining activities: Emphasis on effects on distinct subsidence waters

The increase in NO3− content in surface water caused by intensive mining activities in Huainan City, China, has attracted considerable attention owing to the deterioration of water quality and the degradation of ecosystems in recent years. The Huainan mining area, which is highly disturbed by anthropogenic activities, was selected as a typical observation area, and the surface water was classified as open subsidence water (OSW), closed subsidence water (CSW), and river water (RW). Moreover, the hydrochemical parameters and the δ15N and δ18O values of nitrate were employed to quantitatively trace the sources and biochemical transformation of NO3−, and the contribution ratios of different NO3− sources were estimated using the stable isotope analysis in R based on the Bayesian model. There was evident nitrification in the study area, but no significant denitrification has occurred. A substantial portion of δ15N–NO3- demonstrated complex sources of NO3−. Compared with those of CSW, the NO3− compositions of the OSW approached to those of the RW due to river recharge and discharge, and were greatly affected by anthropogenic activities. The proportional contribution of manure and sewage in the OSW was found to be the highest with a mean value of 39.5 % ± 12.3 %, which was followed by that of mine drainage (mean: 22.1 % ± 13.1 %), chemical fertilizer (mean: 17.5 % ± 10.6 %), and soil organic nitrogen (mean: 17.5 % ± 11.6 %). In the RW, the highest mean contribution of manure sewage was 35.2 % ± 9.7 %, which was followed by that of chemical fertilizer (mean: 29.3 % ± 7.2 %), mine drainage (mean: 23.4 % ± 13.0 %), and soil organic nitrogen (mean: 10.9 % ± 8.3 %). In contrast, the contribution of chemical fertilizer to the CSW was the highest with a mean value of 33.9 % ± 13.6 %, which was followed by that of soil organic nitrogen (mean: 26.5 % ± 13.8 %), mine drainage (mean: 18.1 % ± 11.6 %). Therefore, NO3− in the surface water of the mining area primarily originates from chemical fertilizers and manure sewage. In addition, the contribution of mine drainage to nitrate in the study area indicates the potential impact of mining activities on surface water. These findings highlight the value of classifying different types of surface water in tracing NO3− contamination sources, and provide relevant theoretical basis for tracing nitrate sources in other areas.

Biogeochemical cycling of nutrient in the western Bering Sea as revealed by nitrogen isotopic composition of nitrate and suspended particles

The nitrogen isotopic composition of nitrate and suspended particulate nitrogen (PN) in the upper 500 m water column along a transect in the western Bering Sea was analyzed to characterize the nitrogen cycle in summer. The δ15N of nitrate (δ15N–NO3) increases into the surface, in strong correlation with the upward decrease in nitrate concentration, reflecting the effect of nitrate assimilation by phytoplankton. The isotope effect of nitrate assimilation (15εNO3) in the basin and the slope is estimated to be 6.1 ± 2.1‰ and 4.1 ± 0.8‰, respectively. The variation of 15εNO3 in the basin is probably due to the difference in phytoplankton community, while the 15εNO3 in the slope is underestimated by at least 1‰ by nitrification. There are four types of relationship between δ15N of PN (δ15N-PN) and dissolved inorganic nitrogen concentration ([DIN]), i.e., a constant low δ15N-PN with slightly varying [DIN] in the euphotic zone of the basin, a negative correlation in the euphotic zone of the slope, a positive correlation in the depth from the bottom of the euphotic zone to the temperature minimum layer (Tmin layer), and a relatively constant δ15N-PN at varying [DIN] below the Tmin layer. These relationships reflect the effects of utilization of nutrient substrates in surface water and remineralization below the euphotic zone. By combining the spatial variations of δ15N–NO3 with δ15N-PN, we find that primary production in summer is mainly supported by regenerated nutrients, whether in the basin or in the slope. The effect of nitrification is only apparent in the slope due to its high productivity and active remineralization. δ15N-PN is not a good proxy of surface nitrate use, rather, it may reflect regional change of δ15N of ammonium, at least in summer.

Physiological screening of ruderal weed biomonitors of atmospheric nitrogen deposition

Background: Plants take up various species of reactive nitrogen and their different physiological responses to the increase of nitrogen availability can be useful in biomonitoring.&#x0D; Questions: Does atmospheric nitrogen deposition affect the physiology of ruderal weeds? Which species are most responsive to the nitrogen deposition?&#x0D; Studied species: Eleven ruderal weeds.&#x0D; Study site and dates: Morelia, Michoacán, Mexico. 2019.&#x0D; Methods: Under scenarios of 10, 20, 40 and 80 kg N ha-1year-1, we quantified plant responses of biomass production, nitrate reductase activity, chlorophyll content, photosynthetic efficiency, δ15N, nitrogen and carbon content.&#x0D; Results: Total biomass production increased with the rate of nitrogen deposition for Bidens pilosa , Chloris gayana, Lepidium virginicum , and Pennisetum setaceum, as chlorophyll content in B. pilosa , C. gayana , and L. virginicum. In turn, the below- to above-ground biomass ratio decreased for B. pilosa and C. gayana, as photosynthetic efficiency in C. gayana , L. virginicum , and Chloris pycnothrix . Nitrate reductase activity was only affected in L. virginicumm , C. gayana , and T. officinale. &#x0D; With the exception of C. pycnothrix , the nitrogen content increased, while the carbon augmented in C. gayana , C. pycnothrix, and P. setaceum . The C/N ratio was reduced in B. pilosa , C. gayana, Chloris virgata, P. setaceum, and T. officinale . The δ15N was increased in B. pilosa, C. gayana, C. virgata and P. setaceum .&#x0D; Conclusions: Bidens pilosa, C. gayana, L. virginicum, and P. setaceum were the species with more affected variables to nitrogen deposition, which could be useful in the biomonitoring.

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.

Evaluation of stable isotope ratios (δ15N and δ18O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types

Stable isotope ratios of nitrate are a powerful tool to evaluate aquatic environment stress from treated and untreated sewage. However, there is generally a lack of knowledge on the change in stable isotope ratios within wastewater treatment plants. We investigated nitrogen and oxygen stable isotope ratios (δ15N and δ18O) of nitrate in four types of advanced treatment processes operated in parallel; (A) extended aeration activated sludge, (B) anaerobic-anoxic-aerobic (A2O), (C) recycled nitrification-denitrification, and (D) modified Bardenpho. The results exhibited spatial variation of δ15N and δ18O for nitrate within the treatment steps. The changes in δ15N and δ18O may result from the reactor conditions (aerobic, anoxic, and anaerobic) and the order of these processes. As decreasing nitrate concentration in the anoxic stages, the δ15N/δ18O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ15N and δ18O signatures were attributed to process performance in regard to nitrogen removal. In particular, the modified Bardenpho process has higher nitrogen removal efficiency over other processes, producing effluent with lower nitrate concentration and higher stable isotopes (δ15N: 23.6 to 25.5‰, δ18O: 2.8 to 4.5‰). We concluded that the stable isotope signatures mirrored the treatment efficiency and effluent characteristics.

Dissolved Organic Nitrogen Cycling in the South China Sea From an Isotopic Perspective

AbstractDissolved organic nitrogen (DON) is the dominant form of fixed nitrogen in most low and middle latitude ocean surface waters. Here, we report measurements of DON isotopic composition (δ15N) from the west South China Sea (SCS), with the goal of providing new insight into DON cycling. The concentration of DON in the surface ocean is correlated (r = 0.70, p &lt; 0.0001) with chlorophyll a concentration, indicating DON production in these surface waters. The concentration and δ15N of DON fall in a relatively narrow range in the surface ocean (4.6 ± 0.1 μM and 4.3 ± 0.2‰ vs. air, respectively; ±SD), similar to other ocean regions. The mean DON δ15N above 50 m (4.5 ± 0.3‰) is similar to the δ15N of nitrate in the “shallow subsurface” (i.e., immediately below the euphotic zone; 4.6 ± 0.2‰) but is higher than the δ15N of suspended particles in the surface ocean (~2.3‰). This set of isotopic relationships has been observed previously (e.g., in the oligotrophic North Atlantic and North Pacific) and can be explained by the cycling of N between particulate organic nitrogen (PON), DON, and ammonium, in which an isotope effect associated with DON degradation preferentially concentrates 15N in DON. Consistent with this view, a negative correlation (r = 0.70) between the concentration and the δ15N of DON is observed in the upper 75 m, suggesting an isotope effect of ~4.9 ± 0.4‰ for DON degradation. Comparing the DON δ15N data from the SCS with other regions, we find that the δ15N difference between euphotic zone DON and shallow subsurface nitrate δ15N (Δδ15N(DON‐NO3)) rises from ocean regions of inferred net DON production to regions of net DON consumption, with the SCS representing an intermediate case.

Arctic seals as tracers of environmental and ecological change

AbstractKnowledge of species trophic position (TP) is an essential component of ecosystem management. Determining TP from stable nitrogen isotopes (δ15N) in predators requires understanding how these tracers vary across environments and how they relate to predator isotope composition. We used two seal species as a model for determining TP across large spatial scales in the Arctic. δ15N in seawater nitrate (δ15NNO3) and seal muscle amino acids (δ15NAA) were determined to independently characterize the base of the food web and the TP of harp and ringed seals, demonstrating a direct link between δ15NNO3 and δ15NAA. Our results show that the spatial variation in δ15NAA in seals reflects the δ15NNO3 end members in Pacific vs. Atlantic waters. This study provides a reference for best practice on accurate comparison of TP in predators and as such, provides a framework to assess the impact of environmental and human‐induced changes on ecosystems at pan‐Arctic scales.

Nitrogen isotopes in nitrate aerosols collected in the remote marine boundary layer: Implications for nitrogen isotopic fractionations among atmospheric reactive nitrogen species

The nitrogen isotopic composition (δ15N) of atmospheric nitrate aerosols is determined by both the δ15N of its precursor, NOx emissions, and the isotopic fractionations during the atmospheric oxidation of NOx. However, the latter has not been well-understood nor quantified by field observations. In addition, the seasonal variations of this isotopic fractionation have not been determined. To better understand this isotopic fractionation process, in this study, we analyzed the δ15N of nitrate aerosols collected from June 30, 2015 to August 6, 2016 at Baring Head, New Zealand, where the sources of NOx are well-studied. Our results showed that the δ15N values in nitrate aerosols display a clear seasonal variation, with lower δ15N values (−12‰ to ~ -9‰) in the summer (January to March) and higher δ15N values (0‰ to 3‰) in the winter (June–August), while the δ15N values of NOx sources exhibit a narrow range of variation from −10.7 ± 1.4‰ to −9.8 ± 1.4‰. We attribute this discrepancy to the significant and variable isotopic fractionations during the oxidation processes of NOx. We then quantified the isotopic fractionation during 1) the equilibrium and kinetic isotopic fractionations between NO and NO2; and 2) the oxidation of NO2 to nitrate. Our calculations suggest that at Baring Head, the seasonal variations in the oxidation pathways of NO2 are the main driver of the seasonal variations of nitrate δ15N values. Furthermore, the overall isotopic fractionation factors of the oxidation process determined by two models (Kinetic fractionation model and Equilibrium fractionation model) are generally lower in the summer (from +6.3 ± 1.7‰ to +9.5 ± 5.2‰)and higher in the winter (from +15.8 ± 1.9‰ to +17.0 ± 2.4‰).

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.