δ15N Of NO3 Research Articles

The potential of using 15N natural abundance in changing ammonium-N and nitrate-N pools for studying in situ soil N transformations

This study examined the usefulness of 15N natural abundance (δ15N) with in situ core incubation to quantify the predominant N transformation processes in a natural suburban forest of subtropical Australia, which was subjected to prescribed burning. In situ core incubation for 3 days with 20 ml water, or 160.79 ml of 60 mg L−1 NO3−-N surface application, and in situ core with 160.79 ml water but without incubation were set up in Toohey forest for sampling three times as before (once) and after (twice) a prescribed burning. The δ15N of NH4+-N and NO3−-N in the top 5 cm soil before and after the incubation, and δ15N of NO3−-N in the 5–10 cm soil before incubation were compared with each other to examine the soil N mineralisation, nitrification, denitrification, and nitrate leaching processes. The significant decrease in δ15N of NH4+-N after incubation under 20 ml water treatment was ascribed to soil N mineralisation, and the significant decrease in δ15N of NH4+-N and significant increase in δ15N of NO3−-N after incubation with elevated water and nitrate inputs were associated with N mineralisation and nitrification, respectively, 2 months after the burning. The 160.79 ml water treatment also triggered nitrification in the baseline soil cores in both samplings after the burning. Water was crucial to stimulate soil N mineralisation and nitrification, but excessive water depleted labile N pools and reduced N mineralisation and nitrification. Burning effects were hard to separate from the seasonal impacts on soil N cycling processes. The δ15N in soil mineral N pools was sensitive to indicate soil N mineralisation and nitrification processes. Soil water and labile N were determining factors for N transformations in the soil. It is suggested that δ15N combined with soil inorganic N concentrations and net N transformation rates could be used to identify primary N transformation processes. More frequent samplings would be needed to differentiate burning impacts from the seasonal impacts on soil N cycling processes.

Use of multiple isotopic and chemical tracers to identify sources of nitrate in shallow groundwaters along the northern slope of the Qinling Mountains, China

Isotopes and major ion hydrochemistry are used to investigate potential sources of nitrate contamination in shallow aquifers along the northern slope of the Qinling Mountains, west-central China. Locally, nitrate concentrations exceed a remarkable 850 mg/L, over 40 times background levels observed in fresh mountain springs, and a knowledge and understanding of the nitrate pollution sources is essential for the development of aquifer management and protection strategies that will allow the problem to be effectively controlled. Presently, none of the suspected pollutant sources generates concentrations of nitrate that even approach levels observed in the groundwater, and this is clearly a challenge in terms of assigning responsibility for the contamination and developing appropriate management plans. However, stable isotopes of hydrogen and oxygen (δD and δ18O) in study area waters demonstrate that the centrally located Jijiahe Reservoir and Shidi River have been a major source of aquifer recharge in recent times and are an obvious source of the contamination. Moreover, δ15N–NO3– and δ18O–NO3– determinations suggest that, while nitrate concentrations in the river and reservoir are currently relatively low, a local, long-established fertilizer factory that has likely released high-nitrogen waste to these nearby receptors in the past, is the primary pollutant source. On a positive note, it would appear that the majority of the nitrate contamination, albeit serious in terms of drinking water quality, represents a legacy of past practice and should show gradual improvement due to modern-day environmental protection initiatives, improved equipment, better monitoring and significantly enhanced methods of wastewater treatment. However, the isotope data do indicate that the aquifer continues to be impacted by other anthropogenic activities, including agriculture and livestock raising, downstream of the industrial area and in the eastern part of the study area, meaning that monitoring and active remediation strategies need to be continued with vigilance.

Quantitative identification of nitrate sources in a coastal peri-urban watershed using hydrogeochemical indicators and dual isotopes together with the statistical approaches.

Surplus nutrient load and complex migration and transformation processes are the challenges for water quality management in the peri-urban coastal watershed, leading to increasing concerns worldwide. We investigated the spatio-temporal variation of hydrogeochemical parameters in surface water of Jimei Lake watershed, and distinguished the sources and transformation of nitrate-N (NO3--N) using dual isotopes of nitrate (δ15N and δ18O in NO3-) with hydrogeochemical indicators. Principal component analysis (PCA) on hydrogeochemical parameters demonstrated that surface water was seriously polluted by nutrients, especially in the southeast of the downstream. There were signs of seawater intrusion and increased wastewater discharge in the mid-lower reaches with high ammonium concentrations. Nitrification occurred throughout the monitoring period with lower δ15N and δ18O values and NO3- derived from mixed pollution sources. Results of Bayesian model showed that dominant NO3- input originated from manure and sewage (M&S, 71% and 76% in the wet and dry season, respectively) and atmospheric deposition (22% and 16%, respectively). This result implied that the controls and treatment of M&S discharges are essential to alleviate of NO3- pollution. The proposed method is helpful to understand the origins of NO3- and may be suitable to develop measures for the reducing of nitrogen loadings in the peri-urban watershed.

Assessment of sources and transformation of nitrate in the alluvial-pluvial fan region of north China using a multi-isotope approach

A multi-isotope approach and mixing model were combined to identify spatial and seasonal variations of sources, and their proportional contribution to nitrate in the Hutuo River alluvial-pluvial fan region. The results showed that the NO3− concentration was significantly higher in the Hutuo River valley plain (178.7 mg/L) region than that in the upper and central pluvial fans of the Hutuo River (82.1 mg/L and 71.0 mg/L, respectively) and in the river (17.0 mg/L). Different land use types had no significant effect on the groundwater nitrate concentration. Based on a multi-isotope approach, we confirmed that the main sources of groundwater nitrate in different land use areas were domestic sewage and manure, followed by soil nitrogen, ammonia fertilizer, nitrate fertilizer and rainwater, and there were no significant spatial or seasonal variations. Combining δ15N–NO3-, δ18O–NO3- and δ37Cl results can increase the accuracy of traceability. Nitrification could be the most important nitrogen migration and transformation process, and denitrification did not significantly affected the isotopic composition of the nitrate. The SIAR model outputs revealed that the main nitrate pollution sources in groundwater and river water were domestic sewage and manure, accounting for 55.9%–61.0% and 22.6% (dry season), 50.3%–60.4% and 34.1% (transition season), 42.7%–47.6% and 35.6% (wet season 2016) and 45.9%–46.7% and 38.4% (wet season 2017), respectively. This work suggests that the random discharge and disposal of domestic sewage and manure should be the first target for control in order to prevent further nitrate contamination of the water environment.

Nitrate Isotopic Composition in Precipitation at a Chinese Megacity: Seasonal Variations, Atmospheric Processes, and Implications for Sources

AbstractNitrogen stable isotope composition (δ15N) of nitrate (NO3−) deposition can aid in source apportionment of its precursor emissions, nitrogen oxides (NOx), with implications in mitigation policy to address serious air pollution. However, potential δ15N fractionation during atmospheric NO3− formation may hinder accurate quantification of NOx contributions. Previously, NOx photochemical reactions have been suggested to be the dominant δ15N fractionation process in NO3− formation. Here, we have quantified the potential fractionation effects associated with NOx photochemical reactions based upon δ15N‐NO3− measured in 1 year of daily‐based bulk deposition samples in Shenyang, a megacity with distinct seasonal fossil fuel combustion in northeastern China. The mean δ15N was 0.9 ± 4.0‰ and ranged from −4.9 to 8.3‰, with the lowest and highest values observed during summer and winter, respectively. Calculated NOx photochemical equilibrium fractionation may account for up to 42% of the observed seasonal δ15N change in NO3− deposition. However, the relative NOx source contribution trends, estimated based upon a δ15N Bayesian mixing model, were insensitive to NOx photochemical fractionation considerations. Overall, NOx source partitioning based upon averaged year‐long δ15N of bulk NO3− deposition estimated the following relative trend in NOx emission sources: coal combustion > biomass burning = vehicle emissions ≫ soil emissions. Seasonally, the increase of coal combustion emissions from summer to winter drives the seasonality of δ15N in NO3− deposition, indicating a necessity to control NOx emissions from coal combustion to improve wintertime air quality.

Nitrate sources and processes in the surface water of a tropical reservoir by stable isotopes and mixing model

Nitrate is one of the primary nutrients associated with sedimentation and fuels eutrophication in reservoir systems. In this study, water samples from Bukit Merah Reservoir (BMR) were analysed using a combination of water chemistry, water stable isotopes (δ2H–H2O and δ18O–H2O) and nitrate stable isotopes (δ15N–NO3− and δ18O–NO3−). The objective was to evaluate nitrate sources and processes in BMR, the oldest man-made reservoir in Malaysia. The δ15N–NO3− values in the river and reservoir water samples were in the range +0.4 to +14.9‰ while the values of δ18O–NO3− were between −0.01 and +39.4‰, respectively. The dual plots of δ15N–NO3− and δ18O–NO3− reflected mixing sources from atmospheric deposition (AD) input, ammonium in fertilizer/rain, soil nitrogen, and manure and sewage (MS) as the sources of nitrate in the surface water of BMR. Nitrate stable isotopes suggested that BMR undergoes processes such as nitrification and mixing. Denitrification and assimilation were not prevalent in the system. The Bayesian mixing model highlighted the dominance of MS sources in the system while AD contributed more proportion in the reservoir during both seasons than in the river. The use of δ13C, δ15N, and C:N ratios enabled the identification of terrestrial sources of the organic matter in the sediment, enhancing the understanding of sedimentation associated with nutrients previously reported in BMR. Overall, the nitrate sources and processes should be considered in decision-making in the management of the reservoir for irrigation, Arowana fish culture and domestic water supply.

Quantifying depression trapping effect on rainwater chemical composition during the rainy season in karst agricultural area, southwestern China

Atmospheric wet deposition is a vital part of biogeochemical cycles in earth surface environment systems, which is not only controlled by local sources and long-range transported continental substances from natural and anthropogenic sources, but also potentially influenced by the topographical features (e.g. peak clusters and depressions in karst areas). To investigate the depression trapping effect of vertical distribution of rainwater chemical composition and evaluate the effect of trapping by depression on wet deposition flux, rainwater samples from two sites (HR, a hilltop shrubbery area, and LR, inside a depression paddy field area) of a karst agricultural depression in Southwest China, were collected during the rainy season. From these, the major ionic concentrations were measured. The results indicated a variance in pH from 4.8 to 6.4 with a volume-weighted mean (VWM) value of 5.7 at the HR, and, a range of 5.4–6.9 with a VWM of 6.0 at the LR. The VWM concentrations of ions were decreased in the order of SO42- > NH4+ > Ca2+ > NO3- > Mg2+ > K+ > Na+ > Cl− > F− at both sites, and were dominated by SO42-, NH4+, Ca2+, and NO3-, accounting for >87% of the total ions at the two sites. The result of fractional acidity (FA) and neutralization factors (NF) revealed that the relatively high pH values were the result of neutralization of the alkaline substances (NH4+ and Ca2+) rather than the scarcity of acidic materials. Source analysis of major ions showed that anions were predominantly controlled by anthropogenic emission, while cations originated from both terrestrial sources and anthropogenic activity. The vertical distribution of dominant ions (NH4+, NO3-, Ca2+, Mg2+, and SO42-) were ascribed to the relatively weak air convection movement within the depression area (specific atmospheric circulations) and the differences in human interference at two sites (LR, cultivated land; HR, luxuriant vegetation). This is further supported by the more negative rainwater δ15N–NO3- values (−8.5 to −6.0‰), in LR than that in HR (−6.2 to −3.4‰). The depression trapping effect could significantly affect the estimation of wet deposition fluxes (up to 80.6% for Ca2+ and 68.7% for NH4+), which requires consideration in future studies, e.g. N wet deposition.

Spatiotemporal variations in the hydrochemical characteristics and controlling factors of streamflow and groundwater in the Wei River of China

Analysis of hydrochemical characteristics and controlling factors of streamflow and groundwater in arid regions is important for water security. In this study, we collected samples of streamflow and groundwater from the Wei River in China, analyzed their hydrochemical characteristics, and identified the major solute sources using ion concentrations, δ15N–NO3− and δ18O–NO3−. The major downstream ion contents were greater than the corresponding upstream values and the ion content in streamflow during the wet season is much higher than that during the dry season. The water quality during the wet season was unsatisfactory as approximately one third of the water samples were categorized as the worst water quality based on excessive nitrates and carbonate weathering. Rock weathering contributed the greatest proportion of solutes to both streamflow and groundwater. Evaporite dissolution and carbonate weathering dominated solutes in the wet and dry seasons, respectively. Human activities cannot be ignored in certain areas. Fertilizer application accounts for 43% of the total anthropogenic solute inputs. These results point to the increasing impact of agriculture on water quality.

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean

Abstract. The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of < 3 and < 0.3 µmol kg−1, respectively (< 300 m; σ < 26.4 kg−1 m−3). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ15N > 7 ‰; δ18O > 4 ‰) at 400–500 m (26.6–26.7 kg−1 m−3), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg−1 m−3) of the gyre, indicated by an N deficit (N* ∼-1 to −4 µmol kg−1) and by elevated isotopic ratios of nitrate (δ15N > 7 ‰; δ18O > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of < 3 ‰ within the SAMW. In contrast, remineralization of 15N-enriched organic matter from the Arabian Sea indicates higher Δ(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of > 2 µmol kg−1 and a relatively low Δ(15–18) value of < 3 ‰. A mass and isotope budget implies that at least 32 %–34 % of the nitrate in the upper ocean between 20.36 and 23.91∘ S is provided from newly fixed nitrogen, whereas N2 fixation appears to be limited by iron or temperature south of 26∘ S.

Determination sources of nitrates into the Three Gorges Reservoir using nitrogen and oxygen isotopes

Identification of nitrate sources and its transformations are important for the management of large lakes and reservoirs. The Three Gorges Reservoir (TGR) in China is one of the largest reservoirs around the world. In this study, stable isotopes of nitrogen (δ15N–NO3−) and oxygen (δ18O–NO3−) of nitrate in water were used to gain insights into nitrate sources and transformations in the tail area of the TGR. Bayesian mixing model has been conducted to estimate the proportional contribute of nitrate sources. The mixing modelling results indicated that NH4+ fertilizer (range 7–54%) and soil organic nitrogen (range 2–45%) were the dominant NO3−–N sources in the tail area of the TGR during the three season study period. Nitrification contributed a part of NO3−–N in the river water during the dry season. The nitrate from soil solution in the riparian zone with denitrified NO3− might be another major reason for the enrichment of δ15N–NO3− and δ18O–NO3− during the normal season. Reducing the use of chemical nitrogen fertilizers, especially NH4+ fertilizers, and protecting soil from erosion may be effective measures to improve water quality in the TGR.

Spatial and temporal variability in hydrochemistry of a small-scale dolomite karst environment

Dolomite karst systems are less common and less exploited than limestone systems, but still they can significantly contribute to drinking water supply in many countries. The knowledge about spatial and temporal variations of hydrochemical parameters of such systems is much more limited than for limestone karst aquifers. In the present research, 40-year-long observations of hydrochemical parameters gathered from five springs and a pumping well in Waidhofen an der Ybbs (Austria) were used to show the variability of chemical water composition in a small-scale ( ~ 10 km2) dolomite karst system. Integration of classic geochemical methods and multivariate statistical analysis revealed that the hydrofacies correlate directly with the lithofacies (dolomite) in the study area. At the same time, measured concentrations of Ca2+, Mg2+, Na+, Cl−, and HCO3− allow for a classification of the springs in three groups based on their ionic ratios. This classification highlights the spatial and temporal variability that can be encountered in dolomite karst systems, even on small spatial scales, that are relevant for water suppliers. Moreover, temporal observations of hydrochemical parameters show increasing trends of nitrate concentrations in all sampling points, though with different rates. The analysis of the stable isotopes δ15N–NO3 and δ18O–NO3 revealed that nitrification processes in forest floors and mineral soils are the main source of nitrate in most locations investigated. The findings of the present study highlight the necessity of a detailed temporal and spatial distributed monitoring to support water resources management in dolomitic karst aquifers.

Tracing nitrate sources using the isotopic composition of skeletal-bound organic matter from the calcareous green algae Halimeda

The nitrogen (N) isotopic composition (δ15N) of water column nitrate (NO3−) reveals important information about the source(s) of N, and/or the transformation processes occurring during N transport, to a particular environment. However the physical collection of samples for δ15N–NO3− determination can be logistically challenging and does not address questions about historical δ15N–NO3−. Here we adapt the persulphate/denitrifier protocol, previously developed for scleractinian corals, to analyse the δ15N of organic material in the skeletons of the calcareous macroalgae Halimeda (hereafter referred to as δ15N-SOM). Using the newly established protocol, we show that the Halimeda δ15N-SOM can be used as a proxy for the δ15N of water column NO3−. Based on aquarium and field measurements we show that the maximum offset between the Halimeda δ15N-SOM and the δ15N of water column NO3−, as a result of fractionation during uptake, is 1‰. However this was only tested at low NO3− concentrations (< 10 μmol L−1). Our results demonstrate that in low-nutrient environments, Halimeda skeletal material, including historical material obtained from sediment cores, can provide a reliable proxy record of δ15N–NO3− at the base of the food web. This technique holds much promise for reconstructing spatial and temporal patterns of N supply to coral reef systems worldwide.

Nitrogen isotope fractionation during gas-to-particle conversion of NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; to NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;&amp;lt;sup&amp;gt;−&amp;lt;/sup&amp;gt; in the atmosphere – implications for isotope-based NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; source apportionment

Abstract. Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate (pNO3−), the end product of the oxidation of NOx gases (NO + NO2) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO3− to constrain NOx source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the δ15N values of fresh pNO3− (δ15N–pNO3−) in PM2.5 at a rural site in northern China, where atmospheric pNO3− can be attributed exclusively to biomass burning. The observed δ15N–pNO3− (12.17±1.55 ‰; n = 8) was much higher than the N isotopic source signature of NOx from biomass burning (1.04±4.13 ‰). The large difference between δ15N–pNO3− and δ15N–NOx (Δ(δ15N)) can be reconciled by the net N isotope effect (εN) associated with the gas–particle conversion from NOx to NO3−. For the biomass burning site, a mean εN( ≈ Δ(δ15N)) of 10.99±0.74 ‰ was assessed through a newly developed computational quantum chemistry (CQC) module. εN depends on the relative importance of the two dominant N isotope exchange reactions involved (NO2 reaction with OH versus hydrolysis of dinitrogen pentoxide (N2O5) with H2O) and varies between regions and on a diurnal basis. A second, slightly higher CQC-based mean value for εN (15.33±4.90 ‰) was estimated for an urban site with intense traffic in eastern China and integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates for NOx at this site. Based on the δ15N values (10.93±3.32 ‰; n = 43) of ambient pNO3− determined for the urban site, and considering the location-specific estimate for εN, our results reveal that the relative contribution of coal combustion and road traffic to urban NOx is 32 % ± 11 % and 68 %± 11 %, respectively. This finding agrees well with a regional bottom-up emission inventory of NOx. Moreover, the variation pattern of OH contribution to ambient pNO3− formation calculated by the CQC module is consistent with that simulated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), further confirming the robustness of our estimates. Our investigations also show that, without the consideration of the N isotope effect during pNO3− formation, the observed δ15N–pNO3− at the study site would erroneously imply that NOx is derived almost entirely from coal combustion. Similarly, reanalysis of reported δ15N–NO3− data throughout China and its neighboring areas suggests that NOx emissions from coal combustion may be substantively overestimated (by &gt; 30 %) when the N isotope fractionation during atmospheric pNO3− formation is neglected.

Stable isotopes of nitrate reveal different nitrogen processing mechanisms in streams across a land use gradient during wet and dry periods

Abstract. Understanding the relationship between land use and the dynamics of nitrate (NO3-) is the key to constrain sources of NO3- export in order to aid effective management of waterways. In this study, isotopic compositions of NO3- (δ15N–NO3- and δ18O–NO3-) were used to elucidate the effects of land use (agriculture in particular) and rainfall on the major sources and sinks of NO3- within the Western Port catchment, Victoria, Australia. This study is one of the very few studies carried out in temperate regions with highly stochastic rainfall patterns, enabling a more comprehensive understanding of the applications of NO3- isotopes in catchment ecosystems with different climatic conditions. Longitudinal samples were collected from five streams with different agriculture land use intensities on five occasions – three during dry periods and two during wet periods. At the catchment scale, we observed significant positive relationships between NO3- concentrations (p &lt; 0.05), δ15N–NO3- (p &lt; 0.01) and percentage agriculture (particularly during the wet period), reflecting the dominance of anthropogenic nitrogen inputs within the catchment. Different rainfall conditions appeared to be major controls on the predominance of the sources and transformation processes of NO3- in our study sites. Artificial fertiliser was the dominant source of NO3- during the wet periods. In addition to artificial fertiliser, nitrified organic matter in sediment was also an apparent source of NO3- to the surface water during the dry periods. Denitrification was prevalent during the wet periods, while uptake of NO3- by plants or algae was only observed during the dry periods in two streams. The outcome of this study suggests that effective reduction of NO3- load to the streams can only be achieved by prioritising management strategies based on different rainfall conditions.

Reactive and Mixing Processes Governing Ammonium and Nitrate Coexistence in a Polluted Coastal Aquifer

A comprehensive hydrochemical, stable isotope and microbial analyses characterisation has been performed to evaluate the sources of groundwater, nitrogen pollution and degradation processes occurring in an industrial polluted coastal aquifer in the framework of a complex hydrodynamic system. The coexistence of ammonium and nitrate has been observed in almost all the investigated monitoring wells, reaching maximum values of 100 and 200 mg/L for both species. Chloride and potassium concentration coupled with groundwater stable isotopes data show the influence of local and urban recharge and the occurrence of seawater intrusion in areas near the coastline. δ15N–NH4+ values ranging between −4.9 and +14.9% suggest that different processes such as partial nitrification of ammonium, probably anammox activities and sorption, are occurring at the site. The isotope data for NH4+ also showed the existence of the remnant of an old fertilizer plume in the downgradient area. The nitrate isotope data ranging between +9 and +46% and +6 and +26% for δ15N–NO3− and δ18O–NO3−, respectively, suggest that nitrate content is attenuated by denitrification and probably annamox. The fast groundwater flow field is one of the reasons for the coexistence of NH4+ and NO3− in groundwater, since both compounds can penetrate the reducing zone of the aquifer. The influence of leakage of sewage pipelines on the aquifer cannot be discerned due to the complexities of the nitrogen attenuation processes, also influenced by pumping activities.

Sources of reactive nitrogen in marine aerosol over the Northwest Pacific Ocean in spring

Abstract. Atmospheric deposition of long-range transport of anthropogenic reactive nitrogen (Nr, mainly comprised of NHx, NOy and water-soluble organic nitrogen, WSON) from continents may have profound impact on marine biogeochemistry. In addition, surface ocean dissolved organic nitrogen (DON) may also contribute to aerosol WSON in the overlying atmosphere. Despite the importance of off-continent dispersion and Nr interactions at the atmosphere–ocean boundary, our knowledge of the sources of various nitrogen species in the atmosphere over the open ocean remains limited due to insufficient observations. We conducted two cruises in the spring of 2014 and 2015 from the coast of China through the East China seas (ECSs, i.e. the Yellow Sea and East China Sea) to the open ocean (i.e. the Northwest Pacific Ocean, NWPO). Concentrations of water-soluble total nitrogen (WSTN), NO3- and NH4+, as well as the δ15N of WSTN and NO3- in marine aerosol, were measured during both cruises. In the spring of 2015, we also analysed the concentrations and δ15N of NO3- and the DON of surface seawater (SSW; at a depth of 5 m) along the cruise track. Aerosol NO3-, NH4+ and WSON decreased logarithmically (1–2 orders of magnitude) with distance from the shore, reflecting strong anthropogenic emission sources of NO3-, NH4+ and WSON in China. Average aerosol NO3- and NH4+ concentrations were significantly higher in 2014 (even in the remote NWOP) than in 2015 due to the stronger wind field in 2014, underscoring the role of the Asian winter monsoon in the seaward transport of anthropogenic NO3- and NH4+. However, the background aerosol WSON over the NWPO in 2015 (13.3 ± 8.5 nmol m−3) was similar to that in 2014 (12.2 ± 6.3 nmol m−3), suggesting an additional non-anthropogenic WSON source in the open ocean. Obviously, marine DON emissions should be considered in model and field assessments of net atmospheric WSON deposition in the open ocean. This study contributes information on parallel isotopic marine DON composition and aerosol Nr datasets, but more research is required to explore complex Nr sources and deposition processes in order to advance our understanding of anthropogenic influences on the marine nitrogen cycle and nitrogen exchange at land–ocean and atmosphere–ocean interfaces.

Combining pyrosequencing and isotopic approaches to assess denitrification in a hyporheic zone

Hyporheic zones are considered hot spots for numerically vast and phylogenetically diverse microbial communities. However, biogeochemical effects of hyporheic zones have rarely been investigated in detail because of the difficulty in accurately measuring denitrification in these zones. To date, little is known about the hydroecology of hyporheic zones. The effect of changes in hydraulic conditions on the community variations of indigenous microorganisms and water quality was examined based on the depth of the hyporheic zone. In particular, we report on the use of the pyrosequencing technique to elucidate denitrifying bacteria (DNB) community profiles combined with the stable isotope composition of nitrate and hydrological patterns in the hyporheic zones to reveal whether denitrification occurs. δ15N–NO3 and δ18O–NO3 values of nitrate were analyzed to evaluate the transformation processes of nitrate in upwelling and downwelling areas and mixed zones. The isotope values indicated different origins of water in upwelling and downwelling zones and that denitrification occurred predominantly in the upwelling areas. Analyses of microbial communities in the hyporheic zone showed that the new genera, species, and isotope data were associated with the hydrological uniqueness of the hyporheic zones. The 16S rRNA sequences were determined and phylogenetic analysis revealed that the DNB communities distributed and gathered the genus Comamonas denitrificans within the mixing patterns of the hyporheic zones and that the relative scarcity of these microbes in these zones was caused by the lack of appropriate substrates. The delineation of the surface water–groundwater mixing zone was quantitatively determined by systematically combining the hydrological and heat transfer analyses and by comparing denitrifying bacteria communities and N isotope data. This study showed that pyrosequencing and isotopic approaches are useful for evaluating the transformation processes of nitrate at the upwelling and downwelling points of a hyporheic zone.

Identification of Nitrate Sources and Transformation Processes in Midstream Areas:A Case in the Taizi River Basin

This study analyzed the nitrate (NO3-), ammonia nitrogen (NH4+-N), chloride (Cl-), δ15 N-NO3-, δ18O-NO3-, and δ18O-H2O in the surface water to identify the nitrate sources and transformation processes in midstream areas of the Taizi River basin in May and August 2016, corresponding to the dry season and wet season, respectively. The results indicated that the NO3-, Cl-, and NH4+-N concentrations and the δ18O-NO3- in the dry season were greater than those in the wet season, and the δ15 N-NO3- did not show significant seasonal variations based on the Man-Whitney U test. The NO3- was derived mainly from the mixed sources according to the ranges of NO3-/Cl-, δ15 N-NO3-, and δ18O-NO3-. The nitrate sources in the surface water were synthetic fertilizer, soil nitrogen, manure, and sewage in the Xihe River (XR), Lanhe River (LR), and Xiada River (XDR) and soil nitrogen and synthetic fertilizer in the Er dao River (EDR) during the wet season. The sources were synthetic fertilizer and soil nitrogen in the XDR and soil nitrogen, manure, and sewage in the XR, LR, and EDR during the dry season. The significant negative relationship between NO3- and NH4+ and the significant positive relationship between NO3- and δ15 N-NO3- in the wet season indicated that the volatilization and nitrification of soil N might be related to NO3- sources in the wet season. NH4+-N and δ15 N-NO3- increased with decreasing NO3- and Cl- in EDR and LR during the wet season, which indicated denitrification processes occurred. There was a significant relationship between NO3- and Cl-, indicating that mixing processes occurred in the surface water during the two sampling periods. This study will provide a better understanding for nitrate sources related to seasonal variations and transformation processes in hilly areas.

Tracing Nitrogen Biogeochemistry During the Beginning of a Spring Phytoplankton Bloom in the Yellow Sea Using Coupled Nitrate Nitrogen and Oxygen Isotope Ratios

AbstractNitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO3−, and the hydrological conditions. The δ15N and δ18O values for NO3− in the Yellow Sea were more variable in surface waters than in near‐bottom waters and were generally 6.3–8.2‰ for δ15N and 6.2–9.7‰ for δ18O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ15N and δ18O values found were 13.2‰ and 18.8‰, respectively. Both the δ15N and δ18O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ18O relative to δ15N in NO3− in the Yellow Sea reflects contributions from multiple NO3− sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO3− input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO3− representing 74% of total NO3− input in the YS.

Sources of dissolved ions revealed by chemical and isotopic tracers in the Geum River, South Korea

Water samples were collected in the main channel of the Geum River, South Korea, and measured dissolved elemental concentrations and isotopic compositions of nitrate in order to identify the factors controlling water chemistry. Elemental concentrations significantly increased location-wise after the confluence from urban areas, indicating the changes in solute sources from chemical weathering to anthropogenic inputs such as manure, fertilizers, and sewage. In particular, the effect of sewage input is manifested in the concentrations of Cl−, SO4 2−, and Na+, while the NO3 − concentration is influenced mainly by soil inputs with minor contributions from manure and fertilizer because both δ15N–NO3 and δ18O–NO3 indicate NO3 − mostly consists of soil-derived nitrates in the upper reaches but manure/sewage nitrates in the lower reaches. The relative proportion of three factors, Cl−, Ca2++Mg2+, and NO3 −, indicates that water chemistry in the upstream is controlled by the soil weathering but that in the downstream by the sewage. Seasonally, water chemistry during summer is dominated by the soil weathering due to the flushing effect but that during winter by the sewage. This study suggests that the relative proportion of three factors can be used for tracing natural and anthropogenic sources in water chemistry.