Nitrate δ18O Research Articles

Evaluation of trophic transfer in the microbial food web during sludge degradation based on 13C and 15N natural abundance

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) were determined in activated sludge, which was exposed to endogenous conditions for 36 days and contained a wide diversity of organisms across several trophic levels. The aim of this study was to elucidate the fluctuation of δ13C and δ15N through trophic transfer in the microbial consortia. The sludge was evaluated in view of sludge mass, bacterial community, higher trophic organisms, sludge δ13C and δ15N, and δ15N and δ18O of nitrate. The results show that the activated sludge became more enriched with 15N as degradation proceeded. Eventually, the mixed liquor volatile suspended solid concentrations in the activated sludge decreased from 1610 to 710 mg/L and the δ15N of the sludge increased from 8.3‰ to 10.8‰. In contrast, the δ13C values of the sludge were stable. Microscope observations confirmed that consumers such as Rotifera, Tardigrada and Annelida (Aelosoma sp.) were present in the activated sludge for the entire operational period. The abundance of those organisms drastically changed during the operational periods, and the diversity in bacterial community also changed, resulting in community succession. Changes in biotic community, reduction in sludge mass, and increase in δ15N of the sludge occurred during the sludge degradation processes. This implies that the sludge degradation was partly caused by the trophic conversion of the sludge-derived nitrogen in the food web. The δ15N of the sludge can be used as an indicator of the sludge degradation through trophic transfer in wastewater treatment reactors. These findings provide new insights into understanding trophic transfer during microbial community succession and the effects of the feeding process on sludge degradation.

Influence of the bordering shelves on nutrient distribution in the Arctic halocline inferred from water column nitrate isotopes

AbstractThe East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate δ15N and δ18O is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate δ18O indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate δ15N correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the 15N enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual 15N‐enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4–3.8‰, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters.

Tracing the Fate of Atmospheric Nitrate in a Subalpine Watershed Using Δ17O.

Nitrogen is an essential nutrient for life on Earth, but in excess, it can lead to environmental issues (e.g., N saturation, loss of biodiversity, acidification of lakes, etc.). Understanding the nitrogen budget (i.e., inputs and outputs) is essential to evaluate the prospective decay of the ecosystem services (e.g., freshwater quality, erosion control, loss of high patrimonial-value plant species, etc.) that subalpine headwater catchments provide, especially as these ecosystems experience high atmospheric nitrogen deposition. Here, we use a multi-isotopic tracer (Δ17O, δ15N and δ18O) of nitrate in aerosols, snow, and streams to assess the fate of atmospherically deposited nitrate in the subalpine watershed of the Lautaret Pass (French Alps). We show that atmospheric N deposition contributes significantly to stream nitrate pool year-round, either by direct inputs (up to 35%) or by in situ nitrification of atmospheric ammonium (up to 35%). Snowmelt in particular leads to high exports of atmospheric nitrate, most likely fast enough to impede assimilation by surrounding ecosystems. Yet, in a context of climate change, with shorter snow seasons, and increasing nitrogen emissions, our results hint at possibly stronger ecological consequences of nitrogen atmospheric deposition in the close future.

Distinguishing summertime atmospheric production of nitrate across the East Antarctic Ice Sheet

Surface snow and atmospheric samples collected along a traverse from the coast to the ice sheet summit (Dome A) are used to investigate summertime atmospheric production of nitrate (NO3–) across East Antarctica. The strong relationship observed between δ15N and δ18O of nitrate in the surface snow suggests a large (lesser) extent of nitrate photolysis in the interior (coastal) region. A linear correlation between the oxygen isotopes of nitrate (δ18O and Δ17O) indicates mixing of various oxidants that react with NOx (NOx = NO + NO2) to produce atmospheric nitrate. On the plateau, the isotopes of snow nitrate are best explained by local reoxidation chemistry of NOx, possibly occurring in both condensed and gas phases. Nitrate photolysis results in redistribution of snow nitrate, and the plateau snow is a net exporter of nitrate and its precursors. Our results suggest that while snow-sourced NOx from the plateau due to photolysis is a significant input to the nitrate budget in coastal snow (up to ∼35%), tropospheric transport from mid-low latitudes dominates (∼65%) coastal snow nitrate. The linear relationship of δ18O vs. Δ17O of the snow nitrate suggests a predominant role of hydroxyl radical (OH) and ozone (O3) in nitrate production, although a high Δ17O(O3) is required to explain the observations. Across Antarctica the oxygen isotope composition of OH appears to be dominated by exchange with water vapor, despite the very dry environment. One of the largest uncertainties in quantifying nitrate production pathways is the limited knowledge of atmospheric oxidant isotopic compositions.

Mineral N stock and nitrate accumulation in the 50 to 200 m profile on the Loess Plateau

Nitrogen (N) stored in deep profiles is important in assessing regional and/or global N stocks and nitrate leaching risk to groundwater. The Chinese Loess Plateau, which is characterized by significantly thick loess deposits, potentially stores immense stocks of mineral N, posing future threats to groundwater quality. In order to determine the vertical distributions of nitrate and ammonium content in the region, as well as to characterize the potential accumulation of nitrate in the deep loess profile, we study loess samples collected at five sites (Yangling, Changwu, Fuxian, An'sai and Shenmu) through a 50 to 200m loess profile. The estimated storage of mineral N varied significantly among the five sites, ranging from 0.46 to 2.43×104kgNha−1. Ammonium exhibited fluctuations and dominated mineral N stocks within the whole profile at the sites, except for the upper 20–30m at Yangling and Changwu. Measured nitrate content in the entire profile at Fuxian, An'sai and Shenmu is low, but significant accumulations were observed to 30–50m depth at the other two sites. Analysis of δ15N and δ18O of nitrate indicates different causes for accumulated nitrate at these two sites. Mineralization and nitrification of manure and organic N respectively contribute nitrate to the 0–12 and 12–30m profile at Changwu; while nitrification of NH4+ fertilizer, NO3− fertilizer and nitrification of organic N control the nitrate distribution in the 0–3, 3–7 and 7–10m layer at Yangling, respectively. Furthermore, our analysis illustrates the low denitrification potential in the lower part of the vadose zone. The accumulated nitrate introduced by human activities is thus mainly distributed in the upper vadose zone (above 30m), indicating, currently, a low nitrate leaching risk to groundwater due to a high storage capacity of the thick vadose zone in the region.

Atmospheric nitrate export in streams along a montane to urban gradient

Nitrogen (N) emissions associated with urbanization exacerbate the atmospheric N influx to remote ecosystems – like mountains –, leading to well-documented detrimental effects on ecosystems (e.g., soil acidification, pollution of freshwaters). Here, the importance and fate of N deposition in a watershed was evaluated along a montane to urban gradient, using a multi-isotopic tracers approach (Δ17O, δ15N, δ18O of nitrate, δ2H and δ18O of water). In this setting, the montane streams had higher proportions of atmospheric nitrate compared to urban streams, and exported more atmospheric nitrate on a yearly basis (0.35 vs 0.10 kg-Nha−1yr−1). In urban areas, nitrate exports were driven by groundwater, whereas in the catchment head nitrate exports were dominated by surface runoff. The main sources of nitrate to the montane streams were microbial nitrification and atmospheric deposition, whereas microbial nitrification and sewage leakage contributed most to urban streams. Based on the measurement of δ15N and δ18O-NO3−, biological processes such as denitrification or N assimilation were not predominant in any streams in this study. The observed low δ15N and δ18O range of terrestrial nitrate (i.e., nitrate not coming from atmospheric deposition) in surface water compared to literature suggests that atmospheric deposition may be underestimated as a direct source of N.

Modest Gaseous Nitrogen Losses Point to Conservative Nitrogen Cycling in a Lowland Tropical Forest Watershed

Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with characteristically large hydrologic and gaseous losses of inorganic N. However, emerging evidence suggests that some tropical ecosystems can exhibit tight N cycling, with low biologically available losses. In this study, we combined isotopic data with a well-characterized watershed N mass balance to close the N budget and characterize gaseous N losses at the ecosystem scale in a lowland tropical rainforest on the Osa Peninsula in southwestern Costa Rica. We measured δ15N and δ18O of nitrate (NO3 −) in precipitation, surface, shallow and deep soil lysimeters and stream water biweekly for 1 year. Enrichment of both isotopes indicates that denitrification occurs predominantly as NO3 − moves from surface soil down to 15 cm depth or laterally to stream water, with little further processing in deeper soil. Two different isotopic modeling approaches suggested that the gaseous fraction comprises 14 or 32% of total N loss (2.7 or 7.5 kg N ha−1 y−1), though estimates are sensitive to selection of isotopic fractionation values. Gas loss estimates using the mass balance approach (3.2 kg N ha−1 y−1) fall within this range and include N2O losses of 0.9 kg N ha−1 y−1. Overall, gaseous and soluble hydrologic N losses comprise a modest proportion (~ 25%) of the total N inputs to this ecosystem. By contrast, relatively large, episodic hydrologic losses of non-biologically available particulate N balance the majority of N inputs and may contribute to maintaining conservative N cycling in this lowland tropical forest. Similar patterns of N cycling may occur in other tropical forests with similar state factor combinations—high rainfall, steep topography, relatively fertile soils—such as the western arc of the Amazon Basin and much of IndoMalaysia, but this hypothesis remains untested.

Isotope analysis of ammonium and nitrate: A review on measured methods and their application

In the past several decades, a variety of methods have been developed for measuring the isotopic composition of ammonium (δ15N) and nitrate (δ15N and δ18O). This review summarized the advantages and disadvantages of these methods. Nowadays, the most popular method for measu-ring δ15N of ammonium is the combined hypobromite (BrO-) and hydroxylamine (NH2OH) me-thod, while for δ15N and δ18O of nitrate is the denitrifier method and the sodium azide (NaN3) me-thod. These methods convert NH4+ or NO3- into nitrous oxide (N2O) and measure its isotopic compositions, with higher analytical precision because of the lower background concentration of atmospheric N2O. Accordingly, these methods are suitable for the samples with lower N concentration, and normally require 10-60 nmol N. The development of new methods for measuring N isotopic composition has greatly stimulated the studies in nitrogen cycling worldwide.

Effect of wheat-maize straw return on the fate of nitrate in groundwater in the Huaihe River Basin, China

Straw return is becoming a routine practice in disposing of crop residues worldwide. However, the potential effect of such operation on the chemistry of local groundwater is not well documented. Here, shallow groundwater in an area where wheat-maize straw return is practiced was analyzed, and the seasonal changes in the nitrate concentration and the isotope compositions of NO3− and H2O were determined along two flow paths. Measured δD and δ18O in waters indicated that the groundwater was mainly recharged by atmospheric precipitation, while measured δ15N and δ18O in nitrate suggested that the sources for groundwater NO3− included urea fertilizer, soil nitrogen, and sewage/manure. Reduced NO3− concentrations coincided with an enrichment of organic matter in the groundwater of the straw return area, revealing an environmental condition that facilitates nitrate reduction, whereas increased δ15N-NO3− and δ18O-NO3− along the flow path suggested the occurrence of denitrification. Further analyses showed that, compared to the cases in the absence of straw return, as much as 80% and 90% of groundwater nitrate was removed in low and high water seasons in the straw return area, pointing to a potential positive effect of straw return to groundwater quality.

Legacy of contaminant N sources to the NO3\u2212 signature in rivers: a combined isotopic (\u03b415N-NO3\u2212, \u03b418O-NO3\u2212, \u03b411B) and microbiological investigation

Nitrate content of surface waters results from complex mixing of multiple sources, whose signatures can be modified through N reactions occurring within the different compartments of the whole catchment. Despite this complexity, the determination of nitrate origin is the first and crucial step for water resource preservation. Here, for the first time, we combined at the catchment scale stable isotopic tracers (δ15N and δ18O of nitrate and δ11B) and fecal indicators to trace nitrate sources and pathways to the stream. We tested this approach on two rivers in an agricultural region of SW France. Boron isotopic ratios evidenced inflow from anthropogenic waters, microbiological markers revealed organic contaminations from both human and animal wastes. Nitrate δ15N and δ18O traced inputs from the surface leaching during high flow events and from the subsurface drainage in base flow regime. They also showed that denitrification occurred within the soils before reaching the rivers. Furthermore, this study highlighted the determinant role of the soil compartment in nitrate formation and recycling with important spatial heterogeneity and temporal variability.

Accurate and precise quantification of atmospheric nitrate in streams draining land of various uses by using triple oxygen isotopes as tracers

Abstract. Land use in a catchment area has significant impacts on nitrate eluted from the catchment, including atmospheric nitrate deposited onto the catchment area and remineralised nitrate produced within the catchment area. Although the stable isotopic compositions of nitrate eluted from a catchment can be a useful tracer to quantify the land use influences on the sources and behaviour of the nitrate, it is best to determine these for the remineralised portion of the nitrate separately from the unprocessed atmospheric nitrate to obtain a more accurate and precise quantification of the land use influences. In this study, we determined the spatial distribution and seasonal variation of stable isotopic compositions of nitrate for more than 30 streams within the same watershed, the Lake Biwa watershed in Japan, in order to use 17O excess (Δ17O) of nitrate as an additional tracer to quantify the mole fraction of atmospheric nitrate accurately and precisely. The stable isotopic compositions, including Δ17O of nitrate, in precipitation (wet deposition; n = 196) sampled at the Sado-seki monitoring station were also determined for 3 years. The deposited nitrate showed large 17O excesses similar to those already reported for midlatitudes: Δ17O values ranged from +18.6 to +32.4 ‰ with a 3-year average of +26.3 ‰. However, nitrate in each inflow stream showed small annual average Δ17O values ranging from +0.5 to +3.1 ‰, which corresponds to mole fractions of unprocessed atmospheric nitrate to total nitrate from (1.8 ± 0.3) to (11.8 ± 1.8) % respectively, with an average for all inflow streams of (5.1 ± 0.5) %. Although the annual average Δ17O values tended to be smaller in accordance with the increase in annual average stream nitrate concentration from 12.7 to 106.2 µmol L−1, the absolute concentrations of unprocessed atmospheric nitrate were almost stable at (2.3 ± 1.1) µmol L−1 irrespective of the changes in population density and land use in each catchment area. We conclude that changes in population density and land use between each catchment area had little impact on the concentration of atmospheric nitrate and that the total nitrate concentration originated primarily from additional contributions of remineralised nitrate. By using the average stable isotopic compositions of atmospheric nitrate, we excluded the contribution of atmospheric nitrate from the determined δ15N and δ18O values of total nitrate and estimated the δ15N and δ18O values of the remineralised portion of nitrate in each stream to clarify the sources. We found that the remineralised portion of the nitrate in the streams could be explained by mixing between a natural source with values of (+4.4 ± 1.8) and (−2.3 ± 0.9) ‰ for δ15N and δ18O respectively and an anthropogenic source with values of (+9.2 ± 1.3) and (−2.2 ± 1.1) ‰ for δ15N and δ18O respectively. In addition, both the uniform absolute concentration of atmospheric nitrate and the low and uniform δ18O values of the remineralised portion of nitrate in the streams imply that in-stream removal of nitrate through assimilation or denitrification had little impact on the concentrations and stable isotopic compositions of nitrate in the streams, except for a few streams in summer with catchments of urban/suburban land uses.

Role of oxbow lakes in controlling redox geochemistry of shallow groundwater under a heterogeneous fluvial sedimentary environment in an agricultural field: Coexistence of iron and sulfate reduction

This study aimed to extend the knowledge of the vertical distribution of redox conditions of shallow groundwater in heterogeneous fluvial sediments near oxbow lakes. For this study, we revisited the study area of Kim et al. (2009) to examine the redox zoning in details. Three multi-level samplers were installed along a flow path near two oxbow lakes to obtain vertical profiles of the subsurface geology and hydrochemical and isotopic data (δ18O and δD of water, δ15N and δ18O of nitrate, and δ34S of sulfate) of groundwater. Geologic logging showed that characteristics of the heterogeneous subsurface geology are closely related to the pattern of vertical redox zoning. Hydrochemical data in conjunction with nitrogen and sulfur isotope data show that the redox status of groundwater near oxbow lakes is controlled by denitrification, iron reduction, and sulfate reduction. The oxidizing condition of groundwater occurs in the sand-dominant alluvium located in the up-gradient of oxbow lakes, whereas the reducing condition accompanying denitrification, iron reduction, and local sulfate reduction is developed in silt-rich alluvium in and the downgradient of oxbow lakes. The occurrence of sulfate reduction was newly found in this study. However, the vertical profiles of redox-sensitive parameters show that iron reduction and sulfate reduction occur concurrently near oxbow lakes, although the measured redox potentials suggest that thermodynamic conditions are controlled by the stability of Fe2+/Fe-oxides. Therefore, this study shows that the redox condition of groundwater in the iron-rich zone should be carefully interpreted. For this purpose, depth-specific sampling and careful examination of sulfur isotope data will be very useful for identifying the redox processes occurring in the zone with overlapping iron reduction and sulfate reduction in heterogeneous fluvial sediments.

Nitrogen cycling in the deep sedimentary biosphere: nitrate isotopes in porewaters underlying the oligotrophic North Atlantic

Abstract. Nitrogen (N) is a key component of fundamental biomolecules. Hence, its cycling and availability are central factors governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite limited organic matter delivery from overlying water, the extent and modes of nitrogen transformations have not been widely investigated. Here we use the N and oxygen (O) isotopic composition of porewater nitrate (NO3−) from a site in the oligotrophic North Atlantic (Integrated Ocean Drilling Program – IODP) to determine the extent and magnitude of microbial nitrate production (via nitrification) and consumption (via denitrification). We find that NO3- accumulates far above bottom seawater concentrations (~ 21 μM) throughout the sediment column (up to ~ 50 μM) down to the oceanic basement as deep as 90 m b.s.f. (below sea floor), reflecting the predominance of aerobic nitrification/remineralization within the deep marine sediments. Large changes in the δ15N and δ18O of nitrate, however, reveal variable influence of nitrate respiration across the three sites. We use an inverse porewater diffusion–reaction model, constrained by the N and O isotope systematics of nitrification and denitrification and the porewater NO3- isotopic composition, to estimate rates of nitrification and denitrification throughout the sediment column. Results indicate variability of reaction rates across and within the three boreholes that are generally consistent with the differential distribution of dissolved oxygen at this site, though not necessarily with the canonical view of how redox thresholds separate nitrate regeneration from dissimilative consumption spatially. That is, we provide stable isotopic evidence for expanded zones of co-occurring nitrification and denitrification. The isotope biogeochemical modeling also yielded estimates for the δ15N and δ18O of newly produced nitrate (δ15NNTR (NTR, referring to nitrification) and δ18ONTR), as well as the isotope effect for denitrification (15ϵDNF) (DNF, referring to denitrification), parameters with high relevance to global ocean models of N cycling. Estimated values of δ15NNTR were generally lower than previously reported δ15N values for sinking particulate organic nitrogen in this region. We suggest that these values may be, in part, related to sedimentary N2 fixation and remineralization of the newly fixed organic N. Values of δ18ONTR generally ranged between −2.8 and 0.0 ‰, consistent with recent estimates based on lab cultures of nitrifying bacteria. Notably, some δ18ONTR values were elevated, suggesting incorporation of 18O-enriched dissolved oxygen during nitrification, and possibly indicating a tight coupling of NH4+ and NO2− oxidation in this metabolically sluggish environment. Our findings indicate that the production of organic matter by in situ autotrophy (e.g., nitrification, nitrogen fixation) supplies a large fraction of the biomass and organic substrate for heterotrophy in these sediments, supplementing the small organic-matter pool derived from the overlying euphotic zone. This work sheds new light on an active nitrogen cycle operating, despite exceedingly low carbon inputs, in the deep sedimentary biosphere.

Air–snow transfer of nitrate on the East Antarctic Plateau – Part 2: An isotopic model for the interpretation of deep ice-core records

Abstract. Unraveling the modern budget of reactive nitrogen on the Antarctic Plateau is critical for the interpretation of ice-core records of nitrate. This requires accounting for nitrate recycling processes occurring in near-surface snow and the overlying atmospheric boundary layer. Not only concentration measurements but also isotopic ratios of nitrogen and oxygen in nitrate provide constraints on the processes at play. However, due to the large number of intertwined chemical and physical phenomena involved, numerical modeling is required to test hypotheses in a quantitative manner. Here we introduce the model TRANSITS (TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow), a novel conceptual, multi-layer and one-dimensional model representing the impact of processes operating on nitrate at the air–snow interface on the East Antarctic Plateau, in terms of concentrations (mass fraction) and nitrogen (δ15N) and oxygen isotopic composition (17O excess, Δ17O) in nitrate. At the air–snow interface at Dome C (DC; 75° 06' S, 123° 19' E), the model reproduces well the values of δ15N in atmospheric and surface snow (skin layer) nitrate as well as in the δ15N profile in DC snow, including the observed extraordinary high positive values (around +300 ‰) below 2 cm. The model also captures the observed variability in nitrate mass fraction in the snow. While oxygen data are qualitatively reproduced at the air–snow interface at DC and in East Antarctica, the simulated Δ17O values underestimate the observed Δ17O values by several per mill. This is explained by the simplifications made in the description of the atmospheric cycling and oxidation of NO2 as well as by our lack of understanding of the NOx chemistry at Dome C. The model reproduces well the sensitivity of δ15N, Δ17O and the apparent fractionation constants (15ϵapp, 17Eapp) to the snow accumulation rate. Building on this development, we propose a framework for the interpretation of nitrate records measured from ice cores. Measurement of nitrate mass fractions and δ15N in the nitrate archived in an ice core may be used to derive information about past variations in the total ozone column and/or the primary inputs of nitrate above Antarctica as well as in nitrate trapping efficiency (defined as the ratio between the archived nitrate flux and the primary nitrate input flux). The Δ17O of nitrate could then be corrected from the impact of cage recombination effects associated with the photolysis of nitrate in snow. Past changes in the relative contributions of the Δ17O in the primary inputs of nitrate and the Δ17O in the locally cycled NO2 and that inherited from the additional O atom in the oxidation of NO2 could then be determined. Therefore, information about the past variations in the local and long-range processes operating on reactive nitrogen species could be obtained from ice cores collected in low-accumulation regions such as the Antarctic Plateau.

Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs

Abstract. Snowpits along a traverse from coastal East Antarctica to the summit of the ice sheet (Dome Argus) are used to investigate the post-depositional processing of nitrate (NO3−) in snow. Seven snowpits from sites with accumulation rates between 24 and 172 kg m−2 a−1 were sampled to depths of 150 to 300 cm. At sites from the continental interior (low accumulation, < 55 kg m−2 a−1), nitrate mass fraction is generally > 200 ng g−1 in surface snow and decreases quickly with depth to < 50 ng g−1. Considerably increasing values of δ15N of nitrate are also observed (16–461 ‰ vs. air N2), particularly in the top 20 cm, which is consistent with predicted fractionation constants for the photolysis of nitrate. The δ18O of nitrate (17–84 ‰ vs. VSMOW (Vienna Standard Mean Ocean Water)), on the other hand, decreases with increasing δ15N, suggestive of secondary formation of nitrate in situ (following photolysis) with a low δ18O source. Previous studies have suggested that δ15N and δ18O of nitrate at deeper snow depths should be predictable based upon an exponential change derived near the surface. At deeper depths sampled in this study, however, the relationship between nitrate mass fraction and δ18O changes, with increasing δ18O of nitrate observed between 100 and 200 cm. Predicting the impact of post-depositional loss, and therefore changes in the isotopes with depth, is highly sensitive to the depth interval over which an exponential change is assumed. In the snowpits collected closer to the coast (accumulation > 91 kg m−2 a−1), there are no obvious trends detected with depth and instead seasonality in nitrate mass fraction and isotopic composition is found. In comparison to the interior sites, the coastal pits are lower in δ15N (−15–71 ‰ vs. air N2) and higher in δ18O of nitrate (53–111 ‰ vs. VSMOW). The relationships found amongst mass fraction, δ15N, δ18O and Δ17O (Δ17O = δ17O–0.52 × δ18O) of nitrate cannot be explained by local post-depositional processes alone, and are instead interpreted in the context of a primary atmospheric signal. Consistent with other Antarctic observational and modeling studies, the isotopic results are suggestive of an important influence of stratospheric ozone chemistry on nitrate formation during the cold season and a mix of tropospheric sources and chemistry during the warm season. Overall, the findings in this study speak to the sensitivity of nitrate isotopic composition to post-depositional processing and highlight the strength of combined use of the nitrogen and oxygen isotopes for a mechanistic understanding of this processing.

Vertical decoupling of nitrate assimilation and nitrification in the Sargasso Sea

The fraction of phytoplankton growth that leads to the rain of organic carbon out of the sunlit surface ocean (“export production”) is central to the ocean׳s sequestration of atmospheric carbon dioxide. Nitrate assimilation has long been taken as a measure of export production; however, this has recently been questioned by suggestions that much of the nitrate in the sunlit layer (the “euphotic zone”) originates from biological nitrogen (N) already in surface waters. This view has been supported by recent observations of euphotic zone nitrate elevated in δ18O relative to its δ15N, taken as indicative of nitrification (the oxidation of recycled ammonium to nitrite and then nitrate). To evaluate the potential importance of nitrification in the Sargasso Sea euphotic zone, we measured the δ15N and δ18O of seawater nitrate for samples with ≥0.2µM nitrate collected on 18 cruises in the Sargasso Sea, and here we present the first large data set to correct for the low but often measurable concentrations of nitrite. Regardless of season, nitrate (i.e., nitrate-only rather than nitrate+nitrite as is commonly reported) δ15N and δ18O increase in unison from below the base of the euphotic zone toward the surface. This pattern derives from nitrate assimilation by phytoplankton, implying that nitrification is much slower than the upward transport of nitrate into the lower Sargasso Sea euphotic zone. In the twilight zone below the euphotic zone, we observe a rise in nitrate δ18O (relative to deeper waters) that is not accompanied by a rise in δ15N, suggesting nitrification co-occurring with nitrate assimilation. Nitrification, therefore, does not appear to occur in euphotic zone waters overlapping with nitrate assimilation only in the ~150m-thick twilight zone layer below it. In net, the data argue for a simpler N cycle for the Sargasso Sea euphotic zone than has recently been suggested, with the rate of euphotic zone nitrate assimilation approximating that of organic carbon export to the deep ocean.

Regional nitrogen dynamics in the TERENO Bode River catchment, Germany, as constrained by stable isotope patterns

Interactions between hydrological characteristics and microbial activities affect the isotopic composition of dissolved nitrate in surface water. Nitrogen and oxygen isotopic signatures of riverine nitrate in 133 sampling locations distributed over the Bode River catchment in the Harz Mountains, Germany, were used to identify nitrate sources and transformation processes. An annual monitoring programme consisting of seasonal sampling campaigns in spring, summer and autumn was conducted. δ15N and δ18O of nitrate and corresponding concentrations were measured as well as δ2H and δ18O of water to determine the deuterium excess. In addition, precipitation on 25 sampling stations was sampled and considered as a potential input factor. The Bode River catchment is strongly influenced by agricultural land use which is about 70 % of the overall size of the catchment. Different nitrogen sources such as ammonia (NH4) fertilizer, soil nitrogen, organic fertilizer or nitrate in precipitation show partly clear nitrate isotopic differences. Processes such as microbial denitrification result in fractionation and lead to an increase in δ15N of nitrate. We observed an evident regional and partly temporal variation of nitrate isotope signatures which are clearly different between main landscape types. Spring water sections within the high mountains contain nitrate in low concentrations with low δ15NNO3 values of −3 ‰ and high δ18ONO3 values up to 13 ‰. High mountain stream water sub-catchments dominated by nearly undisturbed forest and grassland contribute nitrate with δ15NNO3 and δ18ONO3 values of −1 and −3.5 ‰, respectively. In the further flow path, which is affected by an increasing agricultural land use and urban sewage, we recognized an increase in δ15NNO3 and δ18ONO3 up to 22 and 18 ‰, respectively, with high variations during the year. A correlation seems to exist between the percentage of agricultural land use area and the corresponding δ15NNO3 values for sub-catchments. A shift towards heavier isotope values in stream water samples taken in July 2012 is significant (p-value = 6 · 10−6) compared to samples from March and October 2012. We also see a season-depending impact of microbial denitrification. Denitrification, especially evident in the lowlands, predominantly takes place in the riverbeds. In addition, mixing processes of different nitrate sources and temperature-depending biological processes such as nitrification have to be taken into consideration. Constant-tempered groundwater does not play a noticeable role in the processes of the stream water system. As constrained from oxygen isotope signatures, precipitation associated with low nitrate concentrations does not have an obvious impact on stream water nitrate in the high mountain region.

On the origin of the occasional spring nitrate peak in Greenland snow

Abstract. Ice core nitrate concentrations peak in the summer in both Greenland and Antarctica. Two nitrate concentration peaks in one annual layer have been observed some years in ice cores in Greenland from samples dating post-1900, with the additional nitrate peak occurring in the spring. The origin of the spring nitrate peak was hypothesized to be pollution transport from the mid-latitudes in the industrial era. We performed a case study on the origin of a spring nitrate peak in 2005 measured from a snowpit at Summit, Greenland, covering 3 years of snow accumulation. The effect of long-range transport of nitrate on this spring peak was excluded by using sulfate as a pollution tracer. The isotopic composition of nitrate (δ15N, δ18O and Δ17O) combined with photochemical calculations suggest that the occurrence of this spring peak is linked to a significantly weakened stratospheric ozone (O3) layer. The weakened O3 layer resulted in elevated UVB (ultraviolet-B) radiation on the snow surface, where the production of OH and NOx from the photolysis of their precursors was enhanced. Elevated NOx and OH concentrations resulted in enhanced nitrate production mainly through the NO2 + OH formation pathway, as indicated by decreases in δ18O and Δ17O of nitrate associated with the spring peak. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and found 19 years with double nitrate peaks after the 1950s. Out of these 19 years, 14 of the secondary nitrate peaks were accompanied by sulfate peaks, suggesting long-range transport of nitrate as their source. In the other 5 years, low springtime O3 column density was observed, suggesting enhanced local production of nitrate as their source. The results suggest that, in addition to direct transport of nitrate from polluted regions, enhanced local photochemistry can also lead to a spring nitrate peak. The enhanced local photochemistry is probably associated with the interannual variability of O3 column density in the Arctic, which leads to elevated surface UV radiation in some years. In this scenario, enhanced photochemistry caused increased local nitrate production under the condition of elevated local NOx abundance in the industrial era.

Animal regeneration and microbial retention of nitrogen along coastal rocky shores

Animal contributions to ecosystem productivity via nutrient regeneration are increasingly recognized as significant for ecosystem productivity. The paradigm in coastal upwelling systems, in contrast, remains centered around the idea that nutrient supply is extrinsic and sourced from deep water. Here we tested for both animal contributions to nitrogen regeneration and microbial retention of nitrogen along a gradient of animal abundance in Washington State, USA. Using inferences from the concentration and isotopic composition of nitrogen forms, as well as the stable nitrogen isotopic composition of particulate organic matter, and plant and animal tissue, we found increased ammonium as well as greater stable nitrogen isotope values in these organisms in areas of increased animal abundance. We further tested the effect of the nearshore biota by comparing it with an immediately adjacent offshore area, and found an enriched δ15N value for nitrate and particulate organic matter. Further, an analysis of the dual isotopes of δ15N and the δ18O of nitrate indicated increasing microbial processing in the nearshore. Isoscapes, or the spatial pattern of stable isotopes, revealed that animal and microbial processing of nitrogen leave a consistent signature and are thus a previously overlooked and essential component of nearshore productivity.

WAIS Divide ice core suggests sustained changes in the atmospheric formation pathways of sulfate and nitrate since the 19th century in the extratropical Southern Hemisphere

Abstract. The 17O excess (Δ17O = δ17O−0.52 × δ18O) of sulfate and nitrate reflects the relative importance of their different production pathways in the atmosphere. A new record of sulfate and nitrate Δ17O spanning the last 2400 years from the West Antarctic Ice Sheet Divide ice core project shows significant changes in both sulfate and nitrate Δ17O in the most recent 200 years, indicating changes in their formation pathways. The sulfate Δ17O record exhibits a 1.1 ‰ increase in the early 19th century from (2.4 ± 0.2) ‰ to (3.5 ± 0.2) ‰, which suggests that an additional 12–18% of sulfate formation occurs via aqueous-phase production by O3, relative to that in the gas phase. Nitrate Δ17O gradually decreases over the whole record, with a more rapid decrease between the mid-19th century and the present day of 5.6 ‰, indicating an increasing importance of RO2 in NOx cycling between the mid-19th century and the present day in the mid- to high-latitude Southern Hemisphere. The former has implications for the climate impacts of sulfate aerosol, while the latter has implications for the tropospheric O3 production rate in remote low-NOx environments. Using other ice core observations, we rule out drivers for these changes other than variability in extratropical oxidant (OH, O3, RO2, H2O2, and reactive halogens) concentrations. However, assuming OH, H2O2, and O3 are the main oxidants contributing to sulfate formation, Monte Carlo box model simulations require a large (≥ 260%) increase in the O3 / OH mole fraction ratio over the Southern Ocean in the early 19th century to match the sulfate Δ17O record. This unlikely scenario points to a~deficiency in our understanding of sulfur chemistry and suggests other oxidants may play an important role in sulfate formation in the mid- to high-latitude marine boundary layer. The observed decrease in nitrate Δ17O since the mid-19th century is most likely due to an increased importance of RO2 over O3 in NOx cycling and can be explained by a 60–90% decrease in the O3 / RO2 mole fraction ratio in the extratropical Southern Hemisphere NOx-source regions.