δ18O Of NO3 Research Articles

Differences in Stream Water Nitrate Concentrations between a Nitrogen-Saturated Upland Forest and a Downstream Mixed Land Use River Basin

Nitrogen (N) saturation of upland forests has been assumed to be a substantial N source downstream. However, removal processes of N, including assimilation and denitrification in the downstream area, have not been clarified. To evaluate the N removal processes, nitrate (NO3−) and organic N concentrations, as well as nitrogen isotope ratio (δ15N) and oxygen isotope ratio (δ18O) of NO3− were measured along three rivers of Tatara River Basin, Japan where upland forests have already been N-saturated. Geographic information system (GIS) based topographical analysis was also conducted to evaluate the land use as urban area in relation to topography. In two of the three rivers, NO3− concentrations did not increase from upstream to downstream, despite the potential non-point N sources of urban areas. In another river, NO3− concentrations rather decreased. The values of δ15N and δ18O of NO3− and organic N concentrations suggested the presence of denitrification and assimilation over N pollutants in the river whose watersheds have a lower percentage of urban area. The lower percentage of urban area could be explained by the lower topographic index. This study concluded that the NO3− leaching from upland N-saturated forests was substantially assimilated or denitrified in the downstream area.

Assimilation and nitrification in pelagic waters: insights using dual nitrate stable isotopes (δ15N, δ18O) in a shallow lake

Nitrate dual stable isotopes (δ15N and δ18O of NO3 −) have proven to be a powerful technique to elucidate nitrogen (N) cycling pathways in aquatic systems. We applied this technique for the first time in the pelagic zone of a small temperate meso-eutrophic lake to identify the dominant N cycling pathways, and their spatial and temporal variability. We measured the lake NO3 − δ15N and δ18O signatures over an annual cycle and compared them to that of the watershed. Both δ15N and δ18O of NO3 − in the lake increased during summer relative to the inputs. Relationships between lake NO3 − isotopic composition and concentrations were different across thermal strata with an apparent isotope effect in the epilimnion of 15eepi = 4.6‰ and 18eepi = 10.9‰. We found a strong deviation of the lake NO3 − δ18O and δ15N from the expected 1:1 line for assimilation (slope = 1.73) suggesting that nitrification was co-occurring. We estimated that nitrification could support between 5 and 30% of nitrate-based production during the growing season, but was negligible in early spring and fall, and probably more dominant under ice. We showed that the technique is promising to study N processes at the ecosystem scale in shallow lakes, particularly during winter. Our results suggest that recycled NO3 − could support primary productivity and influence phytoplankton composition in the surface waters of small lakes.

Field Evaluation of Nitrogen Treatment by Conventional and Single-Pass Sand Filter Onsite Wastewater Systems in the North Carolina Piedmont

Excess nitrogen loading has contributed to the impairment of major watersheds across North Carolina. Onsite wastewater systems (OWS) are a potential source of nitrogen to water resources, but more research is needed to determine their actual contributions, especially in the Piedmont region of the state. The objective of this study was to determine the total dissolved nitrogen (TDN) treatment efficiency of four OWS in clayey soils of the North Carolina Piedmont. Two OWS were conventional style, and two were single-pass sand filters. The four volunteered sites with OWS were instrumented with piezometers (27 total) for groundwater collection and analyses. Piezometers were installed within 1.5 m of each OWS and downgradient from the conventional OWS. Septic tank effluent, groundwater from the piezometers, sand filter effluent, and adjacent surface waters were sampled bimonthly (five times) during 2015. Samples were analyzed for TDN, NO3 −-N, NH4 +-N, chloride, dissolved organic carbon, and physical and chemical parameters on each sampling event. Groundwater samples collected 35 m downgradient from the two conventional OWSs had TDN concentrations and masses, on average, of 98 and 70 %, respectively, lower than septic tank effluent. Isotopic analysis of the natural abundance of δ15N and δ18O in NO3 − in groundwater collected at the conventional OWS sites suggests that denitrification was a mass removal mechanism. The sand filter OWS reduced TDN concentrations by an average of 80 % and mass loading by 50 % prior to discharge to surface waters. Nitrogen management regulations in nutrient-sensitive watersheds should consider the contributions from OWS, especially direct discharge systems like sand filters. Improvements in the TDN treatment efficiency of direct discharge OWS would result in immediate surface water quality improvements.

Enzyme‐level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean

AbstractIn the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO3−) have been used to investigate NO3− assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the 15N/14N (δ15N) and 18O/16O (δ18O) of NO3− and nitrite (NO2−) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ15N and δ18O of NO3− + NO2− increase roughly equally, as expected for NO3− assimilation; however, the δ15N of NO3−‐only (measured after NO2− removal) increases more than does NO3−‐only δ18O. Differencing indicates that NO2− has an extremely low δ15N, often < −70‰ versus air. These observations are consistent with the expression of an equilibrium N isotope effect between NO3− and NO2−, likely due to enzymatic NO3−‐NO2− interconversion. Specifically, we propose reversibility of the nitrite oxidoreductase (NXR) enzyme of nitrite oxidizers that, having been entrained from the subsurface during late summer mixed layer deepening, are inhibited by light. Our interpretation suggests a role for NO3−‐NO2− interconversion where nitrifiers are transported into environments that discourage NO2− oxidation. This may apply to surface regions with upwelling, such as the summertime Antarctic. It may also apply to oxygen‐deficient zones, where NXR‐catalyzed interconversion may explain previously reported evidence of NO2− oxidation.

Isotopic identification of soil and permafrost nitrate sources in an Arctic tundra ecosystem

AbstractThe nitrate (NO3−) dual isotope approach was applied to snowmelt, tundra active layer pore waters, and underlying permafrost in Barrow, Alaska, USA, to distinguish between NO3− derived from atmospheric deposition versus that derived from microbial nitrification. Snowmelt had an atmospheric NO3− signal with δ15N averaging −4.8 ± 1.0‰ (standard error of the mean) and δ18O averaging 70.2 ± 1.7‰. In active layer pore waters, NO3− primarily occurred at concentrations suitable for isotopic analysis in the relatively dry and oxic centers of high‐centered polygons. The average δ15N and δ18O of NO3− from high‐centered polygons were 0.5 ± 1.1‰ and −4.1 ± 0.6‰, respectively. When compared to the δ15N of reduced nitrogen (N) sources, and the δ18O of soil pore waters, it was evident that NO3− in high‐centered polygons was primarily from microbial nitrification. Permafrost NO3− had δ15N ranging from approximately −6‰ to 10‰, similar to atmospheric and microbial NO3−, and highly variable δ18O ranging from approximately −2‰ to 38‰. Permafrost ice wedges contained a significant atmospheric component of NO3−, while permafrost textural ice contained a greater proportion of microbially derived NO3−. Large‐scale permafrost thaw in this environment would release NO3− with a δ18O signature intermediate to that of atmospheric and microbial NO3. Consequently, while atmospheric and microbial sources can be readily distinguished by the NO3− dual isotope technique in tundra environments, attribution of NO3− from thawing permafrost will not be straightforward. The NO3− isotopic signature, however, appears useful in identifying NO3− sources in extant permafrost ice.

Coupled nitrate N and O stable isotope fractionation by a natural marine plankton consortium

The stable nitrogen (N) and oxygen (O) isotope ratios (15N/14N and 18O/16O, respectively) of nitrate (NO3-) were measured during incubations of freshly collected seawater to investigate the effect of light intensity on the isotope fractionation associated with nitrate assimilation and possible co-occurring regeneration and nitrification by in situ plankton communities. Surface seawater was collected off the coast of Vancouver, Canada, in late fall and in late summer and was incubated under different laboratory light conditions for 10 and 30 days, respectively. In the late summer experiments, parallel incubations were supplemented with 15NH4+ and H218O tracers to monitor co-occurring nitrification. Differences in irradiance in the fall incubations resulted in reduced nitrate consumption at low light, but had no distinguishable impact on the N-isotope isotope effect (15e) associated with NO3- assimilation, which ranged between 5 and 8‰. The late-summer community incubations, in contrast, showed reduced growth rates at low light and more elevated 15e of 11.9 ± 0.4‰, compared to 8.4 ± 0.3‰ at high-light conditions. The seasonal differences could reflect physiological adaptations of the fall plankton community to reduced irradiance, such that their incubation at low light did not elicit the increase in proportional cellular nitrate efflux required to raise the isotope effect. In both the fall and summer incubations, the ratio of the coincident rises in the δ15N and δ18O of NO3- was comparable to previous monoculture phytoplankton experiments, with a ∆δ18O:∆δ15N of ~1, regardless of light level. A decoupling of ∆δ18O:∆δ15N is expected if nitrification occurs concomitantly with nitrate assimilation. The lack of such decoupling is best explained by the absence of significant nitrification in any of our study’s treatments, an interpretation supported by our inability to identify any tracer 15N and 18O uptake into the NO3- pool in the late-summer community incubations.

Use of isotopic compositions of nitrate in TSP to identify sources and chemistry in South China Sea

NO3− concentration, nitrogen and oxygen isotopic compositions (δ15N and δ18O) of NO3− were measured in total suspended particulates (TSP) at Yongxing Island in the South China Sea (SCS) between Feb. 2013 and Jan. 2014, as well as on two cruises in the northern South China Sea (NSCS). Measurements aimed to identify NO3− sources, and possible chemical formation processes of NO3−. The δ15N and δ18O of NO3− in TSP at Yongxing Island ranged from −2.5 to +4.9‰, and +48.1 to +99.0‰, with annual weighted averages of +1.5‰ and +83.2‰, respectively. Both δ15N and δ18O had higher values in cool months, indicating that NOx sources and oxidants were different between seasons. In cool months, NOx was mainly from anthropogenic sources, particularly from coal combustion in South China, resulting in high nitrogen deposition that was oxidized by O3 to NO3−. In warm months, natural emissions were an important source of NOx. TSP samples in the NSCS had higher NO3− concentrations, higher δ15N and lower δ18O values than samples from Yongxing Island over the same period. This suggests that atmospheric processes caused a decrease in NO3− concentrations and δ15N but increase in δ18O from coast to remote marine. Assuming to oxygen atoms were derived from O3 during transport in cool months, the mean ratio of NO3− formed by NOx to total NO3− was calculated to be 47.9%. This suggests the mean loss ratio of NOx was 89% while the loss ratio of NO3− was 87% during transport between Chinese coastal areas and Yongxing Island in Nov., 2013.

Nitratenitrogen and Oxygen Isotope Characterization of the Shiraz Aquifer (Iran)

The Shiraz aquifer undergoes anthropogenic pressure from local urban, agricultural and industrial activities that result in dissolved nitrate (NO3−)reaching concentrations as high as 149mg/L, well above the 50mg/L guideline defined by the World Health Organisation. We coupled classical chemical and dual isotope (δ15N and δ18O of NO3−) approaches to characterize sources and processes controlling itsbudget. Our data indicate that NO3− in this aquifer is explained by distinct end-members: while mineral fertilizers isotopically show to have no impact, our isotope approach identifies natural soil nitrification and organic NO3− (manure and/or septic waste) as the two main contributors. Isotope data suggest that natural denitrification may occur within the aquifer, but this conclusion is not supported by the study of other chemical parameters.

Quantification of Nitrate Sources to an Urban Stream Using Dual Nitrate Isotopes

Human-engineered landscapes and subsequent altered hydrology affect the fate and transport of reactive nitrogen, particularly in urban watersheds. In this study, we used dual-nitrate isotopes and mixing model analysis (δ(15)N and δ(18)O of NO3(-)) to quantify nitrogen inputs from two sources concentrated in urban systems, sewage and atmospheric deposition. Analysis was conducted on samples collected from Nine Mile Run (Pittsburgh, PA) including over two years of samples collected biweekly and samples collected through the hydrographs of four storm events. Mixing models incorporated uncertainties in the isotopic composition of potential nitrate sources and resolved the relative proportions of nitrate inputs from each source using Bayesian techniques. The results indicate that up to 94% of nitrate in streamwater originated from sewage sources during baseflow conditions. During storms, atmospheric deposition was a substantial nitrate source (∼ 34%) to total event-based nitrate loads, although sewage-derived nitrate remained the dominant source (66%). The potential influence of denitrification was considered by incorporating associated isotopic fractionations into mixing models; up to 19% of sewage-derived samples showed the isotopic effects of denitrification. This study quantitatively delineates proportions of nitrate from different sources to urban streamwater, while incorporating remaining uncertainties in source endmember compositions.

Sources and Processes Affecting Nitrate in a Dam-Controlled Subtropical River, Southwest China

Excess nutrient (N and P) loads are recognized as the major cause of serious water quality problems in China. River systems play a very important role in nitrate (NO3 −) transportation and transformation in the aquatic environment. To understand and clarify the sources and processes affecting NO3 − in river basins, we have examined spatial and temporal variations of concentration and dual-isotopic composition of NO3 − in the dam-controlled Jialing River, a major tributary of the Yangtze River where land use is dominated by agriculture. Water samples were collected in July 2008 and February 2009 from the main channel of the Jialing River and its major tributaries. The δ15N and δ18O of NO3 − range from 1.5 to 11.0 ‰ (average 6.2 ‰) and −5.0 to 11.1 ‰ (average, 1.6 ‰), respectively. NO3 − isotope data and δ18O of water interpreted in combination with hydrological and chemical data suggest that most of the NO3 − input is from nitrification during the rainy season, and discharge of sewage and manure in the upper course and from cities accounts for much of the NO3 − load during the dry season. The construction of cascade dams has led to retention of Si and a decrease in the Si/N ratio, implying that assimilation and/or denitrification may significantly affect NO3 − in the dam area, as demonstrated by NO3 − and dissolved Si concentrations, and $$\updelta^{ 1 5} {\text{N}}_{{{\text{NO}}_{3} }}$$ and $$\updelta^{ 1 8} {\text{O}}_{{{\text{NO}}_{3} }}$$ values. This study indicates that dual-isotopic data can be used to identify NO3 − pollution sources and the processes NO3 − has undergone during its retention and transport in the watershed of the dam-controlled Jialing River.

Nitrate δ 15N and δ 18O evidence for active biological transformation in the Changjiang Estuary and the adjacent East China Sea

The Changjiang Estuary has been considered as one of the most polluted estuaries in the world due to high nitrate (NO 3 − ) input, especially in spring and summer. In this study, δ 15N and δ 18O of NO 3 − , along with other chemical parameters in this area, were measured in spring to evaluate NO 3 − biogeochemical processes. A simple two end-members mixing model was used to examine the relative contribution of the Changjiang River Diluted Water and marine water to NO 3 − sources in the Changjiang Estuary and the adjacent East China Sea. The isotopic signals show that NO 3 − behaved relatively and conservatively in Transect F and Transect P where assimilation was weak possibly due to vertical mixing, while active assimilation and weak nitrification occurred in Transect D. Spatial difference in assimilation was indicated by the ∼1:1 enrichment of δ 15N and δ 18O in the three transects, while spatial difference in nitrification was reflected by deviations of δ 15N and δ 18O from assimilation line. Our results suggest that the input of the Changjiang River Diluted Water promoted NO 3 − assimilation possibly by stratifying the water column which favored the phytoplankton growth.

Are dual isotope and isotopomer ratios of N2O useful indicators for N2O turnover during denitrification in nitrate-contaminated aquifers?

Denitrifying aquifers are sources of the greenhouse gas N2O. Isotopic signatures reflect processes of production and reduction of N2O, but it is not clear to which extent these can be used to quantify those processes. We investigated the spatial distribution of isotopologue values of N2O (δ18O, average δ15N, and 15N site preference, SP) in two denitrifying sandy aquifers to study N2O production and reduction and associated isotope effects in groundwater. For the first time, we combined this approach with direct estimation of N2O reduction from excess-N2 analysis. Groundwater samples were collected from 15 monitoring wells and four multilevel sampling wells and analysed for NO3-, dissolved N2O, dissolved O2, excess N2 from denitrification and isotopic signatures of NO3- and N2O. Both aquifers exhibited high NO3- concentrations with average concentrations of 22 and 15mgNL−1, respectively. Evidence of intense denitrification with associated N2O formation was obtained from mean excess-N2 of 3.5 and 4.3mgNL−1, respectively. Isotopic signatures of N2O were highly variable with ranges of 17.6–113.2‰ (δ18O), −55.4 to 89.4‰ (δ15Nbulk) and 1.8–97.9‰ (SP). δ15N and δ18O of NO3- ranged from −2.1‰ to 65.5‰ and from −5‰ to 33.5‰, respectively.The relationships between δ15N of NO3-, δ15Nbulk and SP were not in good agreement with the distribution predicted by a Rayleigh-model of isotope fractionation. The large ranges of δ18O and SP of N2O as well as the close correlation between these values could be explained by the fact that N2O reduction to N2 was strongly progressed but variable.We confirm and explain that a large range in SP and δ18O is typical for N2O from denitrifying aquifers, showing that this source signature can be distinguished from the isotopic fingerprint of N2O emitted from soils without water-logging. We conclude that isotopologue values of N2O in our sites were not suitable to quantify production or reduction of N2O or the contribution of different processes to the total N2O flux, apparently because these values were not only governed by individual pathways but eventually also by the spatial distribution of substrates and activity within the aquifers. These observations could be explained by the dynamics of N2O production, reduction and transport in water-saturated systems with heterogenic distribution of microbial activity and by a combination of diffusive and enzymatic isotope effects.

Improving the management of nitrate pollution in water by the use of isotope monitoring: the δ15N, δ18O and δ11B triptych

In spite of increasing efforts to reduce nitrogen inputs into ground water from intensive agriculture, nitrate (NO3) remains one of the major pollutants of drinking-water resources worldwide, with NO3 levels approaching the defined limit of 50 mg l−1 in an increasing number of water bodies. Determining the source(s) of contamination in water is an important first step for improving its quality by emission control. The Life ISONITRATE project aimed at showing the benefit of a multi-isotope approach (δ15N and δ18O of NO3, and δ11B), in addition to conventional hydrogeological analysis, to track the origin of NO3 contamination in water. Based on land use and local knowledge, four distinct cases were studied: (1) natural soil NO3, (2) natural denitrification, (3) single source of NO3 pollution and (4) multiple sources of NO3 pollution. Our results show the added value of combining isotope information, compared to knowledge based on local authorities’ experience, land use and the ‘classical’ chemical approach, by efficiently identifying the number and type of NO3 source(s) for each watershed studied.

Cyclic biogeochemical processes and nitrogen fate beneath a subtropical stormwater infiltration basin

A stormwater infiltration basin in north–central Florida, USA, was monitored from 2007 through 2008 to identify subsurface biogeochemical processes, with emphasis on N cycling, under the highly variable hydrologic conditions common in humid, subtropical climates. Cyclic variations in biogeochemical processes generally coincided with wet and dry hydrologic conditions. Oxidizing conditions in the subsurface persisted for about one month or less at the beginning of wet periods with dissolved O2 and NO3− showing similar temporal patterns. Reducing conditions in the subsurface evolved during prolonged flooding of the basin. At about the same time O2 and NO3− reduction concluded, Mn, Fe and SO42− reduction began, with the onset of methanogenesis one month later. Reducing conditions persisted up to six months, continuing into subsequent dry periods until the next major oxidizing infiltration event. Evidence of denitrification in shallow groundwater at the site is supported by median NO3−–N less than 0.016mgL−1, excess N2 up to 3mgL−1 progressively enriched in δ15N during prolonged basin flooding, and isotopically heavy δ15N and δ18O of NO3− (up to 25‰ and 15‰, respectively). Isotopic enrichment of newly infiltrated stormwater suggests denitrification was partially completed within two days. Soil and water chemistry data suggest that a biogeochemically active zone exists in the upper 1.4m of soil, where organic carbon was the likely electron donor supplied by organic matter in soil solids or dissolved in infiltrating stormwater. The cyclic nature of reducing conditions effectively controlled the N cycle, switching N fate beneath the basin from NO3− leaching to reduction in the shallow saturated zone. Results can inform design of functionalized soil amendments that could replace the native soil in a stormwater infiltration basin and mitigate potential NO3− leaching to groundwater by replicating the biogeochemical conditions under the observed basin.

Soil Property Control of Biogeochemical Processes beneath Two Subtropical Stormwater Infiltration Basins

Substantially different biogeochemical processes affecting nitrogen fate and transport were observed beneath two stormwater infiltration basins in north-central Florida. Differences are related to soil textural properties that deeply link hydroclimatic conditions with soil moisture variations in a humid, subtropical climate. During 2008, shallow groundwater beneath the basin with predominantly clayey soils (median, 41% silt+clay) exhibited decreases in dissolved oxygen from 3.8 to 0.1 mg L and decreases in nitrate nitrogen (NO-N) from 2.7 mg L to <0.016 mg L, followed by manganese and iron reduction, sulfate reduction, and methanogenesis. In contrast, beneath the basin with predominantly sandy soils (median, 2% silt+clay), aerobic conditions persisted from 2007 through 2009 (dissolved oxygen, 5.0-7.8 mg L), resulting in NO-N of 1.3 to 3.3 mg L in shallow groundwater. Enrichment of δN and δO of NO combined with water chemistry data indicates denitrification beneath the clayey basin and relatively conservative NO transport beneath the sandy basin. Soil-extractable NO-N was significantly lower and the copper-containing nitrite reductase gene density was significantly higher beneath the clayey basin. Differences in moisture retention capacity between fine- and coarse-textured soils resulted in median volumetric gas-phase contents of 0.04 beneath the clayey basin and 0.19 beneath the sandy basin, inhibiting surface/subsurface oxygen exchange beneath the clayey basin. Results can inform development of soil amendments to maintain elevated moisture content in shallow soils of stormwater infiltration basins, which can be incorporated in improved best management practices to mitigate NO impacts.

Hydrological influences on spatiotemporal variations of δ15N and δ18O of nitrate in a forested headwater catchment in central Japan: Denitrification plays a critical role in groundwater

To elucidate the internal nitrogen cycle and nitrogen input and output, we measured δ15N‐NO3−, δ18O‐NO3−, and NO3− concentrations in rainfall, soil water from an unsaturated soil horizon, groundwater, and stream water at a small forested headwater catchment. In soil water, the mean NO3− concentration did not decrease concomitantly with soil depth, despite a drastic decrease of δ18O‐NO3− with soil depth, indicating immediate consumption of atmospheric NO3− and production of nitrified NO3−. The spatiotemporal distribution of δ15N‐NO3− and δ18O‐NO3− in soil water indicated that plant uptake contributed mainly to NO3− consumption and that denitrification was negligible. In contrast, denitrification played a critical role in groundwater in decreasing the NO3− concentration. Fluctuations of δ15N‐NO3−, δ18O‐NO3−, and NO3− concentration in groundwater, however, differed from expectations for occurrence resulting from denitrification only. A plausible explanation for the lack of a concurrent increase in δ15N and δ18O of NO3− is that nitrification and denitrification occur concurrently in groundwater and that denitrification is more important than nitrification. The amount of denitrification in the groundwater body was controlled primarily by groundwater residence time. The combination of hydrological flow path from groundwater to stream water and denitrification in the groundwater critically control the amount of leaching of NO3− and temporal distribution of NO3− concentration in stream water. The possibility exists that denitrification in groundwater strongly influences nitrogen leaching in forests because denitrification occurred even in this catchment, which consists of weathered granitic bedrock and predominantly typic udipsamment, with low organic matter content in the soil.

Assessment of the Sources of Nitrate in the Changjiang River, China Using a Nitrogen and Oxygen Isotopic Approach

The Changjiang River is the largest freshwater river in China. Here, the sources and variability in nitrate of the Changjiang River are assessed for the first time using dual isotopic approach. Water samples were collected once in August 2006 from the main channel of the Changjiang and its major tributaries. The concentrations and isotopic composition of nitrate were then analyzed for the waters in the Changjiang River. The delta(15)N and delta(18)O of NO(3)(-) ranges from 7.3 per thousand to 12.9 per thousand and 2.4 per thousand to 11.2 per thousand in the Changjiang River waters, respectively. The ranges of isotopic compositions of nitrate suggested that nitrification (including "modified fertilizer") and urban sewage effluent are the major sources of nitrate in the Changjiang River. The high delta(18)O-NO(3)(-) values were observed in the water of the upper reaches, indicated that the current drought might be one important reason for shifting of isotopes in the special sampling period. In addition, there was a strong positive relationship between delta(15)N-NO(3)(-) and delta(18)O-NO(3)(-), which indicated that denitrification added to the enrichment of heavy isotopes of nitrate.

Isotopic ratios in gas‐phase HNO3 and snow nitrate at Summit, Greenland

Measurements of the isotope ratios of snow nitrate and atmospheric gas‐phase HNO3 (15N/14N and 18O/16O) from Summit, Greenland provide new observational constraints on postdepositional processing of snowpack nitrate. The δ15N of NO3− is similar in surface snow and in air sampled 1.5 m above the snow surface during Spring and Summer 2006, ranging between −15 and +6‰ versus N2. The δ18O of NO3−, however, is ∼40‰ higher in surface snow than air samples. Photochemical box modeling of HNO3 chemistry at Summit suggests that the δ18O of the air samples is derived predominantly from gas‐phase HNO3 originating from photolyzed snow nitrate. Measurements of isotopically labeled nitrate in surface snow confirm that photolytic recycling of snowpack nitrate, in which photolyzed nitrate products recombine to form HNO3 that is subsequently redeposited to the snow surface, does occur at Summit. Calculations incorporating these measurements suggest that photolytic recycling of snow nitrate at Summit will have a maximum −7.5‰ change on the δ18O of nitrate buried in snow, while photolytic loss of snow nitrate will decrease the δ18O of buried nitrate by 0.9‰. Photolytic recycling and loss will increase the δ15N of buried snow nitrate, the degree to which depends on the nitrogen isotope fractionation of nitrate photolysis. Finally, a comparison of surface snow and snowpit samples demonstrates the preservation of the seasonal cycle in isotope ratios of snow nitrate.

Drastic Short‐Term Changes in the Isotopic Composition of Soil Nitrate in Forest Soil Samples

Stable isotope signatures in soil‐derived NO3− provide an opportunity for environmental source tracing, but rapid changes in N transformation rates caused by sampling disturbance may create an artifact in the signature of extracted NO3− To study this, we measured net nitrification and ammonification rates and the δ15N and δ18O of NO3− in soils from a watershed in Camels Hump State Forest, Vermont, known to be sensitive to sampling disturbance. Eleven Oa and one A horizon samples (C/N ratio 15–25, pH 3.3–4.2) had NO3− extracted for isotope analysis approximately 30 min after sampling and again after a 2‐ to 3‐d incubation period at 10°C. Net nitrification rates during incubation were rapid and linear (0.9–15.3 μmol L−1 soil h−1), with increases in NO3− detectable within 1 h after sampling. The δ15N and δ18O of soil NO3− changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in δ15NO3− became more depleted after incubation, as much as 13.5‰ lighter. Soils that were initially relatively depleted in δ15NO3− became more enriched after incubation, as much as 16.5‰ heavier. The latter had high net nitrification rates and low final NH4+ concentrations, probably causing the enrichment of 15N in NO3− due to a diminishing substrate pool. The δ18O of soil NO3− showed little change during incubation and changes were not significantly related to changes in the 15N of NO3−, suggesting that denitrification was not a primary mechanism. Added enriched 15NH4Cl (111.9‰) was rapidly incorporated into the soil NO3− pool in a pattern supportive of a mechanism due to increased nitrification rates. Soil sampling disturbance can dramatically alter the isotopic signature of soil NO3−, and the isotopic signature of extracted NO3− may not be a reliable environmental tracer.

Dual isotope analyses indicate efficient processing of atmospheric nitrate by forested watersheds in the northeastern U.S.

Nitrogen from atmospheric deposition serves as the dominant source of new nitrogen to forested ecosystems in the northeastern U.S. By combining isotopic data obtained using the denitrifier method, with chemical and hydrologic measurements we determined the relative importance of sources and control mechanisms on nitrate (NO3−) export from five forested watersheds in the Connecticut River watershed. Microbially produced NO3− was the dominant source (82–100%) of NO3− to the sampled streams as indicated by the δ15N and δ18O of NO3−. Seasonal variations in the δ18O–NO3− in streamwater are controlled by shifting hydrologic and temperature affects on biotic processing, resulting in a relative increase in unprocessed NO3− export during winter months. Mass balance estimates find that the unprocessed atmospherically derived NO3− stream flux represents less than 3% of the atmospherically delivered wet NO3− flux to the region. This suggests that despite chronically elevated nitrogen deposition these forests are not nitrogen saturated and are retaining, removing, and reprocessing the vast majority of NO3− delivered to them throughout the year. These results confirm previous work within Northeastern U.S. forests and extend observations to watersheds not dominated by a snow-melt driven hydrology. In contrast to previous work, unprocessed atmospherically derived NO3− export is associated with the period of high recharge and low biotic activity as opposed to spring snowmelt and other large runoff events.