Signature Of Nitrate Research Articles

A modified procedure for measuring oxygen-18 content of nitrate

Summary Mass spectrometric analysis of O-isotopic composition of nitrate has many potential applications in studies of environmental processes. Through this work, rapid, reliable, precise, broadly applicable, catalyst-free, low-priced and less labor intensive procedure for measuring δ 18 O of nitrate using Isotope Ratio Mass Spectrometer has been developed and implemented. The conditions necessary to effect complete nitrate recovery and complete removal of other oxygen containing anions and dissolved organic carbon (DOC) without scarifying the isotopic signature of nitrate were investigated. The developed procedure consists of two main parts: (1) wet chemistry train for extraction and purification of nitrate from the liquid matrix; (2) off-line pyrolysis of extracted nitrate salt with activated graphite at 550 °C for 30 min. The conditions necessary to effect complete nitrate recovery and complete removal of other oxygen containing compounds were investigated. Dramatic reduction in processing times needed for analysis of δ 18 O of nitrate at natural abundance level was achieved. Preservation experiments revealed that chloroform (99.8%) is an effective preservative. Isotopic contents of some selected nitrate salts were measured using the modified procedure and some other well established methods at two laboratories in Egypt and Germany. Performance assessment of the whole developed analytical train was made using internationally distributed nitrate isotopes reference materials and real world sample of initial zero-nitrate content. The uncertainty budget was evaluated using the graphical nested hierarchal approach. The obtained results proved the suitability for handling samples of complicated matrices. Reduction of consumables cost by about 80% was achieved.

Alanine scanning mutagenesis of a high-affinity nitrate transporter highlights the requirement for glycine and asparagine residues in the two nitrate signature motifs

Common to all of the nitrate nitrite porter family are two conserved motifs in transmembrane helices 5 and 11 termed NS (nitrate signature) 1 and NS2. Although perfectly conserved substrate-interacting arginine residues have been described in transmembrane helices 2 and 8, the role of NSs has not been investigated. In the present study, a combination of structural modelling of NrtA (nitrate transporter from Aspergillus nidulans) with alanine scanning mutagenesis of residues within and around the NSs has been used to shed light on the probable role of conserved residues in the NSs. Models show that Asn(168) in NS1 and Asn(459) in NS2 are positioned approximately midway within the protein at the central pivot point in close proximity to the substrate-binding residues Arg(368) and Arg(87)respectively, which lie offset from the pivot point towards the cytoplasmic face. The Asn(168)/Arg(368)and Asn(459)/Arg(87) residue pairs are relatively widely separated on opposite sides of the probable substrate translocation pore. The results of the present study demonstrate the critical structural contribution of several glycine residues in each NS at sites of close helix packing. Given the relative locations of Asn(168)/Arg(368)and Asn(459)/Arg(87)pairs, the validity of the models and possible role of the NSs together with the substrate-binding arginine residues are discussed.

Isotopic signature of nitrate in river waters of the lower Mississippi and its distributary, the Atchafalaya

AbstractPeriodic summer hypoxia occurring in the Northern Gulf of Mexico has been attributed to large nutrient inputs, especially nitrate‐nitrogen, from the Mississippi–Atchafalaya River system. The 2008 Gulf Hypoxia Action Plan calls for river corridor wetland restoration to reduce nitrate loads, but it is largely unknown how effective riverine wetland systems in the lower Mississippi River are for nitrate removal. We carried out an intensive isotope study to address this question by comparing nitrate isotopic signatures of the well‐channelized Mississippi River with those of the Atchafalaya River, which has extensive floodplains and backwater swamps. We investigated changes in δ15NNO3 and δ18ONO3 for water samples collected biweekly to monthly over a 2‐year period at the Atchafalaya River outlets (Morgan City and Wax Lake) and on the Mississippi River at Baton Rouge. In addition, in situ water quality parameters including temperature, dissolved oxygen and pH were recorded for each sampling date. Waters from both rivers showed moderately high nitrate concentration (>1 mg l−1) and undetectable (< 0.01 mg l−1) nitrite throughout the study period. The Mississippi River had slightly, but significantly higher (p=0.01) mean nitrate concentrations (1.5 mg l−1) and higher δ15NNO3 (7.7‰) than the Atchafalaya (1.1 mg l−1, 7.0‰); while no difference in δ18ONO3 (4.6‰) was found between the rivers. Flux‐weighted mean isotope values were overall lower than mean values for both the Mississippi and Atchafalaya Rivers, with a greater difference between the two rivers (7.4‰ versus 6.5‰, respectively). River flooding and hurricane storm surge also appeared to affect nitrate isotopic values. The lack of large difference in isotopic values between the Atchafalaya and Mississippi Rivers suggests that the majority of nitrate is transported through the Atchafalaya River with relatively little processing, and that riverine floodplains and wetlands are not effective sinks for nitrate, as previously assumed, because of insufficient residence time and well‐oxygenated river waters. Copyright © 2012 John Wiley & Sons, Ltd.

A simple method for the removal of dissolved organic matter and δ15N analysis of NO3– from freshwater

RATIONALEStable isotopes of nitrogen in nitrate (NO3–) are frequently used to identify nitrate sources and to study nitrogen (N) transformation processes, but the measurement methods available are generally rather labor intensive and/or costly, and dissolved organic matter (DOM) can interfere with the δ15N signature of nitrate. We therefore have developed a simple cleanup procedure for freshwater samples with low nitrate and high DOM concentrations.METHODSNitrate and DOM are extracted from a freeze‐dried water sample by using a concentrated sodium hydroxide solution. By the subsequent addition of acetone, two liquid layers are formed, and nitrate migrates into the acetone while DOM remains in the concentrated NaOH solution, thus separating the nitrate from the DOM. For nitrogen isotope analysis, purified nitrate salts are combusted at 1030 °C to produce N2 gas in an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS).RESULTSWith this novel technique up to 99% of DOM could be removed from river water and soil solutions. The method has been tested for sample amounts as small as 4 µmol NO3– with a precision of <0.1‰ (1SD). Nitrate standards are reproduced accurately without any blank correction.CONCLUSIONSThe benefits of this method are the lack of interferences derived from DOM on the δ15N signature and the ease of sample preparation. Copyright © 2012 John Wiley & Sons, Ltd.

Sustained losses of bioavailable nitrogen from montane tropical forests

Humid montane tropical forests are often thought to contain low levels of bioavailable nitrogen. An analysis of the concentration and isotopic signature of nitrate in tropical montane forest streams suggests that these ecosystems may be rich in nitrogen.

Major Issues Regarding the Efficiency of Monitoring Programs for Nitrate Contaminated Groundwater

Major issues regarding the efficiency of moni toring programs for nitrate contaminated groundwater are analyzed in this paper: (i) representativeness of monitoring networks; (ii) correct interpretation of the monitoring data and resulting time series and trends; and (iii) differentiation among the different sources of nitrates in groundwater. Following an overview of the nitrate contamination problem and possible solutions, as well as some of the difficulties found, a relatively straightforward method for assessing monitoring network representativity is presented, namely interpolation standard error assessment. It is shown how nitrate-concentration time series resulting from periodic observations can be corrected with a conservative tracer, in order to avoid misinterpretation and confirm or correct apparent trends. Finally, coupled ¹⁵N and ¹⁸O isotope signatures of nitrate (NO₃⁻) in groundwater are used to differentiate among nitrogen (N) sources, to ensure correct targeting of restoration measures. The case study regards a Nitrate Vulnerable Zone in the south of Portugal, designated in compliance with the European Nitrates Directive, where coastal discharge of nutrient-rich groundwater threatens the good qualitative and ecological status of the Ria Formosa coastal lagoon. Results show that mineral fertilizer is the main source of N in groundwater, and that increases in N load can be masked by dilution phenomena.

Comparing isotopic groundwater age measurements with simulated groundwater ages: example of the Abbotsford–Sumas Aquifer (USA and Canada) and application

AbstractElevated nitrate concentrations have been documented since the 1970s within the trans‐national Abbotsford–Sumas aquifer, situated in the central Fraser Valley of southern British Columbia, Canada, and northern Washington State, United States. Nitrate concentrations in excess of 10 mg/L NO3–N are commonly observed in the monitoring wells. A groundwater model is used in this study to conduct a particle tracking analysis to estimate groundwater travel times and compare these with groundwater ages determined from 3H/3He concentrations measured in several monitoring wells. Groundwater ages estimated from particle tracking show excellent agreement with measured ages (slope=1), although there is scatter (R2=0.76), reflecting the complexity of the travel pathways and ambiguities in groundwater ages because of mixing. An assessment of the efficacy of recommended nutrient management practices, which were implemented in 1992 to reduce nitrogen loading, suggests that wells shallower than about 20 m should be monitoring a decrease in nitrate concentration. However, persistent elevated nitrate concentrations even at shallow depths point to the lingering effects of a remnant manure–nitrate signature in combination with continued high loading.

Oxygen exchange with water alters the oxygen isotopic signature of nitrate in soil ecosystems

Combined oxygen (O) and nitrogen (N) stable isotope analyses are commonly used in the source determination of nitrate (NO3−). The source and fate of NO3− are studied based on distinct O and N isotopic signatures (δ18O and δ15N) of various sources and isotopic effects during NO3− transformation processes, which differ between sources like fertilizer, atmospheric deposition, and microbial production (nitrification). Isotopic fractionation during production and consumption of NO3− further affects the δ18O and δ15N signal. Regarding the δ18O in particular, biochemical O exchange between O from NO3− and H2O is implicitly assumed not to affect the δ18O signature of NO3−. This study aimed to test this assumption in soil-based systems. In a short (24 h) incubation experiment, soils were treated with artificially 18O and 15N enriched NO3−. Production of NO3− from nitrification during the incubation would affect both the 18O and the 15N enrichment. Oxygen exchange could therefore be studied by examining the change in 18O relative to the 15N. In two out of the three soils, we found that the imposed 18O enrichment of the NO3− declined relatively more than the imposed N15–NO3− enrichment. This implies that O exchange indeed affected the O isotopic signature of NO3−, which has important implications for NO3− source determination studies. We suggest that O exchange between NO3− and H2O should be taken into consideration when interpreting the O isotopic signature to study the origin and fate of NO3− in ecosystems.

Impact of diazotrophy on N stable isotope signatures of nitrate and particulate organic nitrogen: case studies in the north-eastern tropical Atlantic Ocean

During two independent cruises in the north-eastern tropical Atlantic Ocean, we applied two different approaches to investigate the impact of diazotrophy on nitrogen stable isotope signatures in nitrate and particulate organic nitrogen (PON) of the food-web constituents. The first approach, used during the Poseidon cruise 348 in the Mauritanian upwelling, investigated the long-term influence of diazotrophy on the natural abundance of δ15N-NO− 3 and PON. The second approach, adopted during the Cape Verde field cruise, applied stable isotope tracer addition experiments. These served to determine the instantaneous transfer of diazotrophic N to the higher trophic level. Both approaches showed that N2 fixation was compatible with the pattern and the magnitude of the isotopic depletion of dissolved NO− 3 during the Mauritanian upwelling cruise, as well as PON in zooplankton and phytoplankton during the Cape Verde cruises. An N-budget using 15N incorporation rates and diazotrophic N2 fixation rates showed that 6 % of the daily N2 fixation was potentially taken up by the mesozooplankton community. Direct grazing accounted for 56 % of gross mesozooplanktonic N incorporation, while 46 % occurred due to channelling through the microbial loop.

Sources of aerosol nitrate to the Gulf of Aqaba: Evidence from δ 15N and δ 18O of nitrate and trace metal chemistry

The nitrogen (N) and oxygen (O) isotopic composition (δ 15N and δ 18O) of water soluble aerosol nitrate was measured in aerosol samples collected in Eilat, Israel, from August 2003 to November 2004. During this period δ 15N values ranged from − 6.9‰ to + 1.9‰ and δ 18O from + 65.1‰ to + 84.9‰ and exhibited strong seasonal variability with higher average δ 15N values observed in the summer and higher δ 18O values in the winter. Nitrate isotopic composition was compared with bulk chemical composition and extractable ion and trace metals on co-collected samples linking nitrate isotopic composition to various sources of aerosols to this region. Atmospheric processes impacting the isotopic signatures of nitrate were also considered. Based on back trajectory analyses, the majority of NO 3 − came from air masses originating over the Mediterranean Sea (34%), Western Europe (20%) and the local Negev desert (19%), which contain a larger anthropogenic imprint compared to southern and eastern air masses which are dominated by mineral dust. The potential role of reactive mineral dust aerosols as a regulator of NO 3 − isotopic composition is considered; however, based on factor analysis, neither δ 15N nor δ 18O were associated with mineral dust components (such as Fe or Al), but rather with anthropogenic indicators such as Cu, Cd, P and Pb. Seasonality in primary NO x cycling reactions driven by seasonal changes in solar radiation, relative humidity and temperature also influence the observed isotopic signatures. The isotope data, together with trace element analysis, suggests that seasonal variations in both δ 15N NO3 and δ 18O NO3 are related to both NO x source and transport processes as well as NO x chemical reactions in the atmosphere. The flux-weighted δ 15N of aerosol NO 3 − in this area averaged − 2.6‰ making aerosol deposition a substantial contributor of low δ 15N nitrogen to the oligotrophic waters of the Gulf of Aqaba. Thus, while the flux of atmospheric N to oligotrophic marine systems is smaller than the upward flux of NO 3 − from deep water, it nonetheless represents an important source of new N having a low δ 15N. Further, if this low δ 15N signature is not considered, it could interfere with N-fixation estimates based on isotopic composition of dissolved nitrate or particulate organic nitrogen. Thus, atmospheric deposition should be constrained for accurate estimates of marine N-fixation when based on δ 15N in the ocean. Indeed, in the Gulf of Aqaba, low upper water δ 15N NO3 values could be related to inputs of atmospheric NO 3 − as well as N-fixation.

A single channel for nitrate uptake, nitrite export and nitrite uptake by Escherichia coli NarU and a role for NirC in nitrite export and uptake

Two related polytopic membrane proteins of the major facilitator family, NarK and NarU, catalyse nitrate uptake, nitrite export and nitrite uptake across the Escherichia coli cytoplasmic membrane by an unknown mechanism. A 12-helix model of NarU was constructed based upon six alkaline phosphatase and beta-galactosidase fusions to NarK and the predicted hydropathy for the NarK family. Fifteen residues conserved in the NarK-NarU protein family were substituted by site-directed mutagenesis, including four residues that are essential for nitrate uptake by Aspergillus nidulans: arginines Arg(87) and Arg(303) in helices 2 and 8, and two glycines in a nitrate signature motif. Despite the wide range of substitutions studied, in no case did mutation result in loss of one biochemical function without simultaneous loss of all other functions. A NarU+ NirC+ strain grew more rapidly and accumulated nitrite more rapidly than the isogenic NarU+ NirC(-) strain. Only the NirC+ strain consumed nitrite rapidly during the later stages of growth. Under conditions in which the rate of nitrite reduction was limited by the rate of nitrite uptake, NirC+ strains reduced nitrite up to 10 times more rapidly than isogenic NarU+ strains, indicating that both nitrite efflux and nitrite uptake are largely dependent on NirC. Isotope tracer experiments with [15N]nitrate and [14N]nitrite revealed that [15N]nitrite accumulated in the extracellular medium even when there was a net rate of nitrite uptake and reduction. We propose that NarU functions as a single channel for nitrate uptake and nitrite expulsion, either as a nitrate-nitrite antiporter, or more likely as a nitrate/H+ or nitrite/H+ channel.

Africa, Asia and the Middle East

Many reconstructions of past biogeochemical states rely on proxies such as δ15N that in turn are affected by the preservation state of organic matter. N-turnover processes in the upwelling system on the Namibian shelf are difficult to reconstruct because lateral particle advection and differential degradation under oxic to anoxic conditions complicate particle tracing in suspended matter and surface sediments as well as the interpretation of sedimentary δ15N records. We analysed the amino acid composition of suspended matter, sinking particles, phytoplankton biomass and surface sediments in order to investigate the degradation pathway of organic matter from the sea surface to the surface sediment. A principal component analysis carried out with the amino acid data set results in a triangular-shaped model. Two branches evolve from phytoplankton biomass on the apex: The suspended matter branch is characterized by relatively enhanced contents of Glu, Ser and Leu, whereas the non-protein amino acids and Lys mark the sinking particle and sediment branch. The results suggest that both pools have taken different diagenetic pathways and that there is only limited exchange between them. The preservation state of organic matter in surface sediments within the oxygen minimum zone is generally better than on the oxic outer shelf, where organic matter preservation depends on water depth.A vertical increase in δ15N ratios from phytoplankton biomass to the surface sediment by ~ 2.5‰ is attributed to early diagenetic degradation in the water column and at the water/sediment interface. The δ15N ratios of sediments are mainly controlled by the N-isotope signature of nitrate on the outer shelf and upper continental slope. Nutrient-enriched deep water ascends at the shelf breaks due to the influence of internal waves and tides or curl-driven shelf break upwelling. The δ15N of upwelling nitrate at the shelf breaks is ~ 3‰ lower than that of nitrate from the inner shelf. This difference in nitrate sources is expressed in a band of decreasing δ15N ratios of surface sediments at the shelf breaks. The hydrodynamic upwelling regime at the shelf breaks flushes significant amounts of suspended matter from the mesopelagic ocean across the shelf break as indicated by amino acid monomer patterns of suspended matter.

Isotopic signature of nitrate in two contrasting watersheds of Brush Brook, Vermont, USA

We used the dual isotope method to study differences in nitrate export in two subwatersheds in Vermont, USA. Precipitation, soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green Mountains of Vermont. These samples were analyzed for the δ15N and δ18O of NO3−. The range of δ15N–NO3− values overlapped, with precipitation −4.5‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ18O of precipitation NO3− (mean 46.8 ± 11.5‰) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3‰) and soil waters (mean 14.5 ± 4.2‰) even during snowmelt periods. Extracted soil solution and streamwater δ18O of NO3− were similar and within the established range of microbially produced NO3−, demonstrating that NO3− was formed by microbial processes. The δ15N and δ18O of NO3− suggests that although the two tributaries have different seasonal NO3− concentrations, they have a similar NO3− source.

Source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments

Abstract. Nitrate input to a river is largely controlled by land use in its catchment. We compared the information carried by the isotopic signatures of nitrate in 12 Baltic rivers, in relation to the vegetation cover, land use, and fertilization of agricultural land of their catchments. We found isotope values in nitrate ranging from −2 to 14‰ for δ15N and 8 to 25‰ for δ18O. The annual variability of riverine nitrate isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, the Vistula and Oder. Nordic rivers with relatively pristine vegetation in their catchments show not only low δ15N values and high δ18O-NO3− but also lower annual variability than rivers draining densely populated land. Seasonal signals were found in all the rivers. We used load weighted nitrate isotope data and data from the three major N sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results of an isotope mixing model (IMM-1) agree reasonably well with the same estimates for agricultural land derived from a Global Land Cover (GLC) data base, with a deviation varying from −16% to +26%. The comparison with an emission model (EM) reveals relatively good agreements for intensively used catchments (−18 to +18% deviation). Rather unsatisfactory agreement was found between the IMM-1 and GLC calculations for pristine catchments (−36 to +50% deviation). Advantages and limitations of the tested model are discussed.

Trends in Nitrate Concentrations and Determination of its Origin Using Stable Isotopes (18O and15N) in Groundwater of the Western Central Valley, Costa Rica

A study was conducted to evaluate long-term trends in nitrate concentrations and to try to identify the origin of nitrate using stable isotopes (15N(NO3-) and 18O(NO3-)) in the aquifers of the western Central Valley, Costa Rica, where more than 1 million people depend on groundwater to satisfy their daily needs. Data from 20 sites periodically sampled for 4 to 17 years indicate an increasing trend in nitrate concentrations at five sites, which in a period ranging from 10 to 40 years, will exceed recommended maximum concentrations. Results of isotopic analysis indicate a correspondence between land use patterns and the isotopic signature of nitrate in groundwater and suggest that urbanization processes without adequate waste disposal systems, followed by coffee fertilization practices, are threatening water quality in the region. We conclude that groundwater management in this area is not sustainable, and that land use substitution processes from agricultural activity to residential occupation that do not have proper sewage disposal systems may cause a significant increment in the nitrate contaminant load.

Nitrogen Removal in a Small Constructed Wetland: An Isotope Mass Balance Approach

The nitrogen (N) removal potential of constructed wetlands is increasingly used to lower the N load from agricultural nonpoint sources to inland and coastal waters. To determine the removal efficiency and key factors limiting wetland N removal, N fluxes were studied in a small constructed wetland in Central Switzerland. With an isotope mass balance approach integrating the natural isotope signature of nitrate (NO3-, ammonium (NH4+), and particulate nitrogen (PN), the N transformations such as assimilation, mineralization, nitrification, and denitrification were quantified. On average, the wetland removed 45 g m(-2) yr(-1) N during the studied 2.5 years, corresponding to a removal efficiency of 27%. Denitrification contributed 94% to the N removal, while only 6% of the removed N accumulated in the sediments. Denitrification was most efficient during periods with an oxic water column overlying anoxic sediments, as NH4+ released during mineralization of sediment organic matter was completely nitrified and subsequently denitrified at the sediment-water interface. During water column anoxia, NH4+ accumulated in the water and fueled assimilation by duckweed and internal recycling. The NO3-N isotope signature in the wetland mainly reflected the mineralization of sediment organic matter and subsequent nitrification, while denitrification at the sediment-water interface produced no fractionation.

Missense mutations that inactivate the Aspergillus nidulans nrtA gene encoding a high-affinity nitrate transporter.

The transport of nitrate into prokaryotic and eukaryotic cells, of considerable interest to agriculture, ecology, and human health, is carried out by members of a distinct cluster of proteins within the major facilitator superfamily. To obtain structure/function information on this important class of nitrate permeases, a collection of chemically induced mutations in the nrtA gene encoding a 12-transmembrane domain, high-affinity nitrate transporter from the eukaryote Aspergillus nidulans was isolated and characterized. This mutational analysis, coupled with protein alignments, demonstrates the utility of the approach to predicting peptide motifs and individual residues important for the movement of nitrate across the membrane. These include the highly conserved nitrate signature motif (residues 166-173) in Tm 5, the conserved charged residues Arg87 (Tm 2) and Arg368 (Tm 8), as well as the aromatic residue Phe47 (Tm 1), all within transmembrane helices. No mutations were observed in the large central loop (Lp 6/7) between Tm 6 and Tm 7. Finally, the study of a strain with a conversion of Trp481 (Tm 12) to a stop codon suggests that all 12 transmembrane domains and/or the C-terminal tail are required for membrane insertion and/or stability of NrtA.

Two perfectly conserved arginine residues are required for substrate binding in a high-affinity nitrate transporter.

This study represents the first attempt to investigate the molecular mechanisms by which nitrate, an anion of significant ecological, agricultural, and medical importance, is transported into cells by high-affinity nitrate transporters. Two charged residues, R87 and R368, located within hydrophobic transmembrane domains 2 and 8, respectively, are conserved in all 52 high-affinity nitrate transporters sequenced thus far. Site-directed replacements of either of R87 or R368 residues by lysine were found to be tolerated, but such residue changes increased the K(m) for nitrate influx from micromolar to millimolar values. Seven other amino acid substitutions of R87 or R368 all led to loss of function and lack of growth on nitrate. No evidence was obtained of R87 or R368 forming a salt-bridge with conserved acidic residues. Remarkably, the phenotype of loss-of-function mutant R87T was found to be alleviated by an alteration to lysine of N459, present in the second copy of the nitrate signature (transmembrane domain 11), suggesting a structural or functional interplay between residues R87 and N459 in the three-dimensional NrtA protein structure. Failure of the potential reciprocal second site suppressor N168K (in the first nitrate signature copy of transmembrane domain 5) to revert R368T was observed. Taken with recent structural studies of other major facilitator superfamily proteins, the results suggest that R87 and R368 are involved in substrate binding and probably located in a region of the protein close to N459.

Effect of storage on the isotopic composition of nitrate in bulk precipitation.

Stable isotopic analysis of atmospheric nitrate is increasingly employed to study nitrate sources and transformations in forested catchments. Large volumes have typically been required for delta18O and delta15N analysis of nitrate in precipitation due to relatively low nitrate concentrations. Having bulk collectors accumulate precipitation over an extended time period allows for collection of the required volume as well as reducing the total number of analyses needed to determine the isotopic composition of mean annual nitrate deposition. However, unfiltered precipitation left in collectors might be subject to microbial reactions that can alter the isotopic signature of nitrate in the sample. Precipitation obtained from the Turkey Lakes Watershed was incubated under conditions designed to mimic unfiltered storage in bulk precipitation collectors and monitored for changes in nitrate concentration, delta15N, and delta18O. Results of this experiment indicated that no detectable nitrate production or assimilation occurred in the samples during a two-week incubation period and that atmospheric nitrate isotopic ratios were preserved. The ability to collect unfiltered precipitation samples for an extended duration without alteration of nitrate isotope ratios is particularly useful at remote study sites where daily retrieval of samples may not be feasible.

Nitrogen isotope hydrochemistry and denitrification within the Chalk aquifer system of north Norfolk, UK

Although there has been much debate over the potential health risks of nitrate in drinking water, there is a real issue of the costs associated with removing nitrate from drinking water supplies. In the Chalk aquifer system of north-east Norfolk, modern contaminants of a mainly agricultural origin produce high levels of nitrate (>15mg/l NO3-N) in the unconfined valley areas, whereas in confined regions the levels of nitrate are low and commonly below detection limits (<0.04mg/l NO3-N). To understand the source and fate of nitrate within this aquifer system, a detailed hydrochemical sampling programme has been completed in the River Bure catchment. Nitrogen isotope values (δ15N) for nitrate within the unconfined and semi-confined zones range between +4‰ and +10‰, characteristic of nitrified soil organic nitrogen. However, many Chalk groundwaters possess high N2/Ar ratios (39–72) indicating a significant contribution to dissolved N2 from denitrification. Denitrification is believed to be occurring within the overlying glacial deposits, providing a mechanism for naturally improving groundwater quality. δ15N values of low-nitrate groundwaters from the confined zone are isotopically light (−3‰ to +4‰), inconsistent with an origin from denitrification: it is suggested that these waters have a pre-anthropogenic nitrate signature.