Definitive 15N NMR evidence that water serves as a source of 'O' during nitrite oxidation by Nitrobacter agilis

Improving the accuracy of nitrogen estimates from nonpoint source in a river catchment with multi-isotope tracers

Climate change can affect precipitation patterns, temperature, and the hydrological cycle, consequently influencing the dynamics of nitrogen (N) within aquatic ecosystems. In this study, multiple stable isotopes (15N-NO3/18O-NO3 and 2H-H2O/18O-H2O) were used to investigate the N sources and flowpath within the Bogang stream in South Korea. Within the vicinity of the stream with complex land use where various N sources were present, four end-members (rainfall, soil, sewage, and livestock) were sampled and examined. Consequently, spatial-temporal variations of the N sources were observed dependent on the type of land use. During the dry season, sewage accounted for the dominant N source, ranging from 62.2 % to 80.2 %. In contrast, nonpoint sources increased significantly across most sites during the wet season (10.3-41.6 % for soil; 6.3-35.2 % for livestock) compared to the dry season (7.7-28.5 % for soil; 6-13.2 % for livestock). However, sewage (78.7 %) remains dominant, representing the largest ratio at the site downstream of the wastewater treatment plant during the wet season. This ratio showed a notable difference from the calculated N loading ratio of 52.2 %, especially for livestock. This suggests that a significant potential for N legacy effects, given that groundwater flow is likely to be the primary hydrological pathway delivering N to rivers. This study will help to develop water resource management strategies by understanding how the interaction between N sources and hydrological process responds to climate change within sub-basins.

Bringing Nitric Oxide to the Molybdenum World-A Personal Perspective

Molybdenum-containing enzymes of the xanthine oxidase (XO) family are well known to catalyse oxygen atom transfer reactions, with the great majority of the characterised enzymes catalysing the insertion of an oxygen atom into the substrate. Although some family members are known to catalyse the "reverse" reaction, the capability to abstract an oxygen atom from the substrate molecule is not generally recognised for these enzymes. Hence, it was with surprise and scepticism that the "molybdenum community" noticed the reports on the mammalian XO capability to catalyse the oxygen atom abstraction of nitrite to form nitric oxide (NO). The lack of precedent for a molybdenum- (or tungsten) containing nitrite reductase on the nitrogen biogeochemical cycle contributed also to the scepticism. It took several kinetic, spectroscopic and mechanistic studies on enzymes of the XO family and also of sulfite oxidase and DMSO reductase families to finally have wide recognition of the molybdoenzymes' ability to form NO from nitrite. Herein, integrated in a collection of "personal views" edited by Professor Ralf Mendel, is an overview of my personal journey on the XO and aldehyde oxidase-catalysed nitrite reduction to NO. The main research findings and the path followed to establish XO and AO as competent nitrite reductases are reviewed. The evidence suggesting that these enzymes are probable players of the mammalian NO metabolism is also discussed.

Identification of nitrate sources in tap water sources across South Korea using multiple stable isotopes: Implications for land use and water management

Stable nitrate isotopes (δ¹⁵N-NO₃ and δ¹⁸O-NO₃) in conjunction with stable water isotopes (δ¹⁸O-H₂O and δD-H₂O) were used to identify nitrogen (N) sources and N-biogeochemical transformation in tap water sources sampled from 11 water purification plants across South Korea. The raw water sources are taken from rivers within the water supply basins, which indicates the quality of tap water is highly dependent on surrounding the land use type. We estimated the proportional contribution of the various N sources (AD: atmospheric deposition; SN: soil nitrogen; CF: chemical fertilizer; M&S: manure/sewage) using Bayesian Mixing Model. As a result, the contribution of N sources exhibited large seasonal and spatial differences, which were related to the type of land use in the water supply basins. Commonly, the M&S and SN were the dominant N source during the dry and wet seasons in almost regions, respectively. However, in the regions with high N loading ratios from urban and industrial sources, the M&S was the dominant N source during both the wet and dry seasons. In addition, the regions were characterized by high NO₃^- concentrations due to the decreased dilution effect of precipitation during the dry seasons. In contrast, the SN was the dominant N source in the regions with high N loading ratios from agricultural areas during both the wet and dry seasons. The NO₃^--N concentration during the wet season was significantly higher than those during the dry season in these regions due to the input of non-point sources with high concentrations. Meanwhile, denitrification and nitrification were observed in the watersheds. It is important to understand the isotope fractionation due to N-biogeochemical transformation for considering the potential misinterpretations of the origin and fate NO₃^-. Collectively, our findings provide a basis on N source control strategies to ensure tap water quality in complex land use areas.

Deciphering the nitrate sources and processes in the Ganga river using dual isotopes of nitrate and Bayesian mixing model

The dual isotopes of dissolved NO₃- (n = 43) has been used to delineate the nitrate sources and N-cycling processes in the Ganga river. The proportional contribution of nitrate from different sources has been estimated using the Bayesian mixing model. The seasonal NO₃- concentration in the lower stretch of the river Ganga varied between 4.1 and 64.1 μM with higher concentration during monsoon and post-monsoon season and lower concentration during the pre-monsoon and winter season. The temporal variation in the isotopic values ranged between +0.0 and +9.6‰ for δ¹⁵N_NO3- and -1.2 to +11.0‰ for δ¹⁸O_NO3-. The spatial NO₃- concentration during the post-monsoon season varied between 23.2 and 57.7 μM, with higher values from the middle and lower values from the lower stretch of the river Ganga. The isotopic ratio during the post-monsoon season varied between -1.0 and +11.3‰ for δ¹⁵N_NO3- and -4.6 to +5.2‰ for δ¹⁸O_NO3-. The temporal dataset from the lower stretch of the river Ganga showed the dominance of nitrate derived from the nitrification of soil organic matter (SOM) (average ∼53.4%). The nitrate contribution from synthetic fertilizers was observed to be higher during the post-monsoon season (34.7 ± 23.4%) compared to that in the monsoon (25.5 ± 19.5%) and pre-monsoon (22.2 ± 19.6%) season. No significant seasonal variations were observed in the nitrate input from manure/sewage (∼13.9%). Spatial samples collected during the post-monsoon season showed higher contribution of synthetic fertilizer in the lower stretch (34.6 ± 22.7%) compared to the middle stretch (21.1 ± 18.2%), which indicates greater influence of the agricultural activity in the lower stretch. The dual isotope study of dissolved NO₃- established that the nitrate in the Ganga river water is mostly derived from the nitrification of incoming organic compounds and is subsequently removed via assimilatory nitrate uptake. The study also emphasises significant nitrification and assimilatory nitrate removal processes operating in the mixing zone of the Ganga river and Hooghly estuary.

Pathways and efficiency of nitrogen attenuation in wastewater effluent through soil aquifer treatment

Soil Aquifer Treatment (SAT) is used to increase groundwater resources and enhance the water quality of wastewater treatment plant (WWTP) effluents. The resulting water quality needs to be assessed. In this study, we investigate attenuation pathways of nitrogen (N) compounds (predominantly NH₄⁺) from a secondary treatment effluent in pilot SAT systems: both a conventional one (SAT-Control system) and one operating with a permeable reactive barrier (PRB) to provide extra dissolved organic carbon to the recharged water. The goal is to evaluate the effectiveness of the two systems regarding N compounds by means of chemical and isotopic tools. Water chemistry (NO₃^-, NH₄⁺, Non-Purgeable Dissolved Organic Carbon (NPDOC), and O₂) and isotopic composition of NO₃^- (ẟ¹⁵N-NO₃^- and ẟ¹⁸O-NO₃^-) and NH₄⁺ (ẟ¹⁵N-NH₄⁺) were monitored in the inflow and at three different sections and depths along the aquifer flow path. Chemical and isotopic results suggest that coupled nitrification-denitrification were the principal mechanisms responsible for the migration and distribution of inorganic N in the systems and that nitrification rate decreased with depth. At the end of the study period, 66% of the total N in the solution was removed in the SAT-PRB system and 69% in the SAT-Control system, measured at the outlet of the systems. The residual N in solution in the SAT-PRB system had an approximately equal proportion of N-NH₄⁺ and N-NO₃^- while in the SAT-Control system, the residual N in solution was primarily N-NO₃^-. Isotopic data also confirmed complete NO₃^- degradation in the systems from July to September with the possibility of mixing newly generated NO₃^- with the residual NO₃^- in the substrate pool.

Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district

The application of N fertilisers to enhance crop yield is common throughout the world. Many crops have historically been, or are still, fertilised with N in excess of the crop requirements. A portion of the excess N is transported into underlying aquifers in the form of NO₃^-, which is potentially discharged to surface waters. Denitrification can reduce the severity of NO₃^- export from groundwater. We sought to understand the occurrence and hydrogeochemical controls on denitrification in NO₃^--rich aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia, a region of extensive cotton and cereal production. Multiple stable isotope (in H₂O, NO₃^-, DIC, DOC and SO₄^2-) and radioactive isotope (³H and ³⁶Cl) tracers were used to develop a conceptual N process model. Fertiliser-derived N is likely incorporated and retained in the soil organic N pool prior to its mineralisation, nitrification, and migration into aquifers. This process, alongside the near absence of other anthropogenic N sources, results in a homogenised groundwater NO₃^- isotopic signature that allows for denitrification trends to be distinguished. Regional-scale denitrification manifests as groundwater becomes increasingly anaerobic during flow from an upgradient basalt aquifer to a downgradient alluvial aquifer. Dilution and denitrification occurs in localised electron donor-rich suboxic hyporheic zones beneath leaking irrigation channels. Using approximated isotope enrichment factors, estimates of regional-scale NO₃^- removal ranges from 22 to 93% (average: 63%), and from 57 to 91% (average: 79%) beneath leaking irrigation channels. In the predominantly oxic upgradient basalt aquifer, raised groundwater tables create pathways for NO₃^- to be transported to adjacent surface waters. In the alluvial aquifer, the transfer of NO₃^- is limited both physically (through groundwater-surface water disconnection) and chemically (through denitrification). These observations underscore the need to understand regional- and local-scale hydrogeological processes when assessing the impacts of groundwater NO₃^- on adjacent and end of system ecosystems.

Analysis of nitrite oxidation process and nitrification performance by nitrogen and oxygen isotope fractionation effect

The N and O isotope fractionation effects in NO₂^--N oxidation and nitrification performance of an activated sludge system treating municipal wastewater are unknown. The nitrifying sludge was cultured under different temperature (33 ± 1 °C, 25 ± 1 °C,and 18 ± 1 °C) and dissolved oxygen (DO: 0.5-1 mg/L, 3-4 mg/L, and 7-8 mg/L). The inverse kinetic isotope effects of N and O (¹⁵ε_NO2 and ¹⁸ε_NO2) were -0.62‰ to -7.08‰ and -0.87‰ to -1.68‰ in the process of NO₂^--N oxidation, respectively. ¹⁵ε_NO3 gradually increased with increasing of temperature (¹⁵ε_NO3-33°C (14.49‰) > ¹⁵ε_NO3-25°C (10.43‰) > ¹⁵ε_NO3-18°C (7.3‰)), while the ¹⁵ε_NO3:¹⁸ε_NO3 was maintained at 1.02-5.32. The increase of temperature improved the nitrification activity, which promoted the fractionation effect, but the change of DO did not highlight this difference. The exchange of NO₂^--N and H₂O (X_NOB) was 32.5 ± 1.5%, and the kinetic isotope effect of H₂O participating in the reaction (¹⁸ε_{k, H2O, 2}) was 22.57 ± 1.79‰, indicating that H₂O was involved in the NO₂^--N oxidation rather than DO. In summary, the elevated temperature enhanced the fractionation effect of NO₂^--N oxidation. This study provides a new perspective to reveal the mechanism of NO₂^--N oxidation, optimize the process of nitrogen removal from wastewater and further control water eutrophication.

Elucidating heterogeneous nitrate contamination in a small basement aquifer. A multidisciplinary approach: NO3 isotopes, CFCs-SF6, microbiological activity, geophysics and hydrogeology

Nitrate contamination of groundwater remains a major concern despite all the measures and efforts undertaken over the last decades to protect water resources. We focused on a small catchment in Brittany (France) facing nitrate pollution with concentrations over the European drinking water standard of 50 mg.L^-1. This is a common situation in catchments where - supposedly effective - measures were applied for reducing the transfer of N to groundwater. At the scale of this small (~100 ha) basement aquifer, nitrate concentrations are very heterogeneous in the groundwater, sampled up to 15-20 m below the soil surface in several observation wells (hereafter referred as piezometers) and up to 110 m deep in a borehole drilled through a faulted area near the Spring (outlet of the catchment). We used complementary and robust approaches for exploring and constraining the driving parameters of nitrate transfer and distribution in groundwater. Detailed geological work and a geophysical electrical resistivity tomography survey identified the lithologies, tectonic structures and weathering layers. This highlighted a complex geological structure with several compartments delimited by faults, as well as the highly variable thickness of the weathered layer. It also illustrated the heterogeneity of the hydrosystem, some compartments appearing to be disconnected from the general groundwater flow. This was confirmed by geochemical analyses and by the mean apparent groundwater residence time based on CFCs-SF6 and noble-gas analyses, locally revealing old and nitrate-free groundwater, and very old water with a recharge temperature below than the current average temperature in the area, reflecting water dating back to the last period of glaciation (-19 to -17 ky). Nitrate isotopes clearly showed denitrification processes in a few piezometers, which was generally supported by microbiology and molecular biology results. This highlighted the presence of functional genes involved in denitrification as well as a capacity of the groundwater microbial community to denitrify when in situ conditions are favourable. This type of combined approach - covering chemistry, isotopic methods, dissolved gases, microbiological activity, geophysics and hydrogeology - appears to be indispensable for implementing the most relevant programme of measures and for accurately assessing their effectiveness, notably by considering the timeframe between implementation of the measures and their impact on groundwater quality.

Identifying causes of poor water quality in a Polish agricultural catchment for designing effective and targeted mitigation measures

The Gowienica Miedwiańska catchment is a small agricultural catchment located in the NW of Poland draining into Lake Miedwie, on which a drinking water source for the city of Szczecin is located. The catchment is characterized by very rich soils. Subsequently, agriculture is intensive and this is thought to influence the poor water quality in the local area. Despite more than 20 years since first programmes of measures towards protection of water quality have been introduced into the catchment, these have not been produced the expected results, and the local farming community cites other sources such as poor sewage management rather that agricultural activity, as responsible for this problem. Evaluation of flow pathways in the catchment and identification of the areas responsible for the highest impact on local water quality was therefore conducted within the EU funded project Waterprotect. The aim of this study was to clarify sources of pollution precisely in space and time, in order to increase trust from stakeholders, so that targeted measures can be used effectively to improve water quality. The study included water quality monitoring, isotopic analysis and numerical flow modelling. Results showed that water quality in the catchment is spatially and temporally variable. 93% of nitrogen loadings into the Miedwie lake have been attributed to agriculture and only 7% to wastewater inputs. The local hydrology and hydrogeology play an important role in the distribution of the impacts from these inputs. As a result, three sub-catchments were identified which are differentiated by dominant pollution source, land use, and hydraulic characteristics. The highest inputs from agriculture have been identified in the most upper sub-catchment and this area have been pointed out as most suitable for implementation of agricultural best management practices towards protection of water quality at a local level.

Stable Isotopes of Water and Nitrate for the Identification of Groundwater Flowpaths: A Review

Nitrate contamination in stream water and groundwater is a serious environmental problem that arises in areas of high agricultural activities or high population density. It is therefore important to identify the source and flowpath of nitrate in water bodies. In recent decades, the dual isotope analysis (δ15N and δ18O) of nitrate has been widely applied to track contamination sources by taking advantage of the difference in nitrogen and oxygen isotope ratios for different sources. However, transformation processes of nitrogen compounds can change the isotopic composition of nitrate due to the various redox processes in the environment, which often makes it difficult to identify contaminant sources. To compensate for this, the stable water isotope of the H2O itself can be used to interpret the complex hydrological and hydrochemical processes for the movement of nitrate contaminants. Therefore, the present study aims at understanding the fundamental background of stable water and nitrate isotope analysis, including isotope fractionation, analytical methods such as nitrate concentration from samples, instrumentation, and the typical ranges of δ15N and δ18O from various nitrate sources. In addition, we discuss hydrograph separation using the oxygen and hydrogen isotopes of water in combination with the nitrogen and oxygen isotopes of nitrate to understand the relative contributions of precipitation and groundwater to stream water. This study will assist in understanding the groundwater flowpaths as well as tracking the sources of nitrate contamination using the stable isotope analysis in combination with nitrate and water.

Dual nitrogen and oxygen isotope fractionation during anaerobic ammonium oxidation by anammox bacteria

Natural abundance of stable nitrogen (N) and oxygen (O) isotopes are invaluable biogeochemical tracers for assessing the N transformations in the environment. To fully exploit these tracers, the N and O isotope effects (¹⁵ε and ¹⁸ε) associated with the respective nitrogen transformation processes must be known. However, the N and O isotope effects of anaerobic ammonium oxidation (anammox), one of the major fixed N sinks and NO₃^- producers, are not well known. Here, we report the dual N and O isotope effects associated with anammox by three different anammox bacteria including "Ca. Scalindua japonica", a putative marine species, which were measured in continuous enrichment culture experiments. All three anammox species yielded similar N isotope effects of NH₄⁺ oxidation to N₂ (¹⁵ε_NH4→N2) ranging from 30.9‰ to 32.7‰ and inverse kinetic isotope effects of NO₂^- oxidation to NO₃^- (¹⁵ε_NO2→NO3 = -45.3‰ to -30.1‰). In contrast, ¹⁵ε_NO2→N2 (NO₂^- reduction to N₂) were significantly different among three species, which is probably because individual anammox bacteria species might possess different types of nitrite reductase. We also report the combined O isotope effects for NO₂^- oxidation (¹⁸E_NO2→NO3) by anammox bacteria. These obtained dual N and O isotopic effects could provide significant insights into the contribution of anammox bacteria to the fixed N loss and NO₂^- reoxidation (N recycling) in various natural environments.

Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification

Measurements of the stable isotope ratios of nitrogen (¹⁵N/¹⁴N) and oxygen (¹⁸O/¹⁶O) in nitrate (NO₃^-) enable identification of sources, dispersal, and fate of natural and contaminant NO₃^- in aquatic environments. The ¹⁸O/¹⁶O of NO₃^- produced by nitrification is often assumed to reflect the proportional contribution of oxygen atom sources, water, and molecular oxygen, in a 2:1 ratio. Culture and seawater incubations, however, indicate oxygen isotopic equilibration between nitrite (NO₂^-) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO₃^{- 18}O/¹⁶O produced during nitrification. To investigate the influence of kinetic and equilibrium effects on the isotopic composition of NO₃^- produced from the nitrification of ammonia (NH₃), we incubated streamwater supplemented with ammonium (NH₄⁺) and increments of ¹⁸O-enriched water. Resulting NO₃^{- 18}O/¹⁶O ratios showed (1) a disproportionate sensitivity to the ¹⁸O/¹⁶O ratio of water, mediated by isotopic equilibration between water and NO₂^-, as well as (2) kinetic isotope discrimination during O atom incorporation from molecular oxygen and water. Empirically, the NO₃^{- 18}O/¹⁶O ratios thus produced fortuitously converge near the ¹⁸O/¹⁶O ratio of water. More elevated NO₃^{- 18}O/¹⁶O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO₃^- reduction.

Hydrogeological Controls on Regional-Scale Indirect Nitrous Oxide Emission Factors for Rivers

Indirect nitrous oxide (N₂O) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF_5r) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF_5r estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, N₂O and nitrate (NO₃^-) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median N₂O-N concentration (3.0 μg L^-1), EF_5r (0.00036), and N₂O-N flux (10.8 kg ha^-1 a^-1). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median N₂O-N concentration (0.8 μg L^-1), EF_5r (0.00016), and N₂O-N flux (2.6 kg ha^-1 a^-1), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high N₂O yield from high permeability chalk contrasting with significantly lower median N₂O-N concentration (0.7 μg L^-1), EF_5r (0.00020), and N₂O-N flux (2.0 kg ha^-1 a^-1) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF_5r can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale N₂O emission estimates.

Nitrogen, Sulfur, and Oxygen Isotope Ratios of Animal- and Plant-Based Organic Fertilizers Used in South Korea

Organic fertilizers are increasingly used in agriculture in Asia and elsewhere. Tracer techniques are desirable to distinguish the fate of nutrients added to agroecosystems with organic fertilizers from those contained in synthetic fertilizers. Therefore, we determined the nitrogen, sulfur, and oxygen isotope ratios of nitrogen- and sulfur-bearing compounds in animal- and plant-based organic fertilizers (ABOF and PBOF, respectively) used in South Korea to evaluate whether they are isotopically distinct. The δN values of total and organic nitrogen for ABOF ranged from +7 to +19‰ and were higher than those of PBOF (generally <+6‰). This suggests that ABOFs have distinct δN values suitable for tracing these fertilizer compounds in the plant-soil-water system, whereas PBOFs have similar δN values to synthetic fertilizers. However, δO values for nitrate (δO) from organic fertilizer samples (<+17.0‰) were consistently lower than those of synthetic nitrate-containing fertilizers. The δS values of total sulfur, organic sulfur compounds (e.g., carbon-bonded sulfur and hydriodic acid-reducible sulfur), and sulfate for ABOFs yielded wide and overlapping ranges of +0.3 to +6.3, +0.9 to +7.2, and -2.6 to +14.2‰, whereas those for PBOFs varied from -3.4 to +7.7, +1.4 to +9.4, and -4.1 to +12.5‰, respectively, making it challenging to distinguish the fate of sulfur compounds from ABOF and PBOF in the environment using sulfur isotopes. We conclude that the δN values of ABOFs and the O values of organic fertilizers are distinct from those of synthetic fertilizers and are a promising tool for tracing the fate of nutrients added by organic fertilizers to agroecosystems.

Isotopic evidence of nitrogen sources and nitrogen transformation in arsenic-contaminated groundwater

High concentrations of naturally occurring arsenic (As) are typically found in young alluvial and deltaic deposits, and high concentrations of ammonium (NH₄⁺) and nitrate (NO₃^-) are often present in groundwater affected by anthropogenic activities. In this study, on the basis of physicochemical characteristics of groundwater and the nitrogen and oxygen isotope composition of NO₃^-, it was inferred that the main sources of NO₃^- in the proximal fan of the Choushui River alluvial fan are likely to be ammonium fertilizers, manure, and septic waste; that in the mid-fan and the distal fan, the possible sources are nitrate fertilizers and marine nitrate. In the proximal fan, the oxidative state obviously promotes microbial nitrification. Denitrification occurs from the upstream region to the downstream region of the Choushui River, and therefore, the decrease in NO₃^- concentration along streams connecting the Choushui River to the ocean appears plausible. High DO concentrations and relatively low values of δ¹⁸O_NO3 in the deeper aquifer of the proximal fan may be attributed to unconfined granular nature and groundwater pumping by agricultural activities. In the mid-fan, NO₃^- assimilation is the dominant response to NO₃^- attenuation, and denitrification is insignificant; however, high concentrations of As, NH4⁺ and Fe and depletion of δ¹⁵N_NO3 imply the occurrence of feammox process. By contrast, denitrification evidently occurs in the distal fan, through assimilation, mineralization, and dissimilatory NO₃^- reduction to NH₄⁺, resulting in depletion of NO₃^- and increase in NH₄⁺ in groundwater. Feammox in the mid-fan and denitrification in the distal fan may be the main processes leading to the release of As from As-bearing Fe oxyhydroxides into groundwater.

Natural and anthropogenic variations in the Po river waters (northern Italy): insights from a multi-isotope approach

Po is the main Italian river and the δ(18)O and δ(2)H of its water reveal a similarity between the current meteoric fingerprint and that of the past represented by groundwater. As concerns the hydrochemisty, the Ca-HCO3 facies remained constant over the last 50 year, and only nitrate significantly increased from less than 1 mg/L to more than 10 mg/L in the 1980s, and then attenuated to a value of 9 mg/L. Coherently, δ(13)CDIC and δ(34)SSO4 are compatible with the weathering of the lithologies outcropping in the basin, while extremely variable δ(15)NNO3 indicates contribution from pollutants released by urban, agricultural and zootechnical activities. This suggests that although the origin of the main constituents of the Po river water is geogenic, anthropogenic contributions are locally significant. Noteworthy, the associated aquifers have the same nitrogen isotopic signature of the Po river, but are characterized by significantly higher NO(-) 3 concentration. This implies that aquifers' pollution is not ascribed to inflow of current river water, and that the attenuation of the nitrogen load recorded in the river is not occurring in the aquifers, due to their longer water residence time and delayed recovery from anthropogenic contamination.

Isotopic overprinting of nitrification on denitrification as a ubiquitous and unifying feature of environmental nitrogen cycling

Natural abundance nitrogen and oxygen isotopes of nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3) provide an important tool for evaluating sources and transformations of natural and contaminant nitrate (NO₃^-) in the environment. Nevertheless, conventional interpretations of NO₃^- isotope distributions appear at odds with patterns emerging from studies of nitrifying and denitrifying bacterial cultures. To resolve this conundrum, we present results from a numerical model of NO₃^- isotope dynamics, demonstrating that deviations in δ¹⁸O_NO3 vs. δ¹⁵N_NO3 from a trajectory of 1 expected for denitrification are explained by isotopic over-printing from coincident NO₃^- production by nitrification and/or anammox. The analysis highlights two driving parameters: (i) the δ¹⁸O of ambient water and (ii) the relative flux of NO₃^- production under net denitrifying conditions, whether catalyzed aerobically or anaerobically. In agreement with existing analyses, dual isotopic trajectories >1, characteristic of marine denitrifying systems, arise predominantly under elevated rates of NO₂^- reoxidation relative to NO₃^- reduction (>50%) and in association with the elevated δ¹⁸O of seawater. This result specifically implicates aerobic nitrification as the dominant NO₃^- producing term in marine denitrifying systems, as stoichiometric constraints indicate anammox-based NO₃^- production cannot account for trajectories >1. In contrast, trajectories <1 comprise the majority of model solutions, with those representative of aquifer conditions requiring lower NO₂^- reoxidation fluxes (<15%) and the influence of the lower δ¹⁸O of freshwater. Accordingly, we suggest that widely observed δ¹⁸O_NO3 vs. δ¹⁵N_NO3 trends in freshwater systems (<1) must result from concurrent NO₃^- production by anammox in anoxic aquifers, a process that has been largely overlooked.

Nitrogen and Oxygen Isotope Effects of Ammonia Oxidation by Thermophilic Thaumarchaeota from a Geothermal Water Stream

Ammonia oxidation regulates the balance of reduced and oxidized nitrogen pools in nature. Although ammonia-oxidizing archaea have been recently recognized to often outnumber ammonia-oxidizing bacteria in various environments, the contribution of ammonia-oxidizing archaea is still uncertain due to difficulties in the in situ quantification of ammonia oxidation activity. Nitrogen and oxygen isotope ratios of nitrite (δ(15)NNO2- and δ(18)ONO2-, respectively) are geochemical tracers for evaluating the sources and the in situ rate of nitrite turnover determined from the activities of nitrification and denitrification; however, the isotope ratios of nitrite from archaeal ammonia oxidation have been characterized only for a few marine species. We first report the isotope effects of ammonia oxidation at 70°C by thermophilic Thaumarchaeota populations composed almost entirely of "Candidatus Nitrosocaldus." The nitrogen isotope effect of ammonia oxidation varied with ambient pH (25‰ to 32‰) and strongly suggests the oxidation of ammonia, not ammonium. The δ(18)O value of nitrite produced from ammonia oxidation varied with the δ(18)O value of water in the medium but was lower than the isotopic equilibrium value in water. Because experiments have shown that the half-life of abiotic oxygen isotope exchange between nitrite and water is longer than 33 h at 70°C and pH ≥6.6, the rate of ammonia oxidation by thermophilic Thaumarchaeota could be estimated using δ(18)ONO2- in geothermal environments, where the biological nitrite turnover is likely faster than 33 h. This study extended the range of application of nitrite isotopes as a geochemical clock of the ammonia oxidation activity to high-temperature environments.

IMPORTANCE

Because ammonia oxidation is generally the rate-limiting step in nitrification that regulates the balance of reduced and oxidized nitrogen pools in nature, it is important to understand the biological and environmental factors underlying the regulation of the rate of ammonia oxidation. The discovery of ammonia-oxidizing archaea (AOA) in marine and terrestrial environments has transformed the concept that ammonia oxidation is operated only by bacterial species, suggesting that AOA play a significant role in the global nitrogen cycle. However, the archaeal contribution to ammonia oxidation in the global biosphere is not yet completely understood. This study successfully identified key factors controlling nitrogen and oxygen isotopic ratios of nitrite produced from thermophilic Thaumarchaeota and elucidated the applicability and its limit of nitrite isotopes as a geochemical clock of ammonia oxidation rate in nature. Oxygen isotope analysis in this study also provided new biochemical information on archaeal ammonia oxidation.

Rainwater, soil water, and soil nitrate effects on oxygen isotope ratios of nitrous oxide produced in a green tea (Camellia sinensis) field in Japan

RATIONALE

The oxygen exchange fraction between soil H(2)O and N(2)O precursors differs in soils depending on the responsible N(2)O-producing process: nitrification or denitrification. This study investigated the O-exchange between soil H(2)O and N(2)O precursors in a green tea field with high N(2)O emissions.

METHODS

The rainwater δ(18)O value was measured using cavity ring-down spectrometry (CRDS) and compared with that of soil water collected under the tea plant canopy and between tea plant rows. The intramolecular (15)N site preference in (β) N(α) NO (SP = δ(15)N(α) - δ(15)N(β)) was determined after measuring the δ(15)N(α) and δ(15)N(bulk) values using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), and the δ(18) O values of N(2)O and NO(3)(-) were also measured using GC/IRMS.

RESULTS

The range of δ(18)O values of rainwater (-11.15‰ to -4.91‰) was wider than that of soil water (-7.94‰ to -5.64‰). The δ(18)O value of soil water at 50 cm depth was not immediately affected by rainwater. At 10 cm and 20 cm depths of soil between tea plant rows, linear regression analyses of δ(18)O-N(2)O (relative to δ(18)O-NO(3)(-)) versus δ(18) O-H(2)O (relative to δ(18)O-NO(3)(-)) yielded slopes of 0.76-0.80 and intercepts of 31-35‰.

CONCLUSIONS

In soil between tea plant rows, the fraction of O-exchange between H(2)O and N(2)O precursors was approximately 80%. Assuming that denitrification dominated N(2)O production, the net (18)O-isotope effect for denitrification (NO(3)(-) reduction to N(2)O) was approximately 31-35‰, reflecting the upland condition of the tea field.

Nitrate isotopic composition reveals nitrogen deposition and transformation dynamics along the canopy-soil continuum of a suburban forest in Japan

RATIONALE

Heavy nitrogen (N) deposition often causes high nitrate (NO3(-)) accumulation in soils in temperate forested ecosystems. To clarify the sources and production pathways of this NO3(-), we investigated NO3(-) isotope signatures in deposition processes along the canopy-soil continuum of a suburban forest in Japan.

METHODS

The stable isotopes of N and oxygen (O) were used to trace the source and transformation dynamics of nitrate (NO3(-)) in two forest stands: a plantation of Cryptomeria japonica (coniferous tree; CJ) and a natural secondary forest of Quercus acutissima (broadleaf, deciduous tree; QA). The NO3(-) and ammonium (NH4(+)) concentrations were measured, as well as the δ(15)N and δ(18)O values of NO3(-), in rainfall, throughfall, stem flow, litter layer water, and soil water (10, 30, and 70 cm depths).

RESULTS

Seasonal variations were observed in the δ(15)N values of throughfall and stem flow NO3(-) at both sites, and in the δ(18)O values of throughfall and stem flow NO3(-) at the QA site. The range in the δ(18)O values of rainfall and throughfall NO3(-) was large (65-70‰) but decreased dramatically to 2-5‰ in soil water at both sites. At the QA site, the δ(18)O values of stem flow NO3(-) decreased to 40‰ during several rain events, especially in the growing season.

CONCLUSIONS

NO3(-) from atmospheric deposition was replaced by microbially generated NO3(-) mainly in the organic horizon and surface portion of the mineral soil under excess N deposition in this suburban forest. Microbial activity, including both immobilization and nitrification in organic-rich horizons near the surface, contributed to incorporating atmospheric NO3(-) quickly into the internal microbial N cycle. We also found evidence of microbial nitrification in the canopy of the QA stand during the growing season.

Isotopic signals of summer denitrification in a northern hardwood forested catchment

Despite decades of measurements, the nitrogen balance of temperate forest catchments remains poorly understood. Atmospheric nitrogen deposition often greatly exceeds streamwater nitrogen losses; the fate of the remaining nitrogen is highly uncertain. Gaseous losses of nitrogen to denitrification are especially poorly documented and are often ignored. Here, we provide isotopic evidence (δ(15)NNO3 and δ(18)ONO3) from shallow groundwater at the Hubbard Brook Experimental Forest indicating extensive denitrification during midsummer, when transient, perched patches of saturation developed in hillslopes, with poor hydrological connectivity to the stream, while streamwater showed no isotopic evidence of denitrification. During small rain events, precipitation directly contributed up to 34% of streamwater nitrate, which was otherwise produced by nitrification. Together, these measurements reveal the importance of denitrification in hydrologically disconnected patches of shallow groundwater during midsummer as largely overlooked control points for nitrogen loss from temperate forest catchments.

Identifying diffused nitrate sources in a stream in an agricultural field using a dual isotopic approach

Nitrate (NO3(-)) pollution is a severe problem in aquatic systems in Taihu Lake Basin in China. A dual isotope approach (δ(15)NNO3(-) and δ(18)ONO3(-)) was applied to identify diffused NO3(-) inputs in a stream in an agricultural field at the basin in 2013. The site-specific isotopic characteristics of five NO3(-) sources (atmospheric deposition, AD; NO3(-) derived from soil organic matter nitrification, NS; NO3(-) derived from chemical fertilizer nitrification, NF; groundwater, GW; and manure and sewage, M&S) were identified. NO3(-) concentrations in the stream during the rainy season [mean±standard deviation (SD)=2.5±0.4mg/L] were lower than those during the dry season (mean±SD=4.0±0.5mg/L), whereas the δ(18)ONO3(-) values during the rainy season (mean±SD=+12.3±3.6‰) were higher than those during the dry season (mean±SD=+0.9±1.9‰). Both chemical and isotopic characteristics indicated that mixing with atmospheric NO3(-) resulted in the high δ(18)O values during the rainy season, whereas NS and M&S were the dominant NO3(-) sources during the dry season. A Bayesian model was used to determine the contribution of each NO3(-) source to total stream NO3(-). Results showed that reduced N nitrification in soil zones (including soil organic matter and fertilizer) was the main NO3(-) source throughout the year. M&S contributed more NO3(-) during the dry season (22.4%) than during the rainy season (17.8%). AD generated substantial amounts of NO3(-) in May (18.4%), June (29.8%), and July (24.5%). With the assessment of temporal variation of diffused NO3(-) sources in agricultural field, improved agricultural management practices can be implemented to protect the water resource and avoid further water quality deterioration in Taihu Lake Basin.

Insights on the marine microbial nitrogen cycle from isotopic approaches to nitrification

The microbial nitrogen (N) cycle involves a variety of redox processes that control the availability and speciation of N in the environment and that are involved with the production of nitrous oxide (N₂O), a climatically important greenhouse gas. Isotopic measurements of ammonium (NH⁺₄), nitrite (NO⁻₂), nitrate (NO⁻₃), and N₂O can now be used to track the cycling of these compounds and to infer their sources and sinks, which has lead to new and exciting discoveries. For example, dual isotope measurements of NO⁻₃ and NO⁻₂ have shown that there is NO⁻₃ regeneration in the ocean's euphotic zone, as well as in and around oxygen deficient zones (ODZs), indicating that nitrification may play more roles in the ocean's N cycle than generally thought. Likewise, the inverse isotope effect associated with NO⁻₂ oxidation yields unique information about the role of this process in NO⁻₂ cycling in the primary and secondary NO⁻₂ maxima. Finally, isotopic measurements of N₂O in the ocean are indicative of an important role for nitrification in its production. These interpretations rely on knowledge of the isotope effects for the underlying microbial processes, in particular ammonia oxidation and nitrite oxidation. Here we review the isotope effects involved with the nitrification process and the insights provided by this information, then provide a prospectus for future work in this area.

Assessment of nitrogen and oxygen isotopic fractionation during nitrification and its expression in the marine environment

Nitrification is a microbially-catalyzed process whereby ammonia (NH(3)) is oxidized to nitrite (NO(2)(-)) and subsequently to nitrate (NO(3)(-)). It is also responsible for production of nitrous oxide (N(2)O), a climatically important greenhouse gas. Because the microbes responsible for nitrification are primarily autotrophic, nitrification provides a unique link between the carbon and nitrogen cycles. Nitrogen and oxygen stable isotope ratios have provided insights into where nitrification contributes to the availability of NO(2)(-) and NO(3)(-), and where it constitutes a significant source of N(2)O. This chapter describes methods for determining kinetic isotope effects involved with ammonia oxidation and nitrite oxidation, the two independent steps in the nitrification process, and their expression in the marine environment. It also outlines some remaining questions and issues related to isotopic fractionation during nitrification.

Microbial nitrogen cycling processes in oxygen minimum zones

Oxygen minimum zones (OMZs) harbor unique microbial communities that rely on alternative electron acceptors for respiration. Conditions therein enable an almost complete nitrogen (N) cycle and substantial N-loss. N-loss in OMZs is attributable to anammox and heterotrophic denitrification, whereas nitrate reduction to nitrite along with dissimilatory nitrate reduction to ammonium are major remineralization pathways. Despite virtually anoxic conditions, nitrification also occurs in OMZs, converting remineralized ammonium to N-oxides. The concurrence of all these processes provides a direct channel from organic N to the ultimate N-loss, whereas most individual processes are likely controlled by organic matter. Many microorganisms inhabiting the OMZs are capable of multiple functions in the N- and other elemental cycles. Their versatile metabolic potentials versus actual activities present a challenge to ecophysiological and biogeochemical measurements. These challenges need to be tackled before we can realistically predict how N-cycling in OMZs, and thus oceanic N-balance, will respond to future global perturbations.

Land-use controls on sources and processing of nitrate in small watersheds: insights from dual isotopic analysis

Studies have repeatedly shown that agricultural and urban areas export considerably more nitrogen to streams than forested counterparts, yet it is difficult to identify and quantify nitrogen sources to streams due to complications associated with terrestrial and in-stream biogeochemical processes. In this study, we used the isotopic composition of nitrate (delta15N-NO3- and delta18O-NO3-) in conjunction with a simple numerical model to examine the spatial and temporal variability of nitrate (NO3-) export across a land-use gradient and how agricultural and urban development affects net removal mechanisms. In an effort to isolate the effects of land use, we chose small headwater systems in close proximity to each other, limiting the variation in geology, surficial materials, and climate between sites. The delta15N and delta18O of stream NO3- varied significantly between urban, agricultural, and forested watersheds, indicating that nitrogen sources are the primary determinant of the delta15N-NO3-, while the delta18O-NO3- was found to reflect biogeochemical processes. The greatest NO3- concentrations corresponded with the highest stream delta15N-NO3- values due to the enriched nature of two dominant anthropogenic sources, septic and manure, within the urban and agricultural watersheds, respectively. On average, net removal of the available NO3- pool within urban and agricultural catchments was estimated at 45%. The variation in the estimated net removal of NO3- from developed watersheds was related to both drainage area and the availability of organic carbon. The determination of differentiated isotopic land-use signatures and dominant seasonal mechanisms illustrates the usefulness of this approach in examining the sources and processing of excess nitrogen within headwater catchments.

Determination of the origin of groundwater nitrate at an air weapons range using the dual isotope approach

Nitrate is one of the most common contaminants in shallow groundwater, and many sources may contribute to the nitrate load within an aquifer. Groundwater nitrate plumes have been detected at several ammunition production sites. However, the presence of multiple potential sources and the lack of existing isotopic data concerning explosive degradation-induced nitrate constitute a limitation when it comes to linking both types of contaminants. On military training ranges, high nitrate concentrations in groundwater were reported for the first time as part of the hydrogeological characterization of the Cold Lake Air Weapons Range (CLAWR), Alberta, Canada. Explosives degradation is thought to be the main source of nitrate contamination at CLAWR, as no other major source is present. Isotopic analyses of N and O in nitrate were performed on groundwater samples from the unconfined and confined aquifers; the dual isotopic analysis approach was used in order to increase the chances of identifying the source of nitrate. The isotopic ratios for the groundwater samples with low nitrate concentration suggested a natural origin with a strong contribution of anthropogenic atmospheric NOx. For the samples with nitrate concentration above the expected background level the isotopic ratios did not correspond to any source documented in the literature. Dissolved RDX samples were degraded in the laboratory and results showed that all reproduced degradation processes released nitrate with a strong fractionation. Laboratory isotopic values for RDX-derived NO(3)(-) produced a trend of high delta(18)O-low delta(15)N to low delta(18)O-high delta(15)N, and groundwater samples with nitrate concentrations above the expected background level appeared along this trend. Our results thus point toward a characteristic field of isotopic ratios for nitrate being derived from the degradation of RDX.

Tracing the sources of nitrate in the Han River watershed in Korea, using delta15N-NO3- and delta18O-NO3- values

The dissolved nitrate concentrations and their nitrogen and oxygen isotopic ratios were analyzed in seasonal samples from Korea's Han River to ascertain the seasonal and spatial variations of dissolved nitrate and its possible sources. Nitrate concentrations in the South Han River (SHR) were much higher than those in the North Han River (NHR), probably because of the more extensive distribution of agricultural fields, residential areas and animal farms in the SHR drainage basin. The nitrogen isotopic composition of dissolved nitrate indicates that nitrate-nitrogen (NO(3)(-)-N) is derived mainly from atmospheric deposition and/or soil organic matter in the NHR but comes principally from manure or sewage, with only a minor contribution from atmospheric deposition or soil organic matter, in the SHR. The oxygen isotopic compositions of dissolved nitrate suggest that most atmospheric nitrate undergoes microbial nitrification before entering the river.

Using delta15N and delta18O to evaluate the sources and pathways of NO3- in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions

The origin of NO(3) (-) yielded in drainage from agricultural grasslands is of environmental significance and has three potential sources; (i) soil organic mater (SOM), (ii) recent agricultural amendments, and (iii) atmospheric inputs. The variation in delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) was measured from the 'inter-flow' and 'drain-flow' of two 1 ha drained lysimeter plots, one of which had received an application of 21 m(3) of NH(4) (+)-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO(3) (-)-N from the two lysimeters were comparable; an initial flush of NO(3) (-)-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to <1 mg NO(3) (-)-N L(-1). The isotopic signature of the delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) during the first two rainfall events showed a great deal of variation over short time-periods from both lysimeters. Isotopic variation of delta(15)N-NO(3) (-) during rainfall events ranged between -1.6 to +5.2 per thousand and +0.4 to +11.1 per thousand from the inter-flow and drain-flow, respectively. Variation in the delta(18)O-NO(3) (-) ranged from +2.0 to +7.8 per thousand and from +3.3 to +8.4 per thousand. No significant relationships between the delta(15)N-NO(3) (-) or delta(18)O-NO(3) (-) and flow rate were observed in most cases although delta(18)O-NO(3) (-) values indicated a positive relationship and delta(15)N-NO(3) (-) values a negative relationship with flow during event 2. Data from a bulked rainfall sample when compared with the theoretical delta(18)O-NO(3) (-) for soil microbial NO(3) (-) indicated that the contribution of rainfall NO(3) (-) accounted for 8% of the NO(3) (-) in the lysimeter drainage at most. The calculated contribution of rainfall NO(3) (-) was not enough to account for the depletion in delta(15)N-NO(3) (-) values observed during the duration of the rainfall event 2. The relationship between delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) from the drain-flow indicated that denitrification was causing enrichment in the isotopes from this pathway. The presence of slurry seemed to cause a relative depletion in delta(18)O-NO(3) (-) in the inter-flow and delta(15)N-NO(3) (-) in the drain-flow compared with the zero-slurry lysimeter. This may have been caused by increased microbial nitrification stimulated by the presence of increased NH(4) (+)-N.

Oxygen exchange between (de)nitrification intermediates and H2O and its implications for source determination of NO3- and N2O: a review

Stable isotope analysis of oxygen (O) is increasingly used to determine the origin of nitrate (NO(3)-) and nitrous oxide (N(2)O) in the environment. The assumption underlying these studies is that the (18)O signature of NO(3)- and N(2)O provides information on the different O sources (O(2) and H(2)O) during the production of these compounds by various biochemical pathways. However, exchange of O atoms between H(2)O and intermediates of the (de)nitrification pathways may change the isotopic signal and thereby bias its interpretation for source determination. Chemical exchange of O between H(2)O and various nitrogenous oxides has been reported, but the probability and extent of its occurrence in terrestrial ecosystems remain unclear. Biochemical O exchange between H(2)O and nitrogenous oxides, NO(2)- in particular, has been reported for monocultures of many nitrifiers and denitrifiers that are abundant in nature, with exchange rates of up to 100%. Therefore, biochemical O exchange is likely to be important in most soil ecosystems, and should be taken into account in source determination studies. Failing to do so might lead to (i) an overestimation of nitrification as NO(3)- source, and (ii) an overestimation of nitrifier denitrification and nitrification-coupled denitrification as N(2)O production pathways. A method to quantify the rate and controls of biochemical O exchange in ecosystems is needed, and we argue this can only be done reliably with artificially enriched (18)O compounds. We conclude that in N source determination studies, the O isotopic signature of especially N(2)O should only be used with extreme caution.

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.

Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method

We report a novel method for measurement of the oxygen isotopic composition (18O/16O) of nitrate (NO3-) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (15N/14N) analysis of nitrate. (1) Here, we address the additional issues associated with 18O/16O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N20 analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N2O. Experiments with 18O-labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N20 product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured 18O/16O ratios of samples for both effects. This is the first method tested for 18O/16O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 microM NO3-), lack of interference by other solutes, and ease of sample preparation.

Source of the oxygen atoms of nitrate in the oxidation of nitrite by Nitrobacter agilis and evidence against a P-O-N anhydride mechanism in oxidative phosphorylation

15N, 18O Tracer studies were applied to the aerobic oxidation of nitrite to nitrate by the chemolithotrophic bacterium, Nitrobacter agilis. It was established that, in conversion of nitrite to nitrate, one oxygen atom of nitrate arose from water and none from O2 or inorganic phosphate. This result confirms that of Kumar et al. [(1983) FEBS Lett. 152, 71-74]. Oxygen exchange between water and inorganic phosphate was small and that between water and nitrite or nitrate or any reaction intermediates between these two was not detected. Oxidation of nitrite was, therefore, effectively irreversible under the conditions employed. The uptake of extracellular phosphate was sufficient to allow significant transfer of 18O from phosphate to nitrate if oxidative phosphorylation had occurred by way of a P-O-N anhydride between phosphate (or ADP) and nitrate. The results are, therefore, inconsistent with the occurrence of a reaction of this type during nitrite oxidation.