Comparison of the silver nitrate and bacterial denitrification methods for the determination of nitrogen and oxygen isotope ratios of nitrate in surface water

Automated rapid triple-isotope (δ15N, δ18O, δ17O) analyses of nitrate by Ti(III) reduction and N2O laser spectrometry

The nitrogen and oxygen (δ15N, δ18O, δ17O) stable isotopic compositions of nitrate (N O 3 - ) are crucial tracers of nutrient N sources and dynamics in aquatic and atmospheric systems. Methods to reduce aqueous N O 3 - to N2O gas (microbial or Cd method) before 15N and 18O isotope analyses require multi-step conversion or toxic chemicals, and 17O in N2O cannot be disentangled by IRMS due to isobaric interferences. This technical note describes the automation of the stable-isotope analyses of nitrate by coupling the new Ti method with a headspace autosampler and an N2O triple-isotope laser analyzer based on off-axis integrated cavity output spectroscopy. The automation yielded accurate and precise results for routine determinations of δ15N, δ18O, and δ17O values for aqueous nitrate in environmental waters. Systematic corrections were required for cavity pressure, N2O concentration and water vapour content to obtain the highest precision for all three isotopic ratios. For the first time, an automated laser-based system facilitates routine low-cost triple isotope analyses in studies where high-temporal resolution isotope analyses of NO3- are required but have been, until now, cost-prohibitive and time-consuming (e.g. atmospheric N pollution).

Challenges in measuring nitrogen isotope signatures in inorganic nitrogen forms: An interlaboratory comparison of three common measurement approaches

RATIONALE

Stable isotope approaches are increasingly applied to better understand the cycling of inorganic nitrogen (N_i ) forms, key limiting nutrients in terrestrial and aquatic ecosystems. A systematic comparison of the accuracy and precision of the most commonly used methods to analyze δ¹⁵ N in NO₃ ^- and NH₄ ⁺ and interlaboratory comparison tests to evaluate the comparability of isotope results between laboratories are, however, still lacking.

METHODS

Here, we conducted an interlaboratory comparison involving 10 European laboratories to compare different methods and laboratory performance to measure δ¹⁵ N in NO₃ ^- and NH₄ ⁺ . The approaches tested were (a) microdiffusion (MD), (b) chemical conversion (CM), which transforms N_i to either N₂ O (CM-N₂ O) or N₂ (CM-N₂ ), and (c) the denitrifier (DN) methods.

RESULTS

The study showed that standards in their single forms were reasonably replicated by the different methods and laboratories, with laboratories applying CM-N₂ O performing superior for both NO₃ ^- and NH₄ ⁺ , followed by DN. Laboratories using MD significantly underestimated the "true" values due to incomplete recovery and also those using CM-N₂ showed issues with isotope fractionation. Most methods and laboratories underestimated the at%¹⁵ N of N_i of labeled standards in their single forms, but relative errors were within maximal 6% deviation from the real value and therefore acceptable. The results showed further that MD is strongly biased by nonspecificity. The results of the environmental samples were generally highly variable, with standard deviations (SD) of up to ± 8.4‰ for NO₃ ^- and ± 32.9‰ for NH₄ ⁺ ; SDs within laboratories were found to be considerably lower (on average 3.1‰). The variability could not be connected to any single factor but next to errors due to blank contamination, isotope normalization, and fractionation, and also matrix effects and analytical errors have to be considered.

CONCLUSIONS

The inconsistency among all methods and laboratories raises concern about reported δ¹⁵ N values particularly from environmental samples.

Characterization of the analytical performance of δ15 N and δ18 O measurements by the silver nitrate method in the framework of nitrate source apportioning

RATIONALE

Nitrate pollution represents one of the most important issues for ground and surface water quality and source identification is essential for developing effective mitigation practices. Nitrate isotopic fingerprinting can be utilized to identify the sources of nitrate pollution in aquifers. However, it is crucial to assess the performances (precision and accuracy) of the analytical procedure applied to measure the δ¹⁵ N and δ¹⁸ O values of nitrates from field samples to correctly apply this tool.

METHODS

Nitrates were extracted from a large number of KNO₃ samples using the AgNO₃ method, and the δ¹⁵ N and δ¹⁸ O values of these nitrate extracts were measured by isotope ratio mass spectrometry. The availability of this dataset, comprising 693 unprocessed quality control (QC) KNO₃ samples and 618 processed samples, allowed us to rigorously quantify the performance of the procedures employed. A salt doping experiment was also performed from which the effects of contaminants on the performance of the method could be ascertained.

RESULTS

The overall instrumental reproducibility for the analysis of unprocessed QC samples was 0.5‰ and 2‰ for δ¹⁵ N and δ¹⁸ O values, respectively, and a strict dependence on signal amplitude was observed. No isotope fractionation was reported for reference samples that were processed according to the "identical treatment" principle (ITP) but normalized by unprocessed reference materials. A significant increase in the standard deviation (SD) was, however, observed compared with that for unprocessed samples. The SD of the processed QC samples allowed us to quantify the reproducibility of the entire procedure as 0.6‰ and 1.0‰ for δ¹⁵ N and δ¹⁸ O values, respectively. This was comparable with the system reproducibility when normalization using processed reference materials was applied according to the ITP.

CONCLUSIONS

Normalization with processed standards is essential to achieve high-precision measurements of the δ¹⁵ N and δ¹⁸ O values of nitrates extracted from unknown samples. This procedure allowed good accuracy to be guaranteed, and precision levels comparable with the observed instrumental performance to be achieved. A salt doping experiment showed a significant influence of the SO₄ ^2- content on the δ¹⁵ N values.

Further optimisation of the denitrifier method for the rapid 15 N and 18 O analysis of nitrate in natural water samples

RATIONALE

This study aims to develop a simplified denitrifier method for the δ¹⁵ N and δ¹⁸ O analysis of nitrate (NO₃ ^- ) in natural water samples combining the method of Zhu et al (Sci Total Environ. 2018; 633: 1370-1378) and the original denitrifier method of Sigman et al (Anal Chem. 2001; 73: 4145-4153). Unlike in the aforementioned methods, the aerobic cultivation was performed without the addition or removal of nitrate in the liquid medium. We remove the nitrate contained in the nutrient medium as N₂ O in the gas phase by an additional purging step after incubation overnight before the water sample is injected. This eliminates the need for another preparation step, thus saving working time.

METHODS

The δ¹⁵ N and δ¹⁸ O values of dissolved NO₃ ^- were determined using a Delta V Plus isotope ratio mass spectrometer coupled to a GasBench II sample preparation device that included a denitrification kit.

RESULTS

After optimising the influencing factors (i.e., purging gas, purging time, and type of crimp seals), the method yielded high accuracy and precision (standard deviations were generally ≤0.7‰ for δ¹⁸ O values and ≤0.3‰ for δ¹⁵ N values), confirming the suitability of this procedure. Finally, the potential applicability of the method was demonstrated by measuring the isotopic composition of NO₃ ^- in natural water samples.

CONCLUSIONS

The denitrifier method for converting NO₃ ^- into N₂ O for isotope analysis was optimised. This allowed the sample preparation time to be further reduced. The complete working time for sample preparation, including all steps, takes 10 min per vial if 60 vials are prepared in one run. The water samples are ready for isotope analysis on the fourth day after preparation has started. Isotope measurements can be performed up to 14 days after preparation.

Comparative evaluation of urban versus agricultural nitrate sources and sinks in an unconfined aquifer by isotopic and multivariate analyses

Nitrate (NO₃^-) is one of the most widespread contaminants in groundwater primarily due to agricultural activities utilizing N-containing fertilizers and the presence of animal wastes. Hydrochemical and nitrate isotope data (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) from the unconfined aquifer in the urban area of Del Campillo city and its surrounding rural area with different land-use types, i.e. individual sanitation systems, agricultural areas and livestock breeding facilities, were generated to investigate the impact of nitrogen pollution sources and to assess N-biogeochemical processes. The Principal Component Analysis of hydrochemical and isotopic data were used to compare the factors that control the groundwater quality and particularly the nitrate concentrations in the urban and the rural area. The results showed that nitrate pollution in the urban area of Del Campillo city originated mainly from the on-site sanitation systems and/or animal domestic wastes, whereas in the rural area nitrate pollution was mostly attributed to a combination of urea-based fertilizers and manure from livestock breeding activities. The aquifer is under oxic to suboxic conditions in the rural area and becomes suboxic in the urban area where the higher supply of organic matter consumes oxygen. As a result, denitrification was more significant in the urban area compared to the rural area, as evidenced by the higher N and O isotope enrichment factor (ε). This work will be used to benchmark the current nitrate contamination status in the region and evaluate effective planning of environmental measures and remediation strategies.

Nitrogen isotopic analysis of nitrate in aquatic environment using cadmium-hydroxylamine hydrochloride reduction

RATIONALE

The nitrogen isotopic ratio of nitrate (δ¹⁵ N-NO₃ ^- value) is a critical parameter for understanding nitrogen biogeochemical cycling in aquatic systems. Current approaches to the determination of δ¹⁵ N-NO₃ ^- values involve time-intensive handling procedures, the use of toxic chemicals and complicated microbial incubation.

METHODS

A chemical reduction method for measuring the δ¹⁵ N-NO₃ ^- values of aquatic samples was established. Nitrate was first quantitatively reduced to nitrite in a column filled with copper-coated cadmium granules, and the produced nitrite further reduced to nitrous oxide gas with hydroxylamine hydrochloride. The nitrogen isotope ratio of the produced nitrous oxide was measured using a continuous-flow isotope ratio mass spectrometer coupled with a purge and cryogenic trap system.

RESULTS

The optimized experimental conditions were: solution acidity, H⁺ concentration of 0.46 M, pH = 0.34; dosage of hydroxylamine, molar ratio of NH₂ OH to NO₂ ^- of 4; reaction temperature, 45°C; and reaction time, 14-16 h. No salt effect was found for this method. The reproducibility of the δ¹⁵ N-NO₃ ^- value for the laboratory standard was better than 0.3‰ for long-term measurements (20 nmol NO₃ ^- requirement).

CONCLUSIONS

This method provides a reliable approach for the determination of δ¹⁵ N-NO₃ ^- values at natural abundance. It provides (1) high measurement accuracy, (2) ease of operation, (3) environmental-friendly procedure (less toxic regents used), and (4) suitability for both freshwater and saline water samples.

Determining δ15N-NO3- values in soil, water, and air samples by chemical methods

Soil, water, and air NO₃^- pollution is a major environmental problem worldwide. Stable isotope analysis can assess the origin of NO_x because different NO_x sources carry different isotope signatures. Hence, using appropriate chemical methods to determine the δ¹⁵N-NO_x values in different samples is important to improve our understanding of the N-NO_x pollution and take possible strategies to manage it. Two modified chemical methods, the cadmium-sodium azide method and the VCl₃-sodium azide method, were used to establish a comprehensive inventory of δ¹⁵N-NO_x values associated with major NO_x fluxes by the conversion of NO₃^- into N₂O. Precision and limit of detection values demonstrated the robustness of these quantitative techniques for measuring δ¹⁵N-NO_x. The standard deviations of the δ¹⁵N-NO₃^- values were 0.35 and 0.34‰ for the cadmium-sodium azide and VCl₃-sodium azide methods. The mean δ¹⁵N-NO₃^- values of river water, soil extracts, and summer rain were 8.9 ± 3.3, 3.5 ± 3.5, and 3.3 ± 2.1‰, respectively. The δ¹⁵N-NO₃^- values of low concentration samples collected from coal-fired power plants, motor vehicles, and gaseous HNO₃ was 20.3 ± 4.3, 5.6 ± 2.78, and 5.7 ± 3.6‰, respectively. There was a good correlation between the δ¹⁵N-NO₃^- compositions of standards and samples, which demonstrates that these chemical reactions can be used successfully to assess δ¹⁵N-NO₃^- values in the environment.

N and O isotope (δ15 Nα , δ15 Nβ , δ18 O, δ17 O) analyses of dissolved NO3- and NO2- by the Cd-azide reduction method and N2 O laser spectrometry

RATIONALE

The nitrogen and oxygen (δ¹⁵ N, δ¹⁸ O, δ¹⁷ O) isotopic compositions of NO₃^- and NO₂^- are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd-azide method was used to convert NO₃^- or NO₂^- to N₂ O for N and triple-O isotopic analyses by N₂ O laser spectrometry. A protocol for laser-based headspace isotope analyses was compared with isotope ratio mass spectrometry. Lasers provide the ability to directly measure ¹⁷ O anomalies which can help discern atmospheric N sources.

METHODS

δ¹⁵ N, δ¹⁸ O and δ¹⁷ O values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N₂ O using the Cd-azide method and headspace N₂ O laser spectrometry. A ¹⁵ N tracer test assessed the position-specific routing of N to the α or β positions in the N₂ O molecule. A data processing algorithm was used to correct for isotopic dependencies on N₂ O concentration, cavity pressure and water content.

RESULTS

NO₃^- /NO₂^- nitrogen is routed to the ¹⁵ N^α position of N₂ O in the azide reaction; hence the δ¹⁵ N^α value should be used for N₂ O laser spectrometry results. With corrections for cavity pressure, N₂ O concentration and water content, the δ¹⁵ N^α_AIR , δ¹⁸ O_VSMOW and δ¹⁷ O_VSMOW values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO₃ ), -1.7 ± 0.1, -26.8 ± 0.8, -14.4 ± 1.1 (USGS34) and +2.6 ± 0.1, +57.6 ± 1.2, +51.2 ± 2.0 (USGS35), in agreement with their values and with the isotope ratio mass spectrometry results. The ¹⁷ O excess for USGS35 was +21.2 ± 9‰, in good agreement with previous results.

CONCLUSIONS

The Cd-azide method yielded excellent results for routine determination of δ¹⁵ N, δ¹⁸ O and δ¹⁷ O values (and the ¹⁷ O excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd-azide chemicals and the lack of automated sampling devices for N₂ O laser spectrometers. The ¹⁵ N-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high ¹⁵ N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of ¹⁷ O.

Interpretation of anthropogenic impacts (agriculture and urbanization) on tropical deltaic river network through the spatio-temporal variation of stable (N, O) isotopes of NO(-)3

For the first time, the dual isotope approach was applied to trace the sources of impacts and to identify the governing biogeochemical processes in a river network in the tropical deltaic region of the Red River (Vietnam). Our long term surveys concluded that water in this river network was severely impacted by anthropogenic activities. Analysis has shown strong spatio-temporal variation of nitrate isotopes; ranges of δ(15)N-[Formula: see text] and δ(18)O-[Formula: see text] were from -5 to 15 ‰ and from -10 to 10 ‰, respectively. Average values of δ(15)N-[Formula: see text] and δ(18)O-[Formula: see text] in the dry season, when fertilizer is applied, were 3.54 and 3.15 ‰, respectively. In the rainy season, the values changed to 6.41 and -2.23 ‰, respectively. Denitrification and biological assimilation were active throughout the year, but were especially enhanced during fertilization time. Mineralization of domestic organic matter and consequent nitrification of mineralized [Formula: see text] were the dominant processes, particularly during the rainy period.

Ion-exchange method in the collection of nitrate from freshwater ecosystems for nitrogen and oxygen isotope analysis: a review

Nitrate (NO3(-)) contamination of freshwater is considered one of the most prevalent global environmental problems. Dual stable isotopic compositions (δ(15)N and δ(18)O) of NO3(-) can provide helpful information and have been well documented as being a powerful tool to track the source of NO3(-) in freshwater ecosystems. The ion-exchange method is a reliable and precise technique for measuring the δ(15)N and δ(18)O of NO3(-) and has been widely employed to collect NO3(-) from freshwater ecosystems. This review summarizes and presents the principles, affecting factors and corresponding significant improvements of the ion-exchange method. Finally, potential improvements and perspectives for the applicability of this method are also discussed, as are suggestions for further research and development drawn from the overall conclusions.

Feeding strategies for groundwater enhanced biodenitrification in an alluvial aquifer: chemical, microbial and isotope assessment of a 1D flow-through experiment

Nitrate-removal through enhanced in situ biodenitrification (EISB) is an existing alternative for the recovery of groundwater quality, and is often suggested for use in exploitation wells pumping at small flow-rates. Innovative approaches focus on wider-scale applications, coupling EISB with water-management practices and new monitoring tools. However, before this approach can be used, some water-quality issues such as the accumulation of denitrification intermediates and/or of reduced compounds from other anaerobic processes must be addressed. With such a goal, a flow-through experiment using 100mg-nitrate/L groundwater was built to simulate an EISB for an alluvial aquifer. Heterotrophic denitrification was induced through the periodic addition of a C source (ethanol), with four different C addition strategies being evaluated to improve the quality of the denitrified water. Chemical, microbial and isotope analyses of the water were performed. Biodenitrification was successfully stimulated by the daily addition of ethanol, easily achieving drinking water standards for both nitrate and nitrite, and showing an expected linear trend for nitrogen and oxygen isotope fractionation, with a εN/εO value of 1.1. Nitrate reduction to ammonium was never detected. Water quality in terms of remaining C, microbial counts, and denitrification intermediates was found to vary with the experimental time, and some secondary microbial respiration processes, mainly manganese reduction, were suspected to occur. Carbon isotope composition from the remaining ethanol also changed, from an initial enrichment in (13)C-ethanol compared to the value of the injected ethanol (-30.6‰), to a later depletion, achieving δ(13)C values well below the initial isotope composition (to a minimum of -46.7‰). This depletion in the heavy C isotope follows the trend of an inverse fractionation. Overall, our results indicated that most undesired effects on water quality may be controlled through the optimization of the C/N ratio determined from the amounts of injected ethanol vs. the amount of nitrate in groundwater, with a smaller C/N ratio causing a lower level of undesired impurities. Furthermore, the authors suggest that the biofilm life-time has a direct effect on microbial population and hence affects biodenitrification performance, influencing the accumulation of nitrite over time.

Determination of nitrate isotopic signature in waters of different sources by analysing the nitrogen and oxygen isotopic ratio

A reference study on both the nitrogen content in waters and nitrogen and oxygen isotopic signatures characterising nitrate from different sources was conducted within the San River catchment area. Three kinds of catchments were studied: (1) forested and uncultivated; (2) artificially fertilised with nitrate; and (3) fertilised with manure and sewage. Moreover, atmospheric water was studied. The obtained values were found to be similar to others in the literature, with the exception of nitrate from the atmosphere, in regard to which influence reflecting the local conditions was to be noted. The isotopic signature of nitrate in the studied water results from the biogeochemical transformation of nitrogen compounds rather than from the mixing of different sources. The obtained results were statistically distinct and can be used as end-member values in further modelling studies connected with the management of nitrate in river waters, especially under middle-eastern European conditions.

Preliminary insights into δ15N and δ18O of nitrate in natural mosses: a new application of the denitrifier method

Natural mosses have been employed as reactive and accumulative indicators of atmospheric pollutants. Using the denitrifier method, the concentration, δ(15)N and δ(18)O of moss nitrate (NO(3)(-)) were measured to elucidate the sources of NO(3)(-) trapped in natural mosses. Oven drying at 55-70 °C, not lyophilization, was recommended to dry mosses for NO(3)(-) analyses. An investigation from urban to mountain sites in western Tokyo suggested that moss [NO(3)(-)] can respond to NO(3)(-) availability in different habitats. NO(3)(-) in terricolous mosses showed isotopic ratios as close to those of soil NO(3)(-), reflecting the utilization of soil NO(3)(-). Isotopic signatures of NO(3)(-) in corticolous and epilithic mosses elucidated atmospheric NO(3)(-) sources and strength from the urban (vehicle NO(x) emission) to mountain area (wet-deposition NO(3)(-)). However, mechanisms and isotopic effects of moss NO(3)(-) utilization must be further verified to enable the application of moss NO(3)(-) isotopes for source identification.

Identification of the nitrate contamination sources of the Brusselian sands groundwater body (Belgium) using a dual-isotope approach

Isotopic fingerprinting is an advanced technique allowing the classification of the nitrate source pollution of groundwater, but needs further development and validation. In this study, we performed measurements of natural stable isotopic composition of nitrate ((15)N and (18)O) in the groundwater body of the Brussels sands (Belgium) and studied the spatial and temporal dynamics of the isotope signature of this aquifer. Potential nitrogen sources sampled in the region had isotopic signatures that fell within the corresponding typical ranges found in the literature. For a few monitoring stations, the isotopic data strongly suggest that the sources of nitrate are from mineral fertiliser origin, as used in agriculture and golf courses. Other stations suggest that manure leaching from unprotected stockpiles in farms, domestic gardening practices, septic tanks and probably cemeteries contribute to the nitrate pollution of this groundwater body. For most monitoring stations, nitrate originates from a mixing of several nitrogen sources. The isotopic signature of the groundwater body was poorly structured in space, but exhibited a clear temporal structure. This temporal structure could be explained by groundwater recharge dynamics and cycling process of nitrogen in the soil-nitrogen pool.

Error assessment of nitrogen and oxygen isotope ratios of nitrate as determined via the bacterial denitrification method

Currently, bacterial denitrification is becoming the accepted method for delta(15)N- and delta(18)O-NO(3)(-) determination. However, proper correction methods with international references (USGS32, USGS34 and USGS35) are needed. As a consequence, it is important to realize that the corrected isotope values are derived from a combination of several other measurements with associated uncertainties. Therefore, it is necessary to consider the propagated uncertainty on the final isotope value. This study demonstrates how to correctly estimate the uncertainty on corrected delta(15)N- and delta(18)O-NO(3)(-) values using a first-order Taylor series approximation. The bacterial denitrification method errors from 33 batches of 561 surface water samples varied from 0.2 to 2.1 per thousand for delta(15)N-NO(3)(-) and from 0.7 to 2.3 per thousand for delta(18)O-NO(3)(-), which is slightly wider than the machine error, which varied from 0.2 to 0.6 per thousand for delta(15)N-N(2)O and from 0.4 to 1.0 per thousand for delta(18)O-N(2)O. The overall uncertainties, which are composed of the machine error and the method error, for the 33 batches ranged from 0.3 to 2.2 per thousand for delta(15)N-NO(3)(-) and from 0.8 to 2.5 per thousand for delta(18)O-NO(3)(-). In addition, the mean corrected delta(15)N and delta(18)O values of 132 KNO(3)-IWS (internal working standard) measurements were computed as 8.4 +/- 1.0 per thousand and 25.1 +/- 2.0 per thousand, which is a slight underestimation for delta(15)N and overestimation for delta(18)O compared with the accepted values (delta(15)N = 9.9 +/- 0.3 per thousand and delta(18)O = 24.0 +/- 0.3 per thousand). The overall uncertainty of the bacterial denitrification method allows the use of this method for source identification of NO(3)(-).