Use of variations in natural nitrogen isotope abundance for environmental studies: a questionable approach

Identification of nitrate sources in groundwater using a stable isotope and 3DEEM in a landfill in Northeast China

The groundwater was sampled in a typical landfill area of the Northeast China. Coupled stable isotope and three dimensional excitation-emission matrix (3DEEM) were applied to dentify diffused NO3(-) inputs in the groundwater in this area. The results indicated that combined with the feature of groundwater hydrochemistry and three-dimensional fluorescence technology can effectively identify the nitrate pollution sources. The nitrate was derived from manure and sewage by δ(15)N and δ(18)O-NO3(-) values of groundwater in the different periods. The excitation-emission matrix fluorescence spectroscopy was further evidence of groundwater DOM mainly which comes from the landfill. The protein-like was very significant at the sampling points near the landfill (SPNL), but only fulvic acid-like appeared at downstream of the landfill groundwater sampling points (DLGSP) in the study area. Partial denitrification processes helped to attenuate nitrate concentration in anaerobic environment.

A stable isotope approach and its application for identifying nitrate source and transformation process in water

Nitrate contamination of water is a worldwide environmental problem. Recent studies have demonstrated that the nitrogen (N) and oxygen (O) isotopes of nitrate (NO3(-)) can be used to trace nitrogen dynamics including identifying nitrate sources and nitrogen transformation processes. This paper analyzes the current state of identifying nitrate sources and nitrogen transformation processes using N and O isotopes of nitrate. With regard to nitrate sources, δ(15)N-NO3(-) and δ(18)O-NO3(-) values typically vary between sources, allowing the sources to be isotopically fingerprinted. δ(15)N-NO3(-) is often effective at tracing NO(-)3 sources from areas with different land use. δ(18)O-NO3(-) is more useful to identify NO3(-) from atmospheric sources. Isotopic data can be combined with statistical mixing models to quantify the relative contributions of NO3(-) from multiple delineated sources. With regard to N transformation processes, N and O isotopes of nitrate can be used to decipher the degree of nitrogen transformation by such processes as nitrification, assimilation, and denitrification. In some cases, however, isotopic fractionation may alter the isotopic fingerprint associated with the delineated NO3(-) source(s). This problem may be addressed by combining the N and O isotopic data with other types of, including the concentration of selected conservative elements, e.g., chloride (Cl(-)), boron isotope (δ(11)B), and sulfur isotope (δ(35)S) data. Future studies should focus on improving stable isotope mixing models and furthering our understanding of isotopic fractionation by conducting laboratory and field experiments in different environments.

Pollution and variation of stream nitrate in a protected high-mountain watershed of Central Taiwan: evidence from nitrate concentration and nitrogen and oxygen isotope compositions

This study analyzed the concentration and stable nitrogen (δ(15)N) and oxygen (δ(18)O) isotopic compositions of water NO (3) (-) , as well as NO (3) (-) concentration and δ(15)N values of soils and manure-sourced fertilizers to assess pollution and variation in stream nitrate at the watershed of the Chi-Chia-Wan Stream (CCWS), a protected high-mountain stream in Central Taiwan. Results indicate a gully (G1) that contributes significantly high NO (3) (-) concentration water (up to 122 mg/L) to trunk water as the major pollution source of CCWS. The high NO (3) (-) concentration gully water has a close relationship with manure-sourced fertilizer with both having compatible enriched δ(15)N values. Results also indicate that water mixing over isotopic fractionation processes such as denitrification or assimilation is the major process accounting for variations in concentrations and isotopic values for stream NO (3) (-) . Incorporation of gully/tributary water of high NO (3) (-) concentration increases both the concentration and isotopic values of trunk water and vice versa for the incorporation of low NO (3) (-) concentration tributary water. Despite G1 contributing high NO (3) (-) concentration water to the trunk water of CCWS, the concentration of the trunk water is only slightly elevated and is still lower than the required water quality standard due to much lower drainage of the gully water compared to trunk water's runoff. In addition to gully or tributary water and rainwater, NO (3) (-) derived from soil is another important contributor to trunk water. The NO (3) (-) contribution of soil to trunk water is greater in summer than in winter. Additionally, NO (3) (-) concentrations in soil from ex-cultivated land are significantly lower than that in cultivated land. This means that NO (3) (-) contribution from ex-cultivated land soil to trunk water is small and demonstrates that the land-recovery plan that has been underway in the studied watershed for sometime is effective.

Nitrogen stable isotopes in streams: effects of agricultural sources and transformations

The nitrogen stable isotope ratio of biological tissue has been proposed as an indicator of anthropogenic N inputs to aquatic ecosystems, but overlap in the isotopic signatures of various N sources and transformations make definitive attribution of processes difficult. We collected primary consumer invertebrates from streams in agricultural settings in Wisconsin, U.S.A., to evaluate the relative influence of animal manure, inorganic fertilizer, and denitrification on biotic delta15N. Variance in biotic delta15N was explained by inorganic fertilizer inputs and the percentage of wetland land cover in the watershed, but not by animal manure inputs. These results suggest that denitrification of inorganic fertilizer is the primary driver of delta15N variability among the study sites. Comparison with previously collected stream water NO3-N concentrations at the same sites supports the role of denitrification; for a given N application rate, streams with high biotic delta15N had low NO3-N concentrations. The lack of a manure signal in biotic delta15N may be due its high ammonia content, which can be dispersed outside the range of its application by volatilization. Based on our findings and on agricultural census data for the entire United States, inorganic fertilizer is more likely than manure to drive variability in biotic delta15N and to cause excessive nitrogen concentrations in streams.

Isotopic evidence of nitrate sources and denitrification in the Mississippi River, Illinois

Anthropogenic nitrate (NO3-) within the Mississippi-Atchafalaya River basin and discharge to the Gulf of Mexico has been linked to serious environmental problems. The sources of this NO3- have been estimated by others using mass balance methods; however, there is considerable uncertainty in these estimates. Part of the uncertainty is the degree of denitrification that the NO3- has undergone. The isotopic composition of NO3- in the Mississippi River adjacent to Illinois and tile drain (subsurface drain) discharge in agricultural areas of east-central Illinois was examined using N and O isotopes to help identify the major sources of NO3- and assess the degree of denitrification in the samples. The isotopic evidence suggests that most of the NO3- in the river is primarily derived from synthetic fertilizers and soil organic N, which is consistent with published estimates of N inputs to the Mississippi River. The 1:2 relationship between delta18O and delta15N also indicate that, depending on sample location and season, NO3- in the river and tile drains has undergone significant denitrification, ranging from about 0 to 55%. The majority of the denitrification appears to have occurred before discharge into the Mississippi River.

Tansley Review No. 95 15 N natural abundance in soil-plant systems

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable X isotopes, ¹⁵ N and ¹⁴ N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ¹⁵ N] deviations from the standard atmospheric N₂ ) depend on isotopic signatures of inputs and outputs, the input-output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization-N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors van. For example, because nitrification discriminates against ¹⁵ N in the substrate more than does N mineralization, NH₄ ⁺ can become isotopically heavier than the organic N from which it is derived. Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ¹⁵ N of a specific N pool is not a constant, and ¹⁵ N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring ¹⁵ N in small and dynamic pools of N in the soil-plant system, and the complexity of the X cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of ¹⁵ N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ¹⁵ N might offer the advantage of giving insights into the N cycle without disturbing the system by adding ¹⁵ N tracer. Such attempts require, however, that the complex factors affecting ¹⁵ N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NO_X , NH₃ , N₂ -fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil-N used (organic N, NH₄ ⁺ , NO₃ ^- ), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ¹⁵ N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non-N₂ -fixing species should differ more than 5% from N derived by N₂ -fixation, and that several non-N₂ -fixing references are used, when data on δ¹⁵ N are used to estimate N_a -fixation in poorly described ecosystems. As well as giving information on N source effects, δ¹⁵ N can give insights into N cycle rates. For example, high levels of N deposition onto previously N-limited systems leads to increased nitrification, which produces ¹⁵ N-enriched NH₄ and N-depleted NO₃ . As many forest plants prefer NH₄ ^- they become enriched in ¹⁵ N in such circumstances. This change in plant ¹⁵ N will subsequently also occur in the soil surface horizon after litter-fall, and might be a useful indicator of N saturation, especially since there is usually an increase in ¹⁵ N with depth in soils of N-limited forests. Generally, interpretation of ¹⁵ N measurements requires additional independent data and modelling, and benefits from a controlled experimental setting. Modelling will be greatly assisted by the development of methods to measure the ¹⁵ N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level ¹⁵ N studies will be necessary to further develop the interpretation of variations in ¹⁵ N in soil-plant systems. Another promising approach is to study ratios of ¹⁵ N: ¹⁴ N together with other pairs of stable isotopes, e.g. ¹³ C: ¹² C or ¹⁸ O:¹⁶ O, in the same ion or molecules. This approach can help to tackle the challenge of distinguishing isotopic source effects from fractionations within the system studied. CONTENTS Summary 179 I. Introduction 180 II. Units, causes of isotope effects, stoichiometry, modelling 181 III. N dynamics and variations in ¹⁵ N abundance in soil-plant systems 183 IV. Applications 189 V. Conclusions and suggestions for future research 197 Acknowledgements 198 References 198.

A selected bibliography of biomedical and environmental applications of stable isotopes. III--15N 1971-1976

A compilation of selected references of the use of 15N in biochemical, pharmacological, clinical and environmental applications for the period 1971--1976 is presented. Author and subject indices have been compiled to enable the reader to make further use of this information.