Contribution of atmospheric nitrate to stream-water nitrate in Japanese coniferous forests revealed by the oxygen isotope ratio of nitrate

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.

Hydrological and pollution processes in mining area of Fenhe River Basin in China

The hydrological and pollution processes are an important science problem for aquatic ecosystem. In this study, the samples of river water, reservoir water, shallow groundwater, deep groundwater, and precipitation in mining area are collected and analyzed. δD and δ¹⁸O are used to identify hydrological process. δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- are used to identify the sources and pollution process of NO₃^-. The results show that the various water bodies in Fenhe River Basin are slightly alkaline water. The ions in the water mainly come from rock weathering. The concentration of SO₄^2- is high due to the impact of coal mining activity. Deep groundwater is significantly less affected by evaporation and human activity, which is recharged by archaic groundwater. There are recharge and discharge between reservoir water, river water, soil water, and shallow groundwater. NO₃^- is the main N species in the study area, and forty-six percent of NO₃^--N concentrations exceed the drinking water standard of China (NO₃^--N ≤ 10 mg/L content). Nitrification is the main forming process of NO₃^-. Denitrification is also found in river water of some river branches. The sources of NO₃^- are mainly controlled by land use type along the riverbank. NO₃^- of river water in the upper reaches are come from nitrogen in precipitation and soil organic N. River water in the lower reaches is polluted by a mixture of soil organic N and fertilizers.

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.

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

This study examines the role of tree canopies in processing atmospheric nitrogen (Ndep ) for four forests in the United Kingdom subjected to different Ndep : Scots pine and beech stands under high Ndep (HN, 13-19 kg N ha(-1)  yr(-1) ), compared to Scots pine and beech stands under low Ndep (LN, 9 kg N ha(-1)  yr(-1) ). Changes of NO3 -N and NH4 -N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ(18) O, Δ(17) O and δ(15) N in NO3 (-) and δ(15) N in NH4 (+) , were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4 -N and NO3 -N concentrations in RF compared to the LN sites. Similar values of δ(15) N-NO3 (-) and δ(18) O in RF suggested similar source of atmospheric NO3 (-) (i.e. local traffic), while more positive values for δ(15) N-NH4 (+) at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N-forms changed after interacting with tree canopies. Indeed, (15) N-enriched NH4 (+) in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndep also at the low Ndep site. Using two independent methods, based on δ(18) O and Δ(17) O, we quantified for the first time the proportion of NO3 (-) in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ(17) O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of Ndep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ(17) O.

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.

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.