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.