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This work reviews applications of stable isotope analysis to the studies of transport and transformation of N species in groundwater under agricultural areas. It summarizes evidence regarding factors affecting the isotopic composition of NO₃⁻, NH₄⁺ and N₂O in subsurface, and discusses the use of ¹¹B, ¹⁸O, ¹³C, ³⁴S, ⁸⁷Sr/⁸⁶Sr isotopes to support the analysis of δ¹⁵N values. The isotopic composition of NO₃⁻, NH₄⁺ and N₂O varies depending on their sources and dynamics of N cycle processes. The reported δ¹⁵N-NO₃⁻ values for sources of NO₃⁻ are: soil organic N – + 3‰–+8‰, mineral fertilizers – − 8‰–+7‰; manure/household waste – + 5‰ to + 35‰. For NH₄⁺ sources, the isotopic signature ranges are: organic matter – + 2.4–+4.1‰, rainwater – − 13.4–+2.3‰, mineral fertilizers – − 7.4–+5.1‰, household waste – + 5–+9‰; animal manure – + 8–+11‰. For N₂O, isotopic composition depends on isotopic …