Isotopic composition of nitrogen species in groundwater under agricultural areas: A review.

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 NO, NH and NO in subsurface, and discusses the use of B, O, C, S, Sr/Sr isotopes to support the analysis of δN values. The isotopic composition of NO, NH and NO varies depending on their sources and dynamics of N cycle processes. The reported δN-NO values for sources of NO are: soil organic N - +3‰-+8‰, mineral fertilizers - -8‰-+7‰; manure/household waste - +5‰ to +35‰. For NH 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 NO, isotopic composition depends on isotopic signatures of substrate pools and reaction rates. δN values of NO are influenced by fractionation effects occurring during denitrification (ɛ=5-40‰), nitrification (ɛ=5-35‰) and DNRA (ɛ not reported). The isotopic signature of NH is also affected by nitrification and DNRA as well as mineralization (ɛ=1‰), sorption (ɛ=1-8‰), anammox (ɛ=4.3-7.4‰) and volatilization (ɛ=25‰). As for the NO, production of NO leads to its depletion in N, whereas consumption - to enrichment in N. The magnitude of fractionation effects occurring during the considered processes depends on temperature, pH, DO, C/NO ratio, size of the substrate pool, availability of electron donors, water content in subsoil, residence time, land use, hydrogeology. While previous studies have accumulated rich data on isotopic composition of NO in groundwater, evidence remains scarce in the cases of NH and NO. Further research is required to consider variability of δN-NH and δN-NO in groundwater across agricultural ecosystems.