RESUMO
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 signatures of substrate pools and reaction rates. δ15N values of NO3- are influenced by fractionation effects occurring during denitrification (É=5-40), nitrification (É=5-35) and DNRA (É not reported). The isotopic signature of NH4+ is also affected by nitrification and DNRA as well as mineralization (É=1), sorption (É=1-8), anammox (É=4.3-7.4) and volatilization (É=25). As for the N2O, production of N2O leads to its depletion in 15N, whereas consumption - to enrichment in 15N. The magnitude of fractionation effects occurring during the considered processes depends on temperature, pH, DO, C/NO3- 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 NO3- in groundwater, evidence remains scarce in the cases of NH4+ and N2O. Further research is required to consider variability of δ15N-NH4+ and δ15N-N2O in groundwater across agricultural ecosystems.