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Tracing the origin and fate of nitrate in the earth surface ecosystems using stable isotopes as natural tracers
This paper summarizes recent advances in using stable isotopes of nitrate to trace the origin and fate of nitrate in the earth surface ecosystems. While nitrogen isotopes have been used as the indicators of nitrate source contributions to the earth surface ecosystems, cautions must be taken for the isotope fractionations during the chemical reactions and/or biological processing of nitrate within the earth surface ecosystems. On the other hand, the triple oxygen isotope composition (Δ17O = {(1 + δ17O) / (1 + δ18O)0.5247} - 1) is a robust tracer to distinguish atmospheric nitrate (NO3-atm) from the other terrestrial nitrate called remineralized nitrate (NO3-re). The triple oxygen isotope composition can be a useful tool to trace the fate of atmospheric nitrate and to evaluate biological processing of nitrate within the earth surface ecosystems. Furthermore, nitrate dynamics in each surface ecosystem, such as gross uptake rate of nitrate through assimilation and gross production rate of nitrate through nitrification, can be quantified using Δ17O data of nitrate, based on both the deposition rate of atmospheric nitrate onto the entire surface of the ecosystem studied and the influx/efflux of both atmospheric and remineralized nitrate in the ecosystem studied.