Rainwater, soil water, and soil nitrate effects on oxygen isotope ratios of nitrous oxide produced in a green tea (Camellia sinensis) field in Japan.

Rationale The oxygen exchange fraction between soil H2O and N2O precursors differs in soils depending on the responsible N2O-producing process: nitrification or denitrification. This study investigated the O-exchange between soil H2O and N2O precursors in a green tea field with high N2O emissions. Methods The rainwater δ18O value was measured using cavity ring-down spectrometry (CRDS) and compared with that of soil water collected under the tea plant canopy and between tea plant rows. The intramolecular 15N site preference in βNαNO (SP = δ15Nα – δ15Nβ) was determined after measuring the δ15Nα and δ15Nbulk values using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), and the δ18O values of N2O and NO3– were also measured using GC/IRMS. Results The range of δ18O values of rainwater (–11.15‰ to –4.91‰) was wider than that of soil water (–7.94‰ to –5.64‰). The δ18O value of soil water at 50 cm depth was not immediately affected by rainwater. At 10 cm and 20 cm depths of soil between tea plant rows, linear regression analyses of δ18O-N2O (relative to δ18O-NO3–) versus δ18O-H2O (relative to δ18O-NO3–) yielded slopes of 0.76–0.80 and intercepts of 31–35‰. Conclusions In soil between tea plant rows, the fraction of O-exchange between H2O and N2O precursors was approximately 80%. Assuming that denitrification dominated N2O production, the net 18O-isotope effect for denitrification (NO3– reduction to N2O) was approximately 31–35‰, reflecting the upland condition of the tea field. Copyright © 2015 John Wiley & Sons, Ltd.