Triple oxygen isotope evidence for the pathway of nitrous oxide production in a forested soil with increased emission on rainy days

Abstract. Continuous increases in atmospheric nitrous oxide (N2O) concentrations are a global concern. Both nitrification and denitrification are the major pathways of N2O production in soil, one of the most important sources of tropospheric N2O. The 17O excess (Δ17O) of N2O can be a promising signature for identifying the main pathway of N2O production in soil. However, reports on Δ17O are limited. Thus, we determined temporal variations in the Δ17O of N2O emitted from forested soil for more than one year and that of soil nitrite (NO2-), which is a possible source of O atoms in N2O. We found that N2O emitted from the soil exhibited significantly higher Δ17O values on rainy days (+0.12 ± 0.13 ‰) than on fine days (−0.30 ± 0.09 ‰), and the emission flux of N2O was significantly higher on rainy days (38.8 ± 28.0 µg N m−2 h−1) than on fine days (3.8 ± 3.1 µg N m−2 h−1). Because the Δ17O values of N2O emitted on rainy and fine days were close to those of soil NO2- (+0.23 ± 0.12 ‰) and O2 (−0.44 ‰), we concluded that although nitrification was the main pathway of N2O production in the soil on fine days, denitrification became active on rainy days, resulting in a significant increase in the emission flux of N2O. This study reveals that the main pathway of N2O production can be identified by precisely determining the Δ17O values of N2O emission from soil and by comparing the Δ17O values with those of NO2-, O2, and H2O in the soil.