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.