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

Continuous increases in atmospheric nitrous oxide (N ₂ O) concentrations are a global concern. Both nitrification and denitrification are the major pathways of N ₂ O production in soil, one of the most important sources of tropospheric N ₂ O. The ¹⁷ O excess (Δ ¹⁷ O) of N ₂ O can be a promising signature for identifying the main pathway of N ₂ O production in soil. However, reports on Δ ¹⁷ O are limited. Thus, we determined temporal variations in the Δ ¹⁷ O of N ₂ O emitted from forested soil for more than one year and that of soil nitrite (NO ₂ ⁻ ), which is a possible source of O atoms in N ₂ O. We found that N ₂ O emitted from the soil exhibited significantly higher Δ ¹⁷ O values on rainy days (+ 0.12 ± 0.13‰) than on fine days (− 0.30 ± 0.09‰), and the emission flux of N ₂ O 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 Δ ¹⁷ O values of N ₂ O emitted on rainy and fine days were close to those of soil NO ₂ ⁻ (+ 0.23 ± 0.12‰) and O ₂ (− 0.44‰), we concluded that although nitrification was the main pathway of N ₂ O production in the soil on fine days, denitrification became active on rainy days, resulting in a significant increase in the emission flux of N ₂ O. This study reveals that the main pathway of N ₂ O production can be identified by precisely determining the Δ ¹⁷ O values of N ₂ O emission from soil and by comparing the Δ ¹⁷ O values with those of NO ₂ ⁻ , O ₂ , and H ₂ O in the soil.