Seasonal change in N2O flux from forest soils in a forest catchment in Japan

Temperate forest soils are one source of nitrous oxide (N2O), which is an important greenhouse gas and the most important ozone-depleting substance. To clarify N2O flux mechanisms in relation to soil temperature, moisture, and nitrification activity, we measured N2O fluxes and net nitrification rates over 3 years at the lower (Japanese cedar) and upper (deciduous broad-leaved trees) parts of a hill slope in a small forest catchment in the northern Kanto region of Japan. The N2O flux was measured by the closed-chamber technique every month, along with soil temperature and water-filled pore space (WFPS). At the lower slope, the N2O flux increased with increasing soil temperature (r2 = 0.383, P < 0.01) owing to an increase in the nitrification rate. At the upper slope, no positive linear correlation of N2O flux with soil temperature, WFPS, or nitrification rate was observed. The low N2O flux at the upper slope during summer was caused by the low summertime WFPS there. We attributed the higher mean N2O fluxes observed at the lower slope (median 2.36 μg N m−2 h−1) than at the upper slope (median 1.10 μg N m−2 h−1) to a high soil moisture during summer season in the surface soil of the lower slope.