One year of continuous measurements of soil CH4 and CO2 fluxes in a Japanese cypress forest: Temporal and spatial variations associated with Asian monsoon rainfall

We examined the effects of Asian monsoon rainfall on CH[4] absorption of water-unsaturated forest soil. We conducted a 1 year continuous measurement of soil CH[4] and CO[2] fluxes with automated chamber systems in three plots with different soil characteristics and water content to investigate how temporal variations in CH[4] fluxes vary with the soil environment. CH[4] absorption was reduced by the “Baiu” summer rainfall event and peaked during the subsequent hot, dry period. Although CH[4] absorption and CO[2] emission typically increased as soil temperature increased, the temperature dependence of CH[4] varied more than that of CO[2], possibly due to the changing balance of activities between methanotrophs and methanogens occurring over a wide temperature range, which was strongly affected by soil water content. In short time intervals (30 min), the responses of CH[4] and CO[2] fluxes to rainfall were different for each plot. In a dry soil plot with a thick humus layer, both fluxes decreased abruptly at the peak of rainfall intensity. After rainfall, CO[2] emission increased quickly, while CH[4] absorption increased gradually. Release of accumulated CO[2] underground and restriction and recovery of CH[4] and CO[2] exchange between soil and air determined flux responses to rainfall. In a wet soil plot and a dry soil plot with a thinner humus layer, abrupt decreases in CH[4]fluxes were not observed. Consequently, the Asian monsoon rainfall strongly influenced temporal variations in CH[4] fluxes, and the differences in flux responses to environmental factors among plots caused large variability in annual budgets of CH[4] fluxes.