Impacts of riparian wetlands on the seasonal variations of watershed‐scale methane budget in a temperate monsoonal forest

Forest soils are considered a methane (CH 4 ) sink because dry soils can oxidize CH 4 ; however, previous studies on CH 4 fluxes in humid temperate forests indicated a high spatial and temporal variability in CH 4 fluxes, especially in CH 4 emissions from wet soils close to riparian zones, which can turn the soil of a whole forest from a CH 4 sink to a CH 4 source. In this study, the spatial and temporal variability of soil CH 4 fluxes was investigated in a Japanese coniferous forest, including a riparian wetland and a hillslope water‐unsaturated forest floor, based on multipoint flux measurements using laser‐based CH 4 analyzers over a period of 2 years. We identified CH 4 emission hot spots (60.2 ± 169.1 nmol m −2  s −1 from 117 sampling points) in the wetland in late summer, while the CH 4 absorption rate in the forest floor was comparatively lower (−1.2 ± 1.4 nmol m −2  s −1 from 119 sampling points). The temporal variability of watershed‐scale CH 4 flux was amplified by a clear seasonal cycle of soil temperature and rainfall pattern under the Asian monsoon climate. The watershed‐scale CH 4 budget showed that the forest turned into a CH 4 source during the summer owing to the high and variable CH 4 emissions from the riparian wetland and the lower part of the hillslope. Overall, our results indicated that CH 4 emissions from small riparian areas are important in controlling forest CH 4 dynamics at a watershed scale.