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

摘要

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 nmolm~(-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±1.4nmolm~(-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.