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

Abstract

Forest soils are considered a methane (CH4) sink because dry soils can oxidize CH4; however, previous studies on CH4 fluxes in humid temperate forests indicated a high spatial and temporal variability in CH4 fluxes, especially in CH4 emissions from wet soils close to riparian zones, which can turn the soil of a whole forest from a CH4 sink to a CH4 source. In this study, the spatial and temporal variability of soil CH4 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 CH4 analyzers over a period of 2 years. We identified CH4 emission hot spots (60.2 ± 169.1 nmol m−2 s−1 from 117 sampling points) in the wetland in late summer, while the CH4 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 CH4 flux was amplified by a clear seasonal cycle of soil temperature and rainfall pattern under the Asian monsoon climate. The watershed-scale CH4 budget showed that the forest turned into a CH4 source during the summer owing to the high and variable CH4 emissions from the riparian wetland and the lower part of the hillslope. Overall, our results indicated that CH4 emissions from small riparian areas are important in controlling forest CH4 dynamics at a watershed scale.