Soil moisture effects on leaf litter decomposition and soil carbon dioxide efflux in wetland and upland forests

This study examined, first, the response of litter decomposition and soil CO₂efflux (R_S) to different soil moisture conditions and, second, the application of various litter decomposition functions in a wetland and upland forest dominated by Japanese alder. One upland site (US) and three wetland sites - a drained site (DS), poorly drained site (PDS), and surface saturated site (SSS) - were selected based on their variation in soil moisture conditions. Litter mass loss, as determined by a 4-yr litter bag incubation, was applied to Olson's simple exponential function, Berg's asymptotic function, and the rational function. The litter decomposition rate constant (yr^-1), which was commonly obtained by the simple exponential function, was highest in PDS (1.181), followed by SSS (0.950), DS (0.922), and US (0.528). The limit value of the litter mass loss, as determined by the asymptotic function was higher in DS (91.7%) and PDS (89.0%) than in SSS (76.9%) and US (70.5%). The rational function provided the most precise fitting of the litter mass loss pattern with few parameters. Periodic saturation and the higher leaf N content in PDS may enhance litter decomposition compared to constant saturation or drained conditions. The R_S(mg C m^-2h^-1) values, periodically measured using a portable infrared gas analyzer, were ranked in the order US (12.6-355.1) = DS (7.1-324.0) > PDS (5.5-220.9) > SSS (0.0-153.8). More hydric conditions probably reduced the vegetation biomass (in contribution to autotrophic R_S) and aerobic microbial activities (in contribution to heterotrophic R_S). The R_Stemperature dependency (Q₁₀) was little affected by soil moisture conditions, ranging from 2.48 to 2.69. It is concluded that the litter decomposition rate and R_Swere highest under periodic saturation and under lower soil moisture conditions, respectively.