Constraining models for methane oxidation based on long-term continuous chamber measurements in a temperate forest soil

Upland soils are thought to be a sink of CH₄, the second most important anthropogenic greenhouse gas, owing to oxidation by methanotrophs. To better understand CH₄ fluxes in upland forests, we quasi-continuously measured CH₄ fluxes using an automated closed chamber system over seven years on a larch plantation in a volcanic soil in Japan. We hypothesized that the long-term data sufficiently can calibrate modules for CH₄ fluxes, and aimed to predict future pathways of CH₄ uptake and their uncertainties in the forest. Based on the observations, a thinning of the overstory only marginally influenced the CH₄ fluxes measured by the chambers. Using the data with a Bayesian method, we calibrated four modules for CH₄ fluxes in forest soils, which were embedded in the process-based ecosystem model VISIT. The modules well reproduced the observed seasonality, annual budgets, and interannual variability in the CH₄ fluxes after calibrating the following parameters: the diffusion coefficient or base CH₄ oxidation rate constant and temperature sensitivity. The CH₄ fluxes were predicted to increase in the future under the RCP8.5 scenario but to decrease under the RCP 2.6 scenario. The contrasting trajectory was caused by rising and decreasing CH₄ concentrations under the RCP 8.5 and 2.6 scenarios, respectively. Furthermore, the magnitudes of the future changes in the fluxes differed in each module because the responses to the changes in the CH₄ concentrations were inconsistent among the modules. The observed CH₄ fluxes increased with increasing atmospheric CH₄ concentration (4.95 mg CH₄ m⁻² d⁻¹ ppm⁻¹), which was greater in magnitude than those in the modules. Considering the uncertainties in the modules and potential confounding effects in the observations, we conclude that further understanding the responses of CH₄ uptake to rising CH₄ concentrations is required.