A monitoring system for evaluating the responses in CH4 uptake to rising CH4 concentrations in forest soils based on a closed chamber system

Upland soils are an important methane (CH4) sink at the global scale, but their responses to rising CH4 concentrations are not well understood. In this study, we developed a system for the in situ evaluation of the responses in CH4 uptake in forest soils to rising CH4 concentrations, and applied this system to a temperate forest in Japan. The system was used to precisely inject air with a high CH4 concentration into an automated closed chamber, and was used to quasi-continuously measure CH4 uptake under higher CH4 concentrations than ambient air up to 2.6 ppm. Artificial diffusion associated with a sudden increase in CH4 concentrations within the chamber was compensated for with a diffusive flux calculated with a multi-layer diffusion model. We also conducted a long-enclosure experiment, in which CH4 concentrations within the chamber were gradually decreased from 3.0 ppm to 1.9 ppm over two to four hours to minimize artificial diffusion. The measured responses in CH4 uptake to CH4 concentration ranged from 0.10 to 2.47 nmol m-2 s-1 ppm-1 or varied from 1.3 to 1.6 times ppm-1 in four plots that chamber experiments were conducted. Based on the CH4 concentration profiles within soils, CH4 oxidation was estimated to occur within the top 10-cm soil. To further obtain accurate estimates in soil responses to rising CH4 concentrations, the effect of artificial diffusion must be minimized in experiments, as this was the greatest source of uncertainty in this study.