Methane uptake in a temperate forest soil using continuous closed-chamber measurements

• CH4 fluxes were continuously measured using dynamic closed chambers at a temperate forest.
• The forest acted as an annual CH4 sink.
• CH4 uptake increased with increasing soil temperature and decreasing soil water content.
• Chamber-based CH4 fluxes were consistent with those by the micrometeorological method.
• Uncertainties by the chamber deployment period and the calculation methods were examined.

Methane (CH4) fluxes were continuously measured in a temperate forest soil using six dynamic closed chambers with a laser-based analyzer. CH4 uptake increased with an increase in soil temperature and a decrease in volumetric water content, where multiple linear regressions using the two variables explained approximately 80% of the seasonal variations. Based on the measurements, our forest acted as an annual CH4 sink with a spatial variation of 25%. The calculated annual CH4 sink significantly decreased by increasing the chamber deployment period: 8–13 mg CH4 m−2 yr−1 min−1. The annual CH4 sink was significantly different using different calculation methods for the initial slope: 898 ± 11 mg CH4 m−2 yr−1 using a linear regression, 924 ± 8 mg CH4 m−2 yr−1 using a quadratic regression, and 975 ± 10 mg CH4 m−2 yr−1 using an exponential model. These results indicate that underestimations due to disturbing the CH4 gradient between chamber headspaces and soils were significant despite of the short deployment periods of 4–10 min. Chamber-based fluxes were consistent with canopy-scale CH4 fluxes by the micrometeorological hyperbolic relaxed eddy accumulation method: sink of 731 ± 38 to 888 ± 87 mg CH4 m−2 yr−1. Continuous dynamic closed chambers with laser-based analyzers are a powerful tool to understand CH4 fluxes in temperate forest soils, as long as a range of uncertainties are carefully evaluated.