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

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) min(-1) using a linear regression, 924 +/- 8 mg CH4 m(-2) yr(-1) min(-1) using a quadratic regression, and 975 +/- 10 mg CH4 m(-2) yr(-1) min(-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) min(-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. (C) 2015 Elsevier B.V. All rights reserved.