Continuous measurement of methane flux over a larch forest using a relaxed eddy accumulation method

Abstract

We measured the methane flux of a forest canopy throughout a year using a relaxed eddy accumulation (REA) method. This sampling system was carefully validated against heat and CO2 fluxes measured by the eddy covariance method. Although the sampling system was robust, there were large uncertainties in the measured methane fluxes because of the limited precision of the methane gas analyzer. Based on the spectral characteristics of signals from the methane analyzer and the diurnal variations in the standard deviation of the vertical wind velocity, we found the daytime and nighttime precision of half-hourly methane flux measurements to be approximately 1.2 and 0.7 μg CH4 m−2 s−1, respectively. Additional uncertainties caused by the dilution effect were estimated to affect the accuracy by as much as 0.21 μg CH4 m−2 s−1 on a half-hourly basis. Diurnal and seasonal variations were observed in the measured fluxes. The biological emission from plant leaves was not observed in our studies, and thus could be negligible at the canopy-scale exchange. The annual methane sink was 835 ± 175 mg CH4 m−2 year−1 (8.35 kg CH4 ha−1 year−1), which was comparable to the flux range of 379–2,478 mg CH4 m−2 year−1 previously measured in other Japanese forest soils. This study indicated that the REA method could be a promising technique to measure canopy scale methane fluxes over forests, but further improvement of precision of the analyzer will be required.