Abstract
Abstract. Methane flux measurements were carried out at a temperate forest (Haliburton Forest and Wildlife Reserve) in central Ontario (45°17´11´´ N, 78°32´19´´ W) from June to October 2011. Continuous measurements were made by an off-axis integrated cavity output spectrometer that measures methane (CH4) at 10 Hz sampling rates. Fluxes were calculated from the gas measurements in conjunction with wind data collected by a 3-D sonic anemometer using the eddy covariance (EC) method. Observed methane fluxes showed net uptake of CH4 over the measurement period with an average uptake flux (±standard deviation of the mean) of −2.7 ± 0.13 nmol m−2 s−1. Methane fluxes showed a seasonal progression with average rates of uptake increasing from June through September and remaining high in October. This pattern was consistent with a decreasing trend in soil moisture content at the monthly timescale. On the diurnal timescale, there was evidence of increased uptake during the day, when the mid-canopy wind speed was at a maximum. These patterns suggest that substrate supply of CH4 to methanotrophs, and in certain cases hypoxic soil conditions supporting methanogenesis in low-slope areas, drives the observed variability in fluxes. A network of soil static chambers used at the tower site showed reasonable agreement with the seasonal trend and overall magnitude of the eddy covariance flux measurements. This suggests that soil-level microbial processes, and not abiological leaf-level CH4 production, drive overall CH4 dynamics in temperate forest ecosystems such as Haliburton Forest.
Highlights
Model DevelopmentM serve) in central Ontario (45◦17 11 N, 78◦32 19 W) from June to October 2011
While local biogenic methane emission sources may be located to the south of the site, a back trajectory analysis indicates that the higher mixing ratios corresponded to air masses that had passed over the populated areas 200 km to the south with anthropogenic emissions on the order of tens of nmol m−2 s−1
Further support for net uptake in the vicinity of the tower comes from the canister measurements at the three levels on the tower, which showed a decrease in mixing ratio from above the forest canopy (32 m) to the near-ground level (2 m), with a vertical gradient ranging from 0 to 100 nmol mol−1
Summary
Model DevelopmentM serve) in central Ontario (45◦17 11 N, 78◦32 19 W) from June to October 2011. Methane fluxes showed a seasonal progression with average rates of uptake increasing from June through September and remaining high in October. This pattern was consistent with a decreasing trend in soil moisture content at the monthly timescale. There was evidence of increased uptake during the day, when the mid-canopy wind speed was at a maximum These patterns suggest that substrate supply of CH4 to methanotrophs, and in certain cases hypoxic soil conditions supporting methanogenesis in low-slope areas, drives the observed variability in fluxes. This suggests that soillevel microbial processes, and not abiological leaf-level CH4 production, drive overall CH4 dynamics in temperate forest ecosystems such as Haliburton Forest
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