Methane and carbon dioxide exchanges between the atmosphere and northern boreal forest soils

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Abstract
CH4 and CO2 fluxes were measured in upland boreal forest soils near Thompson, Manitoba, from May 16 to September 16, 1994. Most sites consumed atmospheric CH4, fluxes ranging from +0.6 to −2.6 mg CH4 m−2 d−1, and emitted CO2 at rates between 0.2 and 26.8 g CO2 m−2 d−1. There was some evidence of episodic CH4 emissions after heavy rainfall from soils which normally consumed CH4. There were two distinct groups: sites in which both CO2 and CH4 exchange was strong (mean 5.2 g CO2 m−2 d−1, −1.0 mg CH4 m−2 d−1); and those which had a weak exchange of both gases (mean 2.5 g CO2 m−2 d−1, −0.2 mg CH4 m−2 d−1). The presence of black spruce trees, a Sphagnum spp. ground cover and a thick organic layer (20–50 cm) characterized the weak exchange group. These characteristics were indicative of colder, wetter conditions with slower N cycling and longer path lengths to the zone of CH4 oxidation. The strong exchange group had either aspen, jack pine, or birch trees; a vascular plant cover; and a thin organic layer (1–5 cm). These characteristics were indicative of warmer, drier conditions with faster N cycling and shorter path lengths to the zone of CH4 oxidation. The seasonal average of both CO2 and CH4 flux from 11 sites could be predicted by regressions involving the amount of soil organic matter (0–5 cm depth) and seasonal temperature at a depth of 20 cm (r2=0.69 and 0.82, respectively). Using these vegetation characteristics and a Landsat image of the Boreal Ecosystem‐Atmosphere Study (BOREAS) northern study area for areal weighting, net gas fluxes were calculated for the upland soils, estimated to be −0.4 mg CH4 m−2 d−1 and 4 g CO2 m−2 d−1.