EFFECT OF FIRE ON SOIL-ATMOSPHERE EXCHANGE OF METHANE AND CARBON-DIOXIDE IN CANADIAN BOREAL FOREST SITES

During the spring and summer of 1994 we monitored soil-atmosphere exch anges of methane and carbon dioxide at upland sites in the Canadian bo real forest near the northern study area (NSA) of the Boreal Ecosystem -Atmosphere Study (BOREAS). The effects of fire on methane and carbon dioxide exchange in black spruce stands developed on clay soils were e valuated by measuring fluxes with dark chambers in unburned stands and stands burned in 1994, 1992, and 1987. Similar measurements were made in jack pine stands developed on sandy soils, one unburned and the ot her burned in 1989. All of the sites were net sinks of atmospheric met hane with median fluxes ranging from -0.3 to -1.4 mg CH4-C m(-2) d(-1) . Median fluxes of carbon dioxide from the forest floor to the atmosph ere ranged between 1 and 2 g C m(-2) d(-1). Both ecosystem characteris tics (e.g., soil and vegetation type) and burning history (time since burn and fire intensity) appear to have some effect on atmospheric met hane consumption and carbon dioxide emission by these forest soils. In general, the jack pine sites were stronger methane sinks and had lowe r carbon dioxide emissions than the black spruce sites. After a few ye ars of recovery, the burned sites tended to be slightly stronger metha ne sinks than unburned controls. Our results suggest that soil CO2 eff luxes from upland black spruce stands may not be immediately impacted by fire, possibly maintained at preburn levels by microbial decomposit ion of labile compounds released as a result of the fire. By 2 years p ostfire there appears to be a significant reduction in sail CO2 flux, due to the loss of tree root and moss respiration and possibly to the depletion of fire-related labile compounds. The observed recovery of s oil respiration rates to preburn levels by 7 years postburn is probabl y due to the respiration of regrowing vegetation and the combined effe cts of elevated soil temperatures (about 4 degrees to 5 degrees C warm er than unburned sites) and improved litter quality on soil microbial activities. We estimate that soil CO2 emissions from recently burned b oreal forest soils in the northern hemisphere could be of the order of 0.35 Pg C yr(-1), which is in good agreement with a previous estimate that was derived in a different manner.