Carbon monoxide fluxes of different soil layers in upland Canadian boreal forests

Abstract

Dark or low-light carbon monoxide fluxes at upland Canadian boreal forest sites were measured on-site with static chambers and with a laboratory incubation technique using cores from different depths at the same sites. Three different upland black spruce sites, burned in 1987, 1992 and 1995 and a control site, were chosen to determine the effects of fire, temperature, soil structure and soil covers on CO fluxes. Three different surfaces were observed at the sites — bare mineral soil with little living moss cover; burned feather mosses 5–30 cm deep; and unburned, living, green feather mosses. The static chamber measurements indicated similar deposition velocities for the burned and unburned feather moss sites [(1.54 ± 0.64) 10^-2 cm s^-1 ; (1.83 ± 0.63).10^-2 cm s^-1 ], but significantly lower rates for sites that had burned down to the mineral soil [(1.08 ± 0.53).10^-2 cm s^-1, excluding data with net CO emission]. This finding was confirmed by results from the incubation measurements and shows that fire intensities determine the long-term, post-fire effect on soil-atmosphere fluxes of CO. Temperature studies with the cores showed that CO consumption rates increased from (2 ± 1)% at – 15 °C to — 13 °C to (43 ± 20)% at 0 °C to 1.5 °C and (68 ± 15)% at 4 °C to 5 °C of the deposition velocity values obtained at 20 °C. This temperature dependence was consistent with results obtained from the static chamber measurements. The temperature range studied and the dark or low-light conditions were representative for the night-time of nearly the whole six snow-free months in the boreal ecosystem. In nearly all cases, deposition velocities determined for cores from the top 5 cm with the incubation technique were the same, within experimental errors, as those determined with the static chambers. Soil CO concentration profiles taken in situ, moreover, did not show any clear trend below 5 cm. Thus we conclude that the top 5 cm of soils are determining the dark soil-atmosphere CO fluxes at these sites. The top 5 cm of soil columns are most exposed to temperature (and probably moisture) variations and are most affected by fires as well.