Biological consumption by aerobic soils is an important, but poorly understood sink for carbon monoxide, a chemically active atmospheric trace gas. We used laboratory experiments to characterize CO consumption in representative soils of the northern boreal forest. This ecozone may be important in atmospheric CO consumption because it occupies 13% of the world's landmass. Soils initially showing net CO consumption emitted CO following biocidal treatment (ethylene oxide or γ-irradiation), indicating that CO consumption was biologically mediated. The use of selective inhibitors (cycloheximide, streptomycin) suggested that both prokaryotes and eukaryotes were responsible. Soil profiles generally indicated net consumption of atmospheric CO to a depth of 15 cm (concentrations decreasing from ∼150 to <20 nl l-1with depth) and a dynamic equilibrium between CO production and consumption in deeper soils (concentrations nearly constant at <20 nl l-1). Soils to a depth of 30 cm showed vigorous CO-consuming activity, suggesting that local CO production provided necessary substrate beneath the 15 cm surface zone of atmospheric influence. Radiotracer experiments demonstrated that only 5-7% of assimilated14CO was incorporated into biomass in 5 cm core sections taken to 30 cm and that <1% of assimilated14CO was incorporated into cellular material by a methanotroph isolated from this soil. Collectively, these data point to nonutilitarian oxidation of CO by a diverse microbial community. Carbon monoxide oxidation increased with increasing temperature over the range 4-34°C, with a Q10of 1.8. Apparent half-saturation constants (8-36 (μl CO l-1), and maximum rates of CO consumption (0.7-2.7 μg g dry soil-1h-1) were comparable to reports for diverse temperate soil environments, pointing to a fundamental similarity among CO-consuming microbial communities in aerobic soils. © 2001 Elsevier Science Ltd. All rights reserved.