Work was conducted in the homogeneous larch forests of the Central Evenkia (N 64°, E 100°). The sample areas were put on the slopes of the southern and northern exposition differing with intensity of solar radiation, thickness of a forest floor, soil moisture and temperature, and thickness of the active soil layer. Linkage of microbial biomass, number of fragments of copies of DNA and heterotrophic activity of soil microbial communities with stocks of the labile and stable organic matter (OM) of the active layer is analyzed. It is shown that the majority of microbial communities of investigated soils is heterotrophic. Heterotrophic activity of this soils is closely bound to the OM labile fraction while stocks of a microbial pool depend on a share of the OM stable fraction. Connecting the quantitative and functional indices of soil microbial communities and quality of OM used by them as a substratum, it is possible to predict the intensity of carbon mobilization in cryogenic ecosystems at climate change.