Abstract— <p>The microbiome, enzymatic activity, microbial biomass carbon, and cellulose decomposition are characterized in the main soil types of the southern Vitim Plateau and the Selenga Highlands. The quantitative and qualitative parameters of microbial communities in various soil types in Western Transbaikalia are characterized. It is found that the abundance of bacteria, actinomycetes, and fungi, as well as enzymatic activity, are closely related to organic carbon (C_org) content, temperature, and soil moisture (<i>r</i> = 0.63–0.99). The highest bacterial abundance and maximum C-biomass values are observed in burozems and humus–hydrometamorphic permafrost soils, characterized by high Corg content. A significant biomass of actinomycete and fungal mycelium is found in dark gray neopodzolized soils. The lowest levels of prokaryotic microorganisms are characteristic of chestnut soils and fine-carbonate chernozems due to their low Corg content and moisture deficit. Catalase intensity varies from low to medium, and its activity varies from 2.9 to 10.8 mL O₂ (1 g/3 min), reaching a maximum in the humus pockets of burozems and a minimum in fine-carbonate chernozems. The highest values of invertase activity are characteristic of burozems and their humus pockets, and the lowest values are characteristic of fine-carbonate chernozems. Soil cellulolytic activity varies from a minimum (21%) in fine-carbonate chernozems to a maximum (78%) in quasigley chernozems. The new data can provide insight into the stability of the microbiome in the studied soils of the region. The research results suggest a significant difference in biological activity between permafrost-affected and long-term seasonally frozen soils and highlight the importance of organic matter as a key factor in shaping soil biological activity.</p>