Wildfires in the north circumpolar region are increasing in response to global warming and raised precipitation irregularity. Beside the short-time effects of wildfires on carbon (C) cycle by CO2 boost, the decreased amounts and availability of remaining organic matter slow down microbial decomposition over mid- and long-term. Our objective was to investigate the effects of low-intensity surface wildfire common in forest-tundra on soil organic matter (SOM) stability. We hypothesized that wildfire crucially increases organic matter stability due to pyrogenic C production, which reduces SOM availability to microorganisms. To prove this hypothesis, we analyzed SOM stability by thermal analysis (thermogravimetry and differential scanning calorimetry) combined with microbial respiration and assessed temperature sensitivity of SOM decomposition. Wildfire in forest-tundra decreased the thermal labile SOM pool by 1.6–1.9 times and increased the most stable SOM pool by 2.1 times in the O-horizon and 1.3 times in the mineral topsoil. Fire increased SOM thermal stability stronger in the O-horizon compared to the mineral topsoil. For the first time, we revealed the relationship between SOM thermal stability and microbial activity in Cryosols. The increased stability of SOM in Cryosols raised temperature sensitivity (Q10) of SOM decomposition by microorganisms, decreased microbial respiration (CO2 efflux) and microbial biomass content. Concluding, wildfires have strong effects on soil and microbial properties, leaving predominantly stable organic matter pools, which are less available for microorganisms.