Transformation of microbial cenoses in soils of light coniferous forests caused by cuttings and fires in the Lower Angara River basin

The influence of surface fires and cutting on the quantitative and functional parameters of microbial cenoses in the soils of light coniferous forests in the Lower Angara River basin was studied. In the litters of soddy-podzolic soils under pine forests, the microbial biomass was 4080–4700 μg C/g; the basal respiration was 17.00–20.32 μg C-CO₂/g/h; and the qCO₂, 4.17–4.33 μg C-CO₂/mg C_mic/h. In the humus-accumulative horizon, these values were 880–1160 μg C/g, 2.48–4.12 μg C-CO₂/g/h, and 2.83–3.55 C-CO₂/mg C_mic/h, respectively. In the litter of the one-year-old felled area, the content of microbial biomass carbon was by two times lower; in the litter of burned plots, it was by 60–70% lower than in the litter of the control area. The intensity of the microbial respiration did not change proportionally to the microbial biomass content, which resulted in an imbalance between the processes of the organic matter mineralization-immobilization towards a release of CO₂ as evidenced by the increase of the qCO₂ values by 2–4 times. In the five-year-old felled area, at the stage of restoring the herbaceous vegetation, a tendency towards the stabilization of the destructive microbiological processes was revealed. In the felled areas, the high number of heterotrophic microorganisms, the reduced oligotrophy of the soil organic horizons, and the more intense microbiological mineralization of the organic matter were observed. The surface fires in the felled areas and forests significantly affected the structure and the number of ecological-trophic groups of microorganisms in the litters, the humus-accumulative horizons, and in the upper mineral soil layers. The maximal structural and functional disturbance in the soil microbial complex was found in the logged areas affected by fires.