In a 30-day incubation experiment, the impact of biochar and plant composts on basal and substrate-induced respiration and the activity of catalase, dehydrogenase, acid phosphatase, and urease enzymes in low-humus agrosoddy-podzolic soil was studied. Biochar was obtained by fast pyrolysis from birch wood at 600°C. Composts from plant materials with different nitrogen contents were formed with and without the addition of biochar. Biochar and composts were added to the soil in an amount of 1%. The application of all the studied ameliorants reliably increased both basal and substrate-induced respiration by 1.5–2.1 times. At the same time, biochar made a significantly greater contribution to the increase in the rate of substrate-induced respiration and the amount of microbial biomass carbon that plant-based composts. Analysis of microbial respiration indices attests to more favorable conditions for the functioning of microbial communities after the addition of biochar and composts into the soil. Increased microbiological activity was manifested in growing activity of the studied enzymes and organic matter mineralization. It is advisable to introduce biochar into the soil only in combination with organic fertilizers or use it as a component in composting organic waste.