The success and feasibility of CO2-sequestration through incorporation of biochar into soils depends strongly on the long-term biochar stability and on the improvements of physical and microbial soil properties. In this study, the stability of two maize-derived biochars (from pyrolysis and hydrothermal carbonization) and of a compost-biochar mixture and their effects on microbial biomass and enzyme activity were determined in two soils during a 57-day incubation. Soil samples amended with biochar increased soil organic carbon (SOC) content by 20 or 40%. Samples amended with hydrothermal biochar showed the largest respiration rates and the largest increase in microbial and enzymatic activity compared with the untreated controls. Carbon and 13C mass balances showed that between 13 and 16% of the added hydrochar was mineralized within 8 weeks. In the arable soil, hydrochar additions greatly stimulated the degradation of SOC, thus inducing positive priming effects. The mineralization of pyrogenic biochar (pyrochar and a pyrochar-compost mixture) was significantly less (1.4–3%) and comparable to the SOC mineralization in the control soils.