This study investigated the effects of maize (Zea mays L.) straw biochar on phosphorus (P) availability in two soils with different P sorption capacities (iron and aluminum dominated slight acid Red earth and calcium dominated alkaline Fluvo-aquic soil). A 42-day incubation experiment was conducted to study how applications of biochar at different rates (0, 2, 4, and 8 % soil, w/w), in combination with and without mineral KH2PO4 fertilizer, affected contents of soil Olsen-P and soil microbial biomass P (SMB-P) and phosphomonoesterase activity. In addition, P sorption characteristics of soils amended with biochar, as well as main properties of the biochar and the soils, were determined. Application of 8 % biochar after 42 days of incubation substantially increased soil Olsen-P from 3 to 46 mg kg−1 in Red earth and from 13 to 137 mg kg−1 in Fluvo-aquic soil and increased SMB-P from 1 to 9 mg kg−1 in Red earth and from 9 to 21 mg kg−1 in Fluvo-aquic soil. The increase was mainly due to high concentrations of P in the ash fraction (77 % of total biochar P). Biochar effect on soil Olsen-P and SMB-P increased by higher biochar application rates and by lower P sorption capacity. Biochar application significantly reduced acid phosphomonoesterase activity in Red earth and alkaline phosphomonoesterase activity in Fluvo-aquic soil due to large amount of inorganic P added. We conclude that maize straw biochar is promising to potentially improve soil P availability in low-P soils, but further research at field scale is needed to confirm this.