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The purpose of the study was to understand the mechanisms of biochar-induced changes in the available content of aluminum (Al), cadmium (Cd), zinc (Zn), copper (Cu), and manganese (Mn) in a wide range of soils. Five soils from different regions of the Czech Republic were incubated for 12 weeks with four rates of biochar (0.5%, 2%, 4%, and 8% w/w). The available concentrations of Al, Cd, Zn, Cu, and Mn were determined on the 7th and 84th day of incubation. There was a significant decline in the available content of Al, Zn, Cu, Mn, and Cd except in the available content of Al in one soil, which is characterized by very low Al content, higher cation exchange capacity (CEC), and neutral pH = 7.0. The decline in the mobile contents of Al, Zn, Cu, Mn, and Cd was significant in all cases of 8% biochar rate. The decline in the content of Al, Zn, Cu, Mn, and Cd was mainly due to the increment in soil pH and increment in CEC, decline in dissolved organic carbon (DOC), and the release of exchangeable Ca2+ and K+ from biochar. The application of high amounts of biochar to soil could increase the available content of some metals like Al. On the other hand, biochar could efficiently reduce the mobility of Al, Zn, Cu, Mn, and Cd in soil, while the decline is mainly caused by biochar-induced changes in soil pH, CEC, DOC, and exchangeable Ca2+ and K+ content of treated soils.