Depending on the geochemical forms, heavy metal (HM) accumulation is one of the most serious environmental problems in the world and poses negative impacts on soil, plants, animals, and humans. Although the use of biochar to remediate contaminated soils is well known, the huge quantities of waste used and its recycling technique to sustain soil in addition to its use conditions are determinant factors for its characteristics and uses. A pot experiment was conducted in a completely randomized block design to evaluate metal forms and their availability under the application of garden waste biochar (GB) pyrolyzed at different temperatures, and a sequential extraction procedure was designed to fractionate Pb, Cd, Zn, and Cu of the contaminated soil. The results show that the TCLP-extractable Pb, Cd, Zn, and Cu were significantly decreased depending on the biochar addition rate, pyrolysis temperature, and tested metal. The acid extractable fraction was significantly decreased by 51.54, 26.42, 16.01, and 74.13% for Pb, Cd, Zn, and Cu, respectively, at the highest application level of GB400 compared to untreated pots. On the other hand, the organic matter bound fraction increased by 76.10, 54.69, 23.72, and 43.87% for the corresponding metals. The Fe/Mn oxide bound fraction was the predominant portion of lead (57.25–62.84%), whereas the acid fraction was major in the case of Cd (58.06–77.05%). The availability of these metals varied according to the application rate, pyrolysis temperature, and examined metals. Therefore, the GB is a nominee as a promising practice to reduce HM risks, especially pyrolyzed at 400 °C by converting the available fraction into unavailable ones.
