Crop residue biochar modifies Pb fractionation and bioavailability to proso millet in calcareous soil

This study evaluated the efficacy of wheat (WB), maize (MB), and cotton (CB) biochars on changing soil Pb fractions, reduced partitioning index (RPI) and the Pb concentration in roots and shoots of Panicum miliaceum in a highly calcareous soil with different Pb-contamination levels (0, 150 and 300 mg kg−1). Among the treatments, WB at 3% (w/w) demonstrated superior performance, reducing soluble plus exchangeable Pb fraction (WsEx) by 36.7% and increasing residual fraction (Res) by 60.3%, while enhancing the RPI by 78%. This was associated with the higher Si content of WB as evidenced by FTIR spectra (1100 cm−1), and higher resultant soil pH values. Contrastingly, applying Pb at higher levels (300 mg kg−1) promoted the formation of less bioavailable fractions. All the biochars significantly decreased Pb content in Panicum miliaceum, with WB showing the highest reduction (23.4% in shoots, 15.8% in roots). In conclusion, WB showed the greatest potential to immobilize Pb in Pb-contaminated calcareous soil under Panicum miliaceum cultivation.