Biochar and hydroxyapatite enhance both phytoextraction and phytostabilization of a heavily Cd-polluted soil using sweet sorghum.

Sweet sorghum has a high tolerance to toxic metals, but its response to soil amendments in Cd-polluted soils remains underexplored. Here, we compared the effects of biochar and hydroxyapatite (HAP) at different doses (0, 0.5%, and 1%, w/w) on the growth, mineral nutrition, stress tolerance, and phytoremediation efficiency of sweet sorghum grown in an agricultural soil heavily polluted by Cd (20.80 mg/kg). In most cases, both amendments greatly increased plant biomass and nutrient uptake of N, P, and K, and enhanced the activities of antioxidative enzymes (peroxidase and superoxide dismutase) in leaves. They also increased soil pH and reduced soil available Cd concentrations, confirming their liming effects and immobilization of Cd. Unexpectedly, both HAP and biochar significantly increased Cd concentration and uptake in the shoots and roots, contradicting prior findings. Shoot Cd uptake was increased by 96% and 57% by 0.5% and 1% biochar, respectively. Root Cd uptake was increased by 50% and 64% by 1% HAP and 0.5% biochar, respectively. Overall, both HAP and biochar improved sweet sorghum growth and mitigated Cd toxicity, and particularly, decreased soil Cd availability while enhancing shoot and root Cd accumulation, indicating a promising strategy for the phytoextraction and phytostabilization of Cd-polluted soils.