Co-application of biochar and hydroxyapatite suppresses lead accumulation in rice via a soil-plant-microbe cascade

Stabilizing soil lead (Pb) with single amendments often requires a trade-off between immobilization durability and cost-effectiveness, restricting their large-scale use. Here, we demonstrate that co-applying hydroxyapatite (HAP) with either rice straw biochar (RBP) or maize straw biochar (MBP) overcomes these limitations through synergistic soil-plant-microbe interactions. Compared to the un-amended control, the combined treatments reduced CaCl₂-extractable Pb by 93.2-95.5% and the acid-soluble Pb fraction by 39.4-41.8%. Spectroscopic analyses confirmed the direct binding of Pb to biochar surfaces, while an amendment-induced increase in soil pH further enhanced Pb immobilization. Consequently, this reduced Pb availability in the rhizosphere was associated with diminished iron plaque formation on root surfaces, ultimately decreasing root Pb accumulation. At the subcellular level in leaves, the combined treatments promoted Pb sequestration within vacuoles and reduced the organelle-bound Pb fraction by 16.1-31.9%. This subcellular shift significantly lowered the leaf-to-grain transfer factor by 49.4-54.3%. Furthermore, the combined treatments (RBP) enhanced soil bacterial alpha diversity and shifted community composition; notably, the enrichment of Bacteroidota and suppression of Desulfobacterota suggest their potential contribution to further Pb stabilization. Through these integrated mechanisms, Pb concentrations in brown rice were reduced below Chinese national food safety limits. These findings highlight that the co-application of biochar and HAP provides a cost-effective, multi-barrier strategy to mitigate soil-to-grain Pb transfer, enabling the sustainable utilization of Pb-contaminated paddy soils.