Efficient Cadmium-Tolerant Phosphate-Solubilizing Bacteria Loaded on Bone-Derived Biochar Effectively Alleviated Cadmium Pollution and Phosphorus Deficiency in Brassica napus L. Plantation

Both cadmium (Cd) contamination and phosphorus deficiency pose major constraints to sustainable crop production. This study loaded highly efficient Cd-tolerant phosphate-solubilizing bacteria (CdTPSB) onto calcium- and phosphorus-rich pig bone biochar (PBB) to construct a functional composite (MPBB), which was subsequently applied in a Brassica napus L. pot experiment. The results revealed that the CdTPSB possessed strong capabilities in phosphate solubilization (383.67–520.19 mg L−1), Cd immobilization (64.67–66.67%), IAA production (113.24–114.27 μg mL−1), and siderophore activity (65.02–89.08%). Batch experiments further showed that MPBB’s efficiencies in phosphate solubilization and Cd immobilization were significantly enhanced, which was putatively attributable to improved CdTPSB colonization and viability as well as to the activation of PBB functional groups. In the pot experiment, relative to the control treatment, MPBB significantly decreased the soil diethylenetriaminepentaacetic acid-extractable Cd content by 50.47% while increasing the soil labile phosphorus content by 3.32 times (p ≤ 0.05). This was correlated with the enrichment of Cd-immobilizing and phosphate-solubilizing bacteria, the upregulation of phosphate-solubilizing functional genes (pqqC and phoD), and enhanced soil enzyme activity. Consequently, Cd content in the shoots of Brassica napus L. was further decreased by 74.00%, which was correlated with increased polysaccharide content and enhanced Cd retention in root cell walls. Overall, this study highlighted MPBB’s potential as an eco-friendly Cd remediation agent and phosphate fertilizer to support the sustainable production of Brassica napus L.