Combining rape biochar with hydroxyapatite: An approach to reduce Cd availability and uptake in maize while enhancing bacterial abundance in alkaline contaminated soil

Safe maize cultivation is at risk due to increasing cadmium (Cd) contamination in alkaline soils, requiring more research on efficient, soil-friendly biochar passivators suitable for specific conditions. Accordingly, a pot experiment was undertaken to investigate the implication of rape straw biochar at 1 % (LB) and 4 % (HB) concentrations, along with the 1 % compounded passivator where the biochar to hydroxyapatite (HAP) ratio is 3:1, on alkaline-contaminated soil. The bioavailable Cd concentrations in the rhizosphere decreased by 12.35 % (HB) and 5.19 % (LB-H), resulting in a corresponding decrease in Cd uptake by maize roots by 43.5 % (HB) and 22.4 % (LB-H). These findings are associated with elevated pH, which decreased Cd's bioavailability in the rhizosphere soil, consequently decreasing Cd enrichment in maize roots. The 16S rRNA sequencing revealed that LB-H has a beneficial effect on soil health, broadening the proliferation of plant growth-promoting bacteria, Actinobacteria (0.45 %), Blastococcus (0.05–0.10 %), and phosphate-solubilizing bacteria, Bacillus (0.045–0.085 %) in the rhizosphere soil. Redundancy and correlation analyses demonstrated that pH and bacterial composition are significant variables driving the immobilization of Cd in the soil. This study underscores the potential of utilizing low-concentration rape biochar (1 %) combined with HAP as an eco-friendly remediation agent to safeguard the production of maize on Cd-contaminated alkaline soil.