Responses of soil physicochemical and biological properties to four‑year in‑situ passivation in Cd‑contaminated agricultural soils

Heavy metal pollution in agricultural soils poses serious threats to the ecological environment and food safety. Soil microorganisms are sensitive indicators of changes in soil quality. Most existing studies explore passivators’ effects on heavy metal bioavailability in contaminated farmland, yet few investigate their impacts on microbial communities. In this study, Illumina MiSeq high-throughput sequencing was used to investigate the responses of soil microbial communities to passivating agents in heavy-metal-contaminated soil through a long-term field experiment with different in situ passivating agents. The results showed that hydroxyapatite plus rice biochar (HR) and montmorillonite (M) produced the best remediation effects, and soil alkaline phosphatase (S-ALP) activity was also relatively high. Different types and concentrations of passivating agents had significant effects on bacterial α- and β-diversity. Treatments of 0.3% HR1, 0.6% M2, 1.2% M3, and 1.2% CA3 significantly reduced bacterial Chao1 and Shannon indices. The relative abundance of Chloroflexi also decreased significantly in the HR1, M2, M3, and CA3 treatments, whereas the relative abundances of Acidobacteriota, Firmicutes, Proteobacteria, Actinobacteriota, and Bacteroidota increased significantly in the treatments with HR1, HR2, HR3, M1, M2, M3, hydroxyapatite (H), wheat biochar (W), rice biochar (R), and CA1, CA2, CA3. These results indicate that passivating agent addition affects the relative abundance of different microbial groups by altering soil physicochemical properties and Cd bioavailability. This study provides field-scale observational data on microbial community shifts following four-year in-situ passivation in Cd-contaminated agricultural soil.