Synergistic Cadmium Immobilization in Paddy Soil by Klebsiella aerogenes Wn Combined with Hydroxyapatite
Phosphate-solubilizing microorganisms (PSM) can immobilize cadmium (Cd) by dissolving insoluble phosphates and inducing the precipitation of stable Cd-phosphate minerals. However, the low bioavailability of soil phosphorus and the relatively low Cd2+ concentrations often limit the efficiency of microbial-induced phosphate precipitation (MIPP). This study proposed a combined strategy using the phosphate-solubilizing bacterium Klebsiella aerogenes Wn (Wn) and hydroxyapatite (HAP) to enhance Cd immobilization in paddy soil. Pot experiments were conducted to evaluate the effects on soil Cd availability and rice grain Cd accumulation, and the underlying mechanisms were investigated. Results showed that the combined treatment stabilized soil pH between 5.8 and 6.7 and electrical conductivity between 66 and 290 μS·cm−1, while increasing available phosphorus by 4%–67%. The optimal treatment (T4: 8 g·kg−1 HAP + 3.75×10^9 cfu·kg−1 Wn) reduced soil available Cd by 40.2% and decreased rice grain Cd to 0.0699 mg·kg−1, which is 65% below the national food safety limit. Microbial community analysis confirmed successful colonization of Wn. Pure culture experiments demonstrated that Wn induced phosphate precipitation, forming a more stable cadmium hydroxyapatite mineral [Ca3.9(Ca4.7Cd0.7)(PO4)6(OH)1.8]. The combined Wn-HAP treatment is an efficient strategy for remediating Cd-contaminated farmland, with significant potential for ensuring agricultural product safety and promoting soil remediation.