Mechanistic insights into mineral-based potassium humate regulating rhizosphere microenvironment and microbial community assembly in paddy soils along a Cadmium contamination gradient

Mineral-based potassium humate (MBPH) is an organic macromolecular compound applied in Cd-contaminated soil remediation, but its regulatory effects on the rice rhizosphere soil microenvironment under Cd pollution remain unclear. A pot experiment was conducted to assess the impacts of MBPH (0, 0.25%, 0.5%, 1% w/w) on rice Cd accumulation, rhizosphere soil chemical properties, available microelements, and microbial communities in slightly (1 mg kg −1 ), moderately (2 mg kg −1 ), and highly (4 mg kg −1 ) Cd-contaminated soils. Results showed that 0.25% low-dose MBPH significantly reduced brown rice Cd by 46.8% and 44.0% in slightly and moderately contaminated soils, respectively; 0.5% and 1% MBPH treatments decreased brown rice Cd to meet China’s national food safety limit in highly contaminated soil. MBPH application significantly increased rhizosphere soil pH, electrical conductivity, organic matter, nitrate nitrogen, available potassium, and available iron, manganese, copper, and zinc to varying degrees across Cd pollution gradients, while reducing ammonium nitrogen and DTPA-extractable Cd in all tested soils. MBPH addition improved microbial richness, drove a significant dose-dependent increase in Cd-tolerant Chloroflexi and decrease in labile organic matter-degrading Proteobacteria, and particularly at the 0.5% dose, enhanced the complexity and modularity of the rhizosphere microbial network and the number of keystone species. Mantel test revealed that ammonium nitrogen, available potassium, and available zinc were core drivers of microbial community assembly in slightly, moderately, and highly contaminated soils, respectively. Overall, MBPH promotes Cd immobilization by reducing Cd bioavailability via rhizosphere property optimization and driving beneficial bacterial community succession.