Synergistic remediation of cadmium/arsenic co-contaminated paddy soils by Alcaligenes eutrophus-loaded biochar: Microbial and geochemical regulation

Cadmium (Cd) and arsenic (As) co-contamination in paddy soils poses a critical threat to rice safety and agricultural sustainability, necessitating effective and eco-friendly remediation strategies. Here, a functional biochar was developed by immobilizing Alcaligenes eutrophus (AE) onto rice straw-derived biochar (BC) to produce Alcaligenes eutrophus -loaded biochar (BA), and its remediation performance was evaluated at 1% and 3% application rates in Cd/As co-contaminated paddy soils. Compared with BC or AE applied alone, the 3% BA treatment (BA3%) induced the greatest improvements in soil properties, notably increasing pH, organic carbon content, and phosphorus and potassium availability. Soil microbial community analysis revealed that BA3% enhanced bacterial α‑diversity by 53.8% and selectively enriched Fe/As transforming taxa, including Geobacteraceae and Sphingomonadaceae . Meanwhile, BA3%-induced soil extracellular polymeric substance enrichment also facilitates synergistic metal(loid)s immobilization. Consequently, BA3% achieved the highest remediation efficiency, reducing available Cd and As by 60.0% and 39.2%, respectively, and thereby decreasing their accumulation in rice tissues. Importantly, only BA3% reduced grain Cd and As concentrations below the Chinese food safety thresholds (GB 2762–2022, 0.2 mg kg −1 ). These shifts corresponded with substantial gains in rice performance, including maximum increases in root, shoot, and grain biomass of 103.6%, 33.2%, and 112.3%, respectively. Structural equation modeling highlights that regulating Cd/As bioavailability as the primary driver of grain safety. Collectively, this work reveals a microbe-biochar synergy, particularly at BA3%, that integrates geochemical immobilization and microbial functional regulation to control Cd/As transfer from soil to rice, offering a mechanistically and sustainable remediation strategy for contaminated paddy systems. • 3% Alcaligenes eutrophu s-loaded biochar (BA3%) effectively improved soil quality. • BA3% effectively reduced Cd and As bioavailability in co-contaminated soils. • BA3% enhanced EPS production and promoted Cd/As immobilization. • Fe- and As-transforming microbial taxa were enriched by BA3% application. • BA3% ensured grain safety and increased rice grain biomass by 112.3%.