Sustainable in-situ arsenic immobilization in paddy soils using magnetic biochar and the role of microbial functional genes.

Arsenic (As) contamination in paddy soils poses a threat to both rice safety and human health. To address this challenge, we developed and evaluated an in-situ remediation approach using magnetic iron oxide biochar (MBC) for paddy soils with varying As contamination levels, aiming to reduce As accumulation in rice grains. MBC significantly altered soil physicochemical properties [pH, dissolved organic carbon (DOC), soil organic matter (SOM), and Free iron oxides (Fe-ox)] at both tillering and maturity stages. Reduced available As by 31-65 % across low (136.68 mg kg-1), moderate (214.22 mg kg-1), and high (254.21 mg kg-1) contamination levels, and suppressed As translocation to rice grains by 62 %, ensuring brown rice As levels below the Chinese national safety threshold (GB 2762-2022, ≤0.35 mg kg-1). The paddy soils' metagenomic analysis revealed MBC-enriched Pseudomonadota and Actinomycetota with arsM and arsC genes, transitioning microbial networks from modular (tillering stage) to interconnected (maturity stage), enhancing arsenic detoxification and organic matter degradation. MBC enables efficient As immobilization and redox transformation, offering a scalable, eco-friendly solution for reconciling soil remediation with safe rice production in various As-contaminated regions.