Synergistic effects of Fe–Mn modified biochar and water management on remediation of Cd and Hg co-contaminated soils

Cadmium (Cd) and mercury (Hg) contamination in paddy soils presents a formidable challenge to global food security and human health, primarily due to their contrasting biogeochemical behaviors under varying water management regimes. This study investigated an integrated remediation strategy combining iron–manganese oxide modified biochar (FMBC) application with two distinct water management regimes (continuous flooding and aerobic irrigation) to synergistically mitigate Cd and Hg accumulation in rice and elucidate their underlying mechanisms, including the impacts on soil microbial communities. Pot experiments revealed that continuous flooding effectively decreased soil Cd bioavailability and promoted iron−manganese oxide plaque (IMP) formation on the root surface, thereby impeding Cd uptake. Conversely, this condition inadvertently promoted the relative abundance of hgcA-containing methylating microorganisms, thereby elevating the risk of Hg methylation. In contrast, aerobic irrigation reduced total Hg (THg) and methylmercury (MeHg) accumulation in rice grains, yet concurrently exacerbated Cd uptake by the plants. The incorporation of FMBC effectively resolved this antagonistic dilemma through dual immobilization mechanisms. Specifically, the combination of continuous flooding and FMBC application minimized grain Cd concentration (0.05 mg kg−1), while simultaneously suppressing the flooding-induced proliferation of hgcA-containing microbes. Conversely, integrating aerobic irrigation with FMBC achieved the lowest grain THg (0.02 mg kg−1) and MeHg (6.89 μg kg−1) levels, while simultaneously restricting aerobic-induced Cd uptake. Overall, the synergistic integration of FMBC with specific water management provides a highly effective and sustainable strategy for mitigating both Cd and Hg bioaccumulation in rice grains, thereby enhancing food safety in co-contaminated paddy ecosystems.