A new study suggests that pairing iron and manganese modified biochar with carefully chosen irrigation strategies can reduce cadmium and mercury buildup in rice, offering a practical path toward safer food production in co-contaminated paddy fields.
Cadmium and mercury are among the most concerning pollutants in agricultural soils. Both can enter rice plants and accumulate in grains, creating potential risks for people who rely on rice as a daily staple. The challenge is especially difficult because the two metals behave differently in paddy fields. Flooding can help reduce cadmium uptake, but it may increase the formation of methylmercury, a highly toxic form of mercury. Aerobic irrigation can lower mercury accumulation, but it may increase cadmium uptake.
In a study published in Biochar, researchers tested whether iron and manganese oxide modified biochar, known as FMBC, could help solve this tradeoff. The team combined FMBC with two water management regimes, continuous flooding and continuous aerobic irrigation, in pot experiments using cadmium and mercury co-contaminated paddy soil.
"Rice farmers and soil managers often face a difficult choice: water conditions that reduce one metal may worsen the other," said corresponding author Yuebing Sun. "Our study shows that engineered biochar can help break this conflict by stabilizing both cadmium and mercury while also reshaping the microbial processes that control methylmercury formation."
The results showed a clear "see-saw" effect of water management. Continuous flooding reduced cadmium bioavailability and promoted iron and manganese plaques on rice roots, which acted as a barrier to cadmium uptake. Under flooding combined with FMBC, the cadmium concentration in rice grains fell to 0.05 mg kg⁻¹, well below China's food safety limit of 0.2 mg kg⁻¹.
However, flooding alone also encouraged the growth of hgcA-containing mercury-methylating microorganisms, which can convert mercury into methylmercury. In contrast, aerobic irrigation reduced total mercury and methylmercury in rice grains, but increased cadmium uptake.
FMBC helped address both sides of the problem. Under aerobic irrigation, FMBC produced the lowest mercury-related risks, with grain total mercury reaching 0.02 mg kg⁻¹ and methylmercury reaching 6.89 μg kg⁻¹. At the same time, it restricted the cadmium increase usually associated with aerobic conditions.
The study found that FMBC worked through multiple mechanisms. Its surface functional groups and iron and manganese oxides helped bind metals in less available forms. It also promoted root plaque formation, strengthened the plant's natural barrier against cadmium, and reduced the relative abundance of mercury-methylating microorganisms.
Health risk assessment further supported the benefits of the combined approach. Flooding plus FMBC sharply reduced cadmium-related dietary risk, while aerobic irrigation plus FMBC achieved the lowest hazard quotients for total mercury and methylmercury.
The authors note that the best management choice may depend on the dominant contaminant risk. For cadmium control, continuous flooding with FMBC was most effective. For mercury and methylmercury control, aerobic irrigation with FMBC performed best. Together, these findings point to a flexible strategy for paddy fields where both metals are present.
"This is not simply adding biochar to soil," Sun said. "It is about matching an engineered amendment with the right water regime so that soil chemistry, root barriers, and microbial communities work together to protect rice quality."
The researchers suggest that future field studies are needed to confirm long-term performance under real farming conditions, but the findings provide a promising foundation for safer rice production in contaminated regions.
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Journal Reference: Sun, T., Yang, W., Sun, Y. et al. Synergistic effects of Fe–Mn modified biochar and water management on remediation of Cd and Hg co-contaminated soils. Biochar 8, 121 (2026).
https://doi.org/10.1007/s42773-026-00616-w
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
