Ferric sulfate-modified biochar as a dual-function amendment for methane mitigation and heavy metal risk control in flooded rice systems

Flooded rice fields are major sources of methane (CH 4 ) emissions and can facilitate the transfer of toxic metals into edible crops, raising concerns for both climate change and food safety. This study investigated ferric sulfate-modified biochar (Fe-BC) as a dual-function soil amendment in paddy systems. A two-season pot experiment was conducted to evaluate its effects on CH 4 emissions, rice yield, and heavy metal uptake. Compared to convential fertilization, Fe-BC application reduced cumulative CH 4 emissions by 34% and yield-scaled emissions by 47%, while increased rice yield by 14%. The mitigation effect was associated with enhanced CH 4 oxidation due to increased the abundance and activity of type II methanotrophs and improved soil redox conditions. Fe-BC also significantly reduced chromium (Cr) concentrations in rice grains by 56%, without increasing soil heavy metal levels. This effect may be related to pH-driven Cr(VI) reduction and immobilization via iron oxides. Physicochemical and microbial analyses further revealed improved DOC and acetate dynamics and stronger methanotroph network connectivity under Fe-BC treatment. These findings suggest that Fe-BC could serve as a promising strategy for simultaneous CH 4 mitigation and trace metal risk reduction in flooded agroecosystems, offering new insights into the design of multifunctional biochar-based technologies for sustainable agriculture. • Fe-modified biochar reduced CH 4 emissions by 34% over two growing seasons • Fe-BC selectively enriched type II methanotrophs, enhancing CH 4 oxidation capacity • Chromium uptake in rice grains decreased by 56% under Fe-BC treatment • Methanotrophs, not methanogens, were key drivers of CH 4 mitigation under Fe-BC