Simultaneous mitigation of cadmium contamination and greenhouse gas emissions in paddy soil by iron-modified biochar

Cadmium (Cd) contamination in agricultural soils is one of the major environmental challenges globally. Biochar is a promising material for mitigating Cd pollution, but it carries the risk of increasing greenhouse gas emissions. Herein, we incorporate iron-based materials into biochar to simultaneously enhance soil nutrients, mitigate heavy metal contamination, and reduce greenhouse gas emissions. The results showed that the iron-modified biochar (FeBC) increased soil available potassium, alkali-hydrolyzable nitrogen and soil organic carbon. All materials promoted the formation of strongly bound Cd (FMO-Cd), with FeBC outperforming standalone iron or biochar by reducing soil Cd bioavailability by 17.0-44.9 %. And the goethite-modified biochar (GBC) further enhanced iron plaque [FeO(OH)] formation, achieving the highest Cd reduction (80.4 %) in rice grains. In addition, except for biochar and zero-valent iron, the other treatments significantly suppressed CH₄ emission and stabilized CO₂ and N₂O. Among them, GBC treatment reduced the relative abundance of the mcrA gene, a CH₄ emission-related gene, by 22.7 %, ultimately leading to the highest reduction in CH₄ emissions (26.3 %). These findings suggest the potential of FeBC as soil amendments to improve soil nutrients and food safety, while reducing greenhouse gas emissions.