Biochar-mediated N-Fe redox coupling for Cd immobilization and N₂O mitigation in flooded paddy soil

Coupled soil N and Fe redox processes influence heavy-metal mobility and greenhouse-gas emissions. As an electron shuttle, biochar can directly mediate N and Fe redox transformations. However, how this function regulates electron allocation within N-Fe redox processes and whether this regulation contributes to Cd immobilization and N₂O mitigation remains unclear. Here, anaerobic incubation experiments combined with electrochemical characterization, electron allocation analysis and metagenomic analysis were conducted in Cd-contaminated paddy soils. Nitrate addition suppressed Fe(III) reduction and amorphous Fe oxide formation in the control treatments, indicating competition between nitrate and Fe(III) reduction. In contrast, biochar increased Fe(III) and nitrate reduction rates by 18-402% and 53-258%, respectively. Biochar also enhanced soil electron-transfer rate and capacity, enriched electroactive microorganisms and extracellular electron-transfer functional genes. The electron allocation analysis showed that biochar increased total quantified electron consumption and suggested that biochar may alleviate competition between nitrate and Fe(III) reduction. Enhanced Fe(III) reduction increased Fe(II) production and may have promoted nitrate-associated Fe(II) oxidation, with amorphous Fe oxide contents increasing by 9-63%. These Fe transformations redistributed Cd into Fe/Mn oxide-bound fractions and decreased CaCl₂-extractable Cd by 29-55%. Within the nitrate reduction pathways, biochar increased absolute electron consumption by dissimilatory nitrate reduction to ammonium (DNRA), while decreasing that by denitrification. These changes were accompanied by 10-49% higher NH₄⁺ concentrations and 13-19% lower cumulative N₂O emissions. These findings highlight the role of biochar-mediated electron transfer in regulating electron allocation within coupled N-Fe redox processes, providing novel insights into concurrent Cd immobilization and N₂O mitigation in paddy soils.