Iron-loaded biochar enhances electron shuttle function to promote paddy soil dissimilatory nitrate reduction to ammonium

Increasing dissimilatory nitrate reduction to ammonium in agricultural soils enhances nitrogen retention. Because this pathway requires more electrons than denitrification, it is often limited by electron availability. Biochar functions as an electron shuttle in paddy soils, but whether enhancing this property further stimulates dissimilatory nitrate reduction to ammonium remains unclear. Here we show that loading redox-active iron onto biochar strengthens its electron-shuttling capacity and increases the rate of dissimilatory nitrate reduction to ammonium by 3.2-fold, raising its contribution from 5.4% to 19.7% of total nitrate reduction. Enhanced electron-shuttling capacity is positively associated with surface iron content and promotes enrichment of bacteria carrying the nrfA gene together with higher nrfA expression. These results demonstrate that redox-active iron enhances biochar-mediated electron transfer, increasing electron flux to nitrate-reducing microorganisms and thereby promoting nitrogen retention while mitigating nitrous oxide emissions.