Redox-active Mn loading enhances the electron-shuttling capacity of biochar to promote DNRA in soil

Enhancing dissimilatory nitrate reduction to NH 4 + –N (DNRA) in agricultural soils provides environmental and agronomic benefits by improving nitrogen retention and reducing nitrogen losses. However, DNRA requires a higher electron input (8 e − per NO 3 − ) than denitrification (5 e − per NO 3 − ), which often results in denitrification dominating nitrate reduction in soils. Although biochar can function as an electron shuttle to facilitate DNRA, it remains unclear whether enhancing its electron-shuttling capacity can further stimulate this nitrogen-conserving pathway. Here, we prepared a biochar with enhanced electron-shuttling capacity by loading redox-active Mn phases onto its surface and evaluated its effects on DNRA in soil. Mn-loaded biochar markedly enhanced electron-shuttling capacity and increased soil DNRA rates by 2.9-fold compared with the control. The electron-shuttling capacity of biochar was positively correlated with surface redox-active Mn content, and both DNRA rates and their contribution to total nitrate reduction increased accordingly. In addition, Mn-loaded biochar increased the relative abundance of nrfA -type bacterial taxa (e.g., Bacteroidota, Ignavibacteriota, and Bacillota) and significantly upregulated nrfA gene expression. These results demonstrate that loading redox-active Mn onto biochar effectively enhances its electron-shuttling function. Reversible Mn redox cycling may facilitate electron transfer, thereby increasing electron availability for nitrate reduction to NH 4 + –N. Meanwhile, Mn-mediated mobilization of native soil dissolved organic carbon (DOC), without increasing carbon loss, may provide supplementary bioavailable electron donors, collectively promoting DNRA and enhancing soil nitrogen conservation, which was accompanied by lower N 2 O emissions.