Redox-active reactions in denitrification provided by biochars pyrolyzed at different temperatures.

The aim of this study was to investigate the role of redox-active components of biochar in the denitrification processes and N2 O reduction. Both biochars pyrolyzed at 300 and 800°C were separated into two redox-active components (i.e., dissolved aromatic moieties and condensed aromatic structure), then applied to study the reduction process of denitrifying bacteria from a paddy soil. Results demonstrated three main pathways of the biochar redox-active components for the denitrification processes and N2 O reduction. The biochar at 300°C and its redox-active components accelerated the first step of denitrification (i.e., NO3 - reduction), attributable to the oxidation of reduced phenolic moities to donate electrons. The biochar at 800°C and both redox-active components decreased total N denitrified because their dominant quinone moieties and electrical conductivity structure served as alternative electron acceptors. All the biochar treatments accelerated the last step of denitrification and decreased N2 O emission by 74.1%-99.9%. As electron donor, the biochar at 300 °Csignificantly increased humic acid-oxidizing and nitrate-reducing bacteria in the nosZ-harbouring bacterial community, which promoted N2 O reduction. The biochar at 800°C as electron sink decreased N2 O production, and as electron shuttle for nosZ-harbouring denitrifying bacteria, its electrical conductivity structure enhanced N2 O reduction.