Abstract: The study investigated the effects of corn straw biochars (BC300, BC400, and BC500), pyrolyzed at 300, 400, and 500 ℃ respectively, on nitrate removal and N₂O emission in aqueous system mediated by the aerobic denitrifying bacterium Paraclostridium benzoelyticum strain KD1 across varying C/N ratios (4, 6, and 8). Following comprehensive biochar characterization and bacterial strain screening, a series of aerobic incubation experiments were conducted using simulated wastewater containing nitrate (15 mg·L^-1 in N). The experimental design included biochar-amended treatments and a non-amended control, with continuous monitoring of N₂O emission and nitrogen species transformation dynamics. Results demonstrate that the selected strain exhibited an average NO₃^--N removal rate of 0.322 mg·L^-1·h^-1 during the initial 24-hour period. At C/N ratios of 4 and 6, biochar amendments significantly enhanced NO₃^--N removal by denitrification while concurrently suppressing N₂O emission, with BC400 showing the most pronounced effects. The superior performance of BC400 was attributed to its relatively higher dissolved organic carbon content and abundant surface redox-active functional groups, which facilitated both denitrification process and N₂O reduction. Additionally, BC400's promotion of dissimilatory nitrate reduction to ammonium within the incubation system may also have contributed to the observed N₂O emission suppression. Contrastingly, at a higher C/N ratio of 8, biochar amendments exhibited an inhibitory effect on NO₃^--N removal through denitrification. In this scenario, the cumulative N₂O emissions from biochar-amended treatments slightly exceeded that of the control, suggesting that sufficient exogenous carbon may exert a more substantial influence on N₂O production and reduction than biochar amendment under these conditions.