In response to the issues of low denitrification efficiency and high N₂O emissions in the biological nitrogen removal process of low C/N municipal wastewater, studies typically address these challenges by adding carbon sources. In this study, the addition of microorganisms enhanced the carbon flow and electron transport for nitrate reduction, significantly improving the denitrification performance of low C/N wastewater and reducing N₂O production. Proteomic analysis was employed to explore the mechanisms underlying this effect. The results revealed that the metabolites produced by the added microorganisms, S. oneidensis MR-1 and B. subtilis, including biosurfactants, heme, and cytochromes, altered the intracellular carbon redistribution in P. denitrificans, leading to an increased carbon flow directed towards nitrate reduction, thus enhancing total nitrogen removal efficiency.