This study investigated the role of biochar addition on promoting microbial nitrogen transformations in intermittent aerated subsurface flow constructed wetlands (SSF CWs) under various influent C/N ratios. The results showed that the wetland with biochar addition achieved relatively higher COD (89–99%), NH ₄ ⁺ -N (97–99%) and TN (46–98%) removals. Biochar addition significantly improved TN removal, particularly under low influent C/N ratios (<7). Besides, the high adsorption characteristic of biochar attributed to the better removal performance of organics and ammonium. Moreover, the characteristics of dissolved organic matter (DOM) released from biochar was exploited by fluorescence excitation-emission spectrophotometry and parallel factor analysis (EEM-PARAFAC) combining with UV–Visible spectroscopy and results demonstrated that the adsorption-desorption of organics on biochar and DOM released from biochar were the main contributing factor for enhanced denitrification in SSF CWs.