Lack of carbon source is the main factor limiting denitri cation of low C/N ratio wastewater in constructed wetlands (CWs). Agricultural waste has been considered as a supplementary carbon source but research is still limited. To solve this problem, ferric carbon (Fe-C) + zeolite, Fe-C + gravel and gravel were used as substrates to build CWs in this experiment, aiming to investigate the effects of different carbon sources (rice straw, corncobs, alkali-heated corncobs) on nitrogen removal performance and microbial community structure in CWs for low C/N wastewater. The results demonstrated that the microbial community and e uent nitrogen concentration of CWs were mainly in uenced by the carbon source rather than the substrate. Alkali-heated corncobs signi cantly enhanced the removal of NO 2 − -N, NH 4 + -N, NO 3 − -N and TN. Carbon sources addition increased microbial diversity. Alkali-heated corncobs addition signi cantly increased the abundance of heterotrophic denitrifying bacteria (Proteobacteria and Bacteroidota). Furthermore, alkali-heated corncobs addition increased the copy number of nirS, nosZ and nirK genes while greenhouse gas uxes were lower than common corncobs. In summary, alkali-heated corncobs can be considered as an effective carbon source.