Nitrogen removal in constructed wetlands (CWs) is often challenged by limited nitrification due to low oxygen transfer and/or limited denitrification due to the lack of carbon source. In this study, a novel tidal-integrated vertical flow constructed wetland (T-IVCW) was developed to treat secondary effluent with low chemical oxygen demand/total nitrogen (C/N) ratio raw sewage as a carbon source. Three different mixing ratios of raw sewage to secondary effluent (3:7, 5:5, and 7:3) on nitrogen removal performance and microbial community were investigated. The results showed that mixing ratios slightly affected NH4+-N removal, but significantly affected TN removal. When the mixing ratio was 7:3 with influent C/N ratio of 2.35, high removal efficiencies of NH4+-N (85.08%) and TN (81.18%) were obtained. Cluster analysis revealed that mixing ratios and vertical variation of local redox conditions were the main drivers of the microbial community. The tidal zone was characterized by the presence of unclassified Xanthomonadaceae, Nitrospira, and Rhodanobacter, while Candidatus Brocadia and Denitratisoma, which involves in anammox and denitrification, dominated the community composition in the saturated zone. These results were further confirmed by the corresponding functional genes (amoA, nxrA, nirS, and anammox). Thus, the partial denitrification-anammox (PDN/AMX) and denitrification were proposed as the major pathways related to nitrogen removal. Furthermore, with the increase of mixing ratio, the PDN/AMX played an increasingly important role in nitrogen removal and accounted for at least 52.91% when the mixing ratio was 7:3. This study provides an alternative strategy for the treatment of low C/N ratio sewage and new insights into nitrogen transformation pathways in CWs.
