Mixotrophic denitrification driven by plant carbon sources and influent sulphate for enhancing nitrogen removal in constructed wetlands

A lab-scale horizontal baffled subsurface-flow constructed wetland (HBSFCW) packed with plant litter (reed straws (RS)) was built as a sustained-release carbon source, after which it was fed with synthetic wastewater that simulated the low C/N ratios in tailwater from wastewater treatment plants (WWTPs). The purpose of its operation was to explore the relationship between the nitrogen removal performance and variations in microbial community composition, and propose an enhanced nitrogen removal strategy for constructed wetlands (CWs). The results showed that plant carbon resources and influent sulphate can induce changes in redox conditions and organic matter concentrations at CWs, thereby influencing the microbial community composition and denitrification performance. High-throughput sequencing revealed the formation of richer microbial communities in additional RS units. Conventional heterotrophic denitrifying genera (Dechloromonas, Thauera and Ignavibacterium), sulphate-reducing genera (Desulfobacter, Desulfobulbus) and a sulphur-based denitrification genus (Sulfurimonas) were directly or indirectly affected by the addition of RS and became dominant in additional RS units. This study suggests that plant litter in HBSFCWs and sulphate from WWTP tailwater provide an opportunity to utilize sulphate as an electron-acceptor and reduced sulphur as an electron-donor for nitrate reduction, thus driving mixotrophic denitrification.