Aquatic macrophytes mitigate the conflict between nitrogen removal and nitrous oxide emissions during tailwater treatments.

Tailwater from wastewater treatment plants (WWTP) usually reduces the nitrogen (N) removal efficiency while simultaneously elevates nitrous oxide (NO) emissions due to the low carbon-nitrogen (C/N) ratio. Conflicts between N removal and NO emissions require mitigation by selecting appropriate aquatic plants for tailwater treatment. In this study, a simulated tailwater mesocosm was established using three aquatic plants including Eichhornia crassipes, Myriophyllum aquaticum and Pistia stratiotes. Results of the N isotope mass balance analysis revealed the considerable contributions from plant uptake and benthic retention to overall N removal. It was demonstrated that the N assimilation efficiency of aquatic plants depended more on the root-shoot ratio rather than on growth rate. Furthermore, aquatic plants indirectly influence microbial N removal and NO emissions by altering the water quality parameters. Additionally, aquatic plants could regulate the N transformation through affecting the structure of bacterial community, including microbial abundance, diversity and association networks. Overall, the study underlined the enormous capacities of E. crassipes and P. stratiotes for N uptake and NO mitigation in tailwater treatment. Utilizing these two aquatic plants for phytoremediation may help mitigate the conflict between tailwater purification and NO production.