Aquatic Macrophytes Mitigate the Conflict between Tailwater Purification and Greenhouse Gas Production

Abstract

High nitrogen (N) load in tailwater from wastewater treatment plants (WWTP) is increasingly a cause for concern. However, the low carbon-nitrogen (C/N) ratio in tailwater usually inhibits the microbial N transformation and then reduces the N removal efficiency. Phytoremediation is a helpful technique for wastewater purification, however, the greenhouse gas effect should be seriously considered under the increasing global warming, since nitrous oxide (N2O) was inevitably produced from N migration and transformation in phytoremediation. Thus, the conflict caused by N removal and N2O emissions during phytoremediation should be mitigated by selecting appropriate aquatic plants for tailwater treatment. In the present study, a simulated tailwater mesocosm was set up and three aquatic plants including Eichhornia crassipes, Myriophyllum aquaticum and Pistia stratiotes were used for phytoremediation, especially for comparing the N removal efficiency and the N2O flux. The results of the 15N isotope mass balance analysis revealed the considerable contributions of plant uptake and benthic retention for overall N removal. Furthermore, we demonstrated that the N assimilation efficiency of aquatic plants depended on the root-shoot ratio rather than the growth rate. Owing to the effects of environmental factors, the aquatic plants could affect the microbial N removal and N2O emissions indirectly by altering the water quality parameters including pH, nitrate (NO3¯) concentration and nitrogen-phosphorus (N/P) ratio. As well, aquatic plants could regulate the N transformation through affecting the bacterial community structure. As the key nodes in association networks, several bacterial phyla involving Firmicutes, Proteobacteria, Planctomycetes, Latescibacteria and Chloroflexi, might play a vital role in bacteria community construction and the N removal function. Overall, we underlined the enormous capacities of E. crassipes and P. stratiotes for N uptake and N2O mitigation in tailwater treatment. Taking these two aquatic plants for phytoremediation may help mitigate the conflict between tailwater purification and greenhouse gas production.

Keywords: Aquatic macrophytes, Phytoremediation, Tailwater, N removal, N2O emissions