Plant species diversity reduces N2O but not CH4 emissions from constructed wetlands under high nitrogen levels.

Constructed wetlands (CWs) have been widely used for treating wastewater. CWs also are the sources of greenhouse gas (GHG) due to high pollutant load. It has been reported that plant species diversity can enhance nitrogen (N) removal efficiency in CWs for treating wastewater. However, the influence of plant species diversity on GHG emissions from CWs in habitats with high N levels still lack research. This study established four species richness levels (1, 2, 3, 4) and 15 species compositions by using 75 simulated vertical flow CWs microcosms to investigate the effects of plant species diversity on the GHG emissions and N removal efficiency of CWs with a high N level. Results showed plant species richness reduced nitrous oxide (N2O) emission and N (NO3--N, NH4+-N, and TIN) concentrations in wastewater, but had no effect on methane (CH4) emission. Especially, among the 15 compositions of plant species, the four-species mixture emitted the lowest N2O and had under-depletion of N (DminTIN < 0). The presence of Oenanthe javanica had a significantly negative effect on the N2O emission but had no effect on N removal efficiency. The presence of Rumex japonicus significantly reduced the N (NO3--N and TIN) concentrations in wastewater but had no effect on the N2O and CH4 emissions. The N concentrations and GHG emissions in the community of R. japonicus × Phalaris arundinacea were as low as those in the four-species mixture. Assembling plant communities with relatively high species richness (four-species mixture) or particular composition (R. japonicus × P. arundinacea) could enhance the N removal efficiency and reduce the GHG emissions from CWs for treating wastewater with a high N level.