Assembling plant diversity mitigates greenhouse gas emissions and achieves high nitrogen removal when treating the low-C/N wastewater by constructed wetlands

The low carbon-to-nitrogen (C/N) ratio in wastewater will inhibit pollutant removal, and more seriously, it will cause an increment of nitrous oxide (N2O) emissions of constructed wetlands (CWs). Raising the C/N ratio of wastewater is an effective way to solve this problem, while it may cause secondary pollution and is costly. Assembling plant diversity promotes N removal, while the effects of plant diversity and increasing C/N ratio on global warming potential (GWP) combined by N2O and methane (CH4) are lack of comparison. In this study, 108 CW microcosms were established to explore the effects of increasing the C/N ratio from 1 to 5 and assembling plant diversity on N removal and GHG emissions. Results showed that when the C/N ratio was 1, (1) increasing species richness reduced N2O and CH4 emissions then reduced the GWP by 70%; (2) the presence of Arundo donax in microcosms reduced GWP by 72%; (3) an A. donax × Tradescantia fluminensis × Reineckia carnea mixture resulted in a high N removal and decreased the GWP per g N removal by 92% with a cost increment of 0.05 USD per m3 wastewater treated; and (4) as the C/N ratio increasing to 5, the GWP per g N removal of monocultures was reduced by 96%, but the cost increased by at least 0.29 USD per m3 wastewater treated. In summary, configuring plant diversity in CWs is an efficient, clean, and cost-effective measure to treat wastewater with a low C/N ratio.