Increasing plant diversity to mitigate net greenhouse effect of wastewater treatment in floating constructed wetlands

Abstract Constructed wetlands (CWs) are an effective wastewater treatment technology but also an important source of greenhouse gases. Converting CW plant biomass to biofuel as an alternative to fossil fuels can reduce carbon dioxide emissions, and plant diversity influences this mitigation effect. However, the impact of plant diversity on net global warming potential combining carbon dioxide and non-carbon dioxide emissions in CWs is less studied, especially in floating CWs where the substrate is absent and both above- and below-ground biomass can be harvested easily for biofuel production. In this paper, four species richness levels were assembled in 90 hydroponic microcosms that simulated floating CWs to explore the effect of plant diversity on global warming potential. The results showed that: (1) increasing species richness increased the aboveground and total biomass but had no effect on belowground biomass, and carbon dioxide emissions were reduced by converting both above- and belowground biomass to biofuel to replace fossil fuels; (2) increasing species richness increased non-carbon dioxide emissions but decreased the overall global warming potential of combined carbon dioxide and non-carbon dioxide emissions; (3) communities containing Oenanthe javanica had lower overall global warming potential; and (4) some species (Oenanthe javanica) had a stronger mitigating effect on global warming potential than species richness. Our observations suggest that it is economically feasible to produce biofuel from hydroponically grown plants in floating CWs, and net energy gain can be achieved. Assembling high species richness and carefully selecting species in floating CWs alongside converting plant biomass to biofuel offers a greener form of wastewater treatment that can contribute to cleaner production strategies.