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Plant species diversity affects plant nutrient pools, however, previous studies have not considered plant nutrient concentrations and biomass simultaneously. In this study, we conducted an experimental system with 90 microcosms simulating constructed wetlands (CWs). Four species were selected to set up a plant species richness gradient (1, 2, 3, 4 species) and fifteen species compositions. The plant biomass, plant N, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations and pools were analyzed. Results showed that, (1) plant species richness increased plant biomass, and the presence of Oenanthe javanicae increased while the presence of Reineckia carnea decreased plant biomass; (2) plant species richness only increased plant K and Mg concentrations of the communities and plant Mg concentration of the species, and the presence of O. Javanica increased while the presence of R. Japonicus decreased plant N and P concentrations of the communities; (3) plant species richness increased plant N, P, K, Ca, and Mg pools, and the presence of O. Javanica increased while the presence of R. Carnea decreased plant N, P, K, Ca and Mg pools; (4) the four-species mixture produced more biomass and nutrient pools than the corresponding highest specific species monocultures. In case the plant uptake can remove nutrients from CWs through harvesting, the results suggest that both nutrient concentrations and biomass must be considered when evaluating the accumulation of nutrients. Assembling plant communities with high species richness (four species) or certain species (such as O. Javanica) is recommended to remove more nutrients from CWs through harvesting.

Maintaining efficient and stable nitrogen (N) removal in constructed wetlands (CWs) that experience disturbance from their influent pollutant variations is crucial. The ammonium/nitrate (NH4+/NO3−) ratio of influent in CWs often varies widely. The N removal and stability in floating CWs have been found to be enhanced by manipulating plant species diversity. However, whether the positive effects occur in sand-based CWs remains unknown. Here, we established sand-based and hydroponic microcosms to investigate the differences in the responses of N removal and stability to plant species diversity under the disturbance of increasing influent NH4+/NO3− ratio in late period of plant growth. Results indicated that, (1) increasing plant species richness enhanced N removal but did not affect N removal stability in sand-based CWs under disturbance; (2) sand-based CWs had 46% higher average N removal stability than floating CWs, but the stability in floating CWs reached that in sand-based CWs at higher species richness levels; (3) under disturbed conditions, floating CWs with Phragmites australis or Typha latifolia achieved N removal and stability equivalent to those in sand-based CWs. This study indicates that, when treating wastewater with a variable NH4+/NO3− ratio, floating CWs with high plant species richness and specific species can achieve a win-win situation for high and stable N removal and bioenergy production.

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.