Plant growth, community structure, and nutrient removal in monoculture and mixed constructed wetlands

The aim of this study was to compare the growth, community structure, and nutrient removal rates between monoculture and mixed wetlands, based on the hypothesis that it depends on the plant species used in the wetlands as to whether monoculture or mixed wetland is superior in plant growth and nutrient removal. Pilot-scale monoculture and mixed constructed wetlands were studied over 4 years. The monoculture wetland had a community height similar to the mixed wetland during the early years but a significantly lower height than the mixed wetland (P < 0.05) during the last year. The mixed wetland also displayed a higher plant density than the monoculture wetland (P < 0.05). The leaf area index in the monoculture wetland was significantly higher in the first year (P < 0.05) and significantly lower in the later years (P < 0.05) than that in the mixed wetland. The monoculture wetland had a similar vertical distribution of below-ground biomass over 4 years, while the mixed wetland showed a significant change in vertical distribution of below-ground biomass in the last 2 years. The monoculture wetland had a larger (P < 0.05) above-ground biomass and a similar leaf biomass in the first year, and a smaller above-ground biomass (P < 0.05) and a smaller leaf biomass (P < 0.05) than the mixed wetland during the latter 2 years. The amount of standing dead mass was smaller in the mixed wetland than in the monoculture wetland (P < 0.05). The mixed wetland exhibited a significantly lower NH₄-N removal rate in the first year (P < 0.05), and significantly higher NH₄-N removal rate in the last year, when compared to the monoculture wetland (P < 0.05). The study indicated that species competition and stubble growth resulted in significant differences between monoculture and mixed constructed wetlands in plant growth, community structure, and nutrient removal rates.