Abstract: Previous studies have shown that the plant diversity in constructed wetlands with sand substrate can improve the removal efficiency of wastewater contaminants. However, the effects of plant diversity in the aquatic system without substrate on nitrogen removal are unknown. In this paper, 45 hydroponic microcosms (53 cm × 37.5 cm × 18.5 cm) with 1, 2, 3, and 4 plant species were installed, and periodically applied with nitrate as the sole nitrogen source at a loading rate of 548.5 g N·m^-2·a^-1 to simulate wastewater. The results showed that species richness had significant effects on the effluent nitrogen removal. The total inorganic nitrogen concentration in the effluent of four species systems (54.3 mg·L^-1) was obviously lower than that of monoculture systems (129.0 mg·L^-1). The community biomass increased significantly with the increase of species number. The total biomass in four species systems (1621.6 g·m^-2) was higher than that in monoculture systems (1032.7 g·m^-2). The average nitrogen removal rate in the hydroponic microcosms was 466.8 g N·m^-2·a^-1, not lower than that in the previously reported fullscale constructed wetlands. Meanwhile, the nitrogen removal rate in four species systems was 13% higher than that in monoculture systems, indicating that the improvement of nitrogen removal efficiency in constructed wetlands could be achieved by increasing plant diversity.

Key words: Grain for Green Program (GPP), carbon cost, carbon leakage, net carbon sequestration, greenhouse gas.