Effects of plant physiological responses under nitrogen stress on pollutant removal in subsurface constructed wetlands

Nitrogen loading directly affects plant metabolism, morphogenesis and rhizosphere effect, and then the pollutant removal performance of constructed wetlands. However, the nitrogen loading standards are significantly different in many design specifications around the world. The physiological responses of plants (Canna indica) and pollutant removal characteristics under different nitrogen stresses (NSs) in subsurface flow constructed wetlands (SFCWs) were investigated. Under appropriate NS of 120 mgN/L, the enhancement of stress resistance and the stabilization of cell structures of Canna indica were sustainable. However, with excessive NSs of 0, 300 and 600 mgN/L, the cell structure was damaged permanently. Appropriate NS could significantly increase the radial oxygen loss and dissolved oxygen concentration in SFCWs by 40.33 % and 43.11 % by persistently enhancing oxygen production (photosynthesis) and delivery capacity (root aerenchyma development) (p < 0.01). Simultaneously, the abundance and diversity of nitrogen-transforming bacteria significantly increased (p < 0.01), the activities of ammonia monooxygenase, periplasmic nitrate reductase and the removal rate of NH4+ - N and NO3- - N almost doubled. Controlling nitrogen loading is very important since NS would lead to the organic and irreversible difference in aerenchyma development in roots throughout the whole growth period of plants, which could persistently affect the removal efficiency and stability of pollutants in SFCWs.