Regulating Denitrification in Constructed Wetlands: The Synergistic Role of Radial Oxygen Loss and Root Exudates

Constructed wetland (CW) is a critical ecological engineering for wastewater treatment and improvement of water quality. Nitrogen (N) removal is one of the vital functions of CWs during operation, and N treatment in CWs is mainly affected by aquatic plants and denitrification carried out by microbes. However, due to their low efficiency and instability in N removal, further applications of CWs are limited. The review provides a view of two basic characteristics of aquatic plants, radial oxygen loss (ROL) and root exudates, and their coupled effect on denitrification processes in CWs. First, the role of aquatic plants in denitrification is presented. The individual roles of ROL and root exudates in regulating denitrification, as well as their interaction in this process, have been discussed. Also, the limitation of conventional techniques to reveal interaction between the plant and the microbes has been highlighted. Further research on coupling regulatory mechanisms of ROL and root exudates may be conducted to develop an optimal wetland design and improve biological N removal. This review offers new insights and directions for improving N removal in CWs by utilizing the synergistic effects of plant ROL and root exudates.