Constructed wetlands (CWs) are extensively used as an economical and environmentally friendly sewage treatment under ecological engineering technology. Knowledge of the oxygen in the rhizosphere is of primary importance in understanding the function and regulation of microbial communities and macrophytes. Numerous studies on radial oxygen loss (ROL) have greatly elucidated the mechanism of contaminant removal in CWs. The main sources of oxygen in CWs are atmospheric reoxygenation, macrophyte transmission, and artificial aeration. However, artificial aeration is very expensive, and atmospheric reoxygenation is limited. Therefore, ROL by macrophytes is an essential and economical approach for oxygen input in CWs. In this review, we attempted to study the role of macrophytes in CWs. We described the mechanism of ROL and summarized the methods for determining ROL. We also investigated the role of ROL in contaminant removal in CWs. This review will provide considerable useful information on the oxygen input of CWs.
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