Constructed wetlands have been the focus of increasing international interest because of their low maintenance, extent, effi ciency and eco-friendly character. However, there is a debate about the importance of vegetation for wastewater treatment in constructed wetlands. The amounts of pollutants taken up by plants are generally low. However, plants can stimulate pollutant removal by the release of root oxygen. As one oxygen source (others may include infl ow carrying of oxygen and aeration), the role of plants in pollutant removal remains unclear. This study investigated the effect of the “dischargeable oxygen release rate” on organic matter and nitrogen removal using an “in-situ test” method. In addition, the constructed wetlands were operated in a conventional way, with several oxygen sources playing a role, including infl ow carrying of oxygen, aeration and oxygen release from the roots of plants. The maximum rates of dischargeable oxygen release were 23.7 μmol/g·h and 72.7 μmol/g·h at hydraulic loading rates of 6.8 mm/h and 17.0 mm/h, respectively. The percentage contributions of plant roots to BOD₅ removal were 0-18.2% and 1.2-24.8%, respectively, at the two hydraulic loading rates. For nitrogen removal, the corresponding percentage ranges were 0.4-21.7% and 4.0-23.5%, respectively. Therefore, the role of vegetation should not be ignored in the process of wastewater purifi cation in constructed wetlands. Root oxygen release and pollutant removal were infl uenced by light and hydraulic loading rates.