Amount of oxygen released by bulrush (Scirpus validus) roots has been quantified based on the radial oxygen loss (ROL) exhibited by the roots, the number and the length of active lateral roots, and the field plant density. It was found that wetland bulrush contains two types of active lateral roots (showing ROL), viz., laterals of brown and white main roots. The two laterals have distinct oxygen release characteristics. Based on the dissolved oxygen (DO) microprofiles of brown and white laterals, the ROLs were found to be approximate to 61 ng O-2 cm(-2) root surface min(-1) and = 68 ng O-2 cm(-2) root surface min(-1) respectively, at bulk 5-day biochemical oxygen demand (BOD5) of 76 mg L-1. The respective average active root lengths of the brown and the white laterals were approximate to 40 and 1676 mum. Based on field and laboratory measurements, the average amount of oxygen released by bulrush was found to be 2.30 mg O-2 m(-2) wetland surface d(-1); of this approximate to 71% is from the white roots. The results of this study indicate that plants do not release enough oxygen to meet the total oxygen demand of bulk wastewater, and therefore, constructed wetlands should be designed as an anaerobic or an aerobic anaerobic hybrid system rather than as an aerobic system. However, the results of this study should be viewed in the background of possible errors (including a reactor design flaw), which might have made the measured oxygen release significantly lower than what plant roots actually release. Further studies are needed to quantify wetland plant oxygen release based on micro-scale measurements. (C) 2004 Wiley Periodicals, Inc.