Diurnal fluctuations in root oxygen release rate and dissolved oxygen budget in wetland mesocosm

To assess the role of plants for oxygen offering in constructed wetlands, this study experimentally evaluated the amount of root oxygen release rate using mass balance method. The mass balance calculation is based on the following components: respiratory oxygen consumption of the roots; oxygen required for degradation of the organic matters; oxygen required for nitrification; and the amount of dissolved oxygen in the influent, effluent and substrate water. Experimental results have demonstrated that the root oxygen release rate was ranged from 20.3 to 58.3gO2/m2.d with average value of 38.4gO2/m2.d, which was affected dramatically by light intensity. Only 35% and 9% of the oxygen released by roots were used in the degradation of organic matters and nitrogen-nitrification, respectively, while 56% was used for roots respiration with little to be released to the surrounding medium. The results also showed that diurnal fluctuation of oxygen release and light intensity followed unimodal distribution. Meanwhile, a better understanding of the DO (dissolved oxygen) budget was proposed. Root oxygen release rate could be described by two fractions. One is “net specific oxygen release rate” and the other is “dischargeable oxygen release rate”.