Effects of Elevated Atmospheric CO2 Concentration on Phragmites australis and Wastewater Treatment Efficiency in Constructed Wetlands

Elevated atmospheric CO2 concentration (e[CO2]) has been predicted to rise to more than 400 ppm by the end of this century. It has received extensive attention with regard to the pros and cons of e[CO2] effects in terrestrial and marine ecosystems, while the effects of e[CO2] on wastewater treatment efficiency in constructed wetlands (CWs) are rarely known. In this study, the atmospheric CO2 concentration was set as 400 ppm (that is, ambient [CO2]) and 800 ppm (that is, e[CO2]). The physiological performance of Phragmites australis and microbial enzyme activities in constructed wetlands in response to e[CO2] were tested. Significantly higher net photosynthetic rate and plant growth were found under e[CO2]. The concentrations of nitrate, total anions, and total ions in the xylem sap of Phragmites australis were reduced, while the uptake of N and P in plants were not affected under e[CO2] condition. In addition, the ammonia monooxygenase activity was reduced, while the phosphatase activity was enhanced by e[CO2]. The increased removal efficiency of chemical oxygen demand and total nitrogen in CWs could be ascribed to the changes in physiological performance of Phragmites australis and activities of microbial enzymes under e[CO2]. These results suggested that the future atmospheric CO2 concentration could affect the wastewater treatment efficiency in CWs, due to the direct effects on plants and microorganisms.