Effects of Plant Material as Carbon Sources on TN Removal Efficiency and N2O Flux in Vertical-Flow-Constructed Wetlands

A nitrate-dominant synthetic wastewater simulating slightly polluted water with low C/N and poor biochemical availability was treated in lab-scale vertical-flow (VF)-constructed wetlands, which had Phragmites australis planted with different types of external carbon sources: Platanus acerifolia leaf litters, P. australis litters, glucose and a blank test with no external carbon sources. A comparison of the TN removal and N2O flux performances among the four wetland reactors indicated higher TN removal efficiencies and N2O release fluxes in the VF wetland columns with external carbon sources, as measured by the percentage removal of TN (P. acerifolia leaf litters 82.49 %, P. australis litters 70.55 %, glucose 62.50 % and blank 46.45 %) and N2O flux (P. acerifolia leaf litters 2275.22 μg · m−2 · h−1, P. australis litters 1920.53 μg · m−2 · h−1, glucose 1598.57 μg · m−2 · h−1 and blank 1192.08 μg · m−2 · h−1). This was primarily because of an improved supply of organic carbon from the external carbon sources for heterotrophic denitrification. And, the nitrogen released from the decomposition of plant materials resulted in the N2O release fluxes to some extent. However, employing P. acerifolia leaf litters and P. australis litters as external carbon sources caused net increases in organics of the final effluent water. Overall, the results not only demonstrated the potential of using external plant carbon sources in VF wetlands to enhance the TN removal efficiency but also showed a risk of excessive organic release and greater N2O flux feedback to global warming. Hence, future studies are needed to optimise the quantity and method for adding external carbon sources to VF-constructed wetlands so that sufficient nitrate removal efficiency is achieved and the N2O flux and organic pollution are minimised.