Root exudates release from Myriophyllum aquaticum and effects on nitrogen removal by constructed wetlands

The root exudates released from macrophytes are critical for pollutant removal and associated microorganisms, but their role in nitrogen removal remains poorly understood in constructed wetlands (CWs). The release of root exudates by Myriophyllum aquaticum was quantified using a hydroponic experiment, and then four treatments with and without environmentally relevant concentrations of root exudates were conducted to investigate the effect of root exudates on nitrogen removal in CWs. The dissolved organic carbon (DOC) release rate of M. aquaticum was 1.1–2.2 mg g−1 h−1, significantly higher than that of Typha orientalis (0.58–0.78 mg g−1 h−1) and Iris pseudacorus (0.13–0.14 mg g−1 h−1). The DOC release rate of M. aquaticum was affected by the ammonium concentration and harvesting frequency. The removal efficiencies of ammonium and total nitrogen were higher in the CWs with plants than in those without plants, confirming the crucial role of plants in plant-microbe interactions and nitrogen removal in CWs. The CWs showed significantly higher ammonium removal efficiencies under the root exudate treatments (mean 90.4% and 69.3%) than the no-root exudate treatments (87.3% and 62.0%), while total nitrogen presented no significant difference. The phylum-level taxonomic classification of microbial sequences detected in the sediments was slightly different between the root exudate and no-root exudate treatments. The results suggested that the root exudate mass naturally released by plants may be insufficient to change the carbon to nitrogen ratios of wastewater. More gene copies were observed in the rhizosphere sediments than the bulk sediments (p < 0.05), indicating that root exudates had a positive effect on microbes distribution within the rhizosphere. Therefore, high levels of root exudates as external carbon sources should be added to enhance nitrogen removal in CWs for carbon-limited wastewater treatment. The present study provided new insights into the role of macrophytes in pollutant removal and management strategy of CWs for wastewater treatment.