Nitrogen removal through collaborative microbial pathways in tidal flow constructed wetlands

Abstract

Constructed wetlands are efficient in removing nitrogen from water; however, little is known about nitrogen-cycling pathways for nitrogen loss from tidal flow constructed wetlands. This study conducted molecular and stable isotopic analyses to investigate potential dissimilatory nitrate reduction to ammonium (DNRA), denitrification, nitrification, anaerobic ammonium oxidation (anammox), and their contributions to nitrogen removal by two tidal wetland mesocosms, PA (planted with Phragmites australis) and NP (unplanted), designated to treat Yangtze River Estuary water. Our results show the mesocosms removed ~22.6% of TN from nitrate-dominated river water (1.19 mg·L−1), with better performance obtained in PA than that in NP, which was consistent with the molecular and stable isotopic data. The potential activities of DNRA, anammox, denitrification and nitrification varied between 0.6 and 1.6, 4.6–37.3, 36.4–305.7, and 463.7–945.9 nmol N2 g−1 dry soil d−1, respectively, with higher values obtained in PA than NP. Nitrification accounted for 94.3–99.4% of NH4+ oxidation, with the rest through anammox. Denitrification contributed to 77.9–90.3% of NOx− reduction, compared to 9.2–21.6% and 0.5–1.5% via anammox and DNRA, respectively; 78.4–90.9% of N2 was produced through denitrification, with the rest via anammox. Pearson correlation analyses suggest NH4+ was the major factor regulating nitrification, while NO3− played an important role in the competition between denitrification and DNRA, and NO2− was a key restrictive factor for anammox. Overall, this study reveals the importance of nitrification, denitrification, anammox and DNRA in nitrogen removal, providing new insight into the nitrogen-cycling mechanisms in natural/artificial tidal wetlands.