Stimulating ammonia oxidizing bacteria (AOB) activity drives the ammonium oxidation rate in a constructed wetland (CW)

Highlights: • Maximizing the boundary effect of riparian zone to improve ammonium oxidation rate. • Increasing water level fluctuations and HRT to enhance the boundary effect in CW. • Stimulating AOB activity drives the ammonium oxidation rate in a CW. • The increased AOB activity was due to the enhanced cell-specific AOB activity. • The enrichment of Nitrosomonas genus in CW dominated in active AOB. An integrated approach to document high ammonium oxidation rate in Guanjinggang constructed wetland (GJG-CW) was performed and the results showed that the substantial ammonium oxidation rate could be obtained by enhancing Ammonia Oxidizing Bacteria (AOB) activity rather than Ammonia Oxidizing Archaea (AOA) activity. In the plant-bed/ditch system, ditch center and plant-bed fringe were two active zones for NH₄⁺-N removal with ammonium oxidation rate peaking at 2.98 ± 0.04 and 2.15 ± 0.02 mg N kg^{− 1} d^{− 1}, respectively. The enhanced AOB activity were achieved by increasing water level fluctuations, extending hydraulic retention time (HRT) and stimulating substrate availability, which subsequently enhanced NH₄⁺-N removal by 34.06% in GJG-CW. However, the high AOB activity was not correlated with high AOB abundance, but was instead mostly determined by specific AOB taxa, particularly Nitrosomonas, which dominated in the active AOB. The increased cell-specific AOA activity and high AOA diversity were also achieved using those engineering measures. Although the AOA activity decreased overall with extended HRT and increased NH₄⁺-N contents in GJG-CW, AOA still played a major role on ammonium oxidation in plant-bed soil. The study illustrated that artificially enhancing AOB activity and certain species in anthropogenically polluted water ecosystems would be an effective strategy to improve NH₄⁺-N removal.