Rhizosphere as hotspot for ammonia oxidation in secondary effluent constructed wetlands: Role of comammox Nitrospira

Secondary effluent constructed wetlands (SECWs) are engineered ecosystems for advanced wastewater treatment, yet the functional roles and survival strategies of complete ammonia oxidizers (comammox) within these systems remain poorly understood, particularly from a rhizosphere view. The results of this study demonstrated that comammox was numerically and functionally dominant (60.4 % to 70.6 %) in SECWs. The rhizosphere acted as a hotspot for ammonia oxidation and N₂O production, compared to the nonrhizosphere. Enhanced nitrification was attributed to radial oxygen loss and humic acid-like compounds in root exudates. Furthermore, variations in comammox community structure and ammonia kinetic properties (K_m(app) = 0.140 ± 0.026 mg N L⁻¹) revealed niche differentiation among comammox species: the r-strategist Nitrospira sp. HN-bin3 thrived over time, whereas the K-strategist Nitrospira nitrificans was outcompeted, with ammonia concentration identified as the main driving factor. These results highlighted the vital but underappreciated role of comammox in the nitrogen cycle of constructed wetlands and provided new insights into their ecological functions and adaptive strategies.