Nitrogen removal performance and microbial response mechanism of bypass constructed wetlands under sulfamethoxazole stress: Based on alternating influent of runoff and lake water

This is the authors' abstract. We don't add key points for this paper.

Sulfamethoxazole (SMX) residues in river water may disturb microbial nitrogen (N) transformation and compromise N-removal stability. Bypass constructed wetlands (CWs) divert polluted water through off-channel wetland units and return treated water to the river, providing an off-line treatment approach without directly modifying the main river channel. However, the N-removal and microbial N-transformation responses of bypass CWs to SMX under alternating runoff and lake-water conditions, particularly differences between planted and unplanted systems, remain poorly understood. In this study, laboratory-scale bypass CWs receiving alternating simulated rainwater runoff and natural lake water were established, including an unplanted CW (U-CW) and a planted CW (P-CW) with mature Canna indica and Phragmites australis , to evaluate N removal, microbial communities, and predicted functional shifts under SMX stress. Under SMX exposure, TN and NO 3 − -N removal increased in U-CW, while NH 4 + -N and NO 2 − -N remained stable. Surface NO 3 − -N accumulation (89.89 mg/kg) suggested that apparent nitrate removal did not reflect net N reduction. In contrast, P-CW maintained stable N removal with high NH 4 + -N removal efficiency (84.24%). SMX stress decreased microbial diversity in U-CW but increased diversity and unique taxa in P-CW. Anaeromyxobacter increased in both systems and was positively correlated with TN and NO 2 − -N removal, while Nitrospira showed opposite responses. PICRUSt2 predictions indicated reduced nitrate-reduction potential in U-CW but maintained nitrate/nitrite reduction and N-assimilation potential in P-CW. Overall, SMX disrupted N-transformation balance in U-CW, whereas P-CW showed greater buffering capacity against SMX stress. These findings highlight the potential of planted bypass CWs for stable treatment of antibiotic-impacted rivers.