Field study on the effects of antibiotic exposure and environmental heterogeneity on nitrogen conversion processes in constructed wetlands.

Antibiotics in wastewater treatment plant effluents may threaten constructed wetlands, but their field-scale effects on nitrogen cycling and microbial resistance remain unclear. This study examines how antibiotic exposure and environmental heterogeneity jointly influence nitrogen transformation, antibiotic resistance gene dissemination, and microbial stability in constructed wetlands. Field data showed that antibiotic levels in constructed wetlands receiving treated wastewater were generally below 500 ng L-1. In antibiotic-exposed constructed wetlands, denitrification rates correlated negatively with sulfonamide concentrations (r = -0.35, p = 0.288), indicating microbial suppression, while nitrate reduction to ammonium increased with sulfonamides (r = 0.64, p = 0.035). Carbon-to-nitrogen (C/N) ratios strongly influenced nitrogen removal, with higher denitrification rates in C/N-high zones (>3/1) than in C/N-limited areas. Notably, even constructed wetlands with negligible antibiotics harbored multidrug resistance genes (e.g., mexF, ceoB, acrB), yet their microbial communities exhibited greater functional stability (Kullback-Leibler divergence: 0.6 vs. 1.3). High environmental complexity in antibiotic-impacted constructed wetlands also preserved community stability without significant C/N and antibiotic resistance gene linkages. These results provide practical insights for optimizing constructed-wetland management to mitigate antibiotic resistance risks while maintaining nitrogen removal efficiency.