Photoperiod-Driven Optimization of Microbial Iron–Nitrogen Cycle for Improved Nitrogen Removal Efficiency in Constructed Wetlands

Different photoperiods can regulate the activity of microbial communities involved in nitrogen and iron metabolism, which in turn impacts the nitrogen removal efficiency of constructed wetlands (CWs). However, the underlying mechanisms of microbial iron–nitrogen cycling under varying photoperiods remain unresolved. Our study revealed that the 15-h/day treatment significantly enhanced the removal efficiency of COD (87.26%), TN (63.32%), and NH4+–N (33.68%). Additionally, Gibbs free energy calculations indicated that the efficiency of nitrogen removal in the Feammox processes was notably influenced by pH levels. Microbial community analysis showed that the 15-h/day treatment stimulated microbial activity related to denitrification and iron transport, fostering a stable microbial community with key genera enriched in nitrogen metabolism. Furthermore, optimized iron availability facilitates essential biochemical pathways, further enhancing nitrogen metabolism. This research underscores the pivotal importance of the photoperiod in regulating microbial interactions and nutrient cycling in CWs, providing insights into optimizing biological nitrogen removal processes.