Effect of Ferrous Iron Addition on Ammonium Nitrogen Removal and Microbial Communities in Horizontal Subsurface Flow Constructed Wetlands

This study explored the influence of ferrous iron (Fe2+) addition on ammonium nitrogen (NH4-N) removal and microbial communities in horizontal subsurface flow constructed wetlands (HSSFs) at different carbon/nitrogen (C/N) ratios. Fe2+ addition could improve NH4-N removal under the C/N ratio of 2.1 and alter the microbial community structure. The microbial diversity and functional genes at the C/N ratio of 2.1 were enhanced by the addition of 50 mg/L Fe2+, with the increases observed in NH4-N and total nitrogen (TN) removal. This indicates the enhancement of nitrogen removal by Fe2+ addition with the presence of high organic matter. However, the enhancement of Fe2+ on chemical oxygen demand removal was not obvious overall. The relative abundance of functional genes and those related to energy metabolism, xenobiotic biodegradation, and metabolism decreased significantly in HSSFs with Fe2+ at C/N ratio of 1.1, whereas they increased in HSSFs with the addition of 50 mg/L Fe2+ at C/N ratio of 2.1.