Microbial response to nitrogen removal driven by combined iron and biomass in subsurface flow constructed wetlands with plants of different ages.

This study investigated the nitrogen removal enhanced by combined iron scraps and plant biomass, and its microbial response in the wetland with different plant ages and temperatures. The results showed that older plants benefitted the efficiency and stability of nitrogen removal, which could reach 1.97 ± 0.25 g m d in summer and 0.42 ± 0.12 g m d in winter. Plant age and temperature were the main factors determining the microbial community structure. Compared with temperature, plant ages affected more significantly on relative abundance of microorganisms such as Chloroflexi, Nitrospirae, Bacteroidetes and Cyanobacteria, and functional genera for nitrification (e.g., Nitrospira) and iron reduction (e.g., Geothrix). The absolute abundance of total bacterial 16S rRNA ranged from 5.22 × 10 to 2.63 × 10 copies g and presented extremely significant negative correlation to plant age, which would lead to a decline in microbial function on information storage and processing. The quantitative relationship further revealed that the ammonia removal was related to 16S rRNA and AOB amoA, while nitrate removal was controlled by 16S rRNA, narG, norB and AOA amoA jointly. These findings suggested that a mature wetland for nitrogen removal enhancement should focus on aging microbes caused by old plants and possible endogenous pollution.