To explore the microbial community structure at different submerged depths, the biomass was taken from the top, middle and bottom layer of the columns respectively. When the initial concentration of NO3-N was 30 mg/L, the biomass samples were numbered as C30-T, C30-M, C30-B. Similarly, when the initial concentration of NO3-N was 50 mg/L, the biomass samples were numbered with C50-T, C50-M, C50-B.