Nitrogen removal performance of bioretention cells under freeze-thaw cycles: Effects of filler structure and microbial community

Cold climates have an adverse effect on the nitrogen-removal capacity of bioretention cells, especially during freeze-thaw cycles (FTCs). To explore the effects of FTCs on the nitrogen removal performance of bioretention cells, this research compared the effects of FTCs on the pore structure and microbial community composition of the filler, and analyzed the nitrogen removal performance of the bioretention cell before (RT), during (FTC) and after (RRT) FTCs. The results demonstrated that RRT filler had a much greater number of pores with equivalent diameter <500 mu m than RT filler, and that RRT had a higher pore volume and pore density than RT. Microbial community analysis revealed that the diversity and richness of the microbial community in FTC were lower than in RT, and the relative abundance of Lacunisphaera, Pseudomonas, and Dokdonella decreased significantly. There was no significant difference in microbial community richness between RRT and RT, however RRT diversity was lower. RRT has a higher relative abundance of nitrifying bacteria (Subgroup_10, Bryobacter, etc.) than RT, but a lower relative abundance of denitrifying bacteria (Pseudomonas, Dokdonella, Arenimonas, etc.). The nitrogen removal efficiency of FTC was inhibited, resulting in a decrease of 13.0 +/- 4.86%, 19.7 +/- 9.17%, and 26.6 +/- 1.74% in the removal rates of ammonia nitrogen(NH4+-N), nitrate nitrogen(NO3--N), and total nitrogen(TN) when compared to RT, respectively. RRT improved nitrification and increased NH4+-N removal rate by 10.3 +/- 2.69% compared to RT. However, because of denitrification inhibition, the nitrogen removal performance of RRT was not able to reach RT levels, and its NO3--N and TN removal rates decreased by 100 +/- 4.70% and 58.3 +/- 3.71%, respectively. This study has demonstrated that FTCs can permanently harm the bioretention cell's filler structure and microbial community, resulting in a significant decrease in the nitrogen removal performance of the bioretention cell designed according to warm climate conditions after experiencing FTCs.