Unconventional microbial mechanisms for the key factors influencing inorganic nitrogen removal in stormwater bioretention columns

Bioretention systems are environmentally friendly measures to control the amount of water and pollutants in urban stormwater runoff, and their treatment performance for inorganic N strongly depends on various microbial processes. However, microbial responses to variations of N mass reduction in bioretention systems are complex and poorly understood, which is not conducive to management designs. In the present study, a series of bioretention columns were established to monitor their fate performance for inorganic N (NH4+ and NO3-) by using different configurations and by dosing with simulated stormwater events. The results showed that NH4+ was efficiently oxidized to NO3-, mainly by ammonia-and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO3- removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NO3- mass reduction through root assimilation and enhancement of microbial NO3- reduction in the rhizosphere. The construction of an organic-rich saturation zone can make the redox potential too low for heterotrophic denitrification to occur, so as to ensure high NO3- mass reduction mainly via stimulating chemolithotrophic NO3- reduction coupled with oxidation of reductive sulfur compounds derived from the bio-reduction of sulfate. In contrast, in the organic-poor saturation zone, multiple oligotrophic NO3- reduction pathways may be responsible for the high NO3- mass reduction. These findings highlight the necessity of considering the variation of N bio-transformation pathways for inorganic N removal in the configuration of bioretention systems.

Automated summary

Bioretention systems are environmentally friendly measures for managing urban stormwater runoff. This study investigated the fate of inorganic nitrogen (NH4+ and NO3-) in bioretention systems under different configurations and stormwater conditions. The results showed that NH4+ was efficiently converted to NO3- by ammonia-oxidizing and nitrite-oxidizing bacteria in the oxic media. However, the pathways for NO3- removal varied depending on the configuration of the bioretention columns. Vegetation improved NO3- reduction through root assimilation and enhancement of microbial activity in the rhizosphere. The presence of an organic-rich saturation zone stimulated chemolithotrophic NO3- reduction coupled with oxidation of reductive sulfur compounds, while an organic-poor saturation zone allowed for multiple oligotrophic NO3- reduction pathways. These findings emphasize the importance of considering the variation in N transformation pathways for designing bioretention systems.