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 (NHand NO) by using different configurations and by dosing with simulated stormwater events. The results showed that NH was efficiently oxidized to NO, mainly by ammonia- and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NOmass reduction through root assimilation and enhancement of microbial NO 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 NO mass reduction mainly via stimulating chemolithotrophic NO 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 NO reduction pathways may be responsible for the high NO 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.