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This study chose Oak woodchips and gravel as media filter to enhance the denitrification in the bioretention system (saturated zone 7.7 L) treating synthetic stormwater runoff. It revealed that the denitrification process mainly occurred during the drying phase and enlarging volume of saturated zones to retain more stormwater during storm event was the direct method to promote nitrogen removal of the bioretention system. Nevertheless, it was noted that the nitrogen and dissolved organic carbon would be released into the effluent during the wetting period. The denitrification rate with different nitrate nitrogen (NO 3 –N) concentrations did not show the obvious change with zero order kinetics constant of 2.91 mg/L∙d on average. Furthermore, it confirmed that woodchips were degraded and converted to volatile fatty acids (VFAs), especially acetic acid as carbon source, further utilized by the denitrifying bacteria, such as Dechloromonas , Acidoborax , Pseudomonas , Denitratisoma and Acinetobacter. In addition, genera of Lachnospiraceae and Lactobacillus , which had the ability to degrade the macromolecular organic components into low molecular VFAs, were observed in the woodchips bioretention system. [Display omitted] • The denitrification mainly occurs at drying phase. • Larger saturated zone trends to better denitrification performance. • VFAs are the main carbon source for denitrifiers. • Lachnospiraceae and Lactobacillus , which degrade macromolecular organic components into low molecular VFAs, are observed. [ABSTRACT FROM AUTHOR]