Bioretention　system　has　been　commonly　recognized　as　an　emerging　management　method　to　control　urban　rainwater　runoff.　However,　nitrogen　conversion　mechanisms　in　bioretention　system　in　response　to　formulated　soil　medium　remains　unknown.　In　this　work,　five　bioretention　columns　with　different　formulated　soil　medium　were　established,　and　results　indicated　that　all　bioretention　columns　achieved　high　NH4+-N　removal　rate,　but　exhibited　significantly　different　NO3--N　removal　performance.　High-throughput　sequencing　results　suggested　that　nitrification　might　be　achieved　by　heterotrophic　nitrifying　bacteria,　while　Pseudomonas　and　Rubellimicrobium　may　be　the　main　denitrifying　bacteria,　which　served　as　anaerobic　and　aerobic　denitrifier,　respectively.　Heterotrophic　nitrification-anaerobic/aerobic　denitrification　may　the　microbial　pathways　to　realize　effective　nitrogen　removal　performance　in　these　bioretention　systems.　Bioretention　system　with　moderate　silt　content　(NC,　10%)　displayed　higher　abundant　potential　denitrifier,　leading　to　better　nitrogen　removal　performance.　Besides,　soil　adsorption　played　an　important　role　in　nitrogen　removal,　particularly　when　activated　carbon　was　added.　This　work　could　provide　guidance　of　formulated　soil　medium　optimization　for　the　application　of　bioretention　systems.　©　2022　The　Institution　of　Chemical　Engineers