Bioretention　systems　have　been　extensively　studied　as　a　highly　efficient　technical　measure　to　tackle　the　global　threat　of　nitrogen　pollution　during　global　rainfall　runoff.　However,　the　migration　and　transformation　of　various　forms　nitrogen　in　bioretention　system　is　unclear.　So,　in　this　paper,　the　bioretention　systems　with　different　flow　regimes　and　planted　configurations　were　designed　to　study　the　nitrogen　removal　performance　and　migration　and　transformation　mechanism.　The　dynamic　changes　of　NH4+-N　and　NO3−-N　were　continuously　monitored　within　60　h　after　rainfall,　and　the　abundance　of　15N　isotopes　in　soil　layer　NH4+-N　was　simultaneously　measured.　The　results　indicated　that　NH4+-N　was　mainly　intercepted　in　soil　layer　in　four　constructed　bioretention　systems　with　similar　removal　efficiencies　(95.42–97.69%).　However,　NO3−-N　was　retained　in　submerged　layer　with　significant　different　removal　efficiencies　(43.03–83.00%).　After　fitting　calculation,　the　nitrification　rate　of　NH4+-N　(0.0626　mg　kg−1　h−1)　in　soil　was　5.31　times　higher　than　that　of　the　accumulation　rate　of　NO3−-N　(0.0118　mg　kg−1　h−1).　During　the　elimination　process　of　residual　NH4+-N　in　soil,　41.46%　removed　by　denitrification　and　plant　absorption　assimilation,　another　57.28%　stored　in　the　form　of　organic　nitrogen　or　inorganic　nitrogen,　only　1.26%　leaked　out.　Based　on　this,　the　content　variation　of　TN,　NH4+-N　and　NO3−-N　could　be　analyzed　by　a　system-wide　and　established　the　nitrogen　balance　model,　which　provides　a　new　insight　into　the　enhancement　of　nitrogen　removal　in　the　bioretention　system.　©　2019　Elsevier　Ltd