This　study　explored　the　quantitative　mechanisms　of　heterotrophic　nitrification-aerobic　denitrification　(HN-AD)　in　a　pilot-scale　two-stage　tidal　flow　constructed　wetland　(TFCW).　The　TFCW　packed　shale　ceramsite　(SC)　and　activated　alumina　(AA)　at　each　stage,　respectively,　and　aimed　to　improve　decentralized　wastewater　treatment　efficiency.　In　start-up　phases,　AA-TFCW　accelerated　NH4+-N　decline,　reaching　transformation　rates　of　6.68　mg　NH4+-N/(L.h).　In　stable　phases,　SC-AA-TFCW　resisted　low-temperatures　(＜13　degrees　C),　achieving　stable　NH4+-N　and　TN　removal　with　effluents　ranging　6.36-8.13　mg/L　and　9.43-14.7　mg/L,　respectively.　The　dominant　genus,　Ferribacterium,　was　the　core　of　HN-AD　bacteria,　simultaneously　removing　NH4+-N　and　NO3--N　by　nitrate　assimilation　and　complete　denitrification　(NO3--N　-＞　N2),　respectively.　The　quantitative　associations　highlighted　importance　of　nitrification,　nitrate　assimilation,　and　denitrification　in　nitrogen　removal.　HN-AD　bacteria　(e.g.,　Lactococcus,　Thauera,　and　Aeromonas)　carried　high-weight　genes　in　quantitative　associations,　including　napAB,　nasA　and　gltBD,　implying　that　HN-AD　bacteria　have　multiple　roles　in　SC-AA-TFCW　operation.