Tannins　play　vital　roles　in　regulating　ecological　processes　in　mangrove　forests.　However,　how　tannins　affect　nitrogen　(N)　cycling　and　microbial　metabolism　in　mangrove　ecosystems　remains　largely　unexplored.　In　this　study,　we　hypothesized　the　types　and　amounts　of　tannins　released　into　seawater　and　sediments　during　leaf　litter　decomposition　differed　among　mangrove　plant　species,　thus　their　effects　on　N　and　microbial　metabolism　also　varied.　The　alterations　of　tannins,　and　environmental　N　and　microbial　metabolism　during　leaf　litter　decomposition　of　Kandelia　obovata,　Avicennia　marina,　and　Sonneratia　apetala　were　evaluated　by　a　microcosm-simulated　tidal　system.　Results　showed　that　total　polyphenols　(TPs)　in　seawater　treated　with　K.　obovata　litter　were　significantly　higher　than　those　in　A.　marina　and　S.　apetala　treatments,　although　the　trends　of　TP　changes　elicited　an　initial　increase　followed　by　a　decrease　during　decomposition.　The　dynamic　changes　in　TPs　reduced　the　seawater　N　concentrations　in　K.　obovata　treatment　but　not　in　A.　marina　and　S.　apetala　treatments.　The　results　of　microbial　metabolism　analysis　revealed　that　leaf　litter　significantly　increased　microbial　metabolic　activities　and　diversities.　The　types　of　carbon　sources　utilized　by　sediment　microorganisms　differed　among　treatments,　with　the　microbes　in　S.　apetala　and　A.　marina　litter　used　more　varieties　of　amino　acids,　lipids　and　sugars　than　those　in　K.　obovata　treatment,　probably　due　to　the　rich　amount　of　hydrolysable　tannins　(HTs)　in　the　first　two　species　while　the　last　species　only　contained　ondensed　tannins　(CTs).　CTs　released　from　K.　obovata　leaf　litter　not　only　bound　nitrogen-containing　macromolecular　compounds　such　as　amino　acids　and　proteins　but　also　carbohydrates　like　polysaccharides,　which　decreased　the　supply　of　C　and　N　to　sediment　microbiota.　These　results　reveal　that　the　release　of　mangrove　tannins　during　leaf　litter　decomposition　is　one　of　the　key　factors　driving　N　cycling,　and　microbial　activities　and　diversities　in　mangrove　wetlands.