Sea-level　rise　is　projected　to　influence　soil　carbon　(C)　stocks　in　tidal　wetland　systems.　Although　soil　C　and　iron　cycling　are　considered　to　be　related　in　tidal　wetlands,　the　empirical　link　between　root　ferric　iron　[Fe(III)]　plaque　formation　and　C　decomposition　in　response　to　salinity　and　flooding　is　scarcely　known.　Here,　we　established　mesocosms　loaded　with　soils　from　a　tidal　freshwater　wetland　and　subjected　to　three　salinity　treatments　(fresh　control,　oligohaline,　mesohaline)　plus　three　flooding　(i.e.,　flooding　water-level　height)　treatments.　Root　Fe(III)　plaque　abundance,　C-degrading　enzyme　activities,　and　potential　C　mineralization　rates　(CMRs)　and　microbial　Fe(III)　reduction　rates　(FeRRs)　were　simultaneously　quantified.　The　oligohaline　treatment　elevated　root　Fe(III)　plaque　abundance　relative　to　fresh　control,　while　the　mesohaline　treatment　suppressed　it.　Owing　to　high　abundance　of　root　Fe(III)　plaque,　microbial　Fe(III)　reduction　predominated　C　mineralization　in　the　oligohaline　treatment　(56　±　7%).　However,　the　importance　of　microbial　Fe(III)　reduction　decreased　by　up　to　25　±　5%　in　the　mesohaline　treatment　due　to　inhibition　by　salinity.　The　potential　CMRs　increased　by　17%　from　fresh　control　to　oligohaline　treatment,　but　declined　by　27%　from　fresh　control　to　mesohaline　treatment.　The　potential　CMRs　were　affected　by　potential　FeRRs　and　C-degrading　enzyme　activities.　The　latter　two　were　associated　with　root　Fe(III)　plaque　abundance.　These　results　together　showed　that　root　Fe(III)　plaque　abundance　was　linked　to　potential　CMRs　in　response　to　salinity.　Flooding　did　not　affect　root　Fe(III)　plaque　abundance　and　had　much　less　of　an　effect　on　potential　CMRs　compared　to　salinity.　Altogether,　root　Fe(III)　plaque　abundance　could　be　as　an　indicator　of　C　decomposition　rates　in　tidal　freshwater　wetland　soils　in　response　to　salinity　and　flooding.　Future　C　decomposition　prediction　models　under　sea-level　rise　could　embed　root　Fe(III)　plaque　abundance　as　an　indicator　for　integrating　plant-microbe-soil　interaction　into　models.　©　2021　Elsevier　Ltd