Rising　sea　levels　have　increased　the　risk　of　intense　flooding　in　tidal　wetlands,　potentially　leading　to　rises　in　soil　iron-bound　organic　carbon　(Fe-OC)　contents　by　inhibiting　microbial　activity.　However,　flooding-induced　Fe-OC　accumulation　may　be　attenuated　by　root　activities　of　tidal　wetland　plants,　which　remains　under-investigated　in　tidal　wetlands.　Here　we　established　manipulative　＇marsh　organ＇　filed　experiments　with　soils　collected　from　an　oligohaline　tidal　wetland　and　introduced　the　indigenous　plant　species　Phragmites　australis　(Cav.)　Trin.　ex　Steud.　These　＇marsh　organ＇　mesocosms　were　then　subjected　to　three　flooding　water-level　treatments　over　a　period　of　3.5　years.　Overall,　root　biomass,　root　porosity,　and　rhizosphere　ferric　iron-to-ferrous　iron　[Fe(III):Fe(II)]　ratio　increased　with　flooding　levels,　indicating　that　enhanced　flooding　promotes　root　oxygen　loss　of　tidal　wetland　plants.　The　abundances　of　Fe-oxidizing　bacteria　(Gallionella)　and　Fe-reducing　bacteria　(Geobacter)　increased,　whereas　the　abundance　of　sulfate-reducing　bacteria　(dsrA　gene)　decreased　with　increased　flooding,　indicating　a　diversion　of　Fe　from　Fe-sulfur　associations　towards　microbially-mediated　Fe　redox　cycling.　The　soil　organic　carbon　(SOC)　pool　did　not　change　with　increased　flooding;　however,　the　Fe-OC-to-SOC　ratio　(fFe-OC)　increased　from　9　to　18%.　The　fFe-OC　was　strongly　related　to　soil　amorphous　Fe(III)　concentrations　and　the　activities　of　soil　C-acquiring　enzymes,　both　of　which　were　affected　by　rhizosphere　Fe(III):Fe(II)　ratios.　Thus,　increased　root　oxygen　loss,　along　with　enhanced　flooding,　facilitated　increases　in　amorphous　Fe(III)　concentrations　and　C-acquiring　enzyme　activity.　Increased　soil　amorphous　Fe(III)　concentrations　further　promoted　Fe-OC　accumulation,　whereas　increased　soil　C-acquiring　enzyme　activities　reduced　the　labile　organic　C　pool.　Overall,　the　dominance　of　the　Fe-OC　pool　increased　under　enhanced　flooding,　owing　to　increased　oxygen　loss　from　the　roots.　Therefore,　we　outlined　that　soil　C　stability　will　increase　in　tidal　wetland　ecosystems　that　are　exposed　to　future　sea-level　rise.　©　2023　Elsevier　Ltd