Aims　Rising　sea　levels　and　the　associated　increase　in　inundation　heights　will　alter　the　carbon　(C)　cycle　in　tidal　marshes.　However,　current　research　primarily　focuses　on　the　impact　of　increased　inundation　on　total　soil　C　stocks,　while　the　effects　on　the　balance　of　C　budget　processes　remain　unclear.　Therefore,　understanding　how　sea　level　rise　affects　the　C　sequestration　capacity　of　tidal　marshes　is　essential　for　predicting　future　impacts.　Methods　To　address　this,　our　study　established　a　“marsh　organ”　experimental　platform　in　the　tidal　marshes　of　the　Minjiang　River　Estuary.　Three　inundation　treatments—CK　(control),　CK　+　20　cm,　and　CK　+　40　cm—simulated　the　current　and　the　projected　sea　level　rise　scenarios　for　the　next　50　and　100　years.　We　measured　the　effects　of　increased　inundation　on　the　net　ecosystem　carbon　dioxide　exchange　(NEE),　gross　primary　productivity　(GPP),　ecosystem　respiration　(ER),　plant　biomass,　plant　photosynthetic　characteristics,　and　soil　physicochemical　properties　of　the　Cyperus　malaccensis　tidal　marshes.　Important　findings　The　results　showed　that　increased　inundation　height　led　to　a　decrease　in　aboveground　biomass　and　an　increase　in　belowground　biomass.　Compared　to　the　CK,　GPP　decreased　by　27%　and　32%,　while　ER　increased　by　20%　and　58%　in　the　CK　+　20　cm　and　CK　+　40　cm　treatments,　respectively.　The　reduction　in　GPP　was　related　to　decreased　aboveground　biomass　and　declining　plant　photosynthetic　characteristics,　such　as　net　photosynthetic　rates,　stomatal　conductance,　intercellular　CO2　concentrations.　The　increase　in　ER　was　associated　with　higher　soil　oxidation-reduction　potential　and　dissolved　organic　carbon　content.　Under　the　CK,　CK　+　20　cm,　and　CK　+　40　cm　treatments,　NEE　was　–539.8,　–102.7,　and　185.6　g　C·m–2·a–1,　respectively.　These　findings　indicate　that　a　20　cm　increase　in　inundation　height　leads　to　an　increase　in　NEE,　demonstrating　a　weakened　carbon　sequestration　capacity　of　the　Cyperus　malaccensis　tidal　marshes.　Furthermore,　a　40　cm　increase　in　inundation　height　results　in　NEE　shifting　from　negative　to　positive,　indicating　a　transition　of　the　ecosystem　from　a　carbon　sink　to　a　carbon　source.　This　research　provides　a　scientific　basis　for　predicting　and　mitigating　the　impacts　of　future　sea　level　rise　on　the　C　cycle　of　tidal　marsh.　©　Chinese　Journal　of　Plant　Ecology.