To　reveal　the　changes　in　the　activities　of　soil　extracellular　enzymes　in　estuarine　tidal　marshes,　we　investigated　seasonal　soil　extracellular　enzyme　activities,　plant　biomass,　soil　bulk　densities,　and　water　content,　as　well　as　soil　organic　carbon　inventories　(including　SOC,　POXC,　DOC,　and　C:N　ratios)　in　the　Cyperus　malaccensis　tidal　marshes　of　the　Min　River　estuary　in　southeast　China.　The　results　showed　that　higher　activities　of　β-glucosidase　(BG),　cellobiohydrolase　(CBH),　phenol　oxidase　(PHO),　and　peroxidase　(PEO)　were　observed　in　the　tidal　brackish　marshes　than　in　the　freshwater　marshes.　For　the　salinity　gradient　(0‰　to　4.2‰),　the　increased　rates　of　activities　of　soil　extracellular　enzymes　BG,　CBH,　PHO,　and　PEO　were　201%,　305%,　493%,　and　252%,　respectively.　The　soil　bulk　densities　and　water　content,　and　porewater　DOC　concentrations　did　not　change　with　changes　in　the　salinity　gradient.　Soil　SOC　content　and　C:N　ratios　decreased　with　increased　soil　salinity.　The　belowground　biomass　of　C.　malaccensis　was　significantly　higher　in　the　brackish　water　sites　than　in　the　freshwater　sites;　however,　the　trend　was　opposite　for　the　aboveground　biomass.　All　activities　of　soil　extracellular　enzymes　were　negatively　correlated　with　C:N　ratios,　and　activities　of　PHO　and　PEO　negatively　correlated　with　POXC　and　SOC　contents.　The　highest　activities　of　BG　and　CBH　appeared　in　the　spring　and　summer,　respectively.　In　conclusion,　salinity　is　an　important　environmental　factor　for　controlling　soil　extracellular　enzyme　activities　and　carbon　cycling　in　the　estuarine　tidal　marshes.　From　the　tidal　freshwater　marshes　to　brackish　water　marshes,　the　increasing　soil　salinity　promotes　the　belowground　biomass　and　activities　of　soil　extracellular　enzymes,　but　depletes　the　soil　organic　carbon　pool　in　the　estuarine　tidal　marshes.　©　2019,　Science　Press.　All　right　reserved.