Low-level　saltwater　intrusion　and　ferric　iron　(Fe(III))　loading　are　predicted　for　many　mineral　tidal　freshwater　ecosystems　under　sea-level　rise.　To　assess　their　effects　on　the　carbon　mineralization　processes,　in-situ　experiments　were　conducted　with　four　treatments:　Fe(III)　amendment,　saltwater　addition,　saltwater　addition　plus　Fe　(III)　amendment,　and　a　control.　Soil　and　porewater　samples　(0-30　cm)　were　collected　to　assess　geochemical　properties　and　soil　incubation　on　Days　460　and　580　after　the　start　of　the　experiment.　Saltwater　addition　increased　the　salinity　from　freshwater　(0.1　mg.g(-1))　to　oligohaline　(＜　2　mg.g(-1))　and　elevated　the　concentrations　of　porewater　Cl-,　SO42-,　and　NH4+.　Fe(III)　amendment　increased　the　concentrations　of　soil　Fe(III),　Fe(II),　and　total　reduced　sulfur　(TRS).　Both　Fe(III)　amendment　and　saltwater　addition　enhanced　the　belowground　biomass　of　marsh　plants,　while　neither　affected　the　total　organic　carbon　(TOC)　and　total　nitrogen　(TN)　pools　or　their　ratios.　Both　Fe(III)　amendment　and　saltwater　addition　improved　the　overall　mineralization　rates　(i.e.,　CO2　plus　CH4　production)　of　the　soil　at　0-10　cm　depths.　Methanogenesis　decreased　under　both　Fe(III)　amendment　and　saltwater　addition.　Strong　negative　relationships　were　observed　between　methanogenesis　rates　and　the　concentrations　of　porewater　Cl-　and　SO42-,　and　soil　Fe(III).　Relative　to　the　individual　Fe(III)　and　saltwater　treatments,　the　combined　treatment　did　not　further　promote　overall　mineralization　rates　or　further　impede　methanogenesis　potential.　Methanogenesis　and　Fe(III)　reduction　co-dominated　the　overall　mineralization　in　the　control　treatment.　Individual　loading　of　Fe(III)　and　saltwater　into　the　soils　induced　the　predominance　of　Fe(III)　and　sulfate　reduction,　respectively.　Under　the　combined　treatment,　microbial　Fe(III)　and　sulfate　reduction　codominated　the　overall　mineralization,　but　restrained　each　other.　The　shifts　in　the　partitioning　of　mineralization　pathways　among　treatments　were　primarily　influenced　by　porewater　Cl-　and　SO42-,　followed　by　Fe(III),　and　then　DOC.　Together,　the　results　suggest　that　the　projected　modest　levels　of　saltwater　intrusion　and　Fe(III)　loading　may　simultaneously　accelerate　the　belowground　C　input　and　mineralization　rates,　shifting　anaerobic　pathways　away　from　CH4　production　towards　Fe(III)　and　sulfate　reduction.