Microbial　carbon　use　efficiency　(CUE)　typically　promotes　soil　organic　carbon　(SOC)　storage　in　terrestrial　ecosystems.　However,　this　relationship　remains　poorly　understood　in　coastal　wetlands,　where　tidal　flooding　creates　unique　environmental　conditions,　facilitates　lateral　transfer　and　SOC　loss,　and　mediates　organic　matter　exchange　between　terrestrial　and　marine　systems.　Here　we　examined　the　CUE-SOC　relationship　across　a　tidal　flooding　gradient　(4-25　%　frequency)　in　a　subtropical　coastal　wetland.　Along　this　gradient,　SOC　decreased　by　65　%　while　microbial　CUE　increased　from　0.24　to　0.32.　This　inverse　relationship　coincided　with　marked　compositional　shifts:　plant　debris　declined　from　57　%　to　18　%,　while　microbial　necromass　increased　from　21　%　to　35　%.　The　enhanced　CUE　was　accompanied　by　increased　turnover　times　alongside　decreased　metabolic　quotient　(qCO2),　C-acquiring　enzyme　activities,　soil　basal　respiration,　and　microbial　biomass　carbon　(MBC).　This　enhanced　efficiency　stemmed　from　substrate-microbe　interactions　rather　than　environmental　stresses,　as　communities　transitioned　from　oligotrophic　taxa　(alpha-proteobacteria,　Basidiomycota)　specializing　in　recalcitrant　terrestrial　substrates　to　copiotrophic　microorganisms　(gamma-proteobacteria,　Bacteroidota,　Ascomycota)　efficiently　metabolizing　labile　marine　compounds.　Contrary　to　terrestrial　patterns,　enhanced　CUE　did　not　promote　SOC　storage　due　to　three　key　mechanisms:　(i)　enhanced　CUE　from　marine　substrates　could　not　compensate　for　declining　plant　debris　accumulation;　(ii)　reduced　microbial　biomass　limited　necromass　formation　despite　higher　CUE;　and　(iii)　metabolic　benefits　from　high　CUE　(reduced　enzyme　activities　and　respiration　rates)　could　not　offset　the　substantial　decrease　in　SOC　inputs.　Our　findings　reveal　distinct　CUE-SOC　relationships　in　coastal　wetlands　compared　to　terrestrial　ecosystems,　highlighting　the　importance　of　considering　both　terrestrial　and　marine　processes　in　understanding　carbon　cycling　in　these　transitional　environments.