Tidal　marshes　serve　as　critical　carbon　(C)　sinks,　yet　face　increasing　threats　from　global　environmental　changes.　While　previous　research　has　documented　how　nitrogen　(N)　loading　and　sea-level　rise　affect　total　C　pools　individually,　their　impacts　on　soil　organic　carbon　(SOC)　stabilization　remain　critically　underexplored,　particularly　when　these　factors　co-occur　in　tidal　marsh　ecosystems.　Through　a　3-yr　field　experiment,　we　analyzed　how　these　factors,　alone　and　combined,　impact　SOC　stabilization　by　examining　SOC　fraction　dynamics.　Results　showed　that　N　loading　increased　particulate　organic　carbon　(POC)　by　18%　and　decreased　mineral-associated　organic　carbon　(MAOC)　by　13%,　reducing　SOC　stabilization.　Conversely,　increased　inundation　raised　MAOC　by　31%　and　decreased　POC　by　19%,　promoting　SOC　stabilization.　The　decreased　MAOC　under　N　loading　stemmed　from　reduced　fungal　necromass　C,　while　the　increased　POC　related　to　lower　phenol　oxidase　activity.　In　contrast,　with　increased　inundation,　MAOC　rose　due　to　iron-bound　organic　C　(Fe-OC)　accumulation,　while　POC　declined　from　increased　phenol　oxidase　activity.　When　both　factors　were　applied　together,　SOC　stabilization　remained　at　control　levels.　This　occurred　because　the　combined　effect　maintained　oxidative　enzyme　activities　and　thus　retained　POC　levels.　The　simultaneous　reduction　in　fungal　necromass　C　and　enhancement　of　Fe-OC　associations　established　complementary　mechanisms　that　maintained　MAOC　at　levels　equivalent　to　control.　Our　findings　reveal　that　N　loading　and　increased　inundation　drive　contrasting　patterns　of　SOC　stabilization,　while　their　combination　produces　uniquely　stabilized　C　dynamics.　This　insight　challenges　single-factor　predictions　and　underscores　the　importance　of　multi-factor　experiments　in　understanding　ecosystem　responses　under　concurrent　global　change　scenarios.　©　2025　Association　for　the　Sciences　of　Limnology　and　Oceanography.