Divergence of microbial carbon use efficiency and soil organic carbon along a tidal flooding gradient in a subtropical coastal wetland

), 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 (α-proteobacteria, Basidiomycota) specializing in recalcitrant terrestrial substrates to copiotrophic microorganisms (γ-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.