Microbial carbon use efficiency (CUE) regulates the partitioning of organic carbon between microbial biomass and CO2 emission, yet its response to climate warming remains poorly understood, especially in naturally regenerating ecosystems like abandoned croplands. Here, we conducted 3 year in situ warming experiments (+1.6 °C) across 12 abandoned cropland sites spanning arid to humid climatic zones in China. Using the 18O-H2O method and molecular characterization, we found that warming significantly reduced microbial CUE in humid areas (aridity index >0.65), but had little effect in arid areas (aridity index <0.65). In humid areas, warming-induced reductions in CUE were driven by increases in fungal biomass, particularly pathotrophic and pathotroph-saprotrophic fungal guilds, together with shifts in dissolved organic carbon (DOC) composition toward more aromatic and recalcitrant compounds. Moreover, changes in CUE under warming were significantly positively correlated with change in DOC, but not with change in soil organic carbon (SOC), suggesting a temporal asynchrony and highlighting the need to consider the active carbon pool when assessing warming's impact on SOC. Our findings reveal a climate-dependent microbial mechanism that may weaken the carbon sequestration potential of humid abandoned croplands under future warming, underscoring the importance of region-specific strategies for soil carbon management.
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