Aridity Modulates Warming Impacts on Microbial Carbon Use Efficiency

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.