From enzymes to efficiency: Strategic shifts in carbon degrading enzyme allocation are associated with the increase in microbial carbon use efficiency with elevation

Forest soils represent a major terrestrial carbon (C) reservoir; however, the mechanisms governing microbial C use efficiency (CUE), a key determinant of soil C persistence, remain poorly understood along environmental gradients. Here, we investigated soil microbial CUE and its regulatory drivers across an elevational gradient in a subtropical montane forest in eastern China using an 18 O-H 2 O labeling approach combined with measurements of soil physicochemical properties, extracellular enzyme activities, and microbial community composition. Microbial CUE increased significantly with elevation, from 0.08 at 256 m to 0.24 at 1252 m, driven by a shift in C allocation from respiration to growth, with lower respiration and higher growth rates while gross C uptake remained stable. This elevational pattern was closely associated with a strategic shift in extracellular enzyme allocation: the ligninase-to-cellulase ratio decreased with elevation, reflecting a strategic shift that favored energy efficient cellulose degradation over costly lignin decomposition. Structural equation modeling revealed that this enzymatic ratio exerted the strongest direct control over microbial CUE, which was mediated by fungal diversity and soil nutrient availability. Our findings demonstrate that elevation-related increases in microbial CUE are associated with a cascade of biotic and abiotic factors converging on enzymatic investment strategies. These results provide mechanistic insights into microbial-mediated C dynamics in montane forests and highlight the need to consider enzymatic allocation strategies in predictive models of soil C dynamics under global change.