Microbial carbon use efficiency, not extracellular enzyme stoichiometry, shows negligible response to alpine tunnel...

Tunnel construction involving soil disturbance and groundwater discharge can alter soil microbial communities and soil organic carbon dynamics. However, the effects of tunnel construction on microbial carbon use efficiency (CUE) and metabolic adaptation mechanisms in alpine ecosystems remain poorly understood. This study combined microbial CUE estimation with extracellular enzyme stoichiometry to evaluate microbial metabolic responses to tunnel construction across an elevation gradient on the eastern Qinghai-Tibet Plateau. We established three replicated plots in disturbed and control areas at elevations of 3240, 3420, and 3600 m with consistent soil and vegetation. Samples were collected from 0 to 10 cm and 10-30 cm depths in August-September 2022. Soil organic carbon compositions, extracellular enzyme activities, microbial biomass, and microbial CUE were measured, with microbial CUE indirectly estimated using a biogeochemical equilibrium model. Given the limited sample size (n = 3), no statistically significant effect of tunnel construction on microbial biomass or microbial CUE was detected. However, tunnel construction significantly increased the ratio of β-1,4-glucosidase and polyphenol oxidase to readily oxidizable carbon by 41% in surface soils (p < 0.05). The CUE estimated from C:N stoichiometry dropped by 5%, while CUE estimated from C:P stoichiometry rose by 5% (p < 0.05 and p < 0.001). Microbial CUE was positively correlated with C-, N-, and P-cycling enzymes. These findings suggest that microorganisms may adjust enzyme allocation and nutrient uptake to buffer disturbance stress on CUE. For environmental management, minimizing soil disturbance and preserving vegetation cover can maintain soil organic carbon stability in alpine tunnel zones.