Soil microbial properties and mineral protection drive carbon use efficiency along elevation gradient

Understanding the elevational variation of soil microbial carbon use efficiency (CUE) can provide a better understanding for soil C-climate change feedback. Here, we used the 18O-H2O tracer method to quantify microbial CUE along an elevation gradient ranging from 840 m to 3400 m in the Eastern Himalaya. We found that soil microbial CUE first increases and then decreases along elevation gradient and reaches a peak in the mid-altitude area, indicating that the environmental conditions at mid-altitude are more conducive to the efficient utilization of C by microorganisms. Climate was the primary factor that affected elevational variations of CUE by altering microbial properties and mineral protection. In areas with relatively climate-adaptive conditions, soils with lower microbial alpha diversity instead provide higher CUE, and fungal phyla, in contrast with bacterial phyla, has more effects on CUE. Also, soil mineral protection indirectly regulates CUE by altering microbial properties, and both of which consequently determine the changes of SOC along elevation gradient. Particularly, the positive correlation between the fraction of particulate organic carbon and microbial CUE indicated that soil microbes tend to utilize and transform more labile C sources along elevation gradient. Overall, the complex interactions between mineral protection and microbial attributes lead to a more complex soil C climate-feedback.