Increasing Microbial Carbon Use Efficiency With Elevation Depending on Growth and Respiration Differently Between Topsoils and Subsoils

Abstract

Microbial carbon use efficiency (CUE) is a key regulator of soil carbon sequestration and nutrient cycling. However, how microbial growth, respiration, and CUE respond to elevation gradients—particularly across soil depths—remains unclear. In this study, we quantified microbial CUE, growth, and respiration using the ¹⁸O–H₂O labeling method in both topsoil (0–20 cm) and subsoil (20–40 cm) along an elevation transect in a tropical montane forest. Microbial growth and CUE increased with elevation, while microbial respiration declined in the topsoil but exhibited no consistent pattern in the subsoil. The elevational pattern of microbial CUE was coregulated by growth and respiration in the topsoil, whereas it was predominantly driven by growth in the subsoil. Microbial CUE was negatively correlated with biomass-specific carbon-, nitrogen-, and phosphorus-acquiring enzyme activities, suggesting that lower nutrient acquisition investment facilitates more efficient carbon utilization. Furthermore, CUE was negatively associated with the enzymatic C:P ratio in the topsoil but positively in the subsoil, indicating that enhanced CUE with elevation results from the stronger alleviation of carbon and phosphorus limitations, respectively. Overall, our results reveal that while microbial CUE consistently increases with elevation across soil profiles, its underlying controls differ fundamentally between topsoil and subsoil.