Temperature regulates microbial carbon use efficiency effects on soil organic carbon storage

Microbial carbon use efficiency (CUE), describing the partitioning of microbe assimilated carbon into microbial growth and respiration, is commonly used in soil carbon models to link microbial activities with the consumption of soil organic carbon (SOC). However, the role of CUE in regulating SOC storage remains debated. Previous studies have reported that a higher CUE could not only favour SOC formation through microbial necromass accumulation, but also trigger SOC losses because an enhancement in enzyme production facilitates SOC decomposition. The former leads to a positive relationship between CUE and SOC, while the latter leads to a negative one. Temperature dependencies introduce additional uncertainties while exploring the SOC-CUE relationship since temperature affects both SOC decomposition and CUE. Based on the meta-analysis and numerical simulations with a mechanistic model (T&C), we examined the relationship between CUE, SOC storage and temperature. Numerical results recover the expected SOC storage decrease with increasing temperature when temperature effects are isolated; however, an increase of SOC storage with decreasing CUE is found once temperature effects are discounted, indicating that SOC storage increase with increasing CUE is likely a by-product of temperature dependencies. In addition, we show that CUE variability plays a more important role in affecting SOC storage at lower temperature. Our study helps refine the understanding of SOC responses in a warming climate.