Digging deeper into microbial carbon use efficiency in soil: Perspectives from microbial growth and respiration

Microbial carbon use efficiency (CUE), a key regulator of soil organic carbon (SOC) dynamics, reflects carbon allocation between microbial growth and respiration. Separate investigation of microbial growth and respiration helps to clarify the role of CUE in the carbon balance and consequently, in the carbon cycle in soil. First, in terms of microbial resource acquisition, the drivers of microbial growth and respiration differ: microbial growth is primarily regulated by synergistic availability of carbon, nitrogen, and phosphorus, whereas respiration is more strongly influenced by substrate quality, such as the carbon-to-nitrogen ratio. Second, synchronized responses of microbial growth and respiration rates under climate change (e.g., warming, drought, elevated CO2) often mask underlying CUE regulatory mechanisms. Thus, we propose: (1) explicitly representing microbial growth and respiration in models to raise the precision of SOC dynamics predictions; and (2) applying approaches such as isotope probing and metagenomic functional annotation to resolve CUE mechanisms from the community to the single-species level. Overall, decoupling CUE into its growth and respiration components establishes a mechanistic framework that explicitly links CUE to microbially mediated SOC dynamics.