Microbial growth rate is a stronger predictor of soil organic carbon than carbon use efficiency

The extent to which microbial processes control soil organic carbon (SOC) dynamics remains uncertain. Carbon use efficiency (CUE)—the fraction of assimilated carbon allocated to growth—has been used as a key parameter, but its relationship with SOC reflects carbon partitioning rather than the absolute magnitude of microbial fluxes. Microbial growth rate could provide a more mechanistic link to SOC accumulation, as it quantifies biomass production and reflects necromass formation. Here we combine a global ¹⁸O–H2O dataset (n = 268 paired observations) with outputs from four land surface models to test whether growth rate predicts SOC more strongly than CUE. In the incubation experiments, growth rates are more closely associated with SOC than CUE, although soil properties and climate explain equal or greater variance. Models reproduce the stronger role of growth rate over CUE but tend to underestimate the abiotic controls. The models also emphasize CUE as the main predictor of the SOC/NPP ratio, in contrast to observations, which indicates the soil’s capacity to retain plant carbon inputs. Together, these findings identify microbial growth rate as a diagnostic that can help bridge models with empirical data and guide a more balanced representation of microbial and mineral controls in SOC projections.