Global prediction of soil microbial growth rates and carbon use efficiency based on the metabolic theory of ecology

Soil microbial growth rate and microbial carbon use efficiency (CUE) are critical parameters of soil microbial carbon metabolism, moderating soil organic carbon (SOC) dynamics. However, global patterns of soil microbial growth rate and microbial CUE are still unresolved. Here, we show that the metabolic theory of ecology (MTE) can be applied to model soil microbial growth rate at the global scale as a function of microbial biomass, temperature, and SOC contents. Rates of soil microbial growth were modeled at depths of 0-30 cm and calibrated against rates measured with the 18O-labeled H2O incubation method. The modeled soil microbial growth rates were strongly driven by temperature. They decreased with latitude and had greater seasonal variations in tundra and boreal forest compared to tropical biomes. Soil microbial CUE (0-30 cm) ranged from 0.25 to 0.63 among global biomes, averaging at 0.43. Modeled annual soil microbial growth rates followed the same global patterns and were on the same order of magnitude as other key ecosystem C fluxes such as net primary productivity, litterfall, and heterotrophic respiration. This indicates a strong functional linkage of aboveground and belowground communities at the global scale. Our MTE-based approach provides the first estimates of global patterns for soil microbial growth rate and microbial CUE and potentially provides a powerful mechanistic framework to incorporate soil microbes into Earth System Models.