Global distribution and drivers of microbial carbon use efficiency for projecting soil organic carbon fates under a changing climate: A meta-analysis

Comprehending the factors influencing microbial carbon use efficiency (CUE), and where CUE is most optimal to soil organic carbon (SOC) storage, are crucial for managing microbial roles in SOC sequestration and model prediction. Yet, establishing a direct mathematical relationship between CUE and SOC might be challenging, with global distributions and controls remaining unresolved, particularly in response to various global changes. Here, we leverage a global synthesis of CUE measurements by 18O-microbial DNA growth, and observed an average CUE across all biomes at 0.3, with the highest in temperate grasslands and deeper soils, and the lowest in tropical forests. Random forest analysis identified climates (aridity index and mean annual temperature: MAT) and soil properties (pH, bulk density and soil C:N ratio) as primary drivers influencing CUE. However, microbial biomass size overall exhibited a smaller effects on CUE, despite its substantial impact in each land use type. We then review how these drivers affecting CUE values may be altered by warming, soil fertilization, altered precipitation and elevated carbon dioxide. Notably, nitrogen additions plays a big role in increasing CUE and promoting SOC contents, while warming effects depend on time-scale, with long-term warming potentially leading to SOC losses with a lower CUE and growth. Moreover, we found that the CUESOC relationship varies across different climates, greatly driven by MAT and soil properties. Higher CUE promots SOC per fine fraction (clay+silt) across the major data points, contrasting with a negative relationship in a subarctic study, where pH is the primary determinate. Consequently, there might be no simple linear relationship between CUE and C in microbial biomass and soil. We conclude by discussing the integration of CUE into SOC models and the necessity of incorporating interactions between CUE and individual drivers for predicting soil carbon-climate change scenarios. Our study underscores the importance of considering microbial CUE and other microbial processes for improving projections of SOC dynamics.