Soil stores more carbon than other terrestrial ecosystems ^1 , 2 How soil organic carbon (SOC) is formed and maintained remains uncertain. ^1 , 3 , making it difficult to understand how it will respond to climate change ^3 , 4 Soil microorganisms are believed to play an important role in the formation of ^{, preservation a lost SOC5,6,7} Although microorganisms influence the accumulation and loss of organic matter in soil in many ways, ^{4 , 6 , 8 , 9 , 10 , 11}Microbial carbon use efficiency (CUE) is an integrative metric that can capture the balance of these processes ^12 , 13 Although CUE has the potential to act as a predictor of variation in SOC deposition, the role of CUE in SOC persistence remains unresolved. ^{7 , 14 , 15}. Here, we investigate the relationship between CUE and SOC retention and interactions with climate, vegetation, and edaphic properties using a combination of global-scale datasets, an explicit microbial process model, data assimilation, deep learning, and meta-analysis. We find that CUE is at least four times more important in determining SOC storage and its spatial variation worldwide than other assessed factors, such as carbon input, decomposition, or vertical transport. Furthermore, CUE exhibits a positive correlation with SOC content. Our findings point to microbial CUE as a major determinant of global SOC storage. Understanding the microbial processes underlying CUE and their environmental dependence may help predict SOC feedbacks to a changing climate. (Feng Tao)