Controls of microbial carbon use efficiency along a latitudinal gradient across Europe

Microbial carbon use efficiency (CUE) describes the partitioning of carbon (C) between respiration and growth, and this defines the soil-atmosphere C balance. Despite its importance, CUE is not properly represented in soil biogeochemical models. Here, we estimated how CUE varied in soil along a continental gradient. Through a structural equation model, we found that bacterial growth, fungal community composition, and SOM quality were the main drivers of CUE variation. Biotic factors controlled CUE directly, while soil properties and climate indirectly controlled CUE via biotic factors. Surprisingly, we found that microbial assimilability of SOM had a negative relationship with CUE. High rates of microbial SOM-use coincided with a greater growth-fraction used for respiration suggesting decoupled catabolism and anabolism, probably due to nutrient limitation. Our study highlights the importance of the microbial community composition to predict CUE and that interactions between bacterial and fungal communities can have implications for CUE.