Contrasting exogenous and endogenous soil microbial carbon use efficiencies under global changes

Microbial carbon use efficiency (CUE) is a critical parameter for controlling soil carbon dynamic. It is divided into microbial exogenous CUE (CUEex) and microbial endogenous CUE (CUEen) based on microbial utilization of carbon derived from organic material and soil organic matter. Global changes encompassing warming, dramatically decreased precipitation (drought) events, anthropogenic nutrient addition, and plant invasion can strongly influence microbial CUEex and CUEen. However, the responses of microbial CUE to global changes remain unclear, which can lead to significant uncertainties when forecasting terrestrial ecosystem carbon cycling under global changes. To address this issue, we analyzed 196 paired microbial CUEex and 97 paired microbial CUEen data and found that microbial CUEex decreased significantly with absolute latitude, while microbial CUEen showed the opposite trend. Warming had significant negative impacts on microbial CUEex and CUEen with decreases of 16.0% and 28.3%, respectively. Decreased precipitation had a mixed effect; increasing microbial CUEex by 7.9% but decreasing microbial CUEen by 14.4%. Nutrient addition had a consistently negative impact on microbial CUEex, decreasing it by 3.9-14.8%. The application of nitrogen, and nitrogen combined with phosphorus and potassium significantly increased microbial CUEen by 32.8% and 43.1%, respectively. Invasive plants significantly decreased microbial CUEex by 11.5%. Global change enhanced the microbial growth rate by 6.5%; however, when combined with added exogenous carbon there was a decrease of 3.2%. Aridity index, soil pH, and soil cation exchange capacity were the primary determinants of microbial CUEex in response to global changes. In contrast, microbial respiration and growth rates, followed by microbial biomass carbon, were found to be the most influential predictors of microbial CUEen. Therefore, it is essential to differentiate the magnitude, direction, and driving factors of microbial CUEex and CUEen in biogeochemical models, to accurately predict terrestrial ecosystem carbon cycling and potential feedbacks to global changes.