Minor effects of nutrient additions on soil microbial carbon use efficiency

The carbon use efficiency (CUE) of soil microbes is defined as the proportion of assimilated carbon (C) allocated to microbial growth than lost through respiration. CUE is a critical parameter in soil C cycling, and its response to external nutrient inputs (including nitrogen [N], phosphorus [P], and combined NP) could determine the change of soil C pool. However, the general pattern of CUE response to nutrient addition remains unclear. By conducting a global meta-analysis of 427 observations, we found that the nutrient addition had no significant overall effect on CUE, whether in the overall nutrient addition experiments or in experiments with different types of nutrient addition. The response of CUE to N addition was regulated by duration, with long-term addition (>10 year) leading to a decrease in CUE. We identified the aridity index as the most important predictor influencing CUE in response to N addition, while ecosystem type was the most key variable for CUE response to both P and NP additions. Global predictions showed future N inputs tends to enhance CUE across most regions, especially in cropland ecosystems. Overall, the effect of nutrient addition on CUE was relatively minor, providing key microbial evidence for understanding future soil C dynamics.