Nutrient additions have minor impact on microbial carbon use efficiency

Soil nutrient and organic matter contents play a pivotal role in shaping microbial carbon use efficiency (CUE) by modulating microbial catabolic and anabolic processes, thereby influencing carbon (C) dynamics in terrestrial ecosystems. However, a comprehensive understanding of how organic matter, nitrogen (N), phosphorus (P) additions, as well as their interactions affect soil microbial CUE remains unclear. Here, we conducted a meta-analysis comprising 531 observations from 153 published studies to investigate the effects of organic matter, N, and P additions on microbial growth, respiration, and CUE. Our results revealed that N addition significantly decreased soil respiration by 6 %, without affecting microbial growth or CUE. P addition showed no significant effects on microbial physiology. Combined N and P additions significantly enhanced microbial growth and respiration by 9 % and 16 %, respectively, but did not influence microbial CUE. Organic matter addition significantly promoted microbial growth, respiration, and CUE by 37 %, 30 %, and 9 %, respectively. The impacts of N addition on microbial CUE varied across ecosystem types and forms of N applied. Soil microbial responses were more pronounced at lower N addition rates and shorter durations but diminished at higher N addition rates and longer durations. Furthermore, the impacts of inorganic nutrients and organic matter additions on microbial CUE strongly depended on soil properties. These findings provide critical insights into microbial physiological responses to nutrient additions and offer a robust foundation for improving Earth System Models, thereby enhancing predictions of C cycling under scenarios of increased nutrient availability.