Microbial Carbon Use Efficiency in Soils Exhibits a Unimodal Relationship With Temperature Across the Agroecosystem

Abstract Microbial carbon use efficiency (CUE), defined as the proportion of assimilated carbon allocated to microbial growth rather than respiration, is an important physiological trait influencing soil carbon cycling. However, the quantitative relationship between temperature and microbial CUE remains poorly constrained, particularly in agricultural ecosystems. Here, we conducted a large‐scale sampling survey spanning approximately 28° of latitude across Chinese croplands and quantified microbial CUE using the substrate‐independent 18 O‐H 2 O method to investigate the response patterns of microbial CUE to temperature and its implications for soil organic carbon (SOC) pools. Microbial CUE exhibited substantial spatial heterogeneity, ranging from 0.20 to 0.60. Across China's north‐south climatic gradient, microbial CUE increased from colder regions, peaked at 0.44 in the warm‐temperate zone, and declined toward warmer regions. In contrast to the commonly proposed negative temperature dependence of microbial CUE, microbial CUE exhibited a unimodal relationship with mean annual temperature across the observed range of 2.28–25.05°C, increasing below 13.67°C and declining above this threshold. Soil total phosphorus was the most important predictor of microbial CUE, whereas the unimodal temperature–CUE relationship remained significant after accounting for variation in soil total phosphorus. Furthermore, under warmer conditions (≥13.67°C), microbial CUE showed a negative statistical association with SOC through mineral‐associated organic carbon, suggesting that higher microbial CUE was not necessarily associated with greater SOC stabilization in cropland soils. This large‐scale empirical study advances our understanding of temperature‐related variation in microbial CUE and provides an empirical basis for improving the representation of microbial physiology in cropland carbon models.