Ecosystems and climate determine terrestrial microbial carbon use efficiency

Microbial carbon use efficiency (CUE) defined as the microbial partitioning of C used in metabolism versus substrate C uptake reflects the microbial-associated accumulation and loss of soil organic carbon (OC). Comprehensively elucidating the global pattern of soil microbial CUE and related environmental drivers is essential for understanding ecosystem C cycling and its responses to global change. We performed a meta-analysis using data from 132 publications containing 690 observations (143 farmlands, 358 forests, and 189 grasslands) throughout the world to investigate the soil microbial CUE and identified the major environmental drivers in three ecosystems. The global average microbial CUE was 0.41 across all ecosystems. The microbial CUE was highest in temperate soils with moderate precipitation and temperatures. The average microbial CUE was highest in farmland ecosystems (0.51) and lowest in forest ecosystems (0.37) (p < 0.01), thereby indicating the higher potential for microbial C sequestration by farmland soils. Except for the experimental conditions, the soil pH and total nitrogen were important for controlling the microbial CUE in farmland and grassland than forest. However, the microbial CUE had a weak positive correlation with the soil OC content. The microbial CUE values estimated by using ¹³C or ¹⁴C labelling, ¹⁸O labelling, and biogeochemical equilibrium modelling also differed significantly, with mean values of 0.58, 0.30, and 0.34, respectively. These different microbial CUE responses depending on the estimation approaches employed could have been due to differences in the measurement time and addition of substrates. Collectively, our results suggest that ecosystems and climate factors dominate global patterns of microbial CUE, so the optimization of key parameters should be prioritized when integrating microbial process parameters in Earth system models.