Responses of soil microbial carbon use efficiency to warming: Review and prospects

Microbial carbon use efficiency (CUE) is an important factor driving soil carbon (C) dynamics. However, microbial CUE could positively, negatively, or neutrally respond to increased temperature, which limits our prediction of soil C storage under future climate warming. Experimental warming affects plant production and microbial communities, which thus can have a significant impact on biogeochemical cycles of terrestrial ecosystems. Here, we reviewed the present research status of methods measuring microbial CUE and the response of microbial CUE to the changes of biotic and abiotic factors induced by warming. Overall, current measurement methods mainly include metabolic flux analysis, calorespirometry, stoichiometric model, 13C and 18O labeling. Differences in added substrate types can lead to an overestimation or underestimation on microbial CUE, particularly when using the 13C labeling method. In addition, changes in the dominant microbial community under warming may also affect CUE. However, there is still uncertainty in CUE characteristics of different microorganisms. Microbial CUE is generally decreased under warming conditions as microbes are subjected to water stress or soil labile organic matter is much more depleted compared to ambient conditions. In contrast, considering that warming increases soil nutrient availability, warming may enhance microbial CUE by alleviating nutrient limitations for microbes. In conclusion, the response of microbial CUE to warming is more complex than expected. The microbial growth and physiological adaptation to environmental stress under warming is one of the main reasons for the inconsistence in microbial CUE response. Finally, we propose five aspects where further research could improve the understanding of microbial CUE in a warmer world, including using new technologies, establishing multi-factor interactive experiments, building a network of experimental research platform for warming, and strengthening studies on response of CUE to warming at different soil depths and on different temporal scales.