Biogeographic patterns and drivers of microbial carbon use efficiency across a 3000-km aridity gradient in the Mongolian Plateau, Northern China

Microbial carbon use efficiency (CUE) characterizes the partitioning of carbon between the anabolic and catabolic pathways of microbial metabolism. The expanding global dryland ecosystems due to climate change affect several metabolic processes such as microbial respiration, enzymatic synthesis degradation, and nutrient partitioning. However, the distribution and regulatory mechanisms of CUE along aridity gradients remain controversial, particularly at broad regional scales. Here, we obtained 90 samples along an approximately 3000-km transect covering 16 forests, 57 grasslands, and 17 desert ecosystems. Further, the CUE used the biogeochemical equilibrium model and identified a critical biogeographic pattern of nonlinear response between CUE and aridity index (AI) at a threshold of 0.181, where low and high AI patterns are discriminated. We also evaluated the metabolic limitations through the enzyme vector model and the drivers of CUE in different patterns. Our results found that the microbial CUE in LAI was significantly lower than HAl and more sensitive to aridity, suggesting that arid regions exhibited weaker microbial C sequestration potentials. Microbial metabolism was C and P limited regardless of the pattern, but they showed a negative effect on CUE only in HAI. Moreover, when comparing the two AI patterns, the key driver in CUE differs substantially. Specifically, high temperatures and low water availability limit microbial CUE in arid regions by affecting soil nutrients, stoichiometry, and plant-microbe competition. Soil nutrients are the primary regulator of microbial CUE in humid regions. Our findings will be vital to clarify the mechanism of microbial-mediated organic carbon sequestration and soil carbon dynamics in different aridity conditions under ongoing and future climate change.