The paucity of investigations of carbon (C) dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Soil microbes are important engines driving terrestrial biogeochemical cycles; their carbon use efficiency (CUE), defined as the proportion of metabolized organic C allocated to microbial biomass, is a key regulator controlling the fate of soil C. It has been theorized that microbial CUE should decline with warming; however, empirical evidence for this response is scarce, and data from deeper soils are particularly scarce. Here, based on soil samples from a whole-soil-profile warming experiment (0 to 1 m, +4 °C) and ¹⁸ O tracing approach, we examined the vertical variation of microbial CUE and its response to ~3.3-y experimental warming in an alpine grassland on the Qinghai–Tibetan Plateau. Microbial CUE decreased with soil depth, a trend that was primarily controlled by soil C availability. However, warming had limited effects on microbial CUE regardless of soil depth. Similarly, warming had no significant effect on soil C availability, as characterized by extractable organic C, enzyme-based lignocellulose index, and lignin phenol–based ratios of vanillyls, syringyls, and cinnamyls. Collectively, our work suggests that short-term warming does not alter microbial CUE in either surface or deep soils, and emphasizes the regulatory role of soil C availability on microbial CUE.

Changes in soil abiotic and biotic properties can be powerful drivers of feedback between plants and soil microbial communities. However, the specific mechanisms by which seasonal changes in environmental factors shape soil microbial communities are not well understood. Here, we collected soil samples from three sites along an elevational gradient (200–1200 m) in subtropical forests with unvarying canopy vegetation. We used an elevation gradient with similar annual precipitation but a clear temperature gradient, and phospholipid fatty acids (PLFAs) were measured to determine the seasonal variations in the composition of soil microbial communities in response to rising temperatures. Our results showed that the abundance of Gram-negative bacteria and total PLFAs were the lowest at low elevations in winter, and the ratio of Gram-positive to Gram-negative bacteria decreased with increasing elevation. However, the biomass of other microbial groups was the highest at medium elevations in summer, with the exception of actinomycetes species and fungi. Regardless of seasonal changes, soil fungal biomass tended to increase with increasing elevation. Moreover, in summer, microbial carbon use efficiency (CUE) increased with increasing elevation, whereas an opposite trend was observed in winter. Redundancy analysis and structural equation modeling showed that the dissolved organic carbon in soil was the main factor affecting the microbial communities along the elevation gradient in winter, whereas in summer, the microbial community structure was driven by shifting nitrogen availability, with both being associated with changing microbial CUE. As such, this study demonstrates distinct seasonal changes in the soil microbial community composition across an elevation gradient that are driven by carbon and nitrogen resource availability and shifts in microbial CUE. Furthermore, our results suggest that the interaction of underground plant roots and microbes drives changes in resource availability, thereby resulting in seasonal variation in soil microbial community composition across an elevation gradient.