Effects of whole-profile warming on microbial carbon and nitrogen use efficiency at different soil depths in an alpine meadow

Microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE) are key parameters determining the fate of C in soils. However, the paucity of investigations of microbial CUE and NUE dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Here, based on soil samples from a whole-soil-profile warming experiment (0–1 m, +4 °C) and stable isotope (¹⁸O and ¹⁵N) tracing approaches, we examined the vertical variation of microbial CUE and NUE and its response to ~3.3-year experimental warming in an alpine grassland on the Qinghai-Tibetan Plateau. Our findings revealed that microbial CUE and NUE decreased along soil depth, a trend that was primarily controlled by soil C availability. We also observed differential warming effects on microbial CUE and NUE. Microbial CUE showed no significant response to warming in either the topsoil or deep soil. However, microbial NUE in the deep soil decreased by 53% under warming compared to non-warmed controls, suggesting that warming drives soil microbes to incorporate less N into their biomass in the topsoil. The decrease in microbial NUE was likely triggered by a reduction in soil N availability in the topsoil. Collectively, our work emphasizes the regulatory role of substrate availability on microbial CUE and NUE.