Soil Carbon Availability Drives Depth-Dependent Responses of Microbial Nitrogen Use Efficiency to Warming

Microbial nitrogen use efficiency (NUE) describes the partitioning of organic N between microbial growth and N mineralization, which is crucial for assessing soil N retention. However, how warming affects NUE along soil depth remains unclear. Based on a whole-soil-profile warming experiment (0 to 100 cm, +4°C) on the Qinghai-Tibetan Plateau, combined with ¹⁸O and ¹⁵N isotope labeling techniques, we determined soil carbon (C) composition, edaphic properties, and microbial parameters. The results showed that NUE declined with soil depth in both control and warming treatments, driven by microbial C limitation. The response of NUE to warming varied with soil depth. Warming reduced topsoil (0–30 cm) microbial N growth, ultimately leading to a decrease in NUE, but had no effect in deep soils (30–100 cm). Jointly, these findings highlight that warming may exacerbate soil N loss in topsoil, and that maintaining microbial C and N availability could be a key strategy for preserving microbial N sequestration under warming conditions.