Microbial assemblies with distinct trophic strategies drive changes in soil microbial carbon use efficiency along vegetation primary succession in a glacier retreat area of the southeastern Tibetan Plateau

Microbial carbon use efficiency (CUE) is a vital physiological parameter in assessing soil carbon turnover. Yet, how microbial communities with distinct trophic strategies regulate soil microbial CUE has remained elusive. Based on the oligotrophic: copiotrophic framework, we here explored the role of microbial taxa with different trophic strategies in mediating microbial CUE (determined by a ¹³C-labeling approach) along the vegetation primary succession in the Hailuogou glacier retreat area of the southeastern Tibetan Plateau. Soil microbial CUE ranged from 0.54 to 0.72 (averaging 0.62 ± 0.01 across all samples), increasing markedly along the vegetation succession. Microbial assemblies with distinct trophic strategies were crucial regulators of soil microbial CUE. Specifically, microbial CUE increased with microbial oligotroph: copiotroph ratios, with oligotroph-dominated stages having a higher microbial CUE than copiotroph-dominated ones. The prevalence of oligotrophic members would therefore be linked to the high soil microbial CUE at late successional stages. Given that oligotrophs predominate in soils with more recalcitrant carbon and because of their higher microbial CUE, we speculate that oligotrophs are likely to promote carbon sequestration in soils. In addition, the responses of soil microbial CUE to fungal oligotroph: copiotroph ratios were stronger than to bacterial ones. Fungal taxa may play a particularly pronounced role in shaping microbial CUE relative to bacterial members. Overall, our results highlighted close associations between microbial trophic strategies and CUE and provide direct evidence regarding how microbial trophic strategies regulate soil microbial CUE. This study is a significant step forward for elucidating the physiological mechanisms regulating microbial CUE and has significant implications for understanding microbial-mediated carbon cycling processes.