Differential responses of soil microbial metabolic limitation and carbon use efficiency to long-term grazing exclusion across three grassland types in Xinjiang.

INTRODUCTION: Long-term grazing exclusion is widely used to restore degraded grasslands, yet whether its effects on soil microbial metabolism-particularly nutrient limitation and carbon use efficiency (CUE)-vary among grassland types remains poorly understood. METHODS: We investigated three typical grassland types in Xinjiang, China: temperate desert (TD), temperate steppe (TS), and mountain meadow (MM), under 12 years of grazing exclusion and free grazing. We measured soil extracellular enzyme activities, assessed microbial nutrient limitation via vector analysis, and calculated microbial CUE. RESULTS: Grazing exclusion produced contrasting effects among grassland types. In TD, all three enzyme activities significantly increased (P < 0.01); in TS, C- and N-acquiring enzymes decreased (P < 0.05); and in MM, only C-acquiring enzyme increased (P < 0.001), accompanied by a significant reduction in CUE (P < 0.01). Grazing exclusion generally intensified microbial C limitation (increased vector length) while alleviating N limitation (vector angles shifted toward 45°), though all vector angles remained below 45°, indicating persistent N limitation. Partial least squares structural equation modeling revealed distinct regulatory pathway: an indirect "soil → dissolved nutrients → microbial biomass" pathway in TD; a direct "soil → microbial biomass" pathway in TS; and a longer cascade pathway in MM that enhanced C limitation but reduced CUE. DISCUSSION: Our findings demonstrate that grazing exclusion effects on microbial metabolism are strongly grassland-type dependent. Grazing exclusion promoted favorable microbial functional responses in TD and TS, but intensified C limitation and reduced CUE in MM. Vector length and angle serve as sensitive indicators of microbial nutrient limitation during restoration. These findings highlight the need for ecosystem-specific restoration strategies in arid and semi-arid regions.