Soil Extracellular Enzyme Stoichiometry and Microbial Nutrient Constraints: Implications for Grassland Sustainability in the Qilian Mountains

Soil extracellular enzymes serve as critical drivers in the cycling of nutrients within ecosystems, and their stoichiometry can effectively reveal the metabolic resource limitations of soil microorganisms. However, extracellular enzyme activities, microbial metabolic characteristics, and their influencing factors in different grassland types in the Qilian Mountains have rarely been studied. This study focuses on alpine meadows (TJs), swampy meadows (HBs), and temperate desert grasslands (DLHs) in the Qilian Mountains. Extracellular enzyme activity and stoichiometric characteristics in the 0–30 cm soil layer were analyzed to explore the limiting factors on microbial metabolism and clarify the main driving factors affecting nutrient limitation. Compared with swampy meadows and temperate desert grasslands, alpine meadows exhibited greater extracellular enzyme activity, as revealed by the results. Statistical analysis revealed that enzyme activity exhibited a significant positive correlation with nitrate nitrogen (NO3−-N), total phosphorus (TP), total potassium (TK), available potassium (AK), and dissolved organic carbon (DOC), while showing a significant negative correlation with soil moisture content (SWC) (p < 0.05). Vector analysis of soil enzymes showed that soil microorganisms in the three grassland types are limited by carbon (C) and phosphorus (P). Among them, DLH microorganisms are highly restricted by carbon, while HB microorganisms are highly restricted by phosphorus. Random forest results showed that total phosphorus (TP), available potassium (AK), nitrogen-to-phosphorus ratio (N: P), nitrate nitrogen (NO3−-N), and readily oxidizable carbon (ROC) contribute significantly to vector length, while total potassium (TK), soil organic carbon (SOC), particulate organic carbon (POC), bulk density (BD), and carbon–nitrogen ratio (C: N) contribute significantly to vector angle. A partial least squares path model (PLS-PM) revealed that although microbial metabolic limitation is influenced by specific soil factors, the comprehensive effect of soil physicochemical properties is the dominant factor regulating microbial carbon and phosphorus limitation. This study provides valuable data and insights that elucidate the metabolic characteristics of soil microorganisms across different grassland types in the Qilian Mountains, thereby improving the mechanistic understanding of soil nutrient cycling and supporting evidence-based strategies for the sustainable management and conservation of these fragile ecosystems.