As a vital ecological security barrier and climate regulator in northwestern China, the Qinghai Lake Basin (QLB) holds significant strategic value for regional ecological conservation and national park development. This study, utilizing land use data from 2010 to 2023 and integrating the InVEST and GeoDetector models, systematically investigated the spatiotemporal dynamics of soil conservation (SC) and its underlying driving mechanisms. The results indicate the following: (1) During the study period, SC services exhibited a gradual declining trend across the selected years, with total SC decreasing from 14.64 × 10³ t to 7.49 × 10³ t. Spatially, high-value areas were predominantly concentrated in the high-altitude forest–grassland ecotones along the basin margins, whereas low-value areas were mainly distributed in the low-altitude zones surrounding the lake and in the northwestern corner of the basin. (2) Among different land use types, forestland, bare land, and grassland constituted the dominant contributing categories, with bare land exhibiting the highest sediment export (SE) intensity (ranging from 8.93 to 19.13 t·hm⁻²·a⁻¹), thereby serving as the primary erosion source area. Along the elevational gradient, the mid-to-high elevation zones (3500–4500 m) contributed the most to total SC, with SC intensity increasing progressively with rising elevation. With respect to slope classes, total SC was predominantly concentrated in the gentle-to-moderate slope zones (5–25°), whereas SC intensity exhibited a non-linear increase with increasing slope steepness. (3) Slope was identified as the single factor with the strongest explanatory power regarding the spatial heterogeneity of SC, and the coupled interactions of “topography–soil–vegetation” exerted a synergistic regulatory effect on SC function. It is recommended that priority attention be given to the mid-to-high elevation contribution zones (3500–4500 m), the total SC contribution zones on gentle-to-moderate slopes (5–25°), and the extremely steep slope areas (>35°) with high erosion risk, so as to implement differentiated soil and water conservation measures. These findings provide a scientific basis for ecological protection, soil and water conservation strategy formulation, and sustainable development in the QLB.

Cold alpine salt-affected soils may retain carbon through mineral protection or suppressed microbial decomposition, but these processes remain difficult to distinguish. We measured soil physicochemical properties, four extracellular enzyme activities, soil organic carbon (SOC), and its dissolved (DOC), particulate (POC), and mineral-associated (MAOC) fractions across five soil types on the Qiangtang Plateau. Multivariate analyses and five XGBoost models interpreted using Shapley additive explanations (SHAP) characterized carbon-enzyme associations and identified leading predictors and nonlinear response transition points. Model performance varied among response variables (CV-R² = 0.4094–0.8815). Soda (SD) soil had a distinct carbon-pool composition, and no significant carbon-enzyme correlations remained after Benjamini–Hochberg correction. pH was the leading predictor of SOC and its fractions, whereas total nitrogen and total phosphorus ranked highest for overall enzyme activity. The fitted SHAP contributions changed from negative to positive between pH 8.50 and 9.00 for the carbon variables and at a total nitrogen concentration of 0.71 g·kg⁻¹ for enzyme activity. These model-dependent transition points indicate that carbon-pool restructuring and nutrient-related changes in enzyme activity occur over different environmental ranges. SD soil contained the highest carbon concentrations but the smallest MAOC proportion and the largest POC and DOC proportions, indicating relative enrichment of labile carbon fractions. By jointly analyzing carbon fractions and enzyme activity within an interpretable XGBoost-SHAP framework, this study helps distinguish high carbon stocks from stable sequestration and shows why total SOC alone may overestimate long-term carbon stability in alpine salt-affected soils.