Soil organic carbon distribution and multi-scale drivers in semi-arid alpine Regions: Implications for carbon storage...

The spatial distribution pattern of soil organic carbon (SOC) directly influences carbon sequestration and climate regulation potential. However, in semi-arid alpine mountain regions, the spatial heterogeneity of SOC and its underlying driving mechanisms remain insufficiently investigated. Furthermore, empirical evidence for the quantitative assessment of soil carbon storage capacity across different vegetation units under natural conditions is still lacking. In this study, six ecological units were established along an altitudinal gradient in the Xiying River Basin of the eastern Qilian Mountains. A two-year field survey on soil and vegetation was conducted under various topographical conditions, including differences in slope, aspect, and elevation. This study measured 11 key soil and vegetation variables, such as SOC content, total nitrogen, and aboveground biomass. Building on this, the concept of 'Soil Carbon Storage Function Stability' (SCSFS) is proposed for the first time, with a cloud model employed to quantitatively assess soil carbon storage in semi-arid, cold, high-altitude regions. The spatial distribution of soil organic carbon density (SOCD) exhibited a pronounced topographic dependence, following the patterns: north-facing slope (5.06 ± 2.25 kg C/m²) > south-facing slope (3.23 ± 1.68 kg C/m²), footslope (4.60 ± 2.03 kg C/m²) > summit (3.98 ± 1.81 kg C/m²) > midslope (3.86 ± 1.91 kg C/m²), and gentle slope (4.39 ± 2.04 kg C/m²) > steep slope (3.89 ± 1.78 kg C/m²). In the 0-20 cm soil layer, SOCD exhibited a unimodal variation with elevation, peaking at 3141-3189 m, which corresponds to the ecotone between Mountain Forest Grasslands (MFG), Sub-alpine scrub meadow (SASM), and Alpine Meadow (AM). The spatial differentiation of SOC is primarily driven by the combined effects of vegetation productivity and hydrothermal conditions, and is further modulated indirectly by soil physicochemical properties (R² = 0.57). Among these factors, the relative contributions follow the order: soil properties (46 %) > climate (22 %) > vegetation (16 %) > topography (15 %). Variations in SCSFS among different ecological units exhibit a non-linear relationship. Specifically, the Mountainous Desert Grassland (MDG) and Montane Dry Scrub Grassland (MDSG) exhibit moderate stability; the MFG, SASM, and AM demonstrate stable conditions; while the Alpine Desert (AD) is characterized by extremely unstable. The transition from MDSG to MFG shows the most rapid increase in SCSFS, whereas the shift from AM to AD shows the most rapid decline. Overall, the region displays a relatively stable SCSFS pattern. This study elucidates the multi-scale driving mechanisms of SOC distribution in semi-arid alpine mountain regions and provides a theoretical foundation for carbon management in alpine ecosystems.