Spatial distribution of soil organic carbon in contrasting management of grasslands in typical steppes of the Mongolian plateau

Soil organic carbon (SOC) is a key indicator of soil quality and carbon storage in grassland ecosystems, yet its spatial variation under contrasting grazing regimes remains insufficiently understood. Focusing on the typical steppe along the Ujimqin-Wenduerhan transect across the China-Mongolia border, this study compared SOC distribution and its major associated factors under sedentary grazing (SG) and four-season nomadic grazing (FSNG). Field surveys, laboratory analyses, principal component analysis (PCA), and partial least squares path modeling (PLS-PM) were used to assess the relationships among grazing regime, vegetation characteristics, soil properties, and SOC. SOC decreased with soil depth under both grazing regimes, while its spatial pattern differed between FSNG and SG. Compared with SG, FSNG was characterized by higher soil total nitrogen (TN) and total phosphorus (TP) and lower bulk density (BD) and soil water content (SWC). PCA indicated that FSNG was more closely related to nutrient and vegetation factors, whereas SG was more strongly linked to soil physical conditions. PLS-PM further showed that soil TN was the strongest direct factor associated with SOC under both grazing regimes. Under FSNG, aboveground biomass (AGB) and pH were positively associated with SOC via soil TN, suggesting a closer link between SOC variation, vegetation inputs, and nutrient availability; under SG, BD was negatively associated with SOC via soil TN, indicating a closer association with soil physical constraints and nitrogen availability. These findings indicate that contrasting grazing regimes are associated with distinct SOC spatial patterns and differences in the relative importance of vegetation, nutrient, and soil physical factors. These findings improve our understanding of SOC variation under contrasting grazing regimes and provide a scientific basis for sustainable grazing management and soil carbon conservation in dryland grasslands.