Slope Aspect Influences the Organic Carbon Content and Stock in Soil Aggregates of Cut Slopes in the Alpine Region of Southwest China

ABSTRACT Road construction in alpine regions generates extensive bare cut slopes, which aggravates soil erosion and leads to substantial loss of soil organic carbon (SOC). Slope aspect modifies hydrothermal conditions, making it a critical regulator of SOC dynamics; however, it is unclear how it affects the physical protection of SOC within various soil aggregates in engineered cut slopes. In this study, we investigated the distribution and drivers of soil‐aggregate‐associated SOC across four slope aspects [south‐, west‐, east‐, and north‐facing (SFS, WFS, EFS, and NFS, respectively)] in the alpine region of Southwest China. Wet sieving for aggregate separation and partial least squares path model (PLS‐PM) for causal inference demonstrated that SOC content and stock in both bulk soil and aggregate fractions systematically varied with slope aspect type, generally following the order NFS > EFS > WFS > SFS. Small macroaggregates (SMA) and microaggregates (MIA) were identified as the dominant SOC reservoirs. Their combined dominance was the most pronounced in SFS, WFS, and EFS. PLS‐PM revealed that slope aspect type did not directly influence aggregate‐associated SOC but indirectly influenced it by affecting soil water content, total nitrogen, and soil organic matter (R 2 = 0.84). These factors governed the composition of soil aggregates and the SOC amount associated with them, which ultimately served as the most direct and decisive controls on total SOC dynamics (R 2 = 0.99). Our findings demonstrated that the stabilization of carbon in SMA and MIA is the central mechanism underlying slope‐aspect‐driven SOC sequestration patterns in these systems. This mechanistic insight offered a scientific basis for designing aspect‐specific restoration strategies to enhance carbon storage and soil quality in engineering and road construction projects in alpine regions.