Impact of initial water content on aggregate stability: a perspective based on soil internal forces

The soil aggregate stability is closely linked soil moisture through its regulation of soil internal forces, such as the van der Waals attractive force, electrostatic repulsion, and surface hydration repulsion. However, the mechanism driving the stability at different initial water content remains unclear. This study conceptualizes a critical water content (CWC) threshold for the transition between stability enhancement and reduction, analyzing this phenomenon from the perspective of soil internal forces. Four initial water contents (ranging from 0.033 to 0.434 g·g −1 ) were used to assess the effect of soil water content on soil internal force. The results reveal the presence of CWC in soil aggregate stability, which was more pronounced in the Yoder and the fast wetting treatment (Le Bissonnais method). Under Yoder’s method, the mean weight diameter ( MWD ) first increased from 1.38 to 1.71 mm as the water content rose from 0.033 to 0.225 g·g −1 , and then decreased to 1.32 mm (p < 0.05). At the same time, the thickening of the water film from 1.1 to 2.8 nm caused the net soil internal force ( P net ) to shift from repulsive (<1.15 nm) to attractive, reaching a peak of 13.9 atm before declining to zero. When water content exceeded 0.225 g·g −1 , P net showed a decreasing trend in attraction, indicating a reduction in aggregate cohesion and stability under high moisture content. Pearson correlation analysis revealed a highly significant correlation between soil internal forces ( P net ) and aggregate stability ( MWD ) (p < 0.01). These findings suggest that initial soil water content influences aggregate stability by modulating soil internal forces, which partially explains the CWC phenomenon.