Moisture‐induced changes in soil pore structure and their impact on erodibility of red soil

Abstract

Soil erodibility is a key inherent property influencing water erosion, with soil moisture driving its temporal variation. However, the underlying mechanisms remain underexplored. This laboratory study investigated how moisture content affects pore structure by using digital imaging techniques and consequently alters the erodibility of a typical red soil (also classified as Ultisols and Acrisols). Soil samples were incubated at four moisture levels: dry (8%), semi‐dry (12%), semi‐wet (23%), and wet (30%). The microstructural characteristics and soil pore distribution were analyzed using nuclear magnetic resonance spectroscopy and environmental scanning electron microscopy. In addition, x‐ray computed tomography scanning was employed to quantify three‐dimensional pore characteristics. Results showed that increasing soil moisture shifted pore size distribution toward smaller ultra‐micropores (≤5 µm), which peaked under the semi‐wet condition. Compared to the wet state, the dry state exhibited significantly larger pore surface area, a higher degree of pore anisotropy, but a notably lower fractal dimension, and poorer connectivity. The variable pore characteristics, including porosity, pore surface area, degree of pore anisotropy, and fractal dimension, are significantly correlated with soil dispersibility and scourability. In summary, dry, loose soil is characterized by large inter‐aggregate pores, a high degree of soil pore anisotropy, and a low fractal dimension, all of which contribute to reduced shear strength and ultimately weaken soil resistance to erosion. Therefore, particular attention should be paid to the vulnerability of poor soil structure under dry conditions during intense rainfall events.