Analysis of the Variation in Soil Physical Properties Influenced by Forest Soil Development Based on Water Retention and Hydraulic Conductivity Functions

Abstract

The structural development of forest soils regulates soil hydrological processes by altering physical properties. This study examined variations in pore characteristics and hydraulic properties across one forest soil profile using simultaneous measurement data of water retention and hydraulic conductivity, including the unsaturated range, obtained with the Continuous Outflow Evaporation Method. Undisturbed (U) and Disturbed (D) soils samples at various depths in a pine forest stand were analyzed to evaluate the effects of soil structural change during forest soil development. The water retention model based on the pore size distribution performed well, while the hydraulic conductivity model had better performance considering the change of tortuosity parameters (α and β). Soil structural development increased saturated water content (θ _s), median pore size (r_m), and saturated hydraulic conductivity (K_s), though no consistent pattern emerged for pore size distribution width (σ). K_s increased during forest soil development linked to the increase of r_m or σ across depth. Although the tortuosity parameters (α and βσ) showed limited depth dependence, compared to D soils, U soils exhibited greater sensitivity of relative hydraulic conductivity (K_r) to effective saturation (S_e). These trends were well captured by the combination of tortuosity parameters α and βσ, highlighting the importance of incorporating tortuosity changes in hydraulic conductivity model for forest soils. Overall, the results enhance our understanding of the influence of soil structure changes during forest soil development to the water retention and hydraulic conductivity across soil layer and provide a basis for improving predictive hydrological modeling in forest ecosystem.