Effects of under-forest economic activities on soil water repellency, soil hydraulic properties and preferential flow in karst forests

The effects of under-forest economic activities (UFE) on soil water repellency, soil hydraulic properties and preferential flow pathways of karst forest soils are not fully understood. To investigate these effects, the soil physical-chemical properties including soil penetration resistance, surface shear strength and soil water repellency, as well as saturated hydraulic conductivity (K_s) and soil water retention curves (SWRCs) at depths of 0–20, 20–40, and 40–60 cm were compared between undisturbed soils affected by human trampling on forest trails formed by UFE. Disk tension infiltrometers and dye tracing experiments were also conducted to quantify the effects of UFE on the field hydraulic properties and preferential flow pathways from the center of forest trails (CF), the edge of forest trails (EF), and undisturbed forestlands (FL). Our results showed that the forest trails formed by under-forest economic activities enhanced water repellency to various degrees. The K_s of CF (37.03 mm h⁻¹) was significantly lower than that of FL (115.29 mm h⁻¹). The SWRC slopes were steeper for the CF and the EF compared to the FL in the 0–20 cm, 20–40 cm and 40–60 cm intervals, and significant differences in van Genuchten model parameters (α, n, m, θ_r, and θ_s) in the 0–20 cm layer confirm that SWRCs were largely influenced by forest trail. In addition, the averaged preferential flow fraction (PF-fr) was significantly higher for CF and EF compared to FL, while the dye coverage (DC) and uniform infiltration depths (UniFr) were significantly lower. These results indicated that preferential flow is largely enhanced by the UFE. This study provides a field-based case study illustrating the negative effects of UFE on land development and degradation.