Preferential flow characteristics and ecohydrological functions of shallow fissure soils in fragile karst ecosystems

Introduction Preferential flow is a key hydrological process controlling rainfall redistribu-tion, subsurface recharge, and ecohydrological functioning in karst ecosys-tems. However, the formation and ecological significance of preferential flow in shallow karst fissure (SKF) soils remain insufficiently quantified. Methods Field dye-tracer experiments with Brilliant Blue FCF were conducted in SKF soils in Xixiu District, Guizhou Province, Southwest China, to characterize vertical flow patterns and identify soil-hydraulic and root controls. Results Preferential flow showed strong vertical heterogeneity: D c was high in the 0–10 cm layer (68.62–98.75%) but decreased rapidly with depth, while maximum stained depth varied from 23.0 to 74.6 cm among fissures, indicating marked spatial variability in subsurface hydraulic con-nectivity. Bulk density (BD) was the strongest negative control on D c ( r = −0.697), whereas total porosity (TP), non-capillary porosity (NCP), saturated hydraulic conductivity (Ks), saturated water content (SWC), capillary water content (CWC), field capacity (FC), root length density (RLD), and root surface area density (RSAD) generally promoted preferential flow development. PLSR further identified BD, SWC, CWC, NCP, TP, FC, and Ks as the main predictors, with VIP values greater than 1. These factors explained approximately 52–55% of the observed variation in D c . The results indicate that preferential flow in SKF soils is jointly regulated by soil structure, hydraulic properties, and root-induced biopores. Discussion Preferential flow provides a dual ecohydrological function by enhancing deep water redistribution and epikarst recharge while potentially increasing water leakage from the shal-low root zone. These findings improve the mechanistic understanding of water movement in karst fissure soils and provide guidance for vegetation restoration and water conservation in rocky desertification regions.