Root-mediated regulation of soil infiltration dynamics in eroded Mollisols

Water infiltration is a fundamental hydrological process in terrestrial ecosystems, influencing surface runoff, soil erosion, plant growth, and groundwater recharge. Although vegetation restoration is widely recognized for improving soil infiltration, how root-soil interactions respond to erosion-induced changes in soil profile stratification and related properties, and how these responses affect the hydrological effectiveness of vegetation restoration, remain poorly understood. In this study, we simulated the topsoil loss due to erosion by constructing soil columns with varying thicknesses of the black soil layers (30 cm, 20 cm, 10 cm, and 0 cm), representing a stage of no erosion, mild erosion, moderate erosion, and severe erosion. This gradient was achieved by layering different proportions of surface black soil and underlying depositional subsoil. We then compared soil infiltration rates and patterns across the erosion gradient under bare soil conditions and with four perennial herbaceous species: two Poaceae (Festuca arundinacea and Bromus inermis) and two Fabaceae (Medicago sativa L. and Astragalus adsurgens). Results showed that in the absence of vegetation, intensified black soil erosion reduced infiltration rates by more than 60 % and shifted infiltration patterns from matrix flow to localized preferential flow. Notably, these preferential flow paths, initiated by unstable cracks, were transient and contributed minimally to overall infiltration. Meanwhile, vertical water infiltration was restricted, while near-surface lateral flow was enhanced, particularly under moderate erosion (10 cm black soil layer), where the effect was most pronounced. The effects of different herbaceous species on infiltration rates and water flow behavior varied depending on root morphology and erosion severity. Fabaceae species, with their well-developed taproot-lateral root systems, consistently increased infiltration rates by13.36 %‒90.91 % across all erosion levels, promoted deep water infiltration and recharge (maximum stained depth increased by > 60 %), suppressed lateral flow (stained width-to-depth ratio decreased by > 30 %), and significantly enhanced the development of stable preferential flow (preferential flow fraction increased by > 100 %) under moderate and severe conditions. In contrast, Poaceae species, characterized by fibrous root systems, improved infiltration rates and promoted preferential flow only in non-eroded and mildly eroded soils, but their effectiveness was generally lower than that of Fabaceae. This study highlights the importance of matching plant types to erosion severity when designing vegetation-based restoration strategies for degraded black soils. Such matching is critical for effectively restoring infiltration capacity and hydrological function. These findings provide valuable insights for developing targeted soil hydrological restoration and water resource management strategies in Mollisols and other regions with similar soil profiles and climatic conditions.