Dry-Sand Layer Regulates Soil Moisture Stratification and Plant Community Assembly in the Desert–Oasis Ecotone

The desert–oasis ecotone is ecologically fragile, yet the profile-scale role of the Dry-Sand Layer (DSL) in regulating soil moisture stratification and plant community assembly remains poorly understood. We systematically investigated six representative sites across the middle Heihe River Basin (with soil types transitioning from silt loam to aeolian sandy soils), combining multi-depth soil hydro-physical profiling (0–120 cm) with comprehensive vegetation sampling (across 90 nested quadrats recording 37 vascular plant species). A distinct DSL commonly formed at 60–80 cm depth across sandy sites, characterized by a bulk density of 1.39 g. cm−3 and a distinct moisture trough (typically < 2.0%), decoupling shallow root zones from subsoil moisture pools (1.10% to 5.09%). This edaphic boundary filtered out deep-rooted species, restricting vegetation to shallow-rooted xerophytic shrubs and grasses (e.g., Reaumuria soongarica (Pall.) Maxim., Aristida adscensionis L.) exhibiting high interspecific niche overlap (>0.86) and clustered spatial patterns (C > 1). Structural equation modeling indicated that subsoil moisture regulated by the DSL directly drives plant community diversity (path coefficient β = +0.54) and coverage (β = +0.61). Overall, the DSL serves as an essential eco-hydrological buffer balancing shallow soil moisture retention and subsoil conservation, indicating that ecological restoration in hyper-arid ecotones should prioritize DSL preservation and native shallow-rooted vegetation.