The hydraulic mechanism and failure threshold for the decline of salt-blocking capacity of buried interlayers in saline-alkali soil

Buried interlayers are an effective measure for ameliorating salt stress in saline-alkali soils under shallow groundwater conditions. Most existing studies have focused on the effects of interlayer materials and emplacement parameters on salt-blocking capacity. However, little attention has been paid to the effective lifespan of this capacity or the structural-hydraulic mechanisms underlying its decline. To address this, we investigated the structural-hydraulic evolution of straw interlayers (SI) and sand interlayers (SaI) under rotary tillage (RT) and no-tillage (NT), elucidated their impacts on water-salt transport, and assessed failure thresholds under progressive capillarization by integrating in situ observations, X-ray Computed Tomography (CT), and numerical modeling. Results revealed a distinct divergence between morphological and functional degradation over the monitoring period. SI exhibited physical collapse (thickness decreased by 87.5%; bulk density increased by 69.9%), but retained its essential capillary barrier capacity. In contrast, SaI showed pore refinement and fine-particle infilling, accompanied under RT by an increase in SSR from 78.8 to 85.2 × 10 −4 t ha −1 d −1 and a 23.2% increase in 0–100 cm soil salt storage. Conversely, NT partly mitigated this degradation-induced salt accumulation. Scenario simulations linked increasing capillary pore proportion (CPP) to increased salinization risk and identified treatment-specific CPP thresholds for functional decline. Overall, this study demonstrated that the buried interlayer is a dynamic soil functional layer rather than a static barrier, with SI-NT representing the optimal design for sustaining barrier longevity. These findings highlight that buried interlayers behave as dynamic functional layers and that their service life should be considered alongside their initial salt-blocking capacity.