Irrigation Regimes Shape Vertical Salt Distribution in Coastal Saline–Alkali Farmlands: A PLS-SEM Analysis of Direct Ion Input and Physicochemical Pathways

Abstract

Soil salinization constrains agricultural sustainability in coastal low plain regions, yet the mechanisms regulating salt spatial heterogeneity under different water management regimes remain unclear. This study investigated soil soluble ion distribution and influencing factors across 0–200 cm profiles (n = 78) under freshwater irrigation (FWI), brackish water irrigation (BWI), and non-irrigation (NOI) in the North China Plain. We hypothesized that irrigation affects soil salinization through dual pathways: direct ion inputs and indirect modification of soil physicochemical properties. Partial least squares structural equation modeling was used to quantify these effects. Results identified irrigation as an important correlate, exerting a significant direct negative effect on salt accumulation (−0.117) and an indirect effect mediated by soil physicochemical properties (0.218). The three regimes produced markedly distinct soluble ion composition and vertical salt distribution. FWI exhibited a bottom-accumulated distribution of soil salinity. BWI was characterized by significant Na+ and Cl− enrichment in the 50–100 cm layer, displaying a mid-profile accumulation pattern despite temporary surface desalination. NOI exhibited the most severe soil sodification, alongside a surface-accumulated profile. These findings reveal a trade-off in coastal water management: brackish water can partially offset freshwater scarcity, but long-term sustainability requires depth-specific salinity monitoring to mitigate subsurface salt accumulation risks.