To address the difficulty of explaining the causes of patchy salinization in the Hetao Irrigation District using traditional two-dimensional frameworks, this study aims to demonstrate that its spatial differentiation is primarily driven by the interaction between the shallow clay layer and the groundwater table. Employing a multi-scale investigation approach based on sampling data from 114 shallow boreholes and 3 typical deep boreholes, and combined with principal component analysis, this research quantitatively explores the control mechanisms of matrix configuration on water and salt transport. The results indicate that the interactive position between the burial depth of the clay layer and the groundwater table directly determines the salt retention patterns: when the clay layer is located within the groundwater fluctuation zone, strong positive feedback is highly prone to inducing severe salt patches (surface mean of 6.17 g/kg); when the clay layer is situated above the phreatic water level, bidirectional retardation occurs, leading to moderate salt accumulation (mean of 2.71 g/kg); under the deep salt storage pattern, the surface undergoes effective desalination (mean of 1.65 g/kg), but a concealed "salt reservoir" forms in the deep layers. This study breaks through traditional two-dimensional cognition, explicitly establishing that the three-dimensional reshaping of water and salt evolution pathways by the dynamic interaction between the clay layer and the groundwater table is the fundamental mechanism for the formation of patchy salinization. These findings provide an important scientific basis for precise spatial prevention, control, and engineering management in the irrigation district.

Understanding the spatial heterogeneity of deep soil organic matter is critical for terrestrial carbon cycling, yet its driving mechanisms remain elusive due to a historical research bias toward surface layers. This study develops a 3D spatial prediction and mechanistic framework for Soil organic matter(SOM) across a 0–200 cm profile in the Mollisols of the Songnen Plain, Northeast China. By integrating systematic sampling with Random Forest modeling and a comprehensive suite of multi-source environmental covariates, we identified pronounced vertical stratification in SOM distribution and its governing factors. Our results reveal that surface SOM is primarily driven by climate, vegetation, and anthropogenic activities, whereas deep soil organic matter is dictated by intrinsic physicochemical properties and surface matrix compositions. Furthermore, significant interaction effects between topography and soil attributes further enhance SOM accumulation. This research clarifies the depth-dependent processes of carbon stabilization, providing a robust scientific basis for the targeted conservation and carbon sequestration enhancement of Mollisols.