Mechanism and Simulation of Water–Heat–Salt Coupling Process Regulated by Tillage Measures and Straw Return in Cold Black Soil

This study investigates the synergistic regulation mechanism of water–heat–salt transport in the black soil of cold regions in Northeast China by combining field monitoring with HYDRUS-2D simulations. Four tillage treatments were evaluated: control group (CK), no-tillage with flat straw mulching (NM), ridge tillage with flat straw mulching (RM), and straw return with rotary tillage (RR). Monitoring data indicated that all straw incorporation treatments significantly improved soil moisture retention capacity. Compared with CK, soil water content under RM increased by 63.93% correspondingly; soil salinity in CK was 5.75–13.68% higher than that in straw-amended treatments. In addition, RM exerted a more prominent regulatory effect on soil temperature fluctuations relative to CK. Simulation results reveal that straw incorporation effectively reduces surface runoff, thereby substantially weakening the driving force for upward salt migration. Structural equation modeling (SEM) quantified path coefficients, revealing that straw incorporation optimizes the soil microenvironment. This integrated approach provides a mechanistic basis for black soil conservation in seasonally frozen regions, identifying RM as the optimal management practice to balance water retention and salt inhibition.