Field and numerical investigation of soil water–salt dynamics under deep vertical rotary tillage in the Hetao Plain

Deep vertical rotary tillage (DVT) holds promise for improving saline–alkali land, yet its effects on the spatiotemporal dynamics of soil water–salt, as well as the underlying mechanisms, remain unclear. A two–year field experiment was conducted on saline silty loam farmland in the Hetao Plain, involving three treatments: traditional rotary tillage (TRT), deep vertical tillage at 25 cm (DVT25), and 50 cm (DVT50). Soil physical and hydraulic properties, soil water and salt content were measured. The HYDRUS–2D model, calibrated and validated with field data, accurately simulated soil water (RMSE ≤ 0.028 cm³/cm³) and salinity (RMSE ≤ 0.103 g/kg). Simulation results showed that DVT enhanced both lateral and vertical water movement during spring irrigation, increased soil water in the tilled layers, and promoted salt leaching. These effects were enhanced with increasing tillage depth. As the growing season of Helianthus annuus L. progressed, DVT increased root water uptake while reducing the upward movement of water from deeper layers. Consequently, soil water content of tilled layer declined more markedly than under TRT. After two growing seasons, the desalinization rates under the DVT25 and DVT50 treatments were 35 % and 42 % higher than those under the TRT treatment, which consequently led to increases of 12 % and 10 % in the average Helianthus annuus L. yield (irrigation water productivity), respectively. However, the yield differences were not statistically significant. The reduction of soil salinity was mainly due to the decreased soil bulk density resulting from DVT, which enhanced soil water during spring irrigation and suppressed soil evaporation. Scenario simulations revealed that, compared to TRT, optimal water-saving threshold, of DVT25 and DVT50 reduced by 3 % and 4 % in slightly salinized soils, and by 7 % and 5 % in moderately salinized soils, respectively. Considering both mechanical costs and water management benefits, DVT25 with spring irrigation amounts of 123.2 mm and 126.8 mm was recommended as the optimal strategy for water–salt regulation in slightly and moderately salinized silty loam soils of the Hetao Plain. In summary, DVT improved soil structure and contributed to salt control and crop yield improvement in the Hetao Plain.