Effects of autumn irrigation timing and amounts on soil water and salt component migration in seasonally frozen soils

In seasonally frozen and salinized regions, the effectiveness of flood irrigation in leaching salts and conserving soil moisture is strongly influenced by irrigation timing, amount and the dynamics of soil water and salt movement during the freeze-thaw period. While existing studies predominantly focus on total salt content, the component-specific migration behavior of salts under varying irrigation timing and amount remains insufficiently explored. To address this issue, a two-year irrigation field experiment was conducted, involving six flood irrigation treatments that combined two irrigation timings and three irrigation amounts. The FREZCHEM model was employed to quantify both soluble and precipitated salt components. Results showed that final soil water and salt contents, as well as their variations, were significantly affected by irrigation timing, while were only slightly influenced by irrigation amounts ranging from 150 to 210 mm. On average, late irrigation treatments (LAITs) increased soil water content by 0.05 cm³ /cm³ and 0.2 cm³ /cm³ more than early irrigation treatments (EAITs) and the no-irrigated treatment, respectively. This was attributed to a reduction in evaporation duration and downward water flux during the leaching period, along with enhanced groundwater recharge driven by stronger upward convection during the freeze period. The onset of soil freezing in LAITs occurred 13 days earlier than in EAITs, although the overall freeze-thaw duration remained similar across treatments. Under the same irrigation timing, larger irrigation amounts delayed the end of the freeze-thaw period. During the leaching period (October to December), soil salt content within the 0–1 m depth decreased by 4.9–56 %. Conversely, during the freeze-thaw period (December to April), salt accumulated by 9.7–26 %, particularly in LAITs, while EAITs exhibited an approximate 16 %–37.8 % improvement in desalination. The main leached components during the leaching period were Ca, Cl, HCO₃, and Na, with reductions linearly correlated with their initial concentrations. In contrast, SO₄ and Na were the predominant accumulating components during the freeze-thaw period due to upward convection driven by thermal and water potential gradients, as well as the precipitation of solution ions. Among all treatments, the LAIT with an irrigation amount of 150 mm demonstrated the most effective performance, exhibiting superior increases in soil water content and higher desalination rates compared to other treatments.