Optimization of irrigation regimes in salinized farmland based on soil water-salt transport and sunflower growth response

Growing season irrigation can contribute to water resource in the representative large-scale arid saline irrigation area in China, such as the Hetao Irrigation District. It is very necessary to decouple the spring (non-growing season) and summer (growing season) irrigation in sustainable agriculture. Conventional practices can rely heavily on the spring flushing to leach salts, followed by disjointed summer irrigation, leading to inefficient water use. It is still unclear about soil water-salt transport patterns. This study aims to integrate these two phases into a coherent system for optimal irrigation. A systematic investigation was made to explore the synergistic effects of spring irrigation quotas and summer drip irrigation schedules on soil salinity profiles and sunflower physiology. An optimal regime was identified to balance the water conservation, salinity control, and yield stability. A field plot experiment was conducted over two consecutive years (2023–2024) at a 1 333 ha experimental station in Longxingchang Town, Wuyuan County, Inner Mongolia, China. There was the semi-arid temperate continental climate (altitude 1 022 m; mean annual temperature 6.1°C; evaporation-to-precipitation ratio >10). A split-plot design was employed to isolate the effects of the spring irrigation (Main plots: S1: 120 mm, and S2: 240 mm) and summer irrigation (Sub-plots). The treatments were compared: border irrigation (B90) against drip irrigation (D90) in 2023; Drip irrigation gradients (D90, D120, and D150) were expanded in 2024. Key parameters were measured, including the soil moisture and salt content in the 0–100 cm profile, sunflower photosynthetic parameters (LAI, P_n, T_r, and G_s), and yield components. The results revealed that there was better balance in soil-water-salt dynamics and crop performance. Soil Salinity Dynamics: The significant vertical stratification was observed in the interaction between spring flushing and summer drip irrigation in soil salinity. High spring irrigation (S2, 240 mm) with fractional drip irrigation (D120) achieved the highest desalination rate in the shallow root zone (0~40 cm). This regime exacerbated the risk of secondary salinization in the deep soil layer (>80~100 cm), where the salt content increased, due to the "piston flow" effect without adequate drainage. Conversely, drip irrigation significantly optimized the horizontal salt distribution, whereas the surface salt accumulation was reduced, compared with the border irrigation. Physiological Mechanisms: Crop physiological responses were linked to the optimal rhizosphere environment. Drip irrigation treatments maintained significantly higher leaf area index (LAI) and net photosynthetic rates (P_n) than border irrigation under identical spring irrigation. Notably, drip irrigation alleviated water stress to enhance the leaf transpiration rate (T_r), stomatal conductance (G_s), and instantaneous water use efficiency (WUE) during the grain-filling stage. The precise water delivery reduced the inhibition of cytokinin activity and root vitality under salt stress. Yield Optimization: There was a threshold effect during irrigation. While the S2 initially provided a low-salt environment, leading to the nutrient leaching and deep-layer salt stress, with an 8.2% yield penalty, compared with moderate spring irrigation. The moderate spring irrigation (S1, 120 mm) and medium drip irrigation (D90) were combined to emerge as the superior strategy. The S1 was sufficient for the desalinated seedling establishment zone (0~40 cm), while the D90 maintained the optimal moisture without the deep percolation, resulting in the highest crop water use efficiency and stable yields. The disconnection between spring and summer irrigation can be resolved for integrated production. Moderate spring irrigation (120 mm) with mulched drip irrigation (90 mm) also achieved the synergistic regulation of soil water and salt. A favorable root zone environment was formed to minimize the deep-layer salt accumulation and water wastage. Consequently, it is recommended as the standard irrigation protocol for the sustainable sunflower production in the saline-alkali lands of the Hetao Irrigation Districts.