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.

Drought has been one of the most harmful abiotic stress to crop growth and development. The growth and physical performance of corn is susceptible to water deficit stress in the middle and late crop stages. Crop straw-derived biochar has been commonly recognized as the soil amendment in agriculture production. The carbon-rich product can be characterized by the loose and porous, high surface area, and oxygen-rich functional groups. The application of biochar in soil can be expected to improve the soil physical and chemical conditions. Better crop physiological performance can also achieve to promote the crop growth and yield. However, it is still unclear on the effects of biochar on photosynthetic fluorescence of corn grown under deficit irrigation in the middle and late stages. Therefore, this study aims to clarify the compensating effect of biochar additions on corn leaf gas exchange and chlorophyll fast fluorescence induction kinetic parameters. The physiological regulation mechanism of corn was also determined to cope with different deficit irrigation regimes. An in-situ test-pit control experiment was carried out in Hohhot, Inner Mongolia, China in May to October in 2023. The 2×3 factorial combinations were set as the biochar rates [0 and 30 t/hm² (B)] and irrigation regimes (field water-holding capacity 95%±5% (well-watered irrigation, WW), 70%±5% (moderate water-deficit irrigation, MD), 50%±5% (severe water-deficit irrigation, SD). A total of six individual treatments (BWW, WW, BMD, MD, BSD, and SD) were then selected. Soil moisture treatment was lasted for 27 days in the heading-filling period. The responsive characteristics and their relationships were determined, including the leaf gas exchange parameters, chlorophyll fast fluorescence induction kinetic parameters, chlorophyll fast fluorescence induction kinetic OJIP curves, abscisic acid and indole acetic acid in corn leaves. The parameters were sampled and then measured at days 3, 13 and 22 after water control. The results showed that the deficit irrigation was induced the synthesis of a large amount of abscisic acid (ABA) in leaves, with the prolongation of water deficit stress time, indicating the decreasing chlorophyll content and limiting photosynthesis. Severe deficit irrigation was resulted in the partial inactivation of photosystem II (PSII), leading to the blocked electron transfer. There was the varying reduction in the fluorescence intensity of I-P phase of chlorophyll fast fluorescence induction kinetic OJIP curves. Compared with the WW, the maximum fluorescence intensity of P phase decreased by 23.55% on average under SD (on the 22nd day of water control), the net photosynthetic rate (P_n), stomatal conductance (g_s), and transpiration rate (T_r) of leaves were also reduced by 42.37%, 41.26% and 34.16%, respectively. In the severe deficit irrigation, the content of leaf ABA decreased by 21.89%-52.15%, respectively, whereas, the content of leaf auxin (IAA) increased by 47.77%-82.63%, under the biochar treatment, compared with non-biochar one. Furthermore, Pn increased by 1.66%-32.63%, when averaged across three samplings. While the I-P phase of chlorophyll fast fluorescence induction kinetic curves was improved significantly with biochar addition. Meanwhile, the maximum quantum efficiency of PSII (F_V/F_M), potential activity of PSII (F_V/F_O), electron transport rate (ψ_o) and photosynthetic performance index (P_IABS) were also improved by biochar addition under deficit irrigation. The application of biochar was improved the efficiency of PSII light energy conversion and electron transfer. The concentration of ABA and IAA in leaves was adjusted to improve the photosynthetic fluorescence performance. The damage of severe water deficit was then alleviated to the chloroplast and PSII of corn. The drought tolerance of corn was enhanced in the middle and late stages. The finding can provide the theoretical and technical support to the carbonization and utilization of straw resources. The regulation mechanism of crop drought resistance can greatly contribute to the efficient utilization of farmland water resources.