The North China Plain (NCP), a major wheat-producing area in China, faces challenges due to insufficient rainfall during the winter wheat growing season. Traditional irrigation practices have led to a decline in groundwater levels. This study addresses the need for water-saving cultivation techniques and efficient nitrogen fertilizer management. Nitrogen is a crucial macronutrient for wheat, impacting grain-filling and photosynthesis, but excessive application leads to losses and reduced efficiency. Previous research suggests that split nitrogen application, rather than solely basal application, enhances yield and nitrogen use efficiency. However, the interaction between irrigation and nitrogen management remains poorly understood, especially under water-saving irrigation regimes. This study aims to optimize split-nitrogen application under a water-saving irrigation technique based on soil moisture measurement in the NCP, investigating its effects on photosynthetic performance, grain-filling capacity, and overall yield efficiency.

Existing literature highlights the critical role of nitrogen in wheat production, emphasizing the importance of optimal application to maximize yield and efficiency. Studies indicate that total basal nitrogen application leads to significant losses through volatilization and low nitrogen utilization efficiency. Splitting nitrogen applications, particularly with top-dressing, improves grain yield and nitrogen use efficiency compared to basal-only application. Research also indicates that the optimal ratio of split application varies depending on total nitrogen application and growth stage. However, limited information exists on optimizing split nitrogen under water-saving irrigation, emphasizing the need for this study to examine the interactive effects of water and nitrogen management on wheat productivity.

A two-year field experiment was conducted in the NCP using the winter wheat cultivar 'Jimai22'. A randomized complete block design with a split-plot arrangement and three replications was employed. Plots were 20 m² each. Two irrigation treatments were implemented: water-saving irrigation (SI, maintaining 70% relative soil water content at jointing and anthesis) and sufficient irrigation (UI, maintaining 90% relative soil water content). Four split-nitrogen treatments were tested, all with a total nitrogen application of 240 kg ha⁻¹ but with varying basal-top-dressing ratios (10:0, 7:3, 5:5, and 3:7). Phosphate and potash fertilizers were applied at constant rates. Photosynthetic parameters (net photosynthetic rate, transpiration rate, intercellular CO2 concentration, instantaneous water use efficiency), chlorophyll fluorescence parameters (Fv/Fm, ФPSII, Qp, ETR), 1000-grain weight, grain-filling parameters (Tmax, Vmax, Vmean, D), grain potential storage capacity (GPSC), storage capacity index (SCI), plant nitrogen concentration, grain yield (GY), nitrogen use efficiency (NUE), and water use efficiency (WUE) were measured. Statistical analysis included ANOVA and correlation analysis using SPSS 13.0.

The 5:5 basal-top-dressing nitrogen ratio (N3) significantly enhanced various parameters compared to other treatments under both SI and UI. Specifically, N3 increased: * Grain yield by 5.27–17.75% (UI) and 5.70–16.72% (SI) compared to N1, N2, and N4. * Nitrogen use efficiency by 5.68–18.78% (UI) and 5.70–16.72% (SI). * Water use efficiency by 5.65–31.02% (UI) and 5.70–16.72% (SI). These increases were attributed to enhanced flag leaf photosynthetic capacity and grain-filling capacity. The N3 treatment maintained higher nitrogen and water use efficiencies. SI increased water use efficiency compared to UI by 9.75% (2016) and 10.79% (2017). Photosynthetic parameters (Pn, Tr, Ci, IWUE) and chlorophyll fluorescence parameters (Fv/Fm, ФPSII, Qp, ETR) were significantly affected by split-nitrogen treatments, with N3 showing the highest values. 1000-grain weight was significantly higher in N3 at later stages after anthesis. Grain-filling parameters (Vmax, Vmean) were also significantly higher for N3. N3 increased plant nitrogen concentration at jointing, anthesis, and maturity stages. Correlation analysis indicated a strong positive relationship between grain yield and photosynthetic and grain-filling parameters.

The findings highlight the importance of optimizing split-nitrogen application and water management for improving winter wheat productivity and resource use efficiency. The superior performance of the 5:5 basal-top-dressing ratio (N3) suggests a balance between early-season nitrogen availability for vegetative growth and later-season availability for grain filling. The lack of significant differences in photosynthetic parameters between SI and UI treatments indicates that sufficient nitrogen under optimized management can effectively utilize water resources under limited water conditions, increasing water use efficiency. The positive correlations between yield, NUE, WUE and photosynthetic and grain-filling parameters emphasize the crucial role of optimizing both photosynthesis and grain-filling capacity in enhancing overall wheat productivity. These findings offer practical implications for sustainable agricultural practices in water-limited regions.

This study demonstrates that a 5:5 basal-top-dressing nitrogen ratio under water-saving irrigation is optimal for maximizing winter wheat grain yield, nitrogen use efficiency, and water use efficiency. The improved performance is attributed to enhanced photosynthetic capacity and grain-filling capacity. This research provides valuable insights for developing sustainable and efficient water and nitrogen management strategies in water-scarce environments. Future research could explore the effects of the optimized regime across different wheat varieties and ecological regions, further elucidating the underlying mechanisms.

The study was conducted on a single wheat cultivar ('Jimai22') and in a specific ecological region (NCP). The findings might not be directly generalizable to other cultivars or regions with different soil and climate conditions. Further research is needed to explore the broader applicability of the optimized management strategy and to investigate the interactions of nitrogen and irrigation across a wider range of wheat varieties and environments.