The experiment was conducted from April 2023 to October 2024 at the Experimental Station of the College of Agriculture, Shihezi University (44°18’N, 85°59′E). The site features a typical continental climate, with multi-year averages of 7.5–8.2°C temperature, 208 mm precipitation, and 1,660 mm evapotranspiration. The soil was irrigated tillage gray desert soil with a medium loamy texture. Figure 1 shows temperature and precipitation during July–October 2023. Basic physicochemical soil properties before sowing are presented in Table 1.

The experiment employed a two-factor split-plot design, with the primary factor being the amount of irrigation water, with three distinct irrigation levels established: The experimental units were designated as W1 (3,360 m³·hm⁻², a reduction of 33.33% compared to conventional irrigation), W2 (4,200 m³·hm⁻², a reduction of 16.67% compared to conventional irrigation), and W3 (5,040 m³·hm⁻², representing the conventional irrigation); the secondary factor is the quantity of nitrogen applied (pure nitrogen), with four nitrogen application levels established: N1 (0 kg·hm⁻²), N2 (105 kg·hm⁻², representing a 46.15% reduction compared to conventional), N3 (150 kg·hm⁻², representing a 23.08% reduction compared to conventional) andN4 (195 kg·hm⁻², conventional nitrogen application). All 12 treatments received uniform applications of P₂O₅ (102 kg·hm⁻²) and K₂O (69 kg·hm⁻²). Each treatment was replicated three times in 20 m² plots (4 m × 5 m). The preceding spring wheat crop and subsequent relay-intercropped soybean (“Haojiang 35” variety) were established using no-till methods. Soybeans were planted at 30 cm row spacing and 5 cm plant spacing. Drip irrigation mirrored the spring wheat system, with one irrigation belt servicing four soybean rows. Eight irrigation events occurred at 7–10 day intervals during the growing season. Nitrogen fertilizer was applied via irrigation water in split doses according to treatment requirements (Table 2).

The data was processed using Microsoft Excel 2016 software, and graphs were plotted using Origin 2022 software. Statistical analyses were conducted using SPSS 27.0, and one-way ANOVA and Duncan’s method were employed for analysis of variance and multiple comparisons.

Under W1 and W2 irrigation, soybean plant height increased quadratically with nitrogen application over 2 years, peaking at N3 before declining (Figure 2). In 2023, plant height under W1N3 exceeded W1N2 and W1N4 by 27.85 and 35.20%, respectively, whereas W2N3 showed 8.15 and 4.17% greater height than W2N2 and W2N4. Similarly in 2024, W1N3 showed 17.98 and 16.19% greater height than W1N2 and W1N4, with W2N3 surpassing W2N2 and W2N4 by 12.53 and 7.07%. At W3 irrigation, W3N4 produced the tallest plants (61.85 cm in 2023; 57.87 cm in 2024), significantly exceeding other treatments.

Over both years, LAI exhibited a unimodal response to nitrogen under each irrigation level, peaking at N3 (Figure 3). In 2023, LAI showed a quadratic response to irrigation, reaching a maximum of 2.60 under W2N3. In 2024, LAI increased significantly with irrigation, attaining 2.24 under W3N3.

Above-ground biomass (AGB) of relay-cropped soybean increased with irrigation under all fertility conditions (Figure 4). In both years, AGB exhibited a quadratic response to nitrogen application, peaking at N3, except under W3 irrigation in 2023. At the W3 irrigation level in 2023, AGB increased linearly with nitrogen application, peaking at N4 (W3N4 treatment).

During late-season growth, vegetative biomass accumulation (stems + leaves) decelerates as nutrients are substantially remobilized from vegetative to reproductive organs, driving rapid pod biomass growth. Total dry matter accumulation was significantly higher for W1N3, W2N3, and W3N4 than for the other treatments.

Chlorophyll SPAD values under different treatments are presented in Table 3. In 2023, SPAD values peaked under W2 (W3 > W1). In 2024, SPAD values increased significantly with irrigation (W3 > W2 > W1). Across both years, SPAD values showed a quadratic response to nitrogen (initial increase followed by decrease), with values ranked N3 > N4 > N2 > N1. ANOVA indicated significant main effects of irrigation, nitrogen, year, and their three-way interaction on SPAD values (p < 0.05), but no significant irrigation × nitrogen interaction.

Figure 5 shows photosynthetic parameter changes in soybean leaves after 2 years of differential treatments. The net photosynthetic rate (Pn) consistently showed a quadratic response to nitrogen application across both years: increasing then decreasing. Values peaked under N3 (N3 > N4 > N2 > N1), reaching maxima at W2N3 (30.3 μmol·m⁻²·s⁻¹ in 2023; 25.58 μmol·m⁻²·s⁻¹ in 2024).

The transpiration rate (Tr) generally increased with nitrogen application across irrigation levels, peaking at N4. However, under W3 irrigation in 2024, Tr showed a parabolic response to nitrogen, peaking at N3 (8.88 μmol·m⁻²·s⁻¹). In 2023, W2N4 recorded the highest Tr (10.66 μmol·m⁻²·s⁻¹).

Stomatal conductance (Gs) typically exhibited a parabolic response to nitrogen application across years, peaking at N3. Exceptions occurred under W1 and W2 irrigation in 2023, where Gs increased linearly with nitrogen and peaked at N4.

Intercellular CO₂ concentration (Ci) generally showed inverse patterns to Gs. Trends varied by treatment: W3 (2023) & W1/W3 (2024): Ci showed a U-shaped trend during growth progression. W1/W2 (2023): Ci decreased with nitrogen application. W2 (2024): Ci increased with nitrogen application.

Under consistent nitrogen application, both effective flower and pod numbers per plant exhibited a quadratic response to increasing irrigation over 2 years (initial increase followed by decrease; Table 4). Optimal values occurred at W2 irrigation, with W2 and W3 showing no significant difference. Under constant irrigation, pod numbers peaked at N3.

For flower numbers in 2023, N3 increased values by 15.06% versus N1 and 0.77% versus N4. In 2024, N3 produced 32.57% more flowers than N1, 13.14% more than N2, but 10.96% fewer than N4.

ANOVA indicated significant main effects of irrigation, nitrogen, and their interactions with year (p < 0.05) on reproductive parameters, but no significant irrigation × nitrogen interaction.

Flower and pod abortion patterns remained consistent across both years (Figure 6). The abortion rate showed a U-shaped response to irrigation: highest under W1, followed by W3 and W2. With increasing nitrogen application, abortion rates reached a minimum at N3, then increased in the order N1 < N2 < N4.

Total flowers and pods showed positive correlations with irrigation levels in both years (Figure 7). At W1 and W3 irrigation, flower numbers increased with nitrogen application. Under W2 irrigation, flowers exhibited a quadratic response to nitrogen, peaking at N3.

Upper-canopy flowers consistently outnumbered lower-canopy flowers. In both years under W2N3: 2023: Flower numbers exceeded W2N1, W2N2, and W2N4 by 37.17, 10.03, and 4.25%, respectively. 2024: Values surpassed W2N1, W2N2, and W2N4 by 45.41, 15.10, and 6.52%.

Pod numbers under W1/W2 irrigation showed quadratic responses to nitrogen (N3 > N4 > N2 > N1), while W3 showed linear increases. W2 level showed both the greatest temporal variation in pod development and the highest fertile pod numbers (W2N3: 25.67 pods/plant in 2023; 30.67 in 2024).

Soybean yield components are presented in Table 5. At W1 and W2 irrigation, pod number per plant under N3 surpassed N1 and N2 by 38.2–52.6% (p < 0.01) and N4 by 6.3–11.7% (p < 0.05). N4 consistently reduced these components relative to N3. Under conventional irrigation (W3), values peaked at N4.

Analysis revealed: Significant main effects of nitrogen on all yield components (p < 0.05). Significant main effects of irrigation on all components except grain number per plant (p < 0.05). Significant water × nitrogen interactions for 100-grain weight and grain yield (p < 0.05).

Soybean yield responses are shown in Table 6. Under W1 and W2 irrigation, grain yield showed a quadratic response to nitrogen, peaking at N3 (significantly different from other N levels: p < 0.05). N4 reduced yields relative to N3. At W3 irrigation, yields peaked at N4.

Optimal yields occurred at: W1N3: 3047.47 kg·ha⁻¹ (2023); 2795.83 kg·ha⁻¹ (2024). W2N3: 4418.82 kg·ha⁻¹ (2023); 3909.17 kg·ha⁻¹ (2024). W3N4: 4477.00 kg·ha⁻¹ (2023); 3941.83 kg·ha⁻¹ (2024).

No significant yield difference existed between W2N3 and W3N4 (p > 0.05). ANOVA indicated significant main effects of nitrogen, irrigation, year, and their interactions on yield (p < 0.05).

Growth metrics directly reflected the physiological responses of relay-cropped soybean to water and nitrogen inputs. Appropriate management enhances crop growth, while excess application inhibits development (Zhang et al., 2022). Studies confirm that moderate water and nitrogen increases promote soybean growth, elevating above-ground biomass and leaf area index (Liao et al., 2022). However, excessive nitrogen prolongs vegetative growth, delays maturity, increases plant height, and suppresses reproductive structure development while raising lodging risk (Gebre and Earl, 2021; Lawlor et al., 2001). Our results align with predecessor (Chi et al., 2023): Moderate nitrogen reduction maintains soybean plant architecture while extending dry matter allocation to pods, thereby enhancing yield. Reproductive-stage nitrogen timing critically influences growth dynamics. Specifically, plant height, leaf area index, and above-ground biomass exhibited quadratic responses to nitrogen under W1/W2 irrigation, peaking at N3. Conversely, under W3 irrigation, plant height peaked at N4, exceeding N3-level performance.

N3 optimized dry matter accumulation (Figure 4), likely due to enhanced nitrogen partitioning to reproductive organs under moderate resource constraints (Worku et al., 2012). In contrast, W3 required higher nitrogen (N4) to achieve similar biomass, indicating luxury consumption. At W1 and W2 irrigation, N3 maximized dry matter accumulation. Coordinated reduction (W2N3) maintained vegetative and reproductive development, achieving yield parity with W3N4 while enhancing resource efficiency and grain yield potential.

The leaf area index (LAI) reflects photosynthetic area size and indicates photosynthetic capacity (Adams et al., 2016; He et al., 2024). Enhanced photosynthetic parameters—including Pn, Tr, Gs, Ci, and chlorophyll content—improve photosynthetic efficiency and increase yield potential (Anten, 2005; Hu et al., 2020). Studies demonstrate that: At fixed irrigation levels, soybean photosynthetic parameters increase with nitrogen application to an optimal threshold, beyond which excess nitrogen reduces chlorophyll concentration and Pn (Gai et al., 2017b). Under optimal nitrogen, photosynthetic parameters increase with irrigation but decline with excessive water (He et al., 2017). Our findings align with this pattern: photosynthetic parameters (Pn, Tr, Gs) decreased or stabilized across treatments except W2N3, which showed significant increases. This response correlates with chlorophyll dynamics, which naturally decline during maturation (Locke and Ort, 2014). Moderate water and nitrogen reduction may: Maintain leaf integrity and delay senescence. Sustain photosynthetic activity (Shafii et al., 2011). Offset yield losses while improving water/nitrogen use efficiency (Ru et al., 2022).

Soybean under W2N3 achieved optimal photosynthetic efficiency at R6, outperforming W3N4 and other treatments. Reduced inputs (relative to high-input regimes) prolonged leaf functional lifespan, maintained higher green leaf area at R6, and sustained superior photosynthetic capacity, enhancing yield potential while conserving resources.

The effective number of flower pods is a key indicator of soybean yield (Board and Kahlon, 2011). Reduced irrigation decreases floral node formation and promotes flower abscission (Atti et al., 2004). Fertilizer application combined with increased irrigation enhances pod development in soybeans (Basal and Szabo, 2020), consistent with our findings. Fertile flower counts in W1 and W2 were significantly lower than in W3 by 27.03 and 5.46%, respectively. Fertile pods decreased by 24.53% in W1 compared to W3 but increased by 1.18% in W2. This reversal resulted from excessive irrigation in W3 prolonging vegetative growth, thereby shortening the reproductive phase and increasing flower abortion. Nitrogen application significantly reduced floral abscission, explaining the enhanced pod formation under optimized W2 inputs. Furthermore, elevated nitrogen rates under reduced irrigation stimulate soybean pod formation (Kinugasa et al., 2012; Li et al., 2024). Similarly, relay-cropped soybean exhibited significantly higher fertile flower and pod counts at N3 and N4 nitrogen levels than at N1 and N2 (p < 0.05). While fertile flower numbers did not differ significantly between N3 and N4 (p > 0.05), pod counts were significantly higher (p < 0.05). This divergence may result from excessive nitrogen inhibiting pod development (Ohyama et al., 2017).

Crop growth and development depend on synergistic water-nitrogen interactions. Imbalanced inputs compromise both yield and quality (Du et al., 2017), making optimal water-nitrogen ratios essential. Appropriate irrigation enhances nitrogen-use efficiency (NUE), while balanced nitrogen application maximizes water-use efficiency (WUE) (Liu et al., 2020; Ye et al., 2013). Prior research confirms that nitrogen application rates must be adjusted precisely according to irrigation levels to maximize soybean yield (Sun et al., 2012).

In this experiment, under consistent irrigation, all three yield components of relay-cropped soybean—pods per plant, grains per plant, and 100-grain weight—exhibited quadratic responses to increasing nitrogen, peaking at the N3 application rate. These findings align with previous studies showing that under water-limited conditions (W1/W2), nitrogen application increases both yield index (YI) and water-nitrogen use efficiency (WNUE). Conversely, excessive irrigation (W3) impairs nitrogen efficacy, reducing soybean yield and WNUE (Purcell and King, 1996; Ray et al., 2006; Salvagiotti et al., 2008; Tamagno et al., 2018).

Relay-cropped soybean yield was optimal under W2N3 and W3N4 treatments, reaching 4477.00 and 4418.82 kg·hm⁻² in 2023, and 3909.17 and 3941.83 kg·hm⁻² in 2024, respectively. The yield parity between W2N3 and W3N4 demonstrates that resource conservation need not compromise productivity (Zhou et al., 2011b). Reducing inputs conserves resources while maintaining comparable yields with higher water-nitrogen utilization efficiency.

W2N3 demonstrates compelling economic viability for Xinjiang farmers. Direct cost reductions of 483.91 CNY·hm⁻² —primarily from water (210.00 CNY·hm⁻²) and nitrogen fertilizer (273.91 CNY·hm⁻²) savings—outweighed minor yield-related revenue losses (75.12–133.81 CNY·hm⁻²), generating a net profit gain of 350.10–408.79 CNY·hm⁻² (urea: 2,800 CNY·t⁻¹; water: 0.25 CNY·m⁻³). These water savings can be achieved using existing drip irrigation infrastructure, widely used in Xinjiang (Li et al., 2022; Lin et al., 2024; Wang et al., 2018). Scaled to Shihezi City’s 13,400 hectares of wheat fields suitable for soybean relay-cropping, W2N3 could reduce regional water withdrawals by 11,300 m³ annually while maintaining near-equivalent soybean production.