Optimizing water and nitrogen levels improve dry matter accumulation, yield formation, and quality of guar beans

Dry matter accumulation (DMA) is a critical factor influencing crop yields. This study investigates the dynamic changes in water and nitrogen optimization on the DMA of guar beans, as well as its impact on yield and quality. Four irrigation levels were established: W1 (1170 m³/ha), W2 (1530 m³/ha), W3 (1890 m³/ha), and W4 (2250 m³/ha), alongside two nitrogen application levels: N1 (30 kg/ha) and N2 (50 kg/ha). The effects of varying water and nitrogen supplies on the growth and yield of guar beans. The Richards model was employed to fit the DMA, while the Structural Equation Modeling (SEM) was utilized to examine the path relationships among water and nitrogen, DMA, yield, and quality. The results indicate that the Richards model effectively simulates the dynamic DMA throughout (R² > 0.995, NRMSE ≤ 6.58%). Optimizing water and nitrogen supply (W3N1 and W3N2) can enhance the proportion of DMA during the rapid growth period, achieving rates of 53.33% and 53.27%, respectively. SEM analysis reveals that optimizing water and nitrogen supply enhances yield formation by increasing leaf area and DMA, resulting in yield increases of 3.46–16.41%. In comparison to galactomannan, the response of soluble sugar content to water and nitrogen regulation is more pronounced. Specifically, the W2N1 of galactomannan exhibits a 19.70% increase relative to W4N2, while the W3N1 of soluble sugar shows a 12.96% increase compared to W1N1. Compared to conventional irrigation, optimizing water and nitrogen has improved irrigation water utilization efficiency and nitrogen fertilizer partial productivity by 38.68% and 15.76%, respectively. Therefore, optimizing the supply of water and nitrogen can enhance production and improved quality by accelerating the DMA rate and optimizing the distribution ratio. This study provides a theoretical basis for high-yield and high-quality water and nitrogen management of guar beans through mulched drip irrigation.