| To investigate the effects of varying water-nitrogen coupling on soybean root growth, yield, and water-nitrogen use efficiency in maize-soybean intercropping, this study conducted a field experiment utilizing the soybean variety Nandou 12 as the subject of research. The experiment established two primary factors: nitrogen application and irrigation, which included three levels of irrigation (W1: normal irrigation at 85% of field capacity, θfc; W2: moderate irrigation at 85% of W1; W3: low irrigation at 70% of W1) and three levels of nitrogen application (N1: normal nitrogen application; N2: moderate nitrogen application at 85% of N1; N3: low nitrogen application at 70% of N1). This design resulted in a total of nine treatments. The study assessed the effects of nitrogen application, irrigation, and their combinations on soybean root growth, yield, water-nitrogen utilization efficiency, and dry matter accumulation and distribution. An analysis employing Partial Least Squares regression (PLS) and the entropy weighting method was used to evaluate the various indicators under each treatment. The results demonstrated that the N2W2 water-nitrogen treatment resulted in optimal soybean root characteristics, with increases of 116.74% in root length, 102.32% in root surface area, and 90.15% in root volume compared to the N1W1 treatment. Notably, significant interaction effects (P<0.05) were observed between water-nitrogen treatments on soybean root morphology and nodule weight, revealing that nitrogen application had a more pronounced effect on soybean root morphology and yield than irrigation. The N2 treatment achieved the highest grain yield, outperforming N1 and N3 by 5.53% to 227.64% and 6.80% to 24.63%, respectively. Meanwhile, the N2W2 water-nitrogen treatment produced an impressive yield of 3.34 t/hm2, while the N1W1 treatment yielded the lowest results in terms of both yield and water-nitrogen use efficiency. Additionally, soybean yield and water use efficiency demonstrated a significant positive correlation (P<0.05) with specific surface area, specific root length, root surface area, root volume, and root length at the podding stage. Notably, nitrogen fertilizer partial productivity was significantly correlated (P<0.05) with root volume and root surface area at the podding stage, with root surface area exerting the greatest influence on grain yield and water-nitrogen use efficiency. The comprehensive evaluation using the entropy method indicated that the N2W1 water-nitrogen treatment achieved a score of 0.253, representing the most effective overall scheme for enhancing soybean yield, root growth, and water-nitrogen use efficiency in intercropping systems. The findings from this study may offer valuable insights for optimizing irrigation and nitrogen application strategies in soybean cultivation within maize-soybean intercropping systems. |