Optimizing nitrogen application increases strip-intercropped maize yields by reshaping root morphology and physiology

The objective of this study was to identify a combination of N application site and application rate that maximized maize yield and nutrient efficiency in soybean–maize strip intercropping systems in southwest China. A two-year, two-factor split-plot experiment was performed, with the N application site (S) and N application rate (N) as the primary factor and the sub-factor, respectively. S1 represents narrow row N application site, while S2, S3, and S4 represent wide row N application site at distances of 10 cm, 20 cm, and 30 cm from the maize plants, respectively. N0 represents 0 kg N ha⁻¹, while N1, N2, and N3 represent 225 kg N ha⁻¹, 300 kg N ha⁻¹, and 375 kg N ha⁻¹, respectively. Results indicated that S3N2 significantly increased maize yield by 19.77% and system yield by 16.92% relative to S1N2. This yield advantage was mainly attributed to increased biomass allocation to ears rather than stems or leaves. Compared with S1N2, S3N2 significantly increased the root dry weight (RDW) in 0 20 cm layer by 95.02%, and root length (RL), root volume (RV), and root surface area (RSA) in 30 50 cm layer by 42.76%, 28.82%, and 26.67%, respectively. Additionally, compared with S1N2, S3N2 significantly increased root sap rate, ammonium N, nitrate N, and root N metabolism enzymes activity, ultimately increasing N harvest index. Interactions between N application site and N application rate significantly increased RDW, RL, RV, RSA, root physiological activity, and improved nutrient use efficiency and strip-intercropped maize yield. In summary, S3N2 is recommended as the N management strategy for soybean–maize strip intercropping systems, as it can achieve the win–win goals of increasing maize yield and improving nutrient efficiency.