Yield improvement in maize through optimized application of nitrogen fertilizer under narrow-wide row intercropping is associated with source-flow-sink trade-offs

• Applying nitrogen at a rate of 300 kg ha −1 at the 20 cm sites on both sides of the wide row enhanced intercropped maize yield by optimizing the source-flow-sink relationship. • Grain filling, root bleeding sap intensity, and soluble sugar content were the most influential factors in determining the intercropped maize yield. • Application of nitrogen of 300 kg ha −1 at the 20 cm sites on both sides of the wide row is recommended as an optimal nitrogen management strategy for intercropped maize. Narrow-wide row soybean-maize intercropping systems can improve crop yield. Particularly, the mechanisms underlying the maize yield advantage associated with the source-flow-sink theory are unclear. Effect of optimized nitrogen (N) fertilizer application strategies on intercrops yield still needs to be further studied. This study revealed the mechanism of wide-row N application strategy to enhance the yield of intercropped maize through the trade-off relationship of source-flow-sink. Field experiments were conducted from 2023 to 2024, in a typical soybean maize intercropping region of the Sichuan Basin, China. The study was contained different fertilizer application sites: narrow rows (A1), wider row sites 10 cm (A2), 20 cm (A3) and 30 cm (A4) away from the maize plants, respectively, and different N rates (N0, 0 kg ha −1 ; N1, 225 kg ha −1 ; N2, 300 kg ha −1 ; N3, 375 kg ha −1 ) . Specifically, compared to A1N2, optimized fertilization A3N2 treatments significantly enhanced the leaf biomass (11.44%), net photosynthetic rate (11.04%), SPAD (13.24%), root bleeding saps intensity (RBSI, 54.1%), number of vascular bundles (VB, 7.0%), soluble sugars (SS, 12.6%) and amino acids (16.3%), apparently, A3N2 also significantly enhanced the root morphology and maize yield (17.4%). Structural equation modeling showed that the N site and N rate interaction effect significantly increased the number of vascular bundles, soluble sugars and amino acids, which promoted root morphology, sped up grain filling (GF) and extended the active grain filling period by 2 d, ultimately leading to higher intercropped maize yields. Random forest modeling also revealed that factors such as GF, RBSI, SS and VB made a main contribution to the intercropped maize yield effect. Collectively, optimization of trade-offs in source-flow-sink relationships facilitates intercropped maize to achieve increased yield at 20 cm wide row distance with 300 kg ha −1 N application rates. These findings enriched the knowledge of source-flow-sink of strip intercropped maize under the condition of optimal N fertilizer application. It offers critical insights for the optimized application of N fertilizers in large-scale field production of strips intercropped maize.