Partial organic fertilizer substitution under optimized planting density enhances maize yield through improved source–sink coordination

Context and objective Under high-density planting, partially substituting organic fertilizer for chemical fertilizer may regulate source–sink relationships, thereby providing a promising strategy for achieving high maize yield. However, the mechanisms through which organic fertilizer substitution interacts with planting density to regulate source–sink relationships in drip-irrigated maize in arid regions remain unclear. This study aimed to determine how organic fertilizer substitution affects canopy structure, leaf physiological performance, source–sink balance, and yield formation in drip-irrigated maize under different planting densities. Methods A three-year field experiment was conducted during the maize-growing seasons from 2023 to 2025. Two planting densities, 70,000 (D1) and 100,000 plants ha⁻¹ (D2), were evaluated in all three years, with a third density of 130,000 plants ha⁻¹ (D3) added in 2025. Four fertilization treatments were applied at each density: no fertilizer (F0), chemical fertilizer alone (F1), and substitution of 17% (F2) or 34% (F3) of total N input with organic fertilizer. Results Increasing planting density increased the leaf area index (LAI) but reduced source capacity on a per-plant basis. The F2 treatment further improved canopy structure by increasing the proportion of leaf area distributed in the middle and upper canopy layers. At the D2 density, F2 increased the maximum leaf area index by 2.39%–15.29% relative to F1 across the three experimental years. Averaged across planting densities, F2 increased SPAD value and net photosynthetic rate by 23.95%–48.96% and 24.75%–86.17%, respectively, compared with F0, and enhanced sink capacity and grain-filling rate. D2F2 showed relatively favorable source–sink matching, particularly in 2023 and 2025, while increasing grain yield per unit area by 7.46%–23.83% relative to D1F1. Partial least squares structural equation modeling showed that the fertilization regime exerted a positive total effect on grain yield, primarily by improving canopy structure and leaf physiological performance and, consequently, optimizing source–sink balance. Conclusions Combining a planting density of 100,000 plants ha⁻¹ with 17% organic fertilizer substitution improved source–sink coordination and supported high maize yield under the tested environments. Implications This study elucidates the physiological mechanisms through which moderate increases in planting density and partial organic fertilizer substitution regulate maize yield by coordinating canopy development, leaf physiological performance, and source–sink relationships. These findings provide a new perspective on the physiological basis of maize yield improvement.