Optimizing Density and Nitrogen Management Enhances Photosynthetic Performance and Compensates for Yield Loss under Water-Limited Maize Production

Reduced nitrogen (N) inputs inevitably limit maize yield and its components, while increased planting density can potentially boost productivity per unit area. This study investigated the compensatory interplay between reduced N application and elevated planting density on grain yield and photosynthetic performance under water-limited conditions. Reducing N decreased grain yield, kernel number per spike, and 1000-kernel weight. Higher planting densities similarly reduced kernel number and weight, yet increased grain yield through a greater spike number per area. Under a 20% irrigation reduction with traditional N, a medium-density treatment (N180D2) increased grain yield by 19.1–41.6% and spike number by 28.0–28.2% compared to a treatment combining the same water reduction with 25% less N at traditional density (N90D1). This compensation was driven by a 23.9–25.7% increase in spike number, which offset a 9.1–11.7% reduction in kernel number per spike. Physiologically, moderate density elevation under reduced irrigation and traditional N maintained superior late-season photosynthetic capacity, with increases of 6.2–7.7% in leaf greenness, 64.4–83.1% in leaf area index, and 19.8–80.7% in crop growth rate relative to low-density, resource-reduced treatments. We conclude that a strategic increase in planting density effectively compensates for yield losses from concurrent water and N reductions by sustaining photosynthetic source strength and dry matter accumulation during critical late-growth stages. These findings provide a practical agronomic framework for maintaining maize productivity in resource-constrained environments.