Split nitrogen application increases grain yield of densely planted summer maize by delaying post-silking leaf...

Increasing planting density can raise summer maize yield, but it also intensifies competition for light, water, and nutrients within the canopy, accelerating post-silking leaf senescence and reducing assimilatory capacity. Drip fertigation enables precise water and nitrogen supply, but the optimal nitrogen application frequency for summer maize remains unclear in the North China Plain. Therefore, a two-year field experiment was conducted in 2024 and 2025 using two planting densities (75,000 and 97,500 plants ha^-¹), a conventional single nitrogen application control, and several split nitrogen treatments under the same total nitrogen rate. Across the two-year average, split nitrogen application increased grain yield and 1000-kernel weight, and the four-split treatment produced the highest grain yield (14.4t ha^-¹). At 75,000 and 97,500 plants ha^-¹, the two-year average grain yield under the four-split treatment increased by 11.8% and 13.0%, respectively, and the 1000-kernel weight increased by 6.0% and 10.0%, respectively, compared with the corresponding control. Based on the two-year average, the four-split treatment also delayed leaf senescence onset by 2.1 and 6.0d, delayed the time to maximum senescence rate by 11.5 and 12.5d, and reduced the mean senescence rate by 18.0% and 20.8%, respectively. Overall, split nitrogen application, especially the four-split regime, delayed post-silking leaf senescence, improved late-season physiology, and increased grain yield in densely planted summer maize.