Enhanced Nitrogen Supply Sustains Post-Silking Canopy Function and Grain Yield of Drip-Irrigated Spring Maize Under Increased Planting Density in Northern Xinjiang

How nitrogen (N) supply should be matched to increased planting density in drip-fertigated maize in arid regions remains unclear. A two-year split-plot field experiment (2024–2025) at Shihezi, northern Xinjiang, China, tested two planting densities (D1, 60,000; D2, 90,000 plants ha−1) and two N rates (N1, 180; N2, 240 kg N ha−1) in film-mulched, drip-irrigated spring maize. D2 increased grain yield by 13.9% over D1, because a 46.8% increase in ear number outweighed decreases in kernels per ear and kernel weight. The yield response to N depended on density (D × N, p < 0.01): N2 increased yield by 13.8% under D2 but by only 3.6% (not significant) under D1. N2 increased N accumulation at silking similarly at both densities, whereas its post-silking effects were much larger under D2, where it reduced the post-silking decline in leaf area index from 67.3% to 51.2% and increased post-silking dry matter accumulation and apparent N uptake by 22.0% and 44.3%. D2N2 produced the highest yield (11.51 t ha−1), whereas D2N1 achieved the highest partial factor productivity of N (56.2 kg kg−1). At this single site, dense planting benefited more from the higher N rate, most evidently after silking; validation across sites, hybrids, and N rates is needed.