Associations Between Nitrogen Application, Canopy Structure and Post-Physiological-Maturity Grain Moisture of Maize in Northwest China’s Irrigated Oasis

Mechanical grain harvesting (MGH) reduces post-harvest losses and improves operational efficiency in maize, but its adoption is limited mainly by grain breakage arising from excessive grain moisture content (GMC) at harvest. To assess the potential and underlying mechanisms of nitrogen (N) fertilizer management in modulating maize grain moisture, a three-year field study (2023–2025) was conducted in the irrigated oasis zone of Northwest China. In 2023–2024, a single-factor experiment on hybrid DK159 tested four N fertilizer application rates (0, 300, 375 and 450 kg/ha); in 2025, the design was expanded to two hybrids (DK159 and XY1225), two planting densities (7.5 × 104 and 11.25 × 104 plants/ha) and an 18-level N gradient (0~765 kg/ha). Elevated N advanced silking by 2~3 d but delayed physiological maturity (PM) by 6~7 d, a ‘promote-early but delay-late’ phenological effect in 2023–2024. The effect of N on GMC was small and hybrid- and density-dependent: under high density, GMC of XY1225 increased by approximately 0.34% per 100 kg N, whereas DK159 responded minimally. N did not consistently alter post-PM grain dehydration rate (GDR), and leaf area index (LAI) at PM showed no significant linear relationship with GDR. Although some N–GMC associations were statistically significant, they were small, partly confounded by N-induced phenological delay, and associative rather than mechanistic, because key canopy physiological variables (microclimate, transpiration, stomatal conductance) were not measured. Interactive hybrid × density effects derive solely from the 2025 dataset and require cautious generalization. We conclude that N management has limited, context-dependent capacity to lower GMC at harvest toward MGH-suitable levels and that PM-measured LAI is not a primary structural determinant of grain dehydration; breeding hybrids with intrinsically rapid dehydration should therefore be prioritized over N management as a tool for lowering GMC.