Optimized planting density and nitrogen improve grain yield and water productivity in drip-fertigated maize through improved canopy function and source–sink balance

Introduction Balanced source–sink relations are essential for achieving high maize yield and water productivity, and maintaining post-silking green leaf area is critical for dry matter accumulation and yield formation in maize ( Zea mays L.). However, the mechanisms by which nitrogen (N) rate and planting density affect yield formation via leaf senescence and source–sink regulation remain unclear. This study aimed to elucidate the respective contributions of post-silking leaf functional decline and source–sink balance to grain yield and water productivity of drip-irrigated maize. Methods A two-year field experiment was conducted in northwest China with three planting densities (LD: 80,000; MD: 100,000; HD: 120,000 plants ha -1 ) and four N rates (N0: 0; N1: 120; N2: 180; N3: 240 kg N ha -1 ). Leaf area duration (LAD), post-silking leaf functional decline, source–sink traits, grain yield, and water productivity were evaluated, and relationships among key variables were analyzed using PLS-SEM. Results Nitrogen application alleviated stress-induced premature leaf functional decline after silking, whereas increasing planting density accelerated the loss of effective leaf function. Both higher planting density and higher N rate significantly increased LAD. Compared with LD, MD and HD increased source growth by 23.0% and 19.4%, sink capacity by 23.9% and 15.2%, sink growth rate by 23.7% and 15.8%, and source–sink difference by 16.2% and 18.2%, respectively, indicating that sink limitation constrained further yield increases at higher densities. PLS-SEM showed that N mitigated premature leaf functional decline, while planting density negatively affected leaf functional maintenance. Planting density indirectly affected LAD via leaf functional decline (63.7% of the effect), whereas N rate and planting density directly influenced LAD (75.6% of the effect). LAD strongly affected source growth parameters (90.7%), which increased grain yield (89.0%) and source–sink parameters (73.4%), ultimately contributing to direct increases in grain yield (81.8%) and water productivity (74.3%). D2N3 achieved the highest grain yield, followed by D2N2, which significantly improved water productivity and irrigation water productivity. Discussion Considering source–sink balance, water saving, and stable yield, D2N2 is recommended. These results improve understanding of how to achieve effective dense planting and high-yield maize cultivation under water-scarce conditions.