Drip Fertigation Optimizes the Spatial Distribution and Translocation of Nitrogen, Thereby Increasing Yields and Improving Water and Nitrogen Use Efficiency in High-Density Summer Maize

Achieving simultaneous improvements in grain yield and water–nitrogen use efficiency remains a major challenge in high-density summer maize production. Therefore, this study investigated how drip fertigation (DI) regulates soil nitrogen spatial distribution, plant nitrogen translocation, and ultimately resource use efficiency. A two-year field experiment (2023–2024) was conducted in Tai’an, Shandong, China, using a split-plot design. Two water–fertilizer management regimes, conventional border irrigation (BI) and drip fertigation (DI), were assigned to the main plots, while eight planting densities (15,000–120,000 plants ha−1) were allocated to the subplots. Two summer maize cultivars, Denghai 605 (DH605) and MY73, were evaluated. Compared with BI, DI significantly increased grain yield as well as water and nitrogen use efficiency. For DH605 and MY73, grain yield increased by 7.3% and 3.8%, respectively, accompanied by increases of 18.4% and 16.3% in WUE and 7.4% and 3.5% in NPFP. DI enhanced nitrogen accumulation within the 0–20 cm root zone while reducing nitrate-N residues in the 20–60 cm soil layer, thereby improving the spatial distribution and availability of root-zone nitrogen. Consequently, DI increased nitrogen translocation from vegetative organs to grains, as reflected by higher NTA, NTE, and NHI values, which promoted grain nitrogen accumulation and improved nitrogen use efficiency. Notably, DI did not significantly affect nitrogen uptake efficiency (NUpE), suggesting that the improvement in nitrogen utilization efficiency (NUtE) was driven primarily by enhanced nitrogen remobilization from vegetative organs to grains rather than by increased nitrogen uptake. Overall, drip fertigation improved grain yield, water use efficiency, and nitrogen use efficiency in high-density summer maize by optimizing root-zone nitrogen availability and promoting post-silking nitrogen translocation to grains.