Effects of Supplemental Drip Fertigation on Grain Yield and Water and Nitrogen Use Efficiencies of Spring Maize in the Dryland Farming Region of the Eastern Loess Plateau

Water–nitrogen mismatch constrains stable and resource-efficient spring maize production in the eastern Loess Plateau. A two-year field experiment was conducted in Jinzhong, Shanxi Province, China, during 2024–2025. Supplemental irrigation was applied three, four, or five times in 2024 and two, three, or four times in 2025, with 15 mm per event. Nitrogen rates were 180, 240, and 300 kg N ha−1 in 2024 and 180 and 240 kg N ha−1 in 2025. Spring maize growth, grain yield, water use efficiency (WUE), partial factor productivity of applied nitrogen (PFP), and soil NO3−–N were evaluated, and Pareto-front analysis identified non-dominated water–nitrogen combinations. Across the tested supplemental irrigation regimes, growth and grain yield generally increased as the number of irrigation events increased. Compared with 180 kg N ha−1, 240 kg N ha−1 generally increased grain yield and WUE, whereas 300 kg N ha−1 provided no additional yield benefit and reduced PFP in 2024. Higher nitrogen rates increased soil NO3−–N concentrations. Under the conditions of this experiment, Pareto analysis showed that four to five irrigation events (60–75 mm) may represent a candidate strategy for balancing WUE and PFP under the dry conditions of 2024, whereas a lower-input supplemental irrigation regime may be considered under the normal-rainfall conditions of 2025. These findings support precipitation-adaptive water and nitrogen management for dryland spring maize.