Long-Term Nitrogen Fertilization Shapes Yield Stability, Nitrogen Use Efficiency, and Apparent Nitrogen Surplus in Spring Maize on the Loess Plateau of China

Indiscriminate long-term nitrogen fertilization threatens sustainable maize production across the Loess Plateau, yet the long-term effects of different N application rates on yield stability, organ-level N allocation, and N balance remain unclear. This study established a 10-year field experiment (2011–2020) with five N rates (0, 90, 180, 270, and 360 kg N ha−1), using the maize cultivar Xianyu 335. Grain yield, organ biomass, and N content at maturity were measured annually. Sustainable yield index (SYI), nitrogen harvest index (NHI), nitrogen use efficiency (NUE), and apparent N surplus (ANS) were also calculated. The results showed that the 10-year average yield peaked at N270 kg ha−1. The N0 treatment exhibited progressive yield decline before stabilizing. SYI was highest at N270 kg N ha−1 (0.836), and NUE decreased continuously with increasing N rate. NHI stayed relatively stable throughout the study period. The ANS at N180 reached 47.0 kg N ha−1, while at N270 and N360 it far exceeded the 80 kg N ha−1 threshold, reaching 129.2 and 222.6 kg N ha−1, thus reflecting a trade-off between yield and environmental risk, not yield maximization alone. In conclusion, when integrating yield, stability, NUE, and ANS, the recommended optimal N rate for spring maize on the Loess Plateau is 180–186 kg ha−1, achieving synergy among high and stable yield, acceptable NUE, and low environmental risk.