Seasonal Water Availability Outweighs Long-Term Phosphorus Fertilization in Controlling Nitrogen Use Efficiency of Maize (Zea mays L.) on a Chernozem

Drought increasingly limits the nitrogen (N) use efficiency (NUE) of maize, but whether long-term phosphorus (P) fertilization buffers this effect is unclear. This study analyzed five seasons (2021–2025) of a long-term N × P split-plot experiment on a Chernozem at Lovrin, Romania, with five N rates (0–200 kg N ha−1) and five P rates (0–160 kg P2O5 ha−1). Season explained 88% of yield variation, and the N response depended on season (Year × N, p < 0.001). N increased yield by 78% in 2023, the season with adequate rainfall around flowering (5.0 t ha−1 at N0 vs. 8.9 t ha−1 at N150), and by 32% in 2024 (2.6 vs. 3.5 t ha−1 at N200); it had no effect in 2021 and 2022; and it decreased yield by 18–23% in 2025 (3.3 vs. 2.6–2.7 t ha−1 at N100–N200). Grain recovery of fertilizer N decreased from 79% (N50) to 48% (N200) in 2023 and was ≤22% in drought seasons. According to the results, long-term contrasting P supply did not significantly affect yield. The EU Nitrogen Expert Panel criteria were met only in favorable seasons, whereas in dry seasons 150–200 kg N ha−1 left surpluses of 95–160 kg N ha−1. Therefore, moderate N rates adjusted to seasonal water availability appear advisable.