Interactive Effects of Nitrogen and Iron Fertilization on Canola (Brassica napus L.) Productivity and Oil Quality Under Arid Conditions: A Maximum Yield Threshold Approach

Excessive nitrogen (N) fertilization poses environmental risks and can induce micronutrient imbalances that limit crop productivity. This two-season field study (2024/2025–2025/2026) investigated the interactive effects of four nitrogen (N) rates (0, 75, 150, and 225 kg N ha−1) and four foliar iron (Fe) concentrations (0, 100, 200, and 300 mg L−1) on Canola (Brassica napus L.) yield and oil quality under the arid conditions of Upper Egypt. The highest N rate (225 kg N ha−1) increased seed yield by 240% and oil yield by 271% compared to the unfertilized control, but substantially reduced Nitrogen Use Efficiency (NUE), with Partial Factor Productivity of Nitrogen (PFPN) declining from 52.5 to 11.2 kg seed kg−1 N. Foliar Fe at 300 mg L−1 enhanced seed yield by an additional 29.9–37.0%. The combined N3Fe3 treatment produced maximum biological (8258 kg ha−1), seed (2892 kg ha−1), and oil (812 kg ha−1) yields, establishing a maximum yield threshold under the tested conditions. However, at the highest N rate, there were no statistically significant differences in oil yield among Fe1 (100 mg L−1), Fe2 (200 mg L−1), and Fe3 (300 mg L−1), indicating that the lowest tested Fe concentration was sufficient to achieve yields comparable to higher rates. The N × Fe interaction was characterized as additive for seed yield but synergistic for leaf iron uptake and translocation. While integrated N-Fe fertilization offers a promising strategy for sustaining Canola productivity in nutrient-poor calcareous soils, the declining N use efficiency at higher N rates and the absence of economic and environmental indicators (nitrate leaching, nitrous oxide (N2O) emissions) highlight the need for further research to identify true optima for arid-zone Canola production.