Foliar Magnesium Application Enhances Fruit External and Interior Quality and Nitrogen Use Efficiency of Tomato (Solanum lycopersicum L.) Simultaneously Under High Nitrogen Supply

Magnesium fertilizer application generally improves both the internal and visual quality of tomato fruits grown in magnesium-deficient soils. However, it remains unclear whether similar effects occur in magnesium-sufficient soils under high nitrogen fertilization. A field experiment was conducted in Wuhan, China, using soil with suitable available Mg content (385.97 mg kg−1) and four nitrogen (N) application rates (0, 100, 200, and 300 kg N ha−1) combined with foliar Mg spraying. This study evaluated tomato yield, nitrogen use efficiency, and fruit quality. Nitrogen application combined with foliar Mg significantly increased yield and biomass. The highest fruit yield was achieved with 200 kg N ha−1 plus foliar Mg, showing a 104.9% increase compared with the control, while the greatest biomass was observed under 300 kg N ha−1 with Mg spraying. Foliar Mg application also enhanced leaf nitrogen accumulation, shoot magnesium accumulation, and nitrogen use efficiency. Furthermore, fruit titratable acidity, vitamin C, total phenols, redness, chroma, and yellowness were significantly improved. Fruit redness was positively associated with sugars, amino acids, vitamin C, and phenolic compounds. Overall, foliar Mg application under 200 kg N ha−1 improved tomato yield, nitrogen use efficiency, and fruit quality.