Objectives  Calcium (Ca) is one of the most important elements affecting fruit quality, and nitrogen availability affects the absorption of calcium by fruit trees. We assessed the effects of different nitrogen application rates on fruit quality, calcium forms, and their relationship to provide basis for improving apple fruit quality through fertilization.

Methods  Five-year-old potted Fuji apple (Malus domestica Borkh cv. Fuji) trees were selected as the test material, while N application rates 0 (N₀), 50 (N₅₀), 100 (N₁₀₀), 200 (N₂₀₀), 300 (N₃₀₀), and 400 (N₄₀₀) kg/hm² were the treatments. Ca (200 kg/hm²) was generally applied to all treatments. Beginning from the 7th day after flower emergence, the fertilizer was applied once every 7 days for a total of 3 times. Fruit samples were collected at fruit maturity stage to determine fruit quality indicators (fruit diameter, pericarp color, fruit firmness, soluble solid, titratable acid, soluble sugar and vitamin C content), total calcium content, and different forms of calcium contents.

Results  N application increased the total Ca content of the Fuji apple fruits. The total Ca content of the fruits in N200 treatment was significantly higher than those in other treatments. With increase in N application rate, the proportion of different forms of Ca in fruits changed, the proportion of water-soluble Ca increased, but the proportion of pectin calcium was constant and calcium phosphate decreased. Fruit shape index, peel brightness, red-green color difference, soluble solids, soluble sugars, and vitamin C content of Fuji apples in N200 treatment increased by 4.46%, 41.07%, 67.12%, 27.02%, 26.71%, and 38.08%, respectively, whereas the titratable acid content (22.49%) in N200 was less than that in N0 treatment. Total Ca content and fruit shape index, peel brightness, red-green color difference, fruit hardness, soluble solids, soluble sugars, vitamin C content and other fruit quality indicators were highly correlated (P<0.01). Ca pectin had the highest effect on fruit shape index, yellow-blue color difference, fruit hardness, titratable acid, soluble sugars, and vitamin C, while water-soluble Ca had the highest effect on peel brightness, red-green color difference, and soluble solids. Ca pectin contributed to all the quality indicators, while water-soluble Ca, calcium phosphate, and calcium oxalate affected different quality indicators in different ways.

Conclusions  N application could increase the total Ca content in fruit and change the proportion of Ca in each form. The suitable N application rate (200 kg/hm²) promotes the highest total Ca content in the Fuji apple fruits. Both pectin Ca and water-soluble Ca have the highest influence on the quality of fruits, while pectin Ca promotes all the quality indices.