Physiological Responses of Apple to Nitrogen Fertilization Regimes: Roles of Calcium Metabolism in Fruit Quality and Bitter Pit Development

Excessive or improper nitrogen (N) fertilization can disrupt calcium (Ca) nutrition in apple trees and induce Ca-related physiological disorders, yet its effects on Ca availability and partitioning remain unclear. This study evaluated the impact of different N fertilization regimes on soil Ca availability, Ca partitioning, and Ca bioavailability in fruit tissues of 10-year-old ‘Fuji’ apple trees, using Ca fractionation analysis combined with multi-criteria decision-making (TOPSIS). High N applied as a single dose (H1) significantly reduced soil water-soluble and exchangeable Ca, while increasing Ca oxalate (CaOx) accumulation in fruit pedicels, particularly at maturity. Although total CaOx in fruit flesh decreased, its relative proportion increased, indicating enhanced Ca sequestration. In contrast, split application of moderate N (M3) maintained more stable soil Ca availability, reduced CaOx accumulation, and improved Ca allocation to fruit tissues. Integrated evaluation ranked treatments as M3 > M1 > H3 > H1. Overall, moderate and split N fertilization reduced Ca sequestration into CaOx, enhanced Ca availability, and improved Ca distribution in fruit tissues, providing a physiological basis for optimizing N management to mitigate Ca-related disorders and improve fruit quality.