Excessive nitrogen application deteriorates integrated citrus fruit quality: A multi-dimensional field study

• High N input enlarged citrus fruits but reduced dry matter and impaired texture. • High N increased cell wall polysaccharides, causing firmer and less elastic fruit. • Proline, arginine, and aspartic acid are key amino acids regulating citrus flavor, yet suppressed by high N. • High N altered ascorbate, sugar, and organic acid metabolism, disrupting flavor balance. • Moderate N achieved optimal fruit quality based on an entropy weight–membership function evaluation. Consumer evaluation of citrus fruit quality is moving beyond the traditional sugar–acid ratio toward a broader set of attributes, including texture, flavor and nutritional value. Yet the effects of excessive nitrogen (N) fertilization, a common practice in citrus orchards, on these integrated quality traits remain poorly understood. Here, we combined a long-term field experiment with integrated metabolomic, appearance, texture, flavor and aroma analyses and membership-function-based quality evaluation to reveal how excessive N drives citrus fruit quality deterioration. Compared with the moderate N treatment, high N increased fruit size but led to N dilution, reducing N per unit volume by 27.8 %; High N also caused accumulation of cell wall components in the segment membrane (protopectin +84.6 %, water-soluble pectin +24.8 %, cellulose +21.3 %, hemicellulose +71.9 %), resulting in higher hardness and elevated vitamin C. In addition, high N decreased the contents and taste activity values (TAVs) of proline (−17.8 %), arginine (−13.6 %) and aspartic acid (−21.6 %), thereby disrupting the sweet–bitter–umami flavor. In this context, metabolomics revealed that high N disrupted amino acid, ascorbate, carbohydrate and organic acid metabolic pathways. Based on entropy-weight analysis, total N per fruit, total free amino acids, arginine and proline (and their TAVs), and N per unit volume were identified as key determinants of fruit quality. Combined entropy weight–membership function evaluation showed that moderate N achieved the highest quality score (0.915), exceeding high N (0.508) and low N (0.132). Overall, these findings demonstrate that excessive N impairs citrus fruit quality by reshaping amino acid, ascorbate and cell wall metabolism.