Objective This study investigated the regulatory effects of reduced nitrogen application combined with organic fertilizer on soil physicochemical properties, fruit quality and yield in Weinan apple orchards, so as to provide theoretical and practical references for formulating cost-saving, high-efficiency, high-quality and high-yield fertilization schemes.

Method Taking apple orchards of Weinan as the research object, five treatments were set up: conventional nitrogen application (CK), 25% nitrogen reduction with common urea plus organic fertilizer (T1), 25% nitrogen reduction with controlled-release urea plus organic fertilizer (T2), 45% nitrogen reduction with common urea plus organic fertilizer (T3), and 45% nitrogen reduction with controlled-release urea plus organic fertilizer (T4). Indicators including soil nitrogen content and forms, as well as fruit appearance, nutritional quality and yield were determined.

Result Compared with CK, treatment T2 significantly increased the contents of ammonium nitrogen and total nitrogen in the 0-30 cm topsoil by 12.89%-23.67% and 18.69%-27.83%, respectively, and significantly decreased nitrate nitrogen content by 14.27%-22.08%. In 30-60 cm subsoil, the contents of ammonium nitrogen and total nitrogen under T2 were significantly increased by 10.24%-21.81% and 16.29%-25.97%, respectively, while nitrate nitrogen content was significantly decreased by 10.86%-18.75%. Compared with CK, treatment T2 significantly increased single fruit weight, fruit firmness, soluble sugar content and vitamin C content by 42.94%, 22.96%, 22.50% and 37.83%, respectively. Treatment T2 exhibited the optimal performance in soil physicochemical properties, fruit quality and yield, which was significantly superior to other treatments; treatment T1 ranked second and was significantly better than CK, T3 and T4; treatments T3 and T4 showed the lowest performance, with no significant difference observed between them. Correlation analysis and principal component analysis revealed that fruit quality and yield indicators were significantly or extremely significantly positively correlated with soil organic matter and total nitrogen contents, and significantly or extremely significantly negatively correlated with soil bulk density. Soil organic matter and total nitrogen contents were identified as the core regulatory factors governing fruit quality and yield.

Conclusion The fertilization mode of 25% nitrogen reduction combined with controlled-release urea and organic fertilizer achieves the best synergistic improvement effect on orchard soil fertility, apple yield and fruit quality.