Organic Fertilizer Substitution Improves Grape Yield and Quality by Regulating Soil Nutrient Availability and Berry Morphological Traits: A Two-Year Field Study

To improve nitrogen (N) fertilizer management in vineyards and support sustainable production, we conducted field experiments in 2021–2022 to evaluate the effects of reduced N fertilization combined with organic fertilizer (OF) on vine growth, fruit quality, soil fertility, and economic returns in Shine Muscat grapes. Six treatments were established: conventional fertilization (CF), four reduced-N treatments combined with OF (0.9N + OF to 0.6N + OF; i.e., 10–40% N reduction), and a blank control (CK). Yield was significantly increased under 0.8N + OF (18.2% in 2021; 96.0% in 2022) and 0.7N + OF (10.8% in 2021; 47.9% in 2022), with 0.8N + OF also delivering the highest economic returns. Fruit quality analysis showed that 0.8N + OF consistently increased total sugar and the sugar–acid ratio, and improved vitamin C content. Substitution ratios >40% led to a decline in economic benefits. Path analysis indicated that vertical diameter and single-berry weight exerted significant positive effects on total yield. OF substitution also improved fruit quality. Soil available nutrients (N, P, and K) and organic matter were primary factors influencing yield; potassium was the key factor regulating sugar accumulation, with the strongest effect on improving flavor coordination. Reducing N by 20–30% combined with OF (particularly the 0.8N + OF treatment) synergistically enhanced photosynthetic efficiency, N utilization, yield and quality, and soil fertility, representing the optimal fertilization strategy.