Optimizing Nitrogen and Planting Density Improves Source-Sink Balance, Nutrient Efficiency, Yield, and Economic Returns in Sweet Potato

Background and aims Optimizing nitrogen (N) input and planting density (PD) is essential for improving sweet potato yield and quality. Nevertheless, their combined effects remain poorly understood. Therefore, this study aimed to identify optimal N and PD combinations and clarify how they influence physiological and economic outcomes. Methods Field experiments were conducted in Haikou and Sanya using four N levels (no N [NN, 0], low N [LN, 60], medium N [MN, 120], and high N [HN, 180] kg N ha ^-1 ) and three PDs (low [LD, 50,000], medium [MD, 62,500], and high [HD, 83,250] plants ha ^-1 ). Results MNMD substantially optimized source-sink fresh and dry biomass partitioning per plant; however, per unit area-based responses differed between Haikou and Sanya. At both sites, MNMD slightly increased fresh and dry-weight root to shoot ratios. Under MNMD at Haikou, carbon (C) and N content, accumulation, allocation, and nitrogen use efficiency indices were highest. At both sites, MNMD resulted in lower C/N ratios. Similarly, yield and its components under MNMD at Haikou were associated with improved storage root quality characteristics. Furthermore, MNMD at Haikou achieved the highest net returns and benefit-cost ratio. Correlation analysis revealed positive relationships between yield and carbohydrate contents, whereas a high storage root C/N ratio negatively affected yield and N use efficiency indices. Structural equation modeling further identified the C/N ratio and root system development as key mediators linking N utilization efficiency with yield, quality formation, and economic outcomes. Conclusions MNMD at Haikou enhances sweet potato productivity, quality, and profitability.