Effects of Biochar Combined with Organic Fertilizer on Soil Properties and the Yield and Quality of Sweet Potato

Biochar and organic fertilizers are recommended for sustaining crop yield and sustainable agricultural development. However, the mechanisms of their combined application on the nutritional, flavor and eating quality and their optimal ratio remain unclear in sweet potatoes. Therefore, this field experiment employed four fertilization treatments: inorganic compound fertilizer (CF), organic fertilizer (OF), and biochar-based organic fertilizer containing 15% biochar (BOF15) or 30% biochar (BOF30). The results revealed that the OF, BOF15, and BOF30 treatments significantly ameliorated soil physical properties, elevated soil pH values and available nutrient contents compared with CF. They also increased sweet potato yield, regulated sucrose-metabolizing enzyme, and enhanced quality. Among them, the BOF15 treatment exhibited the most significant improvement. Compared with CF, BOF15 treatment increased the contents of starch, total soluble sugar, sucrose, and β-carotene by 23.16–31.82%. It also optimized texture by reducing hardness and chewiness while increasing springiness, elevated key volatile compounds (terpene and aldehyde), and received the highest sensory scores for mealiness, aroma, and sweetness. In conclusion, biochar combined with organic fertilizer synergistically improves soil properties and enhances sweet potato yield and quality, with BOF15 being the optimal treatment. This study provides support for optimizing fertilization strategies in high-quality sweet potato production.