Effects of nitrogen, phosphorus, and potassium fertilizers on storage root yield, nutrient use efficiency, and soil nutrient balance of sweetpotato

Nitrogen (N), phosphorus (P), and potassium (K) fertilization is widely used to enhance crop productivity. However, the synergistic effects of combined N, P, and K application on sweetpotato yield and nutrient use efficiency are not fully understood. To address this knowledge gap, a field experiment was conducted with five treatments: control (CK), no N (-N), no P (-P), no K (-K), and full NPK application (NPK). We systematically analyzed storage root yield, yield components, and nutrient accumulation characteristics. Additionally, fertilizer use efficiency and soil nutrient balance were evaluated. The NPK treatment significantly increased storage root yield by 34.8-53.1% compared with single nutrient deficiency treatments. The greatest yield reduction was observed under -P conditions, associated with low soil available P and a disordered N and K allocation ratio (5.10-14.40%). Phosphorus application resulted in high agronomic efficiency (187.79 kg kg^-1 P₂O₅) but low recovery efficiency (0.05-0.25 kg kg^-1 P₂O₅), whereas -N and -K treatments led to soil P surplus (50.25-63.06 kg ha^-1). A logistic model revealed that NPK treatment increased the maximum and average nutrient accumulation rates compared with deficient treatments. Pearson correlation analysis showed significant positive relationships between yield and yield components, as well as nutrient accumulation in storage roots and whole plants. Random forest regression identified P accumulation in storage roots as the most important predictor of yield. In conclusion, combined NPK fertilization enhances both storage root yield and nutrient use efficiency, with targeted P management playing a critical role in achieving high-yield and high-efficiency sweetpotato production.