Effects of controlled-release urea combined with conventional urea on nitrogen uptake, root yield, and quality of Platycodon grandiflorum

Field experiments were conducted with four nitrogen fertilizer treatments to study the effects of controlled-release urea combined with conventional urea on the nitrogen uptake, root yield, and contents of protein, soluble sugar, saponin, zinc (Zn), iron (Fe), magnesium (Mg), and copper (Cu) in Platycodon grandiflorum. Field experiments were conducted with four nitrogen (N) fertilizer treatments: no N fertilization; conventional urea with N rate of 175 kg N ha(-1); conventional urea with N rate of 160 kg N ha(-1); controlled-release urea combined with conventional urea with N rate of 160 kg N ha(-1); controlled-release urea combined with conventional urea with N rate of 135 kg N ha(-1). The results showed that nitrogen application significantly increased the yield of P. grandiflorum compared with the control. Treatment with controlled-release urea combined with conventional urea at 160 kg N ha(-1) provided the highest yield of 7329.58 kg ha(-1). Nitrogen application also increased the contents of soluble sugar, total saponin, protein, Zn, Fe, and Mg but decreased Cu content. Protein, saponin, and Zn contents were significantly higher, but Cu content was lower in P. grandiflorum fertilized with controlled-release urea combined with conventional urea than those fertilized with conventional urea alone. The combination of controlled-release urea with conventional urea at 160 kg N ha(-1) was the optimal treatment under the experimental condition investigated in this study.