Effects of phosphorus application in different soil layers on soil phosphorus fractions and the root of Platycodon grandiflorum

The enhancement of phosphorus (P) acquisition through deep P application has been documented in crops with fibrous root systems, but little is known about this effect on the morphology of a taproot system. A four-year field experiment with P application was conducted using Platycodon grandiflorum with a taproot system as the experimental material at two P rates of 40 and 60 kg ha−1 with middle and deep P application patterns. The biomass and P accumulation of P. grandiflorum were greater with deep P application than with middle P application at the same P rate. Regardless of whether the root diameter being greater or less than 2 mm, the root length, volume and surface area of P. grandiflorum increased due to P application, and they were greater with deep P application than with middle P application at the same P rate. Four-year consecutive P application increased the concentrations of soil NaHCO3-P, HCI-P, NaOH-P and residual-P, resulting in soil P accumulation, especially HCI-P. This is the first report that deep P application was more favorable to the growth of P. grandiflorum with a taproot system than middle P application. And this research provided evidence that deep phosphorus application is also suitable to crops with a taproot system.