Maize P Uptake in Response to Nitrogen Sources

Plants take up nitrogen (N) mostly as ammonium (NH₄⁺) and nitrate (NO₃^-). Recently it has been shown that the application of N-NH₄⁺ along with phosphorus (P) enhanced maize root growth and P absorption, in soil with neutral/alkaline pH. Furthermore, higher N use efficiency was reported when N-NO₃^- was used, despite the higher energy expenditure. The objective of this study was to evaluate the effect of N sources on dry matter, N uptake and P influx by maize as related to the plant root morphology. Ammonium applied as sole N source had no effect on total N concentration in maize shoots but increased sharply NH₄⁺ concentrations. These high NH₄⁺ concentrations were likely phytotoxic, because total dry matter accumulation and leaf area were decreased, respectively, by 50 and 46% as compared with the application of NO₃^- or both sources combined. Ammonium supply as previous or sole N source clearly decreased by 53% the root dry mass; likewise, both length and volume of the roots were twice lower than with nitrate supply, but none treatment affected the averaged diameter of roots. The concentrations of NO₃^- and carbon in plants were not affected by N sources. Maximum P influx by NH₄⁺NO₃^- and NO₃^- acclimated plants was increased by NH₄⁺. In addition, K_m was increased threefold when NH₄⁺NO₃^- was replaced by NH₄⁺ in the nutrient solution. This treatment appears to have inhibited phosphate carrier expression. Ammonium as a sole source of N impairs maize growth, but when applied to healthy plants it increases P influx.