Partitioning of N-uptake from 1-year of fertigation with 15N-urea in pot-lysimeter-grown M.9-grafted apple trees after 3 years of fertigation with unlabelled urea at three rates of N

Abstract

SummaryThe total amounts of N derived from 15N-urea in the organs of 18 newly-bearing “maiden” apple trees and in the soil were determined after 1-year of fertigation with 15N-urea by drip irrigation, scheduled at a soil matric potential of –50 kPa. Prior to this treatment, the 18 “maiden” apple trees had been fertigated with three concentrations of N for 3 years: 17 mg N l–1 (low), 34 mg N l–1 (medium), or 67 mg N l–1 (high), and the same rates of 15N were chosen for treatment comparisons over the following 1 year. The trees were fertiligated with an aqueous stock solution of 15N-labelled urea (1.5 atom % 15N), KH2PO4, and KCl by drip irrigation. The phosphorus (P) and potassium (K) concentrations were constant, and set at 17 mg l–1 and 34 mg l–1, respectively. The total irrigation volume applied per tree was 363 l for the low, 430 l for the medium, and 216 l for the high N treatment.The corresponding supplies of N per tree were 6.17, 14.62, and 14.47 g, respectively. The total amounts of urea-N absorbed per tree were 1.90 g for the low-N, 3.73 g for the medium-N, and 3.21 g for the high-N treatments, with the percentage of N recovery (30.8%, 25.5%, and 22.1%) decreasing with increasing N rate, while the N retained in the soil was 2.92 g (47.3%), 7.40 g (50.6%), and 8.35 g (55.7%), respectively. Fertiliser N contributed more to whole-tree N uptake at increasing rates of N than N uptake from the soil. The total recovery of 15N from each tree-soil system was 78.1% at the low, 76.1% at the medium, and 79.8% at the high N level, with an average of approx. 22.0% of N unaccounted-for. The total amounts of P and K taken up were 2.42 g tree–1 and 14.49 g tree–1 for the low-N, 2.47 g tree–1 and 15.33 g tree–1 for the medium-N, and 1.31 g tree–1 and 8.04 g tree–1 for the high-N treatments, respectively. Our results showed that an N concentration of 34 mg l–1 (medium) resulted in more productive and profitable trees, bearing more fruit of higher quality than the other two N treatments. This concentration of N also improved the development of new leaves and flowers during the early stages of growth in the following Spring.