Absorption, distribution and utilization of soil applied ¹⁵N-urea in young dwarf rootstock ‘Fuji’ apple trees

Objectives Effects of different nitrogen rates on nitrogen absorption, distribution and utilization of young dwarf rootstock ‘Fuji’ apple trees were studied to provide scientific basis for nitrogen management of dwarf rootstock apple orchards.

Methods A pot experiment was carried out using two–year–old dwarf rootstock ‘Fuji’ apple trees as materials. Three levels of ¹⁵N stable isotopes, 0.1 g/kg (N_0.1), 0.2 g/kg (N_0.2) and 0.3 g/kg (N_0.3), were applied in each trees. The whole seedlings were destructively sampled at the end of spring shoot growth (June 23rd) and autumn shoot growth (August 25th), nutrient backflow stage (September 20th) and early defoliation (October 23rd). Total nitrogen was determined by high resolution digital colorimeter autoanalyzer, and the ¹⁵N abundance was determined by MAT253 mass spectrometer.

Results At the early stage of defoliation, the total dry weight and roots biomass of dwarf rootstock ‘Fuji’ apple trees were maximum under the N_0.1 level. The contribution rates of nitrogen derived from fertilizer (Ndff) to the total nitrogen contents in different organs were affected by the application rates. The highest Ndff was in the new vegetative organs aboveground at the spring shoot stop growing stage. From the autumn shoot stop growing stage to early defoliation stage, the Ndff of roots was the highest, and the ¹⁵N absorbed by roots was preferentiallly distributed to the new vegetative organs. However, the ability of nitrogen uptake of apple trees was weakened with the increase of nitrogen rates. Furthermore, the nitrogen absorbed by trees could be quickly transported to new organs under the N_0.1 level at spring shoot stop growth. The ¹⁵N distribution ratio in the leaves was the highest from spring shoot stop growing to nutrient backflow stage. The ¹⁵N distribution ratio of roots under the N_0.1 treatment (33.8%) was significantly higher than those under the N_0.2 treatment (17.0%) and N_0.3 treatment (22.5%) in the early defoliation stage, while about 37.6% nitrogen in leaves back fluxed in fruit trees. The ratios of ¹⁵N utilization increased significantly with the process of growth, and were in order of N_0.1(30.0%) > N_0.2 (27.9%) > N_0.3 (21.7%) at the nutrient backflow stage. Besides, the ¹⁵N absorbed by trees from spring shoot stop growing to nutrient backflow stage accounted for 80% or more of the nitrogen uptake during the whole growth periods.

Conclusions The duration of spring shoot stop growth stage and nutrient backflow stage is the key period of nitrogen requirement for dwarf rootstock ‘Fuji’ apple trees. N_0.1 is suitable for the vegetative growth, nitrogen absorption, utilization and storage of young dwarf rootstock ‘Fuji’ apple trees. In practice, nitrogen fertilizer rate should be controlled and applied based on the practical requirement, thus nitrogen utilization rate will be increased.