Absorption, distribution and utilization of dwarf apple trees to ¹⁵N applied in different growth stages

【Objectives】The effects of nitrogen application in different growth stages on nitrogen absorption, distribution and utilization by dwarfed apple trees were studied to provide scientific basis for appropriate fertilization and improve nitrogen efficiency of dwarfed apple orchards.【Methods】Seedlings of five-year-old Fuji 3/M26/M hupehensis Rehd. were used as materials, and 15N-urea(abundance 10.14%)was used as N resource, a field experiment was conducted with three urea application dates: the bud stage(March 20th), spring shoot growing slowly stage(June 5th)and autumn shoot growing stage(July 10th). 10 g of 15N-urea and 150 g of urea were applied in each sapling. The whole saplings were destructively sampled at the fruit maturity stage(October 15th). Total nitrogen was determined by kjeldahl method and the 15N abundance was determined by ZHT-03 mass spectrometer.【Results】The contribution of N derived from fertilizer(Ndff)to the total N contents in different organs is significantly affected by the application date. The highest Ndff is in roots at the full-bloom stage when the urea is applied at the bud stage, followed by the perennial branches. From the spring shoot slowly growing stage to fruit rapidly swelling stage, the Ndff values of new organs are higher than those of the storage organs. At the fruit premature stage, the Ndff values of different organs are in high level, and at fruit maturity stage the Ndff value of fruits is the highest. When the fertilization is applied at the spring shoot slowly growing stage, the Ndff in roots of the autumn shoot growing stage is the highest, the Ndff values of new organs are at a high level at the fruit maturity stage, and the Ndff of fruits is the highest. The Ndff values of the storage organs(roots and perennial branches)are at a high level in each period under the autumn shoot growing stage fertilization treatment. Therefore, along with the changing of the phenological period,the Ndff values of new organs(annual branches, leaves and fruits)aboveground increase gradually, and reach the peak at the fruit maturity stage, but the 15N absorption ability of fruits is weakened in comparison. The 15N distribution ratios of each organ at the fruit maturity stage are significantly different among different treatments. The 15N distribution ratio in the vegetative organs is the highest when urea is applied at the bud stage, and that is in reproductive organs when the urea is applied at the spring shoot growing slowly stage, and that is in the storage organs when applied at the autumn shoot growing stage. There are significant differences in the total N uptake of plant, the Ndff and their utilization rates with the three application stages at fruit maturity stage. The highest values(86.34 g, 1.38 g and 30.07%)were measured in the saplings applied fertilizer at the spring shoot growing slowly stage, then were at the autumn shoot growing stage(75.64 g, 1.25 g and 27.22%)and the lowest(72.82 g, 1.09 g and 23.63%)were at bud stage fertilization.【Conclusions】Fertilization should be carrid out at the spring shoot growing slowly stage under the tested soil condition. In practice, less nitrogen should be applied in spring, and more top-dressed in early summer when the fruit is rapid swelling, appropriately postponing fertilization will increase the nutrient storage of the current year.