[Nitrogen application technology in dwarfed apple trees.].

N trace technique was used to explore the characteristics of N-urea absorption, utilization, loss and fruit quality of eight-year-old 'Yanfu3'/M26/Malus hupehensis Rehd. seedlings. There were three different nitrogen fertilization treatments: one-time nitrogen application in spring (1), two times nitrogen application (2), the intensive technique of nitrogen application with several times application and reduction in amount (3). The results showed that leaf area, chlorophyll content (SPAD value), photosynthetic rate (P), total N content of leaves and the total biomass per plant (except fruit), as well as the root-shoot ratio of treatment 3 were obviously higher than those in the treatments of 1 and 2. Treatment 3 significantly increased the protective enzymes (SOD, POD and CAT) activities and decreased the malondialdehyde (MDA) content in leaves. Significant differences were found on the N derived from fertilizer (Ndff value) of different organs, with the fruit Ndff showing the highest, followed by the one-year-old branches, leaves and roots of the three treatments. The Ndff values of each organ were the highest in the treatment 3. At fruit maturity stage, total N content of plant in treatment 3 was 93.0 mg·kg, which was obviously higher than that in the treatments 1 (70.2 mg·kg) and 2 (81.9 mg·kg). N nitrogen utilization ratio of treatment 3 was 33.6%, which was obviously higher than that in the treatments 1 (20.4%) and 2 (26.0%). The N loss rate was 46.9% in treatment 3, being obviously lower than that in the treatments 1 (56.5%) and 2 (52.9%). There were significant differences of fruit mass, soluble solid, fruit firmness, soluble sugar content, titratable acids and sugar-acid ratio among different treatments, with the highest values in the treatment 3 and the lowest values in treatment 1.