Abstract:

In order to clarify the reasonable fertilization position of dwarfing interstock apple trees, reduce the waste of nitrogen fertilizer. In 2018 and 2019,we studied the effects of nitrogen application in three horizontal distance of inner ring,middle ring and outer ring of the apple crown projected on soil ¹⁵N distribution,apple tree ¹⁵N absorption and utilization,and yield and quality on 8-year-old‘Yanfu 3’/SH6/Malus robusta using the ¹⁵N isotope tracer technique. The results showed that the root length density of apple fine roots（diameter ≤ 2 mm）in all treatments presented attenuation rules in both horizontal and vertical directions,and it was mainly distributed in the range of 0-100 cm from the horizontal direction of the trunk and 0-40 cm from the vertical direction. Compared with the control group,the fine root length density of the treated area increased significantly. Judging from the increase in fine root length density in the nitrogen application zone of the 0-20 cm soil layer in the vertical direction,nitrogen application in inner ring had the largest increase,1.33 to 1.36 times of the control,followed by nitrogen application in middle ring. Under different treatments,the peak of soil ¹⁵N content in the horizontal direction appeared in the area where the fertilization area was located,and in the vertical direction in 0-20 cm soil layer. The spatial concordance index between the density of apple fine root length and soil ¹⁵N content（RLD-¹⁵N）showed significant differences among different treatments,and the nitrogen application in inner ring was significantly higher than that in middle and outer ring. The application of nitrogen in the inner ring significantly increased the that of the new organs of the tree during the new shoot growing stage. The ¹⁵N utilization rate of the tree was nitrogen application in inner ring > nitrogen application in middle ring > nitrogen application in outer ring. There was no significant difference in ¹⁵N residue rate between nitrogen application in inner ring and in outer ring,but both were higher than that in outer ring. Compared with nitrogen application in outer ring,nitrogen application in inner ring significantly improved fruit yield,soluble sugar content and sugar acid ratio,but there was no significant difference in hardness and titratable acid content. It can be seen that the application of nitrogen in the inner layer of dwarfing interstock apple was conducive to improving the spatial concordance index between the density of apple fine root length and soil ¹⁵N content,increasing the nitrogen content of new-born organs in the early growth period,improving the ¹⁵N utilization efficiency of the tree,and increasing the fruit yield and soluble sugar content. Therefore,in actual production,it is recommended to apply nitrogen fertilizer at the 1/3 crown projection area close to the trunk of adult dwarfing interstock apple trees,that is,approximately 0-75 cm from the horizontal distance of the trunk.

Key words: apple, nitrogen application spaces, fine root length density, soil ¹⁵N content, spatial concordance index, utilization of ¹⁵N, yield, quality