Effect of different nitrogen application levels on 15N-urea absorption, distribution and utilization of‘Kuerlexiangli’pear

Abstract:【Objective】The application of nitrogen fertilizer has a great effect on the growth of fruit trees. Nitrogen deficiency or nitrogen excess limits the growth of fruit trees and the formation of yield. As a regional product,‘Kuerlexiangli’pear is expanding constantly in terms of planting area. Farmers choose to increase nitrogen fertilizer application in order to improve the yield. Excessive nitrogen fertil- izer input not only reduces the use of nitrogen fertilizer by the tree, but also increases production costs and reduces fruit yield and quality. At present, the application amount of nitrogen fertilizer in‘Kuerlexiangli’pear orchards in Xinjiang has reached 300-450 kg·hm-2, which has exceeded the nitrogen absorption limit of fruit trees. With the increase in nitrogen application, nitrogen utilization rate of the tree decreases. Reasonable application of nitrogen fertilizer has a great effect in improving the nitrogen utili- zation efficiency, and it is also of great significance to improve the economic and ecological benefits of the orchards. Therefore, the effects of different nitrogen application levels on the growth and nitrogen absorption, distribution and utilization characteristics in‘Kuerlexiangli’pear trees were studied in this research so as to provide a reference for the rational fertilization and for the improvement of nitrogen  fertilizer utilization efficiency.【Methods】Six-year-old‘Kuerlexiangli’pear trees were used as the ex- perimental material, and 15N tracing was used to explore the characteristics of the growth status. 15N- urea absorption, distribution and utilization characteristics in different organs (taproot, first-order lateral root, second-order lateral roots, trunk, perennial branch, one-year branch, upper leaf, mid terminal lobe, lower leaf, fruit) of the trees under different nitrogen application rates (N0:without nitrogen, N1:low ni- trogen, N2:middle nitrogen, and N3:high nitrogen) were studied.【Results】The biomass and nitrogen ac- cumulation of the trees increased with fruit development, and showed an pattern of N3> N2> N1> N0 un- der different nitrogen application levels. Significant differences were observed in the 15N absorption and regulation ability derived from fertilizer (Ndff value) in different organs under different nitrogen appli- cation rates. The Ndff values of roots, trunk, perennial branches, one-year branches, leaves and fruit in each measurement period were in a pattern of N3>N2>N1. The Ndff values of leaves and roots were higher at new shoot growing stage. The 15N absorbed by roots during fruit swelling stage was preferen- tially transported to the one-year branches, leaves and fruit. At fruit maturity stage, the fruit became the growth center, with the highest Ndff value. With fruit development and growth, the amount of 15N ab- sorbed by the tree from the fertilizer increased. At fruit maturity stage, the 15N amount in the organs oth- er than the fruit was in a pattern of N3 > N2 > N1, and the 15N amount in fruit was highest in N2 (0.477 g). Under different nitrogen application levels, the 15N distribution rate in the roots gradually decreased with the development and growth, and the 15N distribution rate in the trunk and perennial branches de- creased first and then increased, while those of the one-year branches, leaves and fruit gradually in- creased. At fruit maturity stage, the 15N distribution rate in the fruit in N2 (31.79%) was significantly higher than that in N3 (23.86%) and N1 (23.76%). The 15N utilization rate increased significantly with the development and growth, and 15N utilization rate was in the order of N2 (20.19%) > N1 (16.86%) > N3 (15.58%) at fruit maturity stage【. Conclusion】With the increase in nitrogen level and with fruit devel- opment and growth, the biomass, nitrogen accumulation and 15N absorption of‘Kuerlexiangli’pear trees all increased gradually. However, at fruit maturity stage, the growth and nitrogen accumulation of the fruit were the highest at N2 level, and the utilization rate of the fertilizer 15N-urea was also the high- est (20.19% ). Therefore, it is necessary to control the input of nitrogen and determine the optimal amount of nitrogen fertilizer for the production of‘Kuerlexiangli’pear to increase yield as well as nitrogen fertilizer utilization effiency. Based on this study, the nitrogen application rate for the six-yearold‘Kuerlexiangli’pear was recommended to be 20 kg per 666.7 m2.