Abstract:

By measuring the absorption and utilization of nitrogen in navel orange plants，as well as their residual and loss in the soil，this provides a basis for further optimizing nitrogen management in the navel orange production system during the no-bearing stage. The experiment used 2-year-old Newhall and Gannanzao navel orange grafted on trifoliate orange as materials. ¹⁵N isotope tracing technology was used，and labeled nitrogen fertilizer was applied twice in summer. After the autumn shoots matured，plant and soil samples were collected to measure the ¹⁵N abundance of various plant organs and soil layers，and then calculate nitrogen utilization，residue，and loss. The results showed that there is a significant difference in biomass between Newhall and Gannanzao navel oranges，but the proportion of dry matter of each part in the whole plant was consistent. The proportion of dry matter of 1-year-old leaves and branches in the whole plant was over 40%，which was the growth centers of young navel orange trees，and ¹⁵N was mainly distributed to these organs. The allocation rate of ¹⁵N in Newhall is significantly higher than that in Gannanzao in 2-year-old leaves and coarse roots，and significantly lower than that in branches and trunk. The annual utilization rate，soil residual and loss percentage of ¹⁵N labeled urea for young Newhall navel orange trees were 6.45%，75.72%，and 17.83%，respectively，while for Gannanzao，they were 3.36%，54.25%，and 42.39%，respectively. The above results indicate that the utilization efficiency of nitrogen fertilizer applied in summer for young navel orange trees is relatively low，and there are significant differences in the fate of nitrogen in different varieties of navel orange orchards，which may be related to significant differences in biomass（including root architechture）between the two varieties.

Key words: citrus, nitrogen, isotope, utilization, residual in soil