Objectives  The present study investigated the nitrogen demand in honey pomelo tree at different fertilizer application stages under different fertilizer treatments, and provided the theoretical basis for rational management and efficient utilization of nitrogen fertilizer in pomelo orchard.

Methods  Seven-year-old red flesh honey pomelo trees were selected, two kinds of fertilizer application methods were adopted, high fertilizer input was referred to CK and reducing the fertilizer input was refered to fertilizer reduction treatment. The urea amount was divided in ratio of 5∶3∶2, and applied at spring shoot germination stage, fruit stabilization stage and fruit expansion stage, respectively. And 5% ¹⁵N-urea was added in urea at each stage. In the mature stage, samples of organs from various parts of pomelo tree were taken to measure biomass, N concentration and ¹⁵N abundance, and calculate the contribution of N derived from fertilizer (Ndff) to the total N contents in different organs, ¹⁵N distribution, and utilization rate.

Results  In CK, the Ndff of fruit, new shoot leaf with fruit, and new shoot with fruit were higher when ¹⁵N-urea was applied at the spring shoot germination stage. The Ndff of fruit was significantly higher than other organs when ¹⁵N-urea was applied at the fruit stabilizing stage. The Ndff of the new shoot and new shoot leaf without fruit was the highest when ¹⁵N-urea was applied at the fruit expanding stage. In fertilizer reduction treatment, the Ndff of fruit, vegetative organs and storage organs were similar to CK when the ¹⁵N-urea was applied at the spring shoot germination stage and the fruit stabilizing stage. The Ndff of shoot and branch was higher than leaf when ¹⁵N-urea was applied at the fruit expanding stage, which was different from CK. The results of ¹⁵N distribution rate of different organs at the maturity stage were as follows. In CK, the ¹⁵N distribution rate of fruit was highest (52.5%), followed by vegetative organs (25.5%) and storage organs (22.1%) when ¹⁵N-urea was applied at the spring shoot germination stage. When the ¹⁵N application at the fruit stabilizing stage, the fruit still showed the highest ¹⁵N distribution rate (61.3%), but the ¹⁵N distribution rate of storage organs (25.4%) was significantly higher than the vegetative organs (13.3%). The ¹⁵N-urea application at the fruit expanding stage further increased the ¹⁵N distribution rate of storage organs to the maximum (44.0%), followed by the vegetative organs (35.8%). The ¹⁵N distribution rate in fruit was the lowest (20.3%). In fertilizer reduction treatment, the ¹⁵N utilization of different organs were similar to CK when the ¹⁵N-urea was applied at the spring shoot germination stage and the fruit stabilizing stage. When ¹⁵N-urea was applied at the fruit expanding stage, the ¹⁵N distribution rate of storage organs was highest (45.8%), while the ¹⁵N distribution rate of fruit (34.3%) was higher than that of vegetative organs (19.9%). The ¹⁵N utilization rate was the lowest in both treatments when ¹⁵N-urea was applied at the spring shoot germination stage. In CK, the ¹⁵N utilization rate was gradually increased with the delay of the ¹⁵N-urea application stage. In fertilizer reduction treatment, the ¹⁵N utilization rate was the highest (37.5%) when ¹⁵N-urea was applied at the fruit stabilizing stage. These results showed that the ¹⁵N utilization rate was higher for the pomelo tree’s different organs under the fertilizer reduction treatment than CK, and the maximum increase rate of ¹⁵N utilization was in fruit.

Conclusions  By using ¹⁵N trace technique, it was showed that N was mainly distributed to fruit, new shoot leaf and new shoot when fertilizer was applied at the spring shoot germination stage. Nitrogen was mainly distributed to fruit when fertilizer was applied at the fruit stabilizing stage. Nitrogen fertilizer applied at the fruit expanding stage could significantly improve nitrogen accumulation in the storage organs, which was useful to the fruit growth in the next year. Therefore, in the pomelo production, the fertilizer application at the spring shoot germination stage can be reduced to avoid the loss of nitrogen fertilizer. Fertilizer application at the fruit stabilizing stage is critical for fruit nitrogen accumulation. Nitrogen fertilizer at fruit expanding stage is highly recommended to increase nitrogen nutrition storage, which is used for the flower bud differentiation of the next year and improvement of the yield and fruit quality. Appropriate optimization of nitrogen fertilizer dosage is conducive to suppressing the excessive nutrient growth of pomelo trees and improving the nitrogen utilization rate of fruit. The specific optimization ratio still needs further researches.