The Characteristic of Seasonal Nitrogen Absorption and Distribution in Pear Trees from N Fertilizers Applied at Different Growing Stages

A field trial was carried out to study the seasonal nitrogen (N) uptake and distribution in 8-year-old pear trees using labeled N fertilizer. The trees were treated with equal amounts of labeled N ((¹⁵NH₄)₂SO₄) at three different stages, i.e., (1) one month after fruit harvest (post harvest fertilization, PHF, mid-October, 2011), (2) one month before budding (before budding fertilization, BBF, early April, 2012), and (3) one month before fruit harvest (fruit enlargement fertilization, FEF, early August, 2012). The N concentration and N abundance were determined in the flowers, leaves, roots, fruit, phloem and xylem of one-year-old or perennial branches, and the trunk. We found that the ¹⁵N derived from PHF (Ndff) was mainly located in the root (6.13%), while only trace amounts of ¹⁵N occurred in other organs when determined one month before budding. However, ¹⁵N increased to 8.62 Ndff % in roots when determined during the fruit enlargement period, which is 54.5% higher than that derived from BBF. During the same period, the N concentration in perennial organs was reduced by 8.9%-64.7%, which might be due to the growth and development of new organs. However, also during this period, the distribution of ¹⁵N derived from PHF was significantly increased in the leaves and fruit, and in the phloem and xylem of branches and the trunk. Compared with PHF, BBF resulted in a reduced amount of ¹⁵N in all organs. Compared with the roots, leaves, and xylem of one-year-old perennial branches, the ¹⁵N derived from PHF and BBF was mainly distributed in the growing fruit, while that derived from FEF was mainly present in the phloem of one-year-old branches and buds, indicating that FEF application mainly resulted in an increase in the storage of N for the next season’s budding, flowering, and young fruit development, but not for fruit growth in the current season. Thus, we propose that the growth of fruit depends on the amount of N stored in perennial organs, the newly absorbed N from budding to fruit enlargement, during which N absorbed from PHF was significantly higher than that from BBF, and the recycling of N from the roots, leaves, and xylem to the fruit during the period from fruit enlargement to maturation. PHF appears to have a greater impact on the fruit than does BBF.