Storage and Remobilization of Nitrogen by Chinese Jujube (Z. jujuba Mill. var. inermis Rehd) Seedling as Affected by Timing of 15N Supply

Winter jujube orchard nitrogen (N) management aims at increasing N reserves to meet the tree's growth requirements. Fertilization strategies should maximize the efficiency of fertilizers, including the choice of the optimal timing of N supply. 15N-urea was applied to winter jujubes on Jinsixiaozao jujubes rootstock to evaluate the effect of application timing on N-storage and remobilization in mature trees in pot culture. The treatments consisted of ground application before budding (BB), during fruit core-hardening stage (FCH), and fruit rapid-swelling stage (FRS). Nitrogen-use efficiency of treatments were significantly different, which were 2.42% (BB), 9.77% (FCH), and 9.01% (FRS) in the dormant and 5.20% (BB), 16.16% (FCH), and 10.30% (FRS) in the following full-bloom. N supply in the pre-harvest helped to increase N-reserves of trees and then translocate to the new growth organs the following year. The largest amount of 15N was detected in the roots and trunks. In all the treatments, the partition rates were highest in coarse roots, which were 30.43% (BB), 38.61% (FCH), and 40.62% (FRS), respectively. 15N stored in roots and trunks was used by jujube trees to sustain new growth in the following full-bloom. 15N applied before budding resulted in lower Ndff% in perennial organs (trunks and coarse roots) sampled in the following full-bloom, but fine roots had highest Ndff% (1.28%). Other organs recovered similar amount of Ndff%. In contrast, FCH and FRS treatments led to higher Ndff% (4.01–5.15%) in the new growth organs (new growth branches, deciduous spurs, leaves and flowers), but lower Ndff% in perennial branches (1.49–2.89%). With the delay of 15N-urea application time, 15N increased the partitioning to roots. FCH treatment increased N-storage in perennial organ during winter, which should be remobilized to sustain new growth the following spring.