Data_Sheet_1_Nitrogen Use Efficiency, Allocation, and Remobilization in Apple Trees: Uptake Is Optimized With Pre-harvest N Supply.docx

Optimizing the utilization of applied nitrogen (N) in fruit trees requires N supply that is temporally matched to tree demand. We investigated how the timing of N application affected uptake, allocation, and remobilization within 14-year-old “Gala”/M26 apple trees (Malus domestica Borkh) over two seasons. In the 2017–2018 season, 30 g N tree−1 of 5.5 atom% 15N–calcium nitrate was applied by weekly fertigation in four equal doses, commencing either 4 weeks after full bloom (WAFB) (pre-harvest) or 1-week post-harvest, or fortnightly, divided between pre- and post-harvest (50:50 split). Nitrogen uptake derived from fertilizer (NDF) was monitored by leaf sampling before whole trees were destructively harvested at dormancy of the first season to quantify N uptake and allocation and at fruit harvest of the second season to quantify the remobilization of NDF. The uptake efficiency of applied N fertilizer (NUpE) was significantly higher from pre-harvest (32.0%) than from the other treatments (~17%). The leaf NDF concentration, an indicator of N uptake, increased concomitantly only when pre-harvest N was applied. Pre-harvest treated trees allocated more than half of the NDF into fruit and leaves and stored the same amount of NDF into perennial organs as the post-harvest treatment. Subsequent spring remobilization of NDF was not affected by the timing of N fertigation from the previous season. A seasonal effect of remobilization was observed with a decrease in root N status and a reciprocal increase in branch N status at fruit harvest of season two. These findings represent a shift in the understanding of dynamics of N use in mature deciduous trees and indicate that current fertilizer strategies need to be adjusted from post-harvest to primarily pre-harvest N application to optimize N use efficiency. This approach can provide adequate storage N to support early spring growth the following season with no detriment to fruit quality.优化果树施用氮肥（applied nitrogen, N）的利用效率，需使氮素供应与果树的氮需求在时间维度上实现精准匹配。本研究以两个生长季内的14年生"Gala"/M26嫁接苹果树（Malus domestica Borkh，苹果）为试材，探究了施氮时期对植株体内氮素吸收、分配与再转运过程的影响。 2017–2018生长季中，每株树施用30g标记有5.5 atom% 15N的硝酸钙氮肥，采用每周滴灌施肥的方式分为4等份施入，施氮起始时间设置为三类：一是盛花后4周（采前），二是采后1周，三是分采前与采后两个阶段每两周施入一次（各占50%比例）。 本研究通过叶片取样监测肥料来源氮（nitrogen derived from fertilizer, NDF）的累积量：在第一个生长季休眠期对全树进行破坏性采样，以量化氮素吸收与分配情况；在第二个生长季果实采收期，量化NDF的再转运过程。 结果表明，采前施氮处理的氮肥吸收效率（nitrogen uptake efficiency, NUpE）达32.0%，显著高于其他处理（约17%）。作为氮素吸收核心指标的叶片NDF浓度，仅在采前施氮处理下出现同步上升。采前施氮的果树将超过一半的NDF分配至果实与叶片中，其在多年生器官中的NDF储存量与采后施氮处理无显著差异。上一生长季的施氮时期对后续春季NDF的再转运无显著影响。研究还观察到季节效应引发的氮素动态变化：在第二个生长季果实采收期，根系氮素水平下降，而枝条氮素水平呈反向升高。 本研究结果更新了对成熟落叶果树氮素利用动态的认知，指出当前施肥策略需从以采后施氮为主，调整为以采前施氮为主，以优化氮肥利用效率。该施氮模式可为下一生长季的早春生长提供充足的储存氮素，且不会对果实品质产生不利影响。