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

Abstract

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.

Keywords: 15N; application timing; nitrogen uptake; nitrogen use efficiency; partitioning (nitrogen); remobilization (nitrogen); storage (nitrogen).

Figure 1

Nitrogen uptake efficiency (NUpE) in the 2017–2018 season and nitrogen recovery efficiency (NRec) in the 2018–2019 season following different N application treatments. Different uppercase letters denote significant interaction (p < 0.05) between treatments and seasons and different lowercase letters indicate significant differences (p < 0.05) between treatments within a season (error bars represent ±SEM, n = 4).

Figure 2

Amount of nitrogen derived from fertilizer (NDF) and unlabeled N (Native N) for N-treated trees destructively harvested at the 2018 dormancy (excluding fruit and leaf foliage) and the 2019 harvest (including fruit and leaf) under different N fertigation treatments. Error bars represent ±SEM, n = 4.

Figure 3

Amount of nitrogen derived from fertilizer (NDF) and unlabeled N (Native N) of perennial storage organs excluding fruit and leaf for N-treated trees destructively harvested at the 2018 dormancy and the 2019 harvest under different N fertigation treatments. Error bars represent ±SEM, n = 4.

Figure 4

Dynamics of leaf N concentration [N] (A) and concentration of nitrogen derived from fertilizer (NDF) [NDF] (B) for N-treated trees during the 2017–2018 season following different N application timings. Error bars represent ±SEM, n = 4. Fruit was harvested at 22 WAFB.

Figure 5

Proportion (A) and total amount (B) of nitrogen derived from fertilizer (NDF) allocated into different organs for N-treated trees destructively harvested at dormancy of the 2017–2018 season under different N application treatments. Error bars represent ±SEM, n = 4. For each organ, notation indicating the extent of difference between treatments from unpaired two-sample t-test: ns p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 6

Total N in each organ showing composition of native N and nitrogen derived from fertilizer (NDF) at 1 week after fruit harvest of the 2018–2019 season under different N application treatments. Error bars represent ±SEM, n = 4.

Figure 7

Total N contained in the roots, trunk, branches, and seasonal canopy showing the composition of N derived from native N and nitrogen derived from fertilizer (NDF) at winter dormancy of 2017–2018 season and at 1 week after fruit harvest timing of the 2018–2019 season (March 2019). Error bars represent ±SEM, n = 12. For each organ, notation indicating the extent of difference related to the time of sampling from unpaired two-sample t-test: ns p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001.