Timing of urea application affects leaf and root N uptake in young Fuji/M.9 apple trees

SummaryLeaf and root nitrogen (N) uptake was compared at different times of the season in young apple trees. One-year-old potted Fuji/M.9 trees were supplied with 1% 15N-labelled urea either by foliar or soil applications in May, July and September. Trees receiving only water served as controls. The trees were harvested 10 d after 15N application, separated into shoots (leaves and current-year stems), stem (previous-year wood) and roots. Biomass, total N and 15N contents of all tissues were determined. New shoot biomass and total tree biomass increased as the season advanced, while root biomass peaked in July. Leaf N uptake was higher than root uptake in May and September, while root N uptake was higher than leaf uptake in July. Leaf N uptake increased as the season advanced, while root N uptake was highest in July. The lowest 15N recovery (11%) was obtained in May with soil N application, and the highest 15N recovery (48%) was obtained in September with foliar N application. Our results suggest that foliar application of N early in the season, followed by soil N application in mid-season, then foliar application again late in the season is an efficient N management strategy for young trees.