FATE OF N-15-LABELED NITROGEN-FERTILIZER APPLIED TO KIWIFRUIT (ACTINIDIA-DELICIOSA) VINES .2. TEMPORAL CHANGES IN N-15 WITHIN VINES

Temporal changes in the nitrogen (N) and N-15 content of various components of six-year-old kiwifruit (Actinidia deliciosa 'Hayward') vines which had received N-15-labelled ammonium fertilizer were measured. The fertilizer was applied singularly at 100 or 200 kg N ha-1 in early spring (two weeks before bud burst) or split with 100 kg N ha-1 (unlabelled) in early spring and 100 kg N ha (N-15-labelled) ten weeks later. All treatments were applied to vines with a history of either 50 or 200 kg N ha-1 yr-1. The N concentration of leaf and fruit tissue was generally lower in the 100 kg N ha-1 treatment than in the 200 kg N ha-1 treatments and this effect was greater than that of N fertilizer history. During the first 8 weeks after bud burst there was a rapid accumulation of N in leaves (ca. 80 kg N ha-1). Analysis of xylem sap at 4 weeks after bud burst revealed that about 60% of the N utilised for new growth was from remobilisation of N stored within the vines and about 40% from soil and fertilizer N. This was unaffected by rate of N application. Plant uptake of added N-15 was rapid and almost complete within 10 weeks of application in either early spring or early summer. Initially, most N-15 was present in the leaf and root components but these subsequently declined due to translocation into other components of the vine. The decline in leaf N-15 coincided with an equivalent accumulation of N-15 in the fruit The N-15 enrichment of the 'annual' components (leaves, fruit and current-season's shoots) was about twice that of the structural components (one-year-old shoots, cordon, stem and structural roots) during the first year after application. By the third year the N-15 concentration of the annual components had declined to similar levels to that of the structural components. These changes were used to estimate the annual throughput of N from soil in vines which received no N fertilizer in years 2 and 3 at 50% of total above-ground N and only 7% in roots. This was equivalent to about 120 kg N ha-1 vr-1. Where fertilizer N (200 kg ha-1) was applied in year 2 the annual throughput of N from soil and fertilizer was about 170 kg N ha-1.