UPTAKE AND REDISTRIBUTION OF N-15 WITHIN AN ESTABLISHED ASPARAGUS CROP AFTER APPLICATION OF N-15-LABELED NITROGEN-FERTILIZER

The uptake and redistribution of15N within a 6-year-old asparagus (Asparagus officinalis L.) crop were examined for applications of15N-enviched ammonium sulphate (5 g N m-2) either prior to growth of foliage (commonly called ‘fern’), prior to harvest, or early-harvest prior to the main period of spear (newly-emerged, edible, unexpanded shoot) production. During the harvest in spring, 38 kg N ha-1 was removed in harvested spears, but this was small compared to the 710 kg N ha-1 present in crowns and roots. Limited uptake of15N occurred during harvest from the pre-harvest and early-harvest applications (11 and 4% of the15N applied, respectively) and the lack of plant uptake of N from soil was also evident from an accumulation of inorganic N in unfertilized soil during spring. These results indicate that N in spears was derived largely from remobilisation of N stored in the crowns and roots. Most plant uptake of added15N occurred during the first 8 weeks of foliage growth in summer, when 282 kg N ha-l had accumulated in the above-ground foliage. After this 8 week period, foliage from the early-harvest treatment contained 24% of the15N applied. Fifteen weeks later (late autumn), foliage was senescing and the1SN content of senesced foliage in all treatments had declined by 90% due to remobilisation and translocation into the crown and root tissue. Similarly, foliage N had declined from 282 to 24 kg N ha-1 and this remobilised N was equivalent to approximately 40% of the total plant N present prior to foliage growth. During the subsequent spring period, the15N enrichment of spears was about twice that of the crowns and roots. Thus, there was preferential remobilisation of recently-absorbed, stored N for new spear growth. © 1994 Annals of Botany Company.