SummaryMature ‘Kotata’ trailing blackberry plants growing in the alternate-year (AY) system in the field, were treated with ammonium sulfate depleted in 15N in early April, 1997. Based on the whole plant, excluding roots, accumulation of new dry matter over the two year AY production cycle averaged 5.96 kg plant-1. On average, 28, 64 and 8% of this new dry-matter accumulation could be attributed to harvested fruit; loss from the crop as prunings and leaf senescence; and increased dry matter in the crowns for the next growth cycle, respectively. On average, 46% of new dry matter accumulation occurred in the off-year (non-producing). Whole plant (excluding roots) accumulation of new N (labelled and non-labelled) over the two-year AY production cycle averaged 63.7 g N plant” . Over a two-year production cycle, 44% of new N accumulation occurred in the off-year. Of the newly accumulated N, 37% was in harvested fruit, 58% was lost from the crop as prunings, and 5% was accumulated in crowns for the next growth cycle. For both the on-year and the off-year plants fertilized with labelled N, near maximum nitrogen derived from fertilizer (NDFF) accumulation occurred by August. At this time, 45% of the applied fertilizer could be accounted for in the non-root portion of on-year plants. A portion of this maximum on-year accumulation was either lost from the system or translocated to the roots. At the time of maximum accumulation, 39, 37,19, 3 and 2% of the accumulated NDFF was contained in the fruit, laterals, primocanes plus primocane leaves, floricanes, and crowns, respectively. Labelled N applied in the off-year was used primarily for primocane and primocane leaf growth. A large portion of the nitrogen stored in the off-year was used for early growth of floricanes, fruiting laterals and fruit in the following on-year. Excluding roots, by the end of the 1997 season, almost 30% of the applied fertilizer was accounted for in the non-root portion of off-year plants. There was also evidence of remobilization of N among plant tissues at different times during the production cycle. Results of this study suggest fertilizer N is an important N source for fruiting lateral and fruit growth in the on-year and for new primocane and primocane leaf growth in the off-year. N accumulated in the off-year is also an important source for early growth of floricanes, fruiting laterals and fruit the following year.