Timing of N-15 fertiliser application, partitioning to reproductive and vegetative tissue, and nutrient removal by field-grown low-chill peaches in the subtropics

The effect of timing of nitrogen (N) application as N-15-enriched ammonium sulfate (50 kg N/ha) on the growth response and N uptake by vegetative and reproductive tissues was investigated in the low-chill peach cv. Flordagem growing on a krasnozem soil at Alstonville. Nitrogen was applied in late Au gust, late September, late October, mid February, and early May. Tree parts were sampled for N-15 at 4 and 8 weeks after application and after fruit h arvest in December the following season. After fruit yield was measured, tr ees were excavated and divided into parts for dry weight and nutrient conce ntration determinations, and fertiliser N recovery and to estimate tree nut rient removal. Nitrogen enrichment was detected in all plant parts within 4 weeks of N app lication, irrespective of timing, and was greatest in rapidly growing tissu es such as laterals, leaves, and fruit. The most rapid (P < 0.05) N-15 enri chment in vegetative tissues resulted from September, October, and February N applications and for fruit from a September application. The level of en richment 4 weeks after fertiliser N application was similar for vegetative and reproductive tissues. The timing of N application in the first season h ad no effect on fruit yield and vegetative growth the following season. At tree removal, the recovery of fertiliser N in most tree parts increased (P < 0.05) as fertiliser N application was delayed from October to May the previous season. Maximum contribution of absorbed N to whole tree N was 10- 11% for laterals, leaf, and fruit. Data from this study indicate that veget ative and reproductive growth have similar demand for absorbed N, and that uptake of fertiliser N is most rapid when an application precedes a period of rapid growth. Over 2 seasons, recovery of applied fertiliser N was 14.9- 18.0% in the tree, confirming that stored N and the soil N pool are the dom inant sources of tree N. The recovery of fertiliser N from the May applicat ion was 18% even though uptake in all tree parts including roots at 4 weeks after application was very low, indicating that tree fertiliser N uptake o ccurred when growth resumed after the dormant winter period. The low propor tion and recovery of fertiliser N in the tree confirm the lack of immediate influence of applied N to vegetative growth and yield. Annual crop nutrient removal is a sound basis for fertiliser recommendation s, and for the Flordagem orchard (1000 trees/ha), it consisted of fruit plu s 70% of laterals (removed at pruning) plus 20% of leaf. Removal in vegetat ive tissues was relatively low at (kg/ha) 14 N, 1 P, 12 K, 13 Ca, and 2 Mg. The addition of fruit at a yield of 25 t/ha increased total nutrient remov al to (kg/ha) 46 N, 5 P, 54 K, 14 Ca, and 5 Mg.