Springtime (15)Nitrogen uptake, partitioning, and leaching losses from young bearing Citrus trees of differing nitrogen status

(15)Nitrogen uptake, allocation, and leaching losses from soil were quantif ied during spring, for 4-year-old bearing 'Redblush' grapefruit (Citrus x p aradisi Macf,) trees on root stocks that impart contrasting growth rates. N ine trees on either the fast-growing 'Volkamer' lemon (VL) (C. volkameriana Ten & Pasq,) or nine on the slower-growing sour orange (SO) (C. aurantium L.) rootstocks were established in drainage lysimeters filled with Candler fine sand and fertilized with 30 split applications of N, totaling 76, 140, or 336 g.year(-1) per tree. A single application of double-labeled ammoniu m nitrate ((Np4N3)-Np-15-N-15, 20% enriched) was applied at each rate to re plicate trees, in late April, Leaves, fibrous roots, soil, and leachates we re intensively sampled from each treatment over the next 29 days, to determ ine the fate of the (NH4NO3)-N-15-N-15 application. Newly developing spring leaves and fruit formed dominant competitive sinks for N-15, accounting fo r between 40% and 70% of the total N-15 taken up by the various treatments. Large fruit loads intercepted up to 20% of this N-15, at the expense of sp ring flush development, to the detriment of overall tree N status in low-N frees. Nitrogen supply at less than the currently recommended yearly rate o f 380 g/tree exceeded the requirements of 4-year-old grapefruit trees on SO rootstock; however, larger trees an VL rootstock took up the majority of N -15 from this rate over the 29-day period, Nitrogen-use efficiency declined with increasing N rate, irrespective of rootstock, The residual amounts of N-15 remaining in the soil profile under SO trees after this time represen ted a significant N leaching potential from these sandy soils. Therefore, u nder these conditions, present N recommendations appear adequate for rootst ocks that impart relatively fast growth rates to Citrus trees, but seem exc essive for trees on slower-growing rootstock species.