EFFICIENCY OF NITROGEN FERTILIZATION ON CITRUS ORCHARDS

Nitrogen fertilization has been extensively adopted in Brazil and has resulted in increased fruit production of citrus trees. Studies on the fate of applied N fertilizer and validation of the N dose recommendations are scarce both in the Brazilian and the international literature. In the present work, the dynamics of ¹⁵N (ammonium sulfate), applied to soil surface, in the citrus orchard and the response of citrus trees to N fertilization rates were studied. Three- to four-year-old 'Pêra' sweet orange trees grafted on Rangpur lime were fertilized with 300 g of N tree^-1 (as ammonium sulfate) labeled with 4.917 atom % ¹⁵N at fruiting stage to study the fate of N in the soil-orange tree system. Additionally, 0, 150, 300, 450 and 600 g of N per tree, split in three applications, were supplied from early spring to summer. Fruit yield for different N treatments were evaluated. The trees that received ¹⁵N were destructively harvested and separated into trunk, old twigs, new twigs, leaves, fruits and roots. Plant and soil samples were analyzed for ¹⁴N/¹⁵N isotope ratio and total N. The effect of N fertilization on fruit yield, the amounts of ¹⁵N taken up from fertilizer and localization of this nutrient within the parts of the trees, and the ¹⁵N remaining in the soil were determined. The highest fruit yield (30 kg tree^-1) was obtained with N rate of 400 g tree^-1. The efficiency of applied N ranged from 20-27%, with the lowest value corresponding to the application previous to a heavy rainfall. Nitrogen remaining in the soil within the 0.60 m depth layer was 34% of applied N rate. Fruit exported approximately 34% of N from fertilizer accumulated in the whole tree.