Distribution of 15N-labeled fertilizer applied to pecan:: A case study

From March through June 1996, N-15-labeled fertilizer was applied to mature pecan trees [Carya illinoinensis (Wangehn.) K. Koch] in a commercial orchard to determine the fate of fertilizer-n in the tree and in the soil directly surrounding the tree. The concentrations of N-15 and total N were determined within various tissue components and within the soil profile to a depth of 270 cm. By Nov. 1996, elevated levels of N-15 were greatest at depths just above the water table (280 cm), suggesting a substantial loss of fertilizer-N to leaching. Recoveries of N-15 from tissue and soil at the end of 1996 were 19.5% and 35.5%, respectively. Harvest removed 4.0% of the fertilizer-N applied, while 6.5% was recycled with leaf and shuck drop. In 1997, with no additional application of labeled fertilizer, the tissue components continued to exhibit N-15 enrichment. By the end of the 1997 growing season, N-15 levels decreased throughout the soil profile, with the most pronounced reduction at depths immediately above the water table. Estimated recoveries of N-15 from pecan tissues (excluding root) and soil at the end of 1997 were 8.4% and 12.5%, respectively. In 1996 and 1997, N-15 determinations indicated an accumulation of fertilizer-N in the tissues and a loss of fertilizer-N to the groundwater. Early spring growth, flowering, and embryo development used fertilizer-N applied the previous year, as well as that applied during the current year.