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From March through June 1996, ¹⁵N-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 ¹⁵N 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 ¹⁵N were greatest at depths just above the water table (280 cm), suggesting a substantial loss of fertilizer-N to leaching. Recoveries of ¹⁵N from tissue and soil at the end of 1996 were 19.5% and 35.4%, 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 ¹⁵N enrichment. By the end of the 1997 growing season, ¹⁵N levels decreased throughout the soil profile, with the most pronounced reduction at depths immediately above the water table. Estimated recoveries of ¹⁵N from pecan tissue (excluding root) and soil at the end of 1997 were 8.4% and 12.5%, respectively. In 1996 and 1997, ¹⁵N 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.

The recovery of late-season (September) ¹⁵N-labeled fertilizer (N at 55 kg·ha^-1) was followed in mature pecan trees [Carya illinoinensis (Wangehn.) K. Koch] and soil (0-270 cm) from 1996 (application year) through 2001 (end of study). Recovery of late-season applied ¹⁵N was compared to the recovery of six ¹⁵N applications (March through June, N at 221 kg·ha^-1) of a previously reported study. By Nov. 1996, both fertilizer schedules exhibited considerable ¹⁵N accumulation below the rooting zone and just above the water table (280 cm), with 43.4% and 35.3% ¹⁵N recovered from the soil sampling profile of the September and March-June schedules, respectively. ¹⁵Nitrogen recoveries from perennial storage tissues (root and wood) were 20.6% and 10.1% under the September and March-June schedules, respectively. The ¹⁵N recoveries from annual abscission tissues (leaf, shuck, and nut) were 1.4% and 10.6% under the September and March-June schedules, respectively. By the end of the 2001 growing season, 4% and 9% of the ¹⁵N remained in the soil following the September and March-June applications, respectively. Under both fertilizer schedules, >80% of the fertilizer-N was lost to the environment through natural processes and very little was removed during harvest. Nearly 6 years following application, perennial storage of ¹⁵N remained greater in the September application (4.3% of the ¹⁵N applied) than in the March-June application (2.7% of the ¹⁵N applied). Late-season application of fertilizer-N during the kernel filling stage was stored in perennial tissues for use the following year; very little was used for current year growth of annual tissues. Increased accumulation of perennial storage N by late-season application may reduce the depletion of N caused during a heavy-cropping on-year and may moderate the alternate-bearing trend in pecan by providing a greater reservoir of N the following year.