Mature almond trees [ Prunuis dulcis (Mill) D.A. Webb] growing on a very light-textured soil (Delhi sand) were “pulsed” in 1980 with a soil application of 15 N-depleted ammonium sulfate. Leaching of labeled N from the soil and dilution (with unlabeled N carriers) of residual label in the soil minimized uptake of labeled N from this soil in subsequent years. The percent annual depletion (PAD) of labeled N in tissue samples was 50% and represented the percent annual influx of tree N from the soil N pool. Nitrogen assimilated in previous years also represented 50% of total tree N. The fractional contribution of N absorbed in any prior year, relative to the total pool of storage N, may be expressed as 1/(2) x , where x represents the number of years prior to the current year. The PAD, as measured in tissue samples, was greater in trees growing in the Delhi sand than among comparable trees growing in a heavier-textured soil (Yolo silty clay loam). A hyperbolic depletion function was fitted to the data to predict endpoints of tissue labeling. These endpoints were estimated to be 8.5 and 86.1 years on the light- and heavy-textured soils, respectively.
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Weinbaum et al. (1987) studied this question.