Effects of Time of Nitrogen Application and Soil Texture on the Availability of Isotopically Labeled Fertilizer Nitrogen to Reproductive and Vegetative Tissue of Mature Almond Trees

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Abstract
Mature almond [Prunus dulcis (Mill) D.A. Webb] trees growing on light-(Delhi sand) and heavy-textured (Yolo silty clay loam) soils were fertilized with 15N-depleted ammonium sulfate at different times during the year to permit direct measurement of fertilizer N within the trees. The distribution of fertilizer N between vegetative and reproductive organs was monitored during both the year of application, 1980, and the subsequent year. The later that fertilizer N was applied during the season, the less fertilizer N was recovered in the fruit and leaves that year, and the greater its N contribution to these organs was the following year. Isotopic labeling of fruit and leaves appeared to be relatively unaffected by soil texture during the year of fertilizer application. During the subsequent year, however, the recovery of fertilizer N by fruit and leaves was 2-fold greater on the heavy-textured soil than on the light-textured soil. Recovery of labeled N in fruit was relatively low on both soil types following application of fertilizer during the dormant period. Isotopic N was recovered in fruit in both 1980 and 1981 and constituted about 20% to 28% of fruit N at most. About 25% of the applied N was removed in the fruit on the heavy-textured soil over a 2-year period. Up to 1 kg N per tree was removed annually in the harvested fruit.

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