Variation in the natural enrichment of 15N in apple trees and soils

Leaf, wood and soil materials taken from an established apple orchard contained 15N in excess of atmospheric levels. Enrichments were highest (0.380 atom%) in surface soil and lowest in leaves (0.376 atom%). Enrichments varied over the course of a single season, being generally higher in late summer (August) and autumn (November) than in June and July. The patterns of variation in enrichment suggested that assessments of tree to tree variation for leaves should be made; but time variation for surface soil would be more important.