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Summary The effect of nitrogen (N) recycling on tissue δ 15 N was studied using drought‐deciduous shrubs ( Encelia farinosa and E. frutescens ) and deciduous trees ( Quercus rubra and Q. alba ) to determine if isotope discrimination occurs with N reabsorption or reallocation. There was substantial reabsorption of leaf nitrogen before abscission in all species, yet there was no difference in the δ 15 N of living vs abscised leaves, suggesting no isotope discrimination with reabsorption. However, the contribution of stored N for leaf production resulted in a difference in foliar and absorbed nitrogen δ 15 N, suggesting that isotope discrimination occurs with reallocation. The contribution of stored nitrogen to leaf production varied with timing of leaf development: leaves produced first had a δ 15 N that reflected utilization of both absorbed and stored N, yet the contribution of stored N decreased over time. The results from this study as well as previous research on intraplant variation in isotopic composition suggest that caution needs to be exercised when interpreting foliar δ 15 N because it is not always a reliable indicator of whole‐plant δ 15 N.
Kolb et al. (Thu,) studied this question.