N isotope fractionation in tree tissues during N reabsorption and remobilization in Fagus crenata Blume

Background and Motivation: Nitrogen content in tissues of Fagus crenata Blume is key for flowering and seed production. However, there is a lack of information on seasonal intra-plant nitrogen partitioning in this representative tree species typical of heavy snowfall regions in Japan. Therefore, the objective of this study was to elucidate Fagus crenata intra-plant nitrogen movement by means of nitrogen content, nitrogen isotope analysis, and amino acids temporal variability. Materials and Methods: Nitrogen content, isotope ratio, and free amino acids content were measured in coarse roots, sapwood, leaves, and litter in four phenological stages in nine adult Fagus crenata trees and upscaled to the whole-tree level. Results: Nitrogen was reabsorbed to and stored in coarse roots during the pre-abscission stage, as was revealed by the depletion of the δ15N ratio of coarse roots, which coincided with an enrichment of 15N found in leaves. During the post-abscission stage, N was stored in the sapwood, where an enrichment in 15N was found coinciding with the depletion of the δ15N ratio in leaves. It seemed that 15N-enriched nitrogen was initially reabsorbed from leaves to coarse roots during the pre-abscission period, followed by the reabsorption of 15N-enriched nitrogen from leaves to sapwood shortly before leaf abscission. Free amino acids content and their dynamics could mostly explain seasonal δ15N fractionation in leaves, coarse roots, and partially in sapwood. At the whole-tree level, N content stored in coarse roots and sapwood was similar. Furthermore, reabsorbed leaf N accounted for 32% of all nitrogen stored during leaf senescence. Conclusion: We found three phases of nitrogen storage revealed by δ15N fractionation during leaf senescence: (1) reabsorption of leaf 15N-depleted nitrogen to coarse roots, followed by (2) reabsorption of leaf 15N-enriched nitrogen to sapwood and (3) soil 15N-depleted nitrogen uptake to coarse roots. Further, changes in free amino acids, which are the result of enzyme activities involved in amino acids synthesis, partially explained δ15N fractionation in plant tissues