| Summary: | <i>Background and Motivation:</i> Nitrogen content in tissues of <i>Fagus</i> <i>crenata</i> 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 <i>Fagus</i> <i>crenata</i> intra-plant nitrogen movement by means of nitrogen content, nitrogen isotope analysis, and amino acids temporal variability. <i>Materials</i> <i>and</i> <i>Methods:</i> 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 <i>Fagus</i> <i>crenata</i> trees and upscaled to the whole-tree level. <i>Results:</i> Nitrogen was reabsorbed to and stored in coarse roots during the pre-abscission stage, as was revealed by the depletion of the δ<sup>15</sup>N ratio of coarse roots, which coincided with an enrichment of <sup>15</sup>N found in leaves. During the post-abscission stage, N was stored in the sapwood, where an enrichment in <sup>15</sup>N was found coinciding with the depletion of the δ<sup>15</sup>N ratio in leaves. It seemed that <sup>15</sup>N-enriched nitrogen was initially reabsorbed from leaves to coarse roots during the pre-abscission period, followed by the reabsorption of <sup>15</sup>N-enriched nitrogen from leaves to sapwood shortly before leaf abscission. Free amino acids content and their dynamics could mostly explain seasonal δ<sup>15</sup>N 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. <i>Conclusion:</i> We found three phases of nitrogen storage revealed by δ<sup>15</sup>N fractionation during leaf senescence: (1) reabsorption of leaf <sup>15</sup>N-depleted nitrogen to coarse roots, followed by (2) reabsorption of leaf <sup>15</sup>N-enriched nitrogen to sapwood and (3) soil <sup>15</sup>N-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 δ<sup>15</sup>N fractionation in plant tissues.
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