Patterns in δ15N in roots, stems, and leaves of sugar maple and American beech seedlings, saplings, and mature trees

Stable isotopes of nitrogen (N) in plants are increasingly used to evaluate ecosystem N cycling patterns. A basic assumption in this research is that plant delta N-15 reflects the delta N-15 of the N source. Recent evidence suggests that plants may fractionate on uptake, transport, or transformation of N. If the dominant source of plant N is via roots, a difference in delta N-15 by tissue type would suggest fractionation on transport and assimilation of N. In order to evaluate differences between species and plant parts, we measured delta N-15 in root, stem, and leaf tissues of individual sugar maple (Acer saccharum; SM) and American beech (Fagus grandifolia; BE) plants ranging in age from germinants to mature trees at the Hubbard Brook Experimental Forest, New Hampshire (USA). For SM, root delta N-15 > stem delta N-15 > leaf delta N-15; for BE seedlings, root delta N-15 > stem delta N-15 and root delta N-15 > leaf delta N-15. These differences suggest that fractionation occurs during plant transport and assimilation of N. Beech delta N-15 (root, stem, and leaf) was consistently higher than SM delta N-15 for 1-7 year-old seedlings. At one site, we found no differences with age in foliar delta N-15 (range: 4.1-4.8 aEuro degrees) for seedlings, saplings, and trees which suggests that it may be possible to compare foliar delta N-15 of plants of different ages at some sites. However, at another site, foliar and root delta N-15 were higher for trees than 1-2 year-old seedlings. This study suggests that physiological differences in N assimilation and transport processes that differ by species likely control plant delta N-15.