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 δ¹⁵N reflects the δ¹⁵N 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 δ¹⁵N by tissue type would suggest fractionation on transport and assimilation of N. In order to evaluate differences between species and plant parts, we measured δ¹⁵N 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 δ¹⁵N > stem δ¹⁵N > leaf δ¹⁵N; for BE seedlings, root δ¹⁵N > stem δ¹⁵N and root δ¹⁵N > leaf δ¹⁵N. These differences suggest that fractionation occurs during plant transport and assimilation of N. Beech δ¹⁵N (root, stem, and leaf) was consistently higher than SM δ¹⁵N for 1–7 year-old seedlings. At one site, we found no differences with age in foliar δ¹⁵N (range: 4.1–4.8 ‰) for seedlings, saplings, and trees which suggests that it may be possible to compare foliar δ¹⁵N of plants of different ages at some sites. However, at another site, foliar and root δ¹⁵N 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 δ¹⁵N.