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