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

Background/Question/Methods

Increasingly, ¹⁵N measures in plant tissues are used to explain patterns of nitrogen (N) cycling in forest ecosystems. A number of critical assumptions are made in many analyses including that plants reflect the δ¹⁵N of the inorganic N available in the soil. Implicit in this assumption is that there is no fractionation of N on plant uptake or within-plant transport. For systems where root uptake is the only source of plant N, this means that root (and stem and leaf) δ¹⁵N would be the same as soil solution N that was taken up. In order to test these assumptions, we compared δ¹⁵N in root, stem and leaf tissue of individual seedlings of sugar maple (SM) and beech (BE) at the Hubbard Brook Experimental Forest (HBEF) in NH. We measured stem and leaf δ¹⁵N of SM saplings. We also collected paired root and foliage samples from 60 mature SM trees in order to evaluate differences between root and leaf δ¹⁵N within individual trees. Because all of the comparisons were made for individual plants rather than pooled samples or means, we were able to determine the extent to which δ¹⁵N differed with plant part, seedling age, and species.

Results/Conclusions Preliminary results showed that root δ¹⁵N >stem δ¹⁵N >leaf δ¹⁵N for SM and BE seedlings for all ages from cotyledon (root>shoot) to seedlings 7+ years. Similarly, for mature SM, root δ¹⁵N >leaf δ¹⁵N. These results suggest fractionation of N on within- plant transport (unless leaves and stems each have different N sources). For the youngest SM seedlings, root %N decreased with age from 5% for cotyledons to 2% for 3-year-old seedlings; δ¹⁵N did not change with age for these young seedlings. For plants growing at the same site, BE δ¹⁵N was consistently higher than SM δ¹⁵N for all plant parts, leaf, stem, and root. This pattern has been reported for foliage across northeastern North America and may suggest that these two species utilize N sources with different isotopic signatures or that they have differences in internal N physiology. At one site within the HBEF, SM δ¹⁵N values were higher than for another site studied. This suggests that local site characteristics (N cycling rate, species composition) may influence the values of δ¹⁵N. These results may have significant implications for the interpretation of foliar δ¹⁵N data and the underlying assumptions that govern patterns in plant δ¹⁵N.