Friday, August 7, 2009 - 9:00 AM

COS 128-4: Patterns in δ15N in roots, stems, and leaves of sugar maple and beech seedlings, saplings, and mature trees

Linda H. Pardo1, Priscillia Semaoune2, Paul G. Schaberg1, Christopher Eagar3, and Mathieu Sebilo2. (1) USDA Forest Service, (2) Universite Pierre et Marie Curie, (3) USDA Forest Service, NRS

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.