PARENT SESSION
Organized Oral Session 48: Nitrogen cycling in terrestrial ecosystems: New insights from isotopes and experiments
Organizer(s): MC Mack, P Templer, and K Nadelhoffer
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

What drives species differences in foliar ¹⁵N?

Pardo, Linda^*,1, Templer, Pamela², Goodale, C³, Groffman, P⁴, ¹ USDA Forest Service, Burlington, VT² Boston University, Boston, MA³ Cornell University, Ithaca, NY⁴ Institute of Ecosystems Studies, Millbrook, NY

ABSTRACT- Measurements of ¹⁵N natural abundance of plant tissues are often used as a tool for assessing the N cycling status of forest ecosystems. Foliar ¹⁵N is a more useful measure than foliar N concentration alone, because it integrates N cycling processes over space and time, but does not vary year to year, with crown position, or with needle age as much as foliar N concentration. In this synthesis study, we assembled a large dataset of foliar ¹⁵N coupled with measures of N deposition, and soil N cycling (C:N and net nitrification and mineralization) from over 500 plots in northeastern North America. We assessed the prevalence of a previously observed pattern of species rank of foliar ¹⁵N: American Beech (Fagus grandifolia)>Yellow Birch (Betula alleghaniensis) >Sugar Maple (Acer saccharum). We evaluated this pattern in both litter and fresh green foliage at 51 plots where these three species co-occurred across northeastern North America: Catskill and Adirondack Mountains, NY; White Mountains, NH; sites in southeastern Ontario; and eastern Maine. In all plots, sugar maple foliar ¹⁵N was lower than beech and yellow birch, although foliar ¹⁵N of sugar maple varied from -5 to 0 per mil. We evaluated the relative importance of the following factors in influencing species foliar ¹⁵N patterns: 1. ammonium versus nitrate uptake preference, 2. mycorrhizal association, 3. rooting depth, 4. differences in phenology. In addition, we observed differences between species across gradients of N deposition and forest floor C:N. For example, sugar maple and red maple (Acer rubrum) ¹⁵N values were more strongly correlated with N deposition than American beech and yellow birch. American beech and yellow birch foliar ¹⁵N was correlated with forest floor C:N, but not N deposition. Red maple was also more strongly correlated to forest floor nitrification than American beech. These differences in foliar ¹⁵N across species along these gradients underscore the influence that species have on (or express through) N cycling patterns.

Key words: nitrogen deposition, nitrification, soil C:N