Regional Assessment of N Saturation using Foliar and Root ?15 N

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order
to evaluate whether foliar d
15
N may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we
assembled a large dataset on atmospheric N deposition, foliar and root d
15
N and N concentration, soil C:N, mineralization and
nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers
of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar d
15
N than the
regional driver of N deposition. Foliar d
15
N increased non-linearly with nitrification:mineralization ratio and decreased with forest
floor C:N. Foliar d
15
N was more strongly related to nitrification rates than was foliar N concentration, but concentration was more
strongly correlated with N deposition. Root d
15
N was more tightly coupled to forest floor properties than was foliar d
15
N. We
observed a pattern of decreasing foliar d
15
N values across the following species: American beech>yellow birch>sugar maple. Other
factors that affected foliar d
15
N included species composition and climate. Relationships between foliar d
15
N and soil variables were
stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of
lower foliar d
15
N, due to the importance of ammonium deposition in this region. Our results suggest that examining d
15
N values of
foliage may improve understanding of how forests respond to the cascading effects of N deposition.