N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar delta15N 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 delta15Nand 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 delta15N than the regional driver of N deposition. Foliar delta15N increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar delta15N was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root delta15N was more tightly coupled to forest floor properties than was foliar delta15N. We observed a pattern of decreasing foliar delta15N values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar delta15N included species composition and climate. Relationships between foliar delta15N 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 delta15N, due to the importance of ammonium deposition in this region. Our results suggest that examining delta15N values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.
