N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar δ¹⁵N 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 δ¹⁵N 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 δ¹⁵N than the regional driver of N deposition. Foliar δ¹⁵N increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar δ¹⁵N was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root δ¹⁵N was more tightly coupled to forest floor properties than was foliar δ¹⁵N. We observed a pattern of decreasing foliar δ¹⁵N values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar δ¹⁵N included species composition and climate. Relationships between foliar δ¹⁵N 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 δ¹⁵N, due to the importance of ammonium deposition in this region. Our results suggest that examining δ¹⁵N values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.