Regional assessment of N saturation using foliar and root delta N-15

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar delta N-15 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 delta N-15 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 delta N-15 than the regional driver of N deposition. Foliar delta N-15 increased non-linearly with nitrification: mineralization ratio and decreased with forest floor C: N. Foliar delta N-15 was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root delta N-15 was more tightly coupled to forest floor properties than was foliar delta N-15. We observed a pattern of decreasing foliar delta N-15 values across the following species: American beech > yellow birch > sugar maple. Other factors that affected foliar delta N-15 included species composition and climate. Relationships between foliar delta N-15 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 delta N-15, due to the importance of ammonium deposition in this region. Our results suggest that examining delta N-15 values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.