Regional Assessment of N Saturation using Foliar and Root $$\varvec {\delta}^{\bf 15}{\bf N}$$.

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

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