REGIONAL ASSESSMENT OF N SATURATION USING FOLIAR AND ROOT δ15N

Abstract

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.