Regional patterns in foliar 15N across a gradient of nitrogen deposition in the northeastern US

Recent studies have demonstrated that natural abundance 15N can be a useful tool for assessing nitrogen saturation, because as nitrification and nitrate loss increase, δ15N of foliage and soil also increases. We measured foliar δ15N at 11 high-elevation spruce-fir stands along an N deposition gradient in 1987–1988 and at seven paired northern hardwood and spruce-fir stands in 1999. In 1999, foliar δ15N increased from −5.2 to −0.7‰ with increasing N deposition from Maine to NY. Foliar δ15N decreased between 1987–1988 and 1999, while foliar %N increased and foliar C:N decreased at most sites. Foliar δ15N was strongly correlated with N deposition, and was also positively correlated with net nitrification potential and negatively correlated with soil C:N ratio. Although the increase in foliar %N is consistent with a progression towards N saturation, other results of this study suggest that, in 1999, these stands were further from N saturation than in 1987–1988.