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

Recent studies have demonstrated that natural abundance N-15 can be a useful tool for assessing nitrogen saturation, because as nitrification and nitrate loss increase, delta N-15 of foliage and soil also increases. We measured foliar delta N-15 at I I 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 delta(15) N increased from -5.2 to -0.7 parts per thousand with increasing N deposition from Maine to NY. Foliar delta N-15 decreased between 1987-1988 and 1999, while foliar %N increased and foliar C:N decreased at most sites. Foliar delta(15) N 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. Published by Elsevier Ltd.