Ecosystem N Distribution and delta super(15)N during a Century of Forest Regrowth after Agricultural Abandonment

Stable isotope ratios of terrestrial ecosystem nitrogen (N) pools reflect internal processes and input-output balances. Disturbance generally increases N cycling and loss, yet few studies have examined ecosystem delta super(15)N over a disturbance-recovery sequence. We used a chronosequence approach to examine N distribution and delta super(15)N during forest regrowth after agricultural abandonment. Site ages ranged from 10 to 115 years, with similar soils, climate, land-use history, and overstory vegetation (white pine Pinus strobus). Foliar N and delta super(15)N decreased as stands aged, consistent with a progressive tightening of the N cycle during forest regrowth on agricultural lands. Over time, foliar delta super(15)N became more negative, indicating increased fractionation along the mineralization-mycorrhizal-plant uptake pathway. Total ecosystem N was constant across the chronosequence, but substantial internal N redistribution occurred from the mineral soil to plants and litter over 115 years (>25% of ecosystem N or 1,610 kg ha super(-1)). Temporal trends in soil delta super(15)N generally reflected a redistribution of depleted N from the mineral soil to the developing O horizon. Although plants and soil delta super(15)N are coupled over millennial time scales of ecosystem development, our observed divergence between plants and soil suggests that they can be uncoupled during the disturbance-regrowth sequence. The approximate 2ppt decrease in ecosystem delta super(15)N over the century scale suggests significant incorporation of atmospheric N, which was not detected by traditional ecosystem N accounting. Consideration of temporal trends and disturbance legacies can improve our understanding of the influence of broader factors such as climate or N deposition on ecosystem N balances and delta super(15)N.