Ecosystem N distribution and δ15N 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 N-15 over a disturbance-recovery sequence. We used a chronosequence approach to examine N distribution and delta N-15 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 N-15 decreased as stands aged, consistent with a progressive tightening of the N cycle during forest regrowth on agricultural lands. Over time, foliar delta N-15 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(-1)). Temporal trends in soil delta N-15 generally reflected a redistribution of depleted N from the mineral soil to the developing O horizon. Although plants and soil delta N-15 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 2 parts per thousand decrease in ecosystem delta N-15 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 N-15.