Ecosystem N Distribution and d 15 N during a Century of Forest Regrowth after Agricultural Abandonment Jana E. Compton, 1, * Toby D. Hooker, 2 and Steven S. Perakis 3 1 U.S. Environmental Protection Agency, NHEERL, Western Ecology Division, 300 SW 35th St, Corvallis, Oregon 97333, USA; 2 Department of Biology, Utah State University, Logan, Utah 84322, USA; 3 U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97331, USA ABSTRACT Stable isotope ratios of terrestrial ecosystem nitrogen (N) pools reflect internal processes and input–output balances. Disturbance generally in- creases N cycling and loss, yet few studies have examined ecosystem d 15 N over a disturbance- recovery sequence. We used a chronosequence approach to examine N distribution and d 15 N during forest regrowth after agricultural aban- donment. 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 d 15 N decreased as stands aged, con- sistent with a progressive tightening of the N cycle during forest regrowth on agricultural lands. Over time, foliar d 15 N became more negative, indicating increased fractionation along the min- eralization–mycorrhizal–plant uptake pathway. Total ecosystem N was constant across the chro- nosequence, but substantial internal N redistribu- tion 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 d 15 N generally reflected a redistribution of depleted N from the mineral soil to the developing O horizon. Although plants and soil d 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 2& decrease in ecosystem d 15 N over the century scale suggests significant incorporation of atmo- spheric N, which was not detected by traditional ecosystem N accounting. Consideration of tempo- ral trends and disturbance legacies can improve our understanding of the influence of broader factors such as climate or N deposition on eco- system N balances and d 15 N. Key words: d 15 N; soil nitrogen; secondary suc- cession; root biomass; foliar nitrogen; chronose- quence; nitrogen isotopes; ecosystem nitrogen; white pine. INTRODUCTION Trends in nutrient dynamics over the course of ecosystem development are a fundamental re- search area in ecology (Vitousek and Reiners 1975; Bormann and Likens 1994; Crews and others 1995). Long-term trends involve many years of N inputs and losses from ecosystems, the balance of which can be reflected in plant and soil d 15 N (Ho ¨ gberg 1997; Austin and Vitousek 1998). Chan- ges in plant and soil d 15 N during primary succession have been well studied over time scales from sev- eral hundred to several million years (Vitousek and Electronic supplementary material: The online version of this article (doi:10.1007/s10021-007-9087-y) contains supplementary material, which is available to authorized users. Received 14 September 2006; accepted 26 July 2007; published online 18 September 2007. *Corresponding author; e-mail: compton.jana@epa.gov Ecosystems (2007) 10: 1197–1208 DOI: 10.1007/s10021-007-9087-y 1197