Inorganic Nitrogen Losses from a Forested Ecosystem in Response to Physical, Chemical, Biotic, and Climatic Perturbations John D. Aber, 1 * Scott V. Ollinger, 1 Charles T. Driscoll, 2 Gene E. Likens, 3 Richard T. Holmes, 4 Rita J. Freuder, 1 and Christine L. Goodale 5 1 Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA; 2 Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York 13244, USA; 3 Institute of Ecosystem Studies, Millbrook, New York 12545, USA; 4 Biology Department, Dartmouth College, Hanover, New Hampshire 03755, USA; and 5 Woods Hole Research Center, Woods Hole, Massachusetts 02543, USA ABSTRACT Nitrate leaching to streams is a sensitive indicator of the biogeochemical status of forest ecosystems. Two primary theories predicting long-term (decadal) changes in nitrate loss rates (N saturation theory and the nutrient retention hypothesis) both predict increasing dissolved inorganic nitrogen (DIN) losses for watershed 6 (W6), the biogeochemical reference watershed at the Hubbard Brook Experimental For- est (HBEF). Measured values, however, have de- clined substantially since measurements began in the mid-1960s. Are these theories wrong, or are there other important controls on DIN losses at the annual to decadal time scale that have obscured the tendency toward higher losses over time? We tested the individual and combined effects of several forms of disturbance on DIN loss rates from northern hardwood forests by comparing predictions from a relatively simple model of forest carbon, nitrogen, and water dynamics (PnET-CN) with the long-term record of annual DIN loss from W6 at HBEF. Per- turbations tested include interannual climate vari- ation, changes in atmospheric chemistry (CO 2 ,O 3 , N deposition), and physical and biotic disturbances (two harvests, a hurricane salvage, and a defoliation event). No single disturbance caused changes in DIN losses to mimic measured values. Only when run with all of the disturbances combined did the model-predicted pattern of interannual change in DIN loss approach the measured record. Single- disturbance simulations allow an estimation of the role of each in the total pattern of DIN loss. We conclude that DIN losses from W6 were elevated in the 1960s by a combination of recovery from ex- treme drought and a significant defoliation event. N deposition alone, in the absence of other distur- bances, would have increased DIN losses by 0.35 g Nm -2 y -1 . These findings indicate that predictions of DIN losses must take into account the full spec- trum of disturbance events and changes in environ- mental conditions impacting the systems examined. Key words: model; PnET; climate; harvest; defoli- ation; CO 2 ; ozone; N deposition; nitrate; Hubbard Brook Experimental Forest; streams. INTRODUCTION Nitrate leaching to streams is a sensitive indicator of the biogeochemical status of forest ecosystems, and has been used as a primary response variable in studies of both forest management and atmospheric deposition (Gundersen and others 1998; Dise and others 1998; Aber and others 1998; Hornbeck and others 1986, 1997; Pardo and others 1995; Stod- dard 1994; Likens and others 1970). Two primary theories have been developed to predict long-term Received 9 October 2001; Accepted 27 February 2002. *Corresponding author; e-mail: John.aber@unh.edu Ecosystems (2002) 5: 648 – 658 DOI: 10.1007/s10021-002-0203-2 ECOSYSTEMS © 2002 Springer-Verlag 648