The Long-term Effects of Disturbance on Organic and Inorganic Nitrogen Export in the White Mountains, New Hampshire Christine L. Goodale, 1 * John D. Aber, 1 and William H. McDowell 2 1 Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA; and 2 Department of Natural Resources, University of New Hampshire, Durham, New Hampshire 03824, USA ABSTRACT Traditional biogeochemical theories suggest that ecosystem nitrogen retention is controlled by biotic N limitation, that stream N losses should increase with successional age, and that increasing N depo- sition will accelerate this process. These theories ignore the role of dissolved organic nitrogen (DON) as a mechanism of N loss. We examined patterns of organic and inorganic N export from sets of old- growth and historically (80 –110 years ago) logged and burned watersheds in the northeastern US, a region of moderate, elevated N deposition. Stream nitrate concentrations were strongly seasonal, and mean ( SD) nitrate export from old-growth wa- tersheds (1.4  0.6 kg N ha -1 y -1 ) was four times greater than from disturbed watersheds (0.3  0.3 kg N ha -1 y -1 ), suggesting that biotic control over nitrate loss can persist for a century. DON loss av- eraged 0.7 ( 0.2) kg N ha -1 y -1 and accounted for 28 – 87% of total dissolved N (TDN) export. DON concentrations did not vary seasonally or with suc- cessional status, but correlated with dissolved or- ganic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N de- mand but instead were consistent with expected differences in DOC production and sorption. De- spite decades of moderate N deposition, TDN export was low, and even old-growth forests retained at least 65% of N inputs. The reasons for this high N retention are unclear: if due to a large capacity for N storage or biological removal, N saturation may re- quire several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner. Key words: disturbance; dissolved organic carbon; dissolved organic nitrogen; fire; logging; nitrate leaching; nitrogen saturation; nitrogen retention; old growth; succession. INTRODUCTION Human activities have vastly changed the global nitrogen (N) cycle (Galloway and others 1995; Vi- tousek and others 1997). Nitrogen deposition to the eastern United States has increased at least five- to 10-fold since preindustrial times, and even higher N deposition rates occur in western Europe (Galloway and others 1995; Holland and others 1999). In or- der to anticipate the downstream consequences of this large change in N inputs, it is important to understand the mechanisms that control ecosystem N retention. Traditional biogeochemical theories suggest that ecosystem N losses are controlled by varying degrees of biotic N limitation, with N losses increasing as N availability exceeds plant and mi- crobial demand. Essentially: Received 27 April 1999; accepted 30 May 2000. *Corresponding author’s current address: Carnegie Institution of Washington, Department of Plant Biology, 260 Panama St., Stanford, California 94305, USA; e-mail: christy@jasper.stanford.edu Ecosystems (2000) 3: 433– 450 DOI: 10.1007/s100210000039 ECOSYSTEMS © 2000 Springer-Verlag 433