H.: The long-term effects of disturbance on organic and inorganic nitrogen export in the White Mountains

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 (6 SD) nitrate export from old-growth wa-tersheds (1.4 6 0.6 kg N ha21 y21) was four times greater than from disturbed watersheds (0.3 6 0.3 kg N ha21 y21), suggesting that biotic control over nitrate loss can persist for a century. DON loss av-eraged 0.7 (6 0.2) kg N ha21 y21 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