The long-term effects of disturbance on organic and inorganic nitrogen export in the White Mountains, New Hampshire

Traditional biogeochemicaI 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 deposition 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 watersheds (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 averaged 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 successional status, but correlated with dissolved organic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N demand but instead were consistent with expected differences in DOC production and sorption. Despite 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 require several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner.