The Long-term Effects of Disturbance on Organic and Inorganic Nitrogen Export in the White Mountains, New Hampshire

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 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. Received 27 April 1999; accepted 30 May 2000.     

Effects of Holocene Alnus Expansion on Aquatic Productivity, Nitrogen Cycling, and Soil Development in Southwestern Alaska

Numerous pollen records provide evidence for the widespread range expansion of Alnus throughout Alaska and adjacent Canada during the middle Holocene. Because Alnus can fix atmospheric N₂, this vegetational change probably had a profound effect on N availability and cycling. To assess this effect, we analyzed a sediment core from Grandfather Lake in southwestern Alaska for a suite of geochemical indicators, including elemental composition, biogenic silica (BSi) content, and carbon (C) and nitrogen (N) isotopes of organic matter. These data, in conjunction with a pollen record from the same site, are used to infer biogeochemical processes associated with the mid-Holocene Alnus expansion. The increase in Alnus pollen percentages from 10% to 70% circa 8000-7000 BP (¹⁴C years before present) suggests the rapid spread of Alnus shrub thickets on mountain slopes and riparian zones in the Grandfather Lake region. Coincident with this vegetational change, the mean value of the sediment BSi content increases from 20.4 to 106.2 mg/g, reflecting increased diatom productivity within the lake as a result of Alnus N₂ fixation in the watershed soils and the associated N flux to the lake. Elevated aquatic productivity at this time is also supported by increased percentages of organic C and N, decreased C:N ratios, and decreased values of δ ¹³C. Furthermore, the δ ¹⁵N values of sediments increase substantially with the establishment of Alnus shrub thickets, suggesting enhanced N availability and accelerated N cycling within the lake and its watershed. Superimposed on a general trend of soil acidification throughout the postglacial period, soil acidity probably increased as a result of the Alnus expansion, as can be inferred from decreasing ratios of authigenic base cations to allogenic silica (Si) and increasing ratios of authigenic aluminum (Al) to allogenic Si. The ultimate cause of these mid-Holocene ecosystem changes was an increase in effective moisture in the region. Received 21 July 2000; accepted 3 January 2001.