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TECHNICAL REPORT
Landscape and Watershed Processes

Biogeochemistry of a Treeline Watershed, Northwestern Alaska

U.S. Geological Survey, 240 W. Prospect Rd., Ft. Collins, CO 80526

* Corresponding author (robert_stottlemyer{at}usgs.gov)

Received for publication September 18, 2000. Since 1950, mean annual temperatures in northwestern Alaska have increased. Change in forest floor and soil temperature or moisture could alter N mineralization rates, production of dissolved organic carbon (DOC) and organic nitrogen (DON), and their export to the aquatic ecosystem. In 1990, we began study of nutrient cycles in the 800-ha Asik watershed, located at treeline in the Noatak National Preserve, northwestern Alaska. This paper summarizes relationships between topographic aspect, soil temperature and moisture, inorganic and organic N pools, C pools, CO₂ efflux, growing season net N mineralization rates, and stream water chemistry. Forest floor (O2) C/N ratios, C pools, temperature, and moisture were greater on south aspects. More rapid melt of the soil active layer (zone of annual freeze–thaw) and permafrost accounted for the higher moisture. The O2 C and N content were correlated with moisture, inorganic N pools, CO₂ efflux, and inversely with temperature. Inorganic N pools were correlated with temperature and CO₂ efflux. Net N mineralization rates were positive in early summer, and correlated with O2 moisture, temperature, and C and N pools. Net nitrification rates were inversely correlated with moisture, total C and N. The CO₂ efflux increased with temperature and moisture, and was greater on south aspects. Stream ion concentrations declined and DOC increased with discharge. Stream inorganic nitrogen (DIN) output exceeded input by 70%. Alpine stream water nitrate (NO^-₃) and DOC concentrations indicated substantial contributions to the watershed DIN and DOC budgets.

Abbreviations: DOC, dissolved organic carbon • DON, dissolved organic nitrogen • DIC, dissolved inorganic carbon • DIN, dissolved inorganic nitrogen