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Seasonal Relationships Between Precipitation, Forest Floor, and Streamwater Nitrogen, Isle Royale, Michigan

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

David Toczydlowski

Dep. of Biological Sciences, Michigan Technological Univ., Houghton, MI 49931

*Corresponding author.

ABSTRACT

The Upper Great Lakes receive large amounts of precipitation-NH⁺₄ and moderate NO^-₃ inputs. Increased atmospheric inorganic N input has led to concern about ecosystem capacity to utilize excess N. This paper summarizes a 5-yr study of seasonal N content and flux in precipitation, snowpack, forest floor, and streamwater in order to assess the source of inorganic N outputs in streamflow from a small boreal watershed. Average precipitation N input was 3 kg ha^-1 yr^-1. The peak snowpack N content averaged 0.55 kg ha^-1. The forest floor inorganic N pool was 2 kg ha^-1, eight times larger than monthly precipitation N input. The inorganic N pool size peaked in spring and early summer. Ninety percent of the forest floor inorganic N pool was made up of NH⁺₄-N. Forest floor inorganic N pools generally increased with temperature. Net N mineralization was 15 kg ha^-1 yr^-1, and monthly rates peaked in early summer. During winter, the mean monthly net N mineralization rate was twice the peak snowpack N content. Streamwater NO^-₃ concentration peaked in winter, and inorganic N output peaked in late fall. Beneath the dominant boreal forest species, net N mineralization rates were positively correlated (P < 0.05) with streamwater NO^-₃ concentrations. Forest floor NO^-₃ pools beneath alder [Alnus rugosa (Du Roi) Spreng] were positively correlated (P < 0.01) to streamwater NO^-₃ output. At the watershed mouth, streamwater NO^-₃ concentrations were positively correlated (P < 0.05) with precipitation NO^-₃ input and precipitation amount. The relatively small snowpack N content and seasonal precipitation N input compared to forest floor inorganic N pools and net N mineralization rates, the strong ecosystem retention of precipitation N inputs, and the seasonal streamwater NO^-₃ concentration and output pattern all indicated that little streamwater NO^-₃ came directly from precipitation or snowmelt.

Received for publication January 22, 1998.

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