Seasonal relationships between precipitation, forest floor, and streamwater nitrogen, Isle Royale, Michigan

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.