Species change and nitrogen retention in forests of the northeastern U.S.
Lovett, Gary*,1, Arthur, Mary2, Weathers, Kathleen1, Fitzhugh, Ross3, Templer, Pamela4, 1 Institute of Ecosystem Studies, Millbrook, NY2 University of Kentucky, Lexington, KY3 University of Illinois, Urbana, IL4 University of California, Berkeley, CA
ABSTRACT- In the forested watersheds of the northeastern US, retention of atmospheric nitrogen (N) deposition by vegetation and soils provides a major benefit by protecting surface waters from nitrate leaching. The amount of N retention is highly variable among watersheds and is primarily controlled by the rate of nitrification in forest soils. Nitrification rates are strongly influenced by the tree species composition of the watershed forest. In the Catskill Mountains of southeastern New York State, soils collected from single-species plots of five dominant tree species (sugar maple (Acer saccharum), red oak (Quercus rubra), yellow birch (Betula alleghaniensis), eastern hemlock (Tsuga canadensis), and American beech (Fagus grandifolia)) differ widely in net nitrification rates. Approximately 40% of the variation in net nitrification is explained by differences soil C:N ratio among the plots, but oak-dominated plots have near-zero net nitrification rates across a range of soil C:N ratios from 19-25. Plots dominated by sugar maple consistently have low soil C:N and high net nitrification rates. Overall, the species effect on net nitrification rates appears to be mediated by a combination of differences in N mineralization, soil C:N ratio, and specific nitrification inhibitors. Given that species composition has a strong influence on nitrification and nitrate leaching, we predict that species change will substantially alter the nitrogen-retention capacities of watersheds. Species change in the northeastern US is happening rapidly because of the effects of introduced forest pests, climate change, selective harvest, and, in some places, soil acidification. Changes that favor oak-dominated forests will result in lower nitrate leaching to surface waters, and changes that favor maple-dominated forests will result in higher nitrate leaching.
Key words: tree species, species change, nitrogen, nitrification
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