Biogeochemical Controls on Altered Nitrogen Cycling in the Third Decade of Whole - Watershed N Deposition

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This research will study nitrogen cycling in two paired, forested watersheds of the northeastern U.S. Scientists will experimentally increase the abundance of the rare, naturally occurring, stable isotope of nitrogen (15N) to track the movement of nitrogen from precipitation through trees, microbes, soils, soil solutions and finally into streams. This research will be carried out in two adjacent forested watersheds at the Bear Brook Watershed in Maine. One watershed has been treated with nitrogen fertilizer for 22 years to mimic air-pollution effects on forests and streams - one of the longest studies of its kind in the world. Nitrogen dynamics in forested watersheds are of intense scientific interest because nitrogen is typically the most limiting plant nutrient in land-based ecosystems. And yet, during the last half of the 20th century, emissions of nitrogen and sulfur from fossil-fuel combustion and other sources created "acid rain," causing the leaching of soil calcium and aluminum. These emissions also resulted in excess nitrogen inputs to forested watersheds, a condition referred to as nitrogen saturation, which yields increased amounts of nitrogen in streams draining forests. In the 21st century, nitrogen dynamics altered by climatic change will also influence forest health and downstream ecosystems.