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

这项研究将研究美国东北部两对森林流域的氮循环科学家将通过实验增加稀有，天然存在的稳定同位素氮（15 N）的丰度，以跟踪氮从降水通过树木，微生物，土壤，土壤溶液最终进入溪流的运动。 这项研究将在缅因州的熊溪流域的两个相邻的森林流域进行。 一个流域已经用氮肥处理了22年，以模拟空气污染对森林和河流的影响-这是世界上同类研究中时间最长的研究之一。 森林流域的氮素动态具有强烈的科学兴趣，因为氮素通常是陆地生态系统中最具限制性的植物营养素。 然而，在世纪后半叶，化石燃料燃烧和其他来源排放的氮和硫造成了“酸雨”，导致土壤中钙和铝的浸出。这些排放还导致了过量的氮输入到森林流域，这种情况被称为氮饱和，这会增加森林排水溪流中的氮含量。在世纪，气候变化改变的氮动态也将影响森林健康和下游生态系统。

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.