Nitrogen Cycling in Watersheds

For about 25 years, Maine scientists have been spreading fertilizer on a forested watershed east of the town of Aurora. Why? Not to make the forest in the Bear Brook Watershed grow faster – though this fertilizer contains nitrogen, the most common limiting nutrient in our forests. Instead, the forest where this fertilizer spreading occurs is a research watershed where, in the late 1980s, scientists began studying the effects of acid rain. Acid rain is created when a mixture of sulfur and nitrogen compounds, produced primarily by the burning of fossil fuels, is washed out of the atmosphere and deposited on watersheds - often watersheds remote from where the fossil fuels were burned. When acid rain brings excess sulfur and nitrogen to a watershed, lakes and streams become more acidic; and everything on the land, from the smallest soil organisms to largest trees, become stressed.

In the 1980s, scientists determined that sulfur was the major acidifying compound in acid rain and, as a result, implementation of government laws and regulations in the 1990s reduced the amount of sulfur coming from power plants, and therefore raining down on watersheds. However, nitrogen levels in acid rain have not been reduced – partly because there are more sources of nitrogen than sulfur, and partly because nitrogen was not targeted for substantial reduction by those laws and regulations. Nitrogen is usually a limiting nutrient for plants; historically this has been particularly true in Northeastern forests that have survived with very limited amounts of nitrogen. And yet, additional nitrogen in a watershed can acidify streams and lakes without being any less limiting to plant growth. So, what does happen - to trees, microbes, streams and lakes- if too much nitrogen from these human-caused sources is deposited on a landscape that is used to having not enough nitrogen? And because ecological systems like watersheds are complex, for different parts of the system ‘too much’ means different amounts.

Bear Brook scientists are beginning important new research this year to track nitrogen through two side-by-side watersheds and figure out how it moves through the watershed - through the trees, soils, waters and where it ends up. The scientists will also look at the speed of nitrogen movement and whether a changing climate will have an effect on the speed of nitrogen cycling. The project uses two watersheds – one where the fertilizer, ammonium sulfate, has been added for 22 years, and one where no ammonium sulfate has been added. The fertilizer is rich in a stable isotope of nitrogen, enabling the scientists to track where the nitrogen goes within the watershed and how it is processed. This “paired watershed” experimental design permits scientists to investigate what happens in both an acidified and an un-acidified (reference) watershed through time.  Because the watersheds are right next to each other and most of their characteristics are nearly identical, the paired watershed design allows scientists to see how things work now (reference watershed) and how they work in a changed system (acidified).

Acadia Learning is developing a nitrogen citizen science project to accompany the research. We will help the Bear Brook project scientists understand more about how nitrogen moves in other forested watersheds in different seasons of the year – information that will help them interpret patterns in nitrogen through time and space.