For more than 30 years, scientists in Europe and North America have recognized that acid rain could spur the loss of nutrients that are important for growing trees. Nutrients moving out of the soil into lakes and streams could also affect water quality. Nevertheless, observations that such losses have occurred have often been dismissed as extreme cases or as a result of natural changes in forested landscapes.
At the Bear Brook Watershed in Hancock County, Maine, a research team led by University of Maine scientists has now documented that under carefully controlled conditions, treating a watershed with additional acids accelerates the loss of two critical nutrients, calcium and magnesium.
"No one else has shown this at an ecosystem scale in this region," says Ivan Fernandez, UMaine professor of soil science and lead author of the paper. "It shows that we can experimentally induce (nutrient) depletion in a Maine forest with modest treatment." Co-authors were Lindsey Rustad of the USDA Forest Service; Stephen A. Norton and Steve Kahl, both of UMaine; and Bernard J. Cosby of the University of Virginia.
The Bear Brook Watershed Manipulation project began in the mid-1980s on land now owned by International Paper with funding from the U.S. Environmental Protection Agency. Located on Lead Mountain in Down East Maine, the site includes two side-by-side forested watersheds. Scientists constructed concrete weirs on each stream at the base of each watershed in collaboration with the U.S. Geological Survey. They installed continuous monitoring equipment to track changes in hydrology and water quality. In 1989, they began a bi-monthly routine of spreading ammonium sulfate, a commercial fertilizer, on the West Bear watershed to mimic high levels of acid rain. Subsequent studies have focused on changes to soil, water and vegetation on both the treated and untreated reference watersheds. What the Bear Brook research does not yet conclusively show, Fernandez adds, is whether the loss of soil nutrients is being balanced by gains from other processes in the untreated watershed. "It seems clear that the treatments have exceeded the natural supplies of nutrients in the treated watershed," says Fernandez. "Because there are no historical data on soils for comparisons, conclusions about the untreated watershed will require more time." Understanding the full nutrient picture in the untreated East Bear watershed would provide information that is representative of actual conditions in Maine and the Northeast.
"We can infer what is occurring from stream chemistry, and indeed, there appears to be a slow loss of base cations (nutrients) that may or may not be balanced by soil weathering processes," adds Fernandez. "Our treatment watershed suggests that whether it is happening or not across the Maine landscape, it will definitely happen with a little push."
An ongoing synthesis of data from acid rain research sites in North America and Europe includes the Maine research group and the Bear Brook site. Almost none of the watersheds show evidence of increasing nutrient concentrations in soils and surface waters, but many show evidence of a decreasing trend, says Fernandez. Results from the synthesis are still being developed.
The loss of nutrients due to acid rain is likely a regional phenomenon, although consequences for New England's forests, lakes and streams vary across the landscape. These effects may become increasingly important to forest health if predicted climate warming occurs, Fernandez adds. Acidic inputs of nitrogen and sulfur are likely to interact with temperature and moisture changes in forested ecosystems.