Acid rain works fast, study finds
March 12, 1999
Acid rain dissolves forest nutrients much faster than previously believed, posing a threat to future forest productivity, according to a study conducted by researchers from three universities in the southeastern United States.
The study shows that acid rain accelerated the nutrient-robbing acidification of forest soils by 38 percent at the Calhoun Experimental Forest in South Carolina, where soil data was collected over the past 30 years.
"We found a dramatic increase in acidity and a steady depletion of nutrients over this 30-year period," said Daniel Markewitz, a soil scientist in the University of Georgia's Warnell School of Forest Resources.
Acid rain forms when emissions of sulfur dioxide and nitrogen oxide -- primarily from electric utility plants and automobile emissions -- react with water and oxygen in the atmosphere to produce acidic compounds. These compounds float to the ground, either in a dry form as gas particles or as precipitation, and strip the soil of nutrients and minerals as they percolates downward, gradually limiting forest productivity.
Acid rain also hampers the soil's ability to buffer trees from toxic substances such as aluminum and other heavy metals.
Researchers have been uncertain about the effects of acid rain because they have trouble distinguishing between human-made and natural sources of soil acidity. The study at the Calhoun Experimental Forest provided researchers with a rare opportunity to obtain reliable data because the forest service began recording soil data in 1957.
"Most studies on acid rain have looked at the soil from one to 10 years or less," said Markewitz, "where we couldn't reliably quantify the chemical changes in the soil exchange complex."
To reliably compare the contribution of various acid sources, researchers analyzed what they call the hydrogen ion budget for the Calhoun forest. This involves calculating net gains and losses in hydrogen by measuring accumulated nutrients, tree root respiration and organic acids produced from the breakdown of pinestraw, roots and litter.
"The great advantage of hydrogen ion budgets is that they integrate information about many chemical and biological processes into a single parameter for comparison," said Markewitz. "In addition, we had the great advantage of soil samples archived from all collection years to evaluate changes in the soil exchange complex and the potential accumulation of sulfate, the predominant component of acid rain within the soil profile."
Changes from 1962 to 1990 were dramatic. Soil pH, the measure of a compound's relative acidity or alkalinity, decreased by as much as one unit in the top 14 inches of soil and by half that amount in the lower 14 inches. A one pH unit decrease indicates a 10-fold increase in hydrogen concentration.
"This study shows us that as we continue to push growth in managed forests, we will have to add not just major nutrients like nitrogen and phosphorous, but also minerals such as calcium and potassium to ensure sustainable production," said Markewitz.
Researchers did find a decrease in soil sulfate levels over the past decade. They attribute the lower levels to stricter air quality standards, particularly the use of low-sulfur coal.
"Soil scientists predicted this improvement more than a decade ago," said Markewitz, "and it has shown us that following sound science will benefit the health of the forest ecosystems in the long run."
The findings of this study were published in the October 1998 issue of The Soil Science Society of America Journal.
Copyright 1999, Environmental News Network, All Rights Reserved
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