Acid Rain Threatens Future Productivity Of Forests, Research Shows

"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. The researchers, who also include Daniel D. Richter, of Duke University and H. Lee Allen and J. Byron Urreggo, of North Carolina State University, conducted the work at the Calhoun Experimental Forest near Spartanburg, South Carolina. One of the few long-term studies to look at the effects of acid rain on a forested site, findings were published in the October 1998 issue of The Soil Science Society of America Journal.

Acid rain forms when emissions of sulfur dioxide and nitrogen oxide react with water and oxygen in the atmosphere to produce acidic compounds. These compounds float to the ground, either in dry form as gas particles or in wet form as rain, fog or snow. The vast majority of sulfur and nitrogen emissions come from electric utility plants, automobiles and other forms of transportation.

Acid rain doesn't usually kill trees directly. Instead, it strips the soil of nutrients and minerals as it percolates downward, gradually limiting productivity. It also hampers the soil's ability to buffer trees from toxic substances such as aluminum and other heavy metals. Markewitz said the uncertainty among scientists about acid rain's effects stems from the difficulty in distinguishing between natural and human-made sources of soil acidity.

"The soil in most places throughout the southeast has been altered -- tilled, fertilized and limed over long periods, from decades to centuries," he said. "Also, much of the acid in soils, especially in an already acid forested environment, is produced in the natural processes of tree growth and the breakdown of soil organic matter."

Markewitz said soil acidity must be measured repeatedly over long periods to get reliable data. But few studies of this type have been conducted in the U.S. The Calhoun Experimental Forest, where U.S. Forest Service scientists began recording soil data in 1957, provided a rare opportunity to study the relative importance of acid rain on soil acidification in a growing forest.

"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. But how damaging is that change to the future productivity of the forest?

"The effects of acid rain are incipient and so is the level of concern about it," said Markewitz. "But 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 phosphorus, but also minerals such as calcium and potassium to ensure sustainable production."

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-sulfer 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."