Acid rain threatens future productivity of forests

The damaging effects of acid rain appear far sooner than scientists knew and threaten the future productivity of forests by quietly dissolving their food reserves in the soil, according to collaborative research by scientists from three US universities.

Acid rain was shown to accelerate the nutrient-robbing acidification of forest soils by a startling 38 percent at a site in South Carolina, where soil data were collected over three decades.

“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 sulphur 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 sulphur and nitrogen emissions come from power stations and traffic.

Acid rain strips the soil of nutrients and minerals as it percolates downward, gradually limiting productivity. It also hampers the soil’s ability to protect trees from toxic substances such as 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 southeast United States has been altered – tilled, fertilised 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 US. The Calhoun Experimental Forest, where US Forest Service scientists began recording soil data in 1957, provided an opportunity to study the relative importance of acid rain on soil acidification in a growing forest.

To reliably compare the contribution of various acid sources, researchers analysed 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.

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 sulphate levels over the past decade. They attribute the lower levels to stricter air quality standards, particularly the use of low-sulphur 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.”

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