Is Acid Rain a Thing of the Past?
Calcium levels in the soil were still low, but aluminum in surface soils had begun to disappear—at least in Vermont, New Hampshire, and Maine; New York soils still sported high levels of the metal. “The way the soils were recovering was not really the way we expected,” Lawrence says.
Lawrence suggests a two-step explanation. First, less acid rain means less aluminum dissolving from minerals and circulating in the soil. Second, surface soils are being replenished by decaying plant matter, which has low levels of aluminum and is essentially diluting the concentration of the metal in soil. “This is a response to the declining acid rain levels,” Lawrence says. “It’s just being driven more by the plants than it is the geology.”
Calcium is not rebounding in the soil because the rocks at these sites, which are typical of the region, are not rich in the nutrient and weather very slowly, says Lawrence. That’s one reason the soils take so long to recover. In fact, calcium can buffer soils against some of the worst consequences of acid rain, but now—because there is so little calcium left to stand in the way of harmful chemical reactions such as the ones that mobilize aluminum— these soils “are actually more sensitive to acid rain today than they were 25 years ago,” he says. On their way to recovery these soils are hanging by a precarious thread.
The study “is the first to hint that the deterioration of northeastern U.S. soils from acidic deposition has finally bottomed out,” says Brenden McNeil, a biogeochemist at West Virginia University in Morgantown, who was not involved in the work. He points out, however, that the impacts of acid rain extend beyond northeastern spruce forests to areas where the extent of the damage and the status of the recovery remain unknown. A 2012 global acidification assessment reports, for example, that in Canada and Western Europe, sulfur dioxide emissions have declined at about the same rate as in the United States, but in places like China, sulfur dioxide emissions are now reaching levels that haven’t been seen in the U.S. since 1970. Even in the region studied, McNeil says, the subtle improvements in soil are “not near as dramatic as the reductions in emissions”—a sign that clearing the air of sulfur dioxide is just the first milestone on a long road to recovery.