Acid rains falling over a large portion of Europe and the northeastern United States. Sulfur oxides and nitrogen oxides are, injected into the atmosphere in large quantities by the burning of fossil fuels. Those oxides change in chemical reactions in the atmosphere into strong acids--sulfuric and nitric, respectively. Unless they are neutralized in reactions with alkaline compounds in the atmosphere, those acids eventually return to Earth in rain. Rainfall in unpolluted areas is normally slightly acid (pH usually is between 5.5 and 6.5) because water and carbon dioxide combine in the atmosphere to form carbonic acid, a weak acid. In the 1970s, the pH of rain over much of the northeastern United States was less than 4, values from 3 to 3.5 were not unusual, and one rain with a measured pH of 2.1 was recorded. (Dunlop, Stewart and Michael Jackson, 1991)
Acid rain may be even more damaging to forests than we van thought. Not only does acid rain wilt and destroy trees' leaves and needles, but new research shows it also leaches their life-sustaining metal nutrients from topsoil.
The news comes from University of California-Riverside scientists who experimented on trees in Chile's pristine forests--far removed from industrial pollutants--to learn how forest ecosystems are supposed to work if left alone. The scientists were surprised to discover that trees obtain their essential metal elements (such as potassium, magnesium, and calcium) almost exclusively from atmospheric deposits in the soil, not from weathered rocks buried deep in the soil. And they were alarmed to learn just how small a pool of these nutrients is available to nourish the trees.
The small size of this upper soil nutrient pool has important implications for industrially influenced forests in the northeastern United States and in Europe. These forests may be more vulnerable to the effects of acid rain than we had previously thought........
