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Acid Deposition Methodology: Current Conditions

Acid deposition occurs when emissions of sulfur dioxide (SO₂) and oxides of nitrogen (NO_x) react with atmospheric water, oxygen, and oxidants to form acidic compounds. Mild solutions of nitric and sulfuric acids are formed and fall as acid precipitation. Sulfur and nitrogen deposition was first described as a problem in Europe in the early 19^th century and has been studied extensively in North America since the 1970s (Blancher 1991). Sulfur and nitrogen deposition can impair tree growth in several ways. They can leach calcium and magnesium from soils where base cation stores are very low, and the ability of the ecosystems to retain sulfur or nitrogen is minimal (McLaughlin and others 1998). Acid deposition may also involve the release of toxic elements such as aluminum from the soil, adversely affecting biological processes and living organisms (Malmer 1976). Nutrient loss and soil degradation have been observed in some hardwood forests (Swank and Vose 1997). However, pine, hardwood, and mixed (oak-pine) forests experience slower losses of base cation nutrients and degradation because of their ability to buffer sulfur and nitrogen deposition. These forests generally have large calcium pools that increase their ability to buffer acid deposition.

There is a wide range of sulfate deposition rates across the South (National Atmospheric Deposition Program 2000) (fig. 18.1). The mean regional sulfate deposition for 1999 was 11 pounds per acre, which is a 13-percent decrease in sulfur deposition from 1994 (National Atmospheric Deposition Program 2000). The highest regional sulfur values are in North Carolina and Tennessee (fig. 18.1). They are produced primarily in industrialized States in the northern part of the South.

Currently, forests in the South are exposed to a wide range of nitrogen deposition rates (National Atmospheric Deposition Program 2000) (fig. 18.2). The mean regional nitrogen deposition for 1999 was 10 pounds per acre, a 10-percent decrease in nitrogen deposition from 1994 (National Atmospheric Deposition Program 2000). The highest regional nitrogen values are generally located in the northern part of the South (fig. 18.2). Their sources are emissions from all 31 States east of the Mississippi River (Nash and others 1992).

For this discussion, the South has been divided into nine forest types according to various factors that include geographic location, precipitation, minimum and maximum air temperature, and soil conditions (more or less sensitive to acid precipitation) (fig. 18.3). Sensitive soils have low base cation stores, and the ecosystem has a low ability to retain sulfur or nitrogen, or both. Less sensitive soils are ones with high concentrations of base cations, high buffering capacity to sulfur and nitrogen deposition, and, normally, nitrogen deficiency. Within the region, the high-elevation spruce-fir forests are most sensitive to sulfur and nitrogen deposition. The least sensitive ecoregions are those covered primarily by hardwood, pine, and oak-pine forests. The sensitivity of a given region to acid precipitation depends on the ability of the rocks and soils to neutralize or buffer the acid. Soils derived from granite, which are low in calcium, are highly sensitive. Soils derived from limestone, which are high in calcium, are much more capable of buffering the acid.