Three decades of observed soil acidification in the calhoun experimental forest: Has acid rain made a difference?

Three decades of repeated soil sampling from eight permanent plots at the Calhoun Experimental Forest in South Carolina allowed us to estimate the rate of soil acidification, the chemical changes in the soil exchange complex, and the natural and anthropogenic sources of acidity contributing to these processes. During the first 34 yr of loblolly pine (Pinus taeda L,) forest growth, soil pH, (pH in 0.01 M CaCl2) decreased by 1 unit in the upper 0- to 15-cm of soils and by 0.4 and 0.3 units in the 15- to 35- and 35- to 60-cm layers, respectively. Throughout the 0- to 60-cm horizon, base ration depletion averaged 1.57 kmol(c) ha(-1) yr(-1) and effective and total acidity increased by 1.26 and 3.28 kmol(c) ha(-1) yr(-1), respectively. A forest H+ budget estimated for these decades indicated that 38% of soil acidification was due to acid deposition, while 62% of soil acidification was attributed to the internal functioning of the ecosystem. Soil samples archived during the three-decade experiment also document decreases in soil-adsorbed SO42-, presumably in response to decreasing atmospheric inputs in recent years.