Influence of acidic atmospheric deposition on soil solution composition in the Daniel Boone National Forest, Kentucky, USA

Acid atmospheric deposition may enter an environmental ecosystem in a variety of forms and pathways, but the most common components include sulfuric and nitric acids formed when rain water interacts with sulfur (SO_x) and nitrogen (NO_x) emissions. For many soils and watersheds sensitive to acid deposition, the predominant chronic effect appears to be a low pH, loss of base cations, and a shift in the mineral phase controlling the activity of Al³⁺ and/or SO₄^2- in solution. Soil solutions from lysimeters at various depths were taken at two sites in the Daniel Boone National Forest, Kentucky, USA, to evaluate potential impacts caused by acid deposition. The sites chosen were in close proximity to coal-burning power plants near Wolfe and McCreary counties and contained soils from the Rayne and Wernock series, respectively. Physicochemical characteristics of the soils revealed that both sites contained appreciable amounts of exchangeable acidity in the surface horizons, and that their base saturation levels were sufficiently low to be impacted adversely by acidic inputs. Soil solution data indicated that the sites were periodically subjected to relatively high NO₃^- and SO₄^2- inputs, which may have influenced spatial and temporal variation in Al and pH. As a consequence, the formation of Al-hydroxy-sulfate minerals such as jurbanite, alunite and basaluminite were thermodynamically favored over gibbsite. Given these conditions, longterm changes in soil solution chemistry from acid deposition are acknowledged.