Understanding Water Quality Responses to Long-Term Acidic Deposition in a High-Elevation Southern Appalachian Watershed: A Focus on Soil Watershed Processes

Noland Divide Watershed (NDW), locating in the Great Smoky Mountains National Park, is characterized as one of the watersheds to receive some of the highest acidic deposition in the US. It has been continuously monitored for deposition, soil water and stream water chemistry weekly and biweekly, since 1991. The long-term trend analysis over past 17 years (1991–2007) found that decreasing sulfate and proton in the precipitation, however stream sulfate concentration, pH and ANC did not show the same significant trends. In contrast, atmospheric deposition of nitrogen was found to be increasing over time, but stream nitrate concentration was observed to be declining, apparently due to increasing overstory vegetation uptake. This long-term study has found mean annual detention of sulfate, nitrate and ammonium of 60%, 3% and 95% respectively in the watershed. Sulfate is accumulated in soil matrix by soil adsorption. Ammonium was mainly converted to nitrate by soil mineralization and nitrification at surface soil layer, and combined with deposited nitrate to be uptaken by plant. Despite the decreasing nitrate concentration and stable sulfate concentration, stream recovery from acidification is not observed in the NDW, due to the depletion of base cations with decreasing stream base cation concentrations.