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

摘要

Noland Divide Watershed (NDW), locating in the Great Smoky Mountains NationalPark, is characterized as one of the watersheds to receive some of the highest acidicdeposition in the US. It has been continuously monitored for deposition, soil waterand stream water chemistry weekly and biweekly, since 1991. The long-term trendanalysis over past 17 years (1991-2007) found that decreasing sulfate and proton inthe precipitation, however stream sulfate concentration, pH and ANC did not showthe same significant trends. In contrast, atmospheric deposition of nitrogen was foundto be increasing over time, but stream nitrate concentration was observed to bedeclining, apparently due to increasing overstory vegetation uptake. This long-termstudy 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 soiladsorption. Ammonium was mainly converted to nitrate by soil mineralization andnitrification at surface soil layer, and combined with deposited nitrate to be uptakenby plant. Despite the decreasing nitrate concentration and stable sulfate concentration,stream recovery from acidification is not observed in the NDW, due to the depletionof base cations with decreasing stream base cation concentrations.