Critical loads of sulfur and nitrogen and modeled effects of deposition reduction for forested ecosystems of Great Smoky Mountains National Park

Abstract

Great Smoky Mountains National Park (GSMNP) is subject to high levels of sulfur (S) and nitrogen (N) deposition, which can adversely affect forest vegetation and aquatic biota. We used multiple chemical criteria to calculate critical loads for S and N deposition (CL (S+N)) and nutrient N (CLnutN) and used the Very Simple Dynamic (VSD) model to predict the effects of deposition reduction scenarios on critical thresholds for four forested sites in GSMNP. Critical loads were exceeded for current deposition at three of four sites using critical thresholds of aluminum to base cations (Al:Bc) = 0.1 or no decrease in base saturation but were not exceeded using the chemical criteria of Al = 0.2 meq L-1, Al:Bc = 1.0, and pH = 4.2. With deposition reductions of 48 percent S and 56 percent nitrate (NO3‾), neither the critical thresholds of acid neutralizing capacity (ANC) = 20 μeq L-1 nor a decrease in base saturation was achieved. With deposition reductions of 90 percent S and 90 percent NO3‾, ANC = 100 μeq L-1 was achieved at a single site. Historical ANC values affected a site's ability to achieve ANC critical thresholds. The critical threshold for soil solution NO3‾ was exceeded for all but the most stringent deposition scenarios (–90 percent S and –90 percent NO3¯). Deposition reductions of 90 percent for NO3‾ and 80 percent for ammonium (NH4+) were not sufficient to lower deposition below the CLnutN at all sites. Data indicate that upper sites at GSMNP are N saturated; to protect these sites from acidification, the more protective chemical criteria of no decrease in base saturation and Al:Bc = 0.1 should be used when determining critical loads. When choosing chemical criteria in deposition reduction modeling, care should be taken to ensure that the criteria chosen will protect sensitive ecosystem elements.