Understanding the acid-base chemistry of soil and the soil processes related to the release or retention of sulfate and nitrate is important in order to predict watershed recovery from long-term acid deposition. Soils were sampled from the Noland Divide Watershed (NDW), a small, high-elevation watershed in the Great Smoky Mountains National Park receiving high rates of acid deposition over several decades. Soil samples were measured for chemical properties related to acidification and used to conduct sulfate adsorption and nitrogen (N) incubation experiments. Shallow soil was higher in acidic and basic ions than deeper soils, and the mean effective cation exchange capacity was 8.07, 5.06, and 3.57 cmol ckg -1 in the A, Bw, and Cb horizons, respectively. In all three soil horizons, the base saturation was equal to or below 7% and the ratio of Ca/Al was below 0.01, indicating that the NDW is very sensitive to acid deposition. Based on results from sulfate adsorption isotherms, the NDW has not reached its maximum sulfate adsorption saturation and is likely able to retain further additions of sulfate. Desorption of sulfate from NDW soils is expected if sulfate concentrations in soil solution drop below 50 μeq L -1 but is highly dependent on soil pH and organic carbon content. Total soil organic N was 500 times greater than inorganic N in the A soil horizon, and net N mineralization and nitrification remained constant during a 28-day incubation indicating a large reservoir of N substrate for soil microbes. Nitrogen experiment results suggest that nitrate export from the watershed is largely controlled by biological processes rather than by nitrate deposition flux. Soil data collected in this study contributes to our understanding of biogeochemical processes affecting the response of acid-impacted ecosystems such as the NDW to future changes in atmospheric deposition.
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Acknowledgments This research was funded in part by the United States Department of Interior, National Park Service Cooperative Agreement Grant No. 1443-CA-5460-98-006 (Amendment 10), and the United States Environmental Protection Agency through the University of Tennessee Natural Research Policy Center, United States Environmental Protection Agency Grant No. EM-83298901-1. Special thanks are given to Galina Melnichenko for her instruction and measurement of organic nitrogen. We are thankful for the support of Dr. Nancy Finley, Natural Resource Research Director at the Great Smoky Mountain National Park. We appreciate the help of Joe Parker, Keil Neff, and Lee Mauney in field sampling work.
- Acidic deposition
- Biogeochemical processes
- Soil characteristics
- Southern Appalachian
- Sulfate adsorption