Soil Nitrification and Mineralization Rates Along an Elevation Gradient in the Great Smoky Mountains National Park

The Great Smoky Mountains National Park (GRSM) is an area sensitive to acid deposition. Although reports indicate there have been reductions of acid deposition in the eastern United States, water quality in streams has not recovered to perceived natural levels. Coupled soil biogeochemical processes of nitrification and nitrogen mineralization can acidify soil water and play a key role in the fate of nitrogen-based acid deposition and observed stream acidification. Characterizing nitrogen decomposition rates at different elevations improves our understanding of the potential effects of acid deposition and soil interactions with acid ions. Soil chemical properties and potential reaction rates for nitrification and mineralization among 36 sites in three GRSM watersheds were characterized by 28-day laboratory incubation experiments. In addition, relationships were identified by comparing soil chemistry to watershed characteristics including site location, soil characteristics, and geomorphic factors. Nitrification rates ranged between 1 and 177 μ[mu]eq kg-1 dry soil day-1, and mineralization rates ranged between 2 and 339 μ[mu]eq kg-1 dry soil day-1. For the three watersheds combined, mineralization and nitrification rates were significantly correlated with elevation. Mineralization was increasing at a rate of 0.1578 / 0.0816 μ[mu]eq kg-1 dry soil day-1 m-1 in the A and B/C soil horizons, and nitrification at 0.1269 / 0.0425 μ[mu]eq kg-1 dry soil day-1 m-1, in the A and B/C soil horizons. For individual watersheds, Cosby and the West Prong of the Little Pigeon shared this significant positive correlation while the Noland Divide watershed did not because sample sites were only located at higher elevations. Soil horizon class played a key role in controlling the nitrogen cycle processes, where the A soil horizon was found to be more dependent on total organic nitrogen, and the B/C soil horizon was more dependent on organic matter. Nitrification and mineralization rates were not correlated with site slope, organic matter to total organic nitrogen ratio, and A soil horizon depth. The study results illustrate that nitrification and mineralization play a significant part of the soil biogeochemical process that govern episodic stream acidification response in the GRSM.

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