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United States Department of Agriculture

Agricultural Research Service

Research Project: Assessing the Potential Causes of Soil and Water Acidification in the Mulberry River Watershed, Arkansas

Location: Poultry Production and Products Safety Research

Project Number: 6022-63000-003-02
Project Type: Specific Cooperative Agreement

Start Date: Aug 01, 2014
End Date: Jun 30, 2019

Objective:
Investigate the effect of increasing pine density (decreasing hardwood density) and atmospheric nitrogen deposition on soil water and stream pH in the Mulberry River and its tributaries.

Approach:
The Mulberry River was listed in 2008 on the Arkansas 303(d) list of impaired waterbodies due to low pH. The river is located in the Boston Mountain Ecoregion in Northwestern Arkansas. This region is characterized by shallow soils with low to moderate amounts of calcium carbonate, which make them poorly buffered against acid-base reactions. The objective of this research is to explore the significance of two major watershed changes and their effect on river pH. One potential cause of acidification is the shift in dominate forest species from deciduous hardwood forest to coniferous pine forest. The chemical properties of the leaf litter from these trees differs considerably, and an increase in pine litter could explain the increased soil and water acidity. Another potential cause of acidification is the atmospheric deposition of ammonium from high-intensity animal agriculture operations. The deposition of ammonium does not directly lower the pH of soil and water. However, this ammonium is quickly nitrified by soil and water bacterial communities, and the process of nitrification results in the production of hydrogen ions, thereby lowering the pH. Our approach is first to determine if pine density or nitrogen concentrations and isotopic composition correlate with lowered pH in the Mulberry River. We will derive a tributary sampling program focused on tributaries that have differing densities of second-growth pine in their watersheds. We will also use the linear distance to major animal agriculture operations to select sample sites that are more and less likely to have substantial atmospheric nitrogen deposition. Once the sampling locations have been identified, we will sample streams on a monthly basis for two years. Field instrumentation will be used to measure stream temperature, specific conductivity, dissolved oxygen, and pH. Grab water samples will be collected and analyzed for dissolved organic and inorganic carbon, acidity, alkalinity, total nitrogen and phosphorus, dissolved organic nitrogen and phosphorus, nitrate plus nitrite, ammonium, soluble reactive phosphorus and soluble metals. In addition, we will measure the isotopic composition of nitrate (N-15 concentration) in order to source track the nitrogen from human or animal waste. We will develop regression models between watershed metrics such as percent pine cover and in-stream pH conditions in order to test the hypothesis that increased pine densities are lowering stream pH. We will also develop regression models between other in-stream measurements such as nitrate concentration and nitrate N-15 concentration in order to test the hypothesis that lower stream pH is correlated with an increase in the signature of animal waste in stream water.

Last Modified: 12/19/2014
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