Skip to main content
ARS Home » Southeast Area » Fayetteville, Arkansas » Poultry Production and Product Safety Research » Research » Research Project #431997

Research Project: Effects of Tree Species, Ammonia Deposition, and Acid Rain on Soil Acidity in the Mulberry River Watershed, Arkansas

Location: Poultry Production and Product Safety Research

Project Number: 6022-63000-006-007-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Jul 1, 2017
End Date: Jun 30, 2022

To determine the relative effects of forest species, atmospheric ammonia deposition and acid rain on soil acidification in the Mulberry River Watershed in Arkansas.

The Mulberry River, which is located in the Boston Mountain Ecoregion of northwest (NW) Arkansas, was listed in 2008 on the Arkansas 303(d) list of impaired waterbodies due to low pH. The objective of this research is to determine the amount of acidity generated by three potential sources; (1) converted hardwood forests to pine, (2) atmospheric deposition of ammonia, and (3) acid rain. Large areas of the watershed are being converted from hardwood to pine forest and the chemical properties of leaf litter of these trees differ considerably, hence, an increase in pine forests could explain the increased soil and water acidity. Another potential cause of soil acidification is the atmospheric deposition of ammonium from animal agriculture, since roughly 700 million broiler chickens are grown in NW Arkansas. Ammonium deposition does not directly lower the pH of soil and water. However, when ammonium is nitrified in the soil or water by bacteria in the process of nitrification, it results in the production of hydrogen ions, thereby lowering the pH. While acid rain has been greatly reduced since the passage of the Clean Air Act, it still exists and may also be a cause of acidification. We will first determine the effects of forest species on soil acidification. Soil acidification measurements will be made on soils in existing hardwood forests and adjacent paired pine forests of various age, stand densities, etc., located within the same or similar soil types. This research will be conducted within the Mulberry River Watershed at ten sites. At each of these 10 sites soil samples will be taken from 10 locations in pine forest and from 10 matching locations in deciduous forest. Three soil samples will be taken from each site, including; (1) leaf litter, (2) top 5 cm of mineral soil, and (3) 5-15 cm of mineral soil. Soil samples will be analyzed for pH, acid neutralizing capacity, water soluble and exchangeable aluminum (Al), calcium (Ca), magnesium (Mg), Na, K, ammonium, nitrate, chloride, sulfate, and organic carbon. Soils will also be analyzed for total acidity, cation exchange capacity, and percent base saturation. Significant differences at each site will be determined using a paired t test. We will also 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.