Saturday, 15 July 2006
170-6

Phosphorus Sequestration Ability of Soil in the Upper North Bosque River Watershed.

Landon Darilek, David Weindorf, and Anil Kumar Somenahally. Tarleton State Univ, 402 E. Clifton St., Stephenville, TX 76401-4918

The Upper North Bosque River Watershed is an impaired watershed in Erath County due to high concentrations of dairy effluent runoff. This runoff is high in water-soluble Phosphorus (P), a major contributor to eutrophication. Dairies are looking for economical solutions for managing large volumes of manure in environmental friendly methods. Current practices include spreading effluent from ponds via pivot systems to fields, which has, over time, contributed to a build-up of P in these fields. Previous studies suggest that some soils have physical and chemical properties which would give them an ability to sequester P more effectively than others. A study was conducted at Tarleton State University to test the P sequestration ability of the fifteen most extensive soil series in the Upper North Bosque River Watershed. From each soil series, samples were collected by taking 3 cores at a depth of 1-18cm (topsoil), and from the same core-sites, three cores were made at a depth of 19-36cm (subsoil). Samples were placed in 60cm columns with a 7.62cm diameter to a height of 36cm. They were packed with oven-dried and mechanically ground soil by being tapped lightly on a table until no settling could be observed. Three columns were constructed for each soil series and were flushed with 250mL of manure effluent containing 52.9 mg/kg P concentration and then flushed with 1L deionized H2O. Leachate was collected from the bottom of the columns, filtered through a 0.45-ìm membrane, acidified with HCl, centrifuged, and analyzed for total P using inductively-coupled plasma atomic-emission spectroscopy. Samples were analyzed for particle size, pH, electric conductivity, organic matter percent, calcium carbonate percent, bulk density, particle density, and porosity to determine which parameters contributed to P sequestration. Ninety three percent of the soils tested showed an increase in post-leached P concentration and four soils showed particular affinity for sequestering P while some soils seemed to dissolve P as effluent was added. The soil series that proved effective at sequestering P were the Lewisville, Purves, Dugout and Bracket. All of these soils have appreciable clay content with very high calcium carbonate percentages and high cation exchange capacities. These series are distributed widely across the county and comprise more than one-fourth of the soils in Erath County, providing viable alternatives for manure application.

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