Phosphorus Transport and Variability in Two Louisiana Coastal Plain Soils
Soil phosphorus (P) build-up from long-term application of poultry litter may increase P loss in surface and subsurface water and lead to eutrophication. Anoxic conditions during water saturation may aggravate the problem by increasing P solubility and mobility. However, the same level of soil P at different locations may not yield equal runoff P if the capacity of the soil to retain P varies across the landscape. This study examined: 1) effects of soil water oxygenation and concentration of soil organic matter (SOM) on development of anoxic conditions leading to decreased P sorption and increased mobility in a Ruston (fine-loamy, siliceous, thermic Typic Paleudult) soil; and 2) the spatial variability of P sorption parameters (Langmuir sorption maximum, initial isotherm slope and sorption at solution concentration = 1 mg/L) in Ruston and Darley (fine, kaolinitic, thermic Typic Hapludult) soils.
Batch Langmuir isotherms were developed for Ruston pasture (high SOM) and forest (low SOM) soils. Phosphorus movement through duplicate 5 cm long x 4.6 cm diameter columns of water-saturated soils with oxic or anoxic input solution was compared to P mobility predicted using the batch isotherms. The Eh of effluent (flow velocity ~ 2.5 cm/hr) indicated rapid development of reducing conditions regardless of input oxgenation. However, lower Eh values occurred in the pasture soil. Similar Ehs developed in the batch systems so that redox effect on P sorption was implictly included in transport predictions. Accordingly, predicted and measured elution of a 200 mg/L P pulse were generally consistent, particularly for the higher SOM pasture soil. Whether discrepancies between predicted and measured P elution were due to precipitation reactions was examined using MINTEQ, but results were inconclusive.
Surface 0 to 5 cm samples of Ruston and Darley soils were taken on square grids (60 x 60 m and 30 x 30 m, respectively) and Langmuir isotherms developed for every location. Soil pH and SOM were also determined. Isotherm parameters were spatially correlated and well-described by exponential or Gaussian semivariograms. Correlations of P sorption parameters with pH or SOM were inconsistent between sites except for the relationship of P sorption maximum with SOM.
Advisor:Jim Wang; Gary Breitenbeck; Lewis Gaston
School:Louisiana State University in Shreveport
School Location:USA - Louisiana
Source Type:Master's Thesis
Date of Publication:11/12/2003