The role of iron-rich sediment in the biogeochemical cycling of phosphorus in Georgia Piedmont impoundments
Parker, Amanda Kaye
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Phosphorus cycling in lake systems can be significantly affected by the iron cycle. Iron oxides bind inorganic phosphorus via ligand exchange. Bt horizon soil in the Georgia Piedmont is high in oxidized metals, especially iron and manganese. Impoundments in the Georgia Piedmont receive sediment laden with phosphorus and iron from tributary flow. We present here studies that explore the role of transported sediment in the biogeochemical cycling of phosphorus in Lake Lanier, a Georgia Piedmont impoundment. X-ray diffraction analysis of Lake Lanier surface water demonstrate that newly formed iron oxides are present in surface water samples. Experiments to investigate the capacity of iron oxyhydroxide, Piedmont soil from the Bt horizon, and Lake Lanier sediments to adsorb phosphate were conducted in a series of treatments. Lake Lanier sediments had the greatest capacity to sorb phosphate in all treatments, and sorbed all the phosphate in solution in most experiments. We conducted laboratory investigations to determine if sediment bound phosphorus would be available for uptake by natural cultures of lake phytoplankton. Biological uptake of radiolabelled P in the sediment treatment was not evident. Alkaline desorption experiments were conducted with Bt horizon soil. Desorption of phosphate occurred at pH values greater than 8. The higher the pH the greater the desorption of phosphate from Bt horizon soil. Transported sediment plays a more significant role in the phosphorus cycle in impoundments of the Georgia Piedmont than in the north temperate systems where the traditional phosphorus cycling paradigm was developed. The photosynthetic elevation of the local pH may increase local phosphate availability by releasing phosphate from oxidized metal complexes, and thus may be a mechanism for niche construction by phytoplankton in poorly buffered surface waters.