Phosphorus transformations in stream bank sediments in Iowa, USA, at varying redox potentials

PurposeStream bank erosion is one of the main sources of suspended sediments in stream water, and it often carries phosphorus with it. With a controlled laboratory study, we investigated the changes in different forms of P in stream bank sediments at varying redox potentials. We hypothesized that there could be increases in labile forms of solid-phase P under low redox conditions.Materials and methodsSediment samples representing four major stratigraphic units were collected from alluvial banks of Walnut Creek in Jasper County, Iowa: Camp Creek, Roberts Creek, and Gunder members of the Holocene-age De Forest Formation and pre-Illinoian till, a glacial deposit that is older than 500,000years. We compared three treatments: no anaerobic incubation, anaerobic incubation, and anaerobic incubation with addition of glucose. After each treatment, soil P fractions were determined and classified as water-extractable P, labile P, slowly cycling P, stable P, and residual P. Labile P is extractable with 0.5M NaHCO3, slowly cycling P is extractable with 0.1M NaOH or 1M HCl, stable P is extractable with concentrated HCl, and residual P is associated with acid-insoluble compounds in the soil.Results and discussionRedox potential of the equilibrated systems was significantly less when sediments were incubated anaerobically than when they were not incubated, especially when glucose was added to the anaerobic incubation. Variations in redox potential did not significantly affect the total P, organic or inorganic stable P, or residual P in the sediments. In contrast, we found that the amount of inorganic labile P tended to increase at the same time that slowly cycling P associated with Fe decreased.ConclusionsThe younger sediments, higher in the stratigraphic profile and with more organic matter, had greater labile P and slowly cycling P that was associated with Fe, reflecting a greater potential to contribute to elevated levels of P in the stream water if subjected to low redox potential in the stream environment. The forms of P in the four bank sediments were redistributed among the fractions by reducing conditions. There was an increase in the concentration of inorganic labile P, which coincided with a decrease in slowly cycling P associated with Fe, especially in the Holocene-age sediments. These sediments appear to have a greater potential to contribute to elevated levels of P in the stream water when they are subjected to low redox potential in the stream environment.