Sedimentary Phosphorus Dynamics in a Coastal Louisiana Prograding Delta

A number of studies have quantified phosphorus (P) concentration and sediment-water column exchange undergoing changes in redox condition in the Louisiana delta plain. However, there is not information on the potential role of bacteria in P transformations in Louisiana coastal wetlands. The main objective of this work was to evaluate sedimentary P dynamics by identifying dominant forms of P in sediments and determine the magnitude of P mobilization during bacterially mediated redox reactions. Sediment samples were collected from three sites (ridge, interior, and channel) characterized by variation in the vegetation and elevation in the currently prograding Wax Lake Delta. Sediments were incubated with and without the bacterium Shewanella putrefaciencs CN32 (Sp-CN32), which is an iron reducing bacteria that consumes dissolved oxygen and leads to reduction of Fe III to Fe II thereby releasing PO₄^3- bound to Fe III oxide. Our results revealed that P release in sediments spiked with Sp-CN32 was significantly higher in all sediments when compared to sediments in natural condition. P release from sediment spiked with Sp-CN32 significantly increased from 0.064 to 1.460 umoles/g in the sediment sampled at high elevation (ridge) and from 0.079 to 2.407 moles/g in the medium elevation area (interior) of the island. This increase in P release was significantly correlated with an increase in Fe (II) concentrations. The extent of P release was proportionally higher than the Fe (II) measured in all sediments. The P/Fe ratio steadily increased for the first 24 h followed by a rapid decrease. This temporal trend suggests a lag in Fe (III) reduction and an initial bacterial uptake and release of loosely sorbed NaHCO₃–P pool when Sp-CN32 respired on O₂ as the terminal electron acceptor. This study contributes to our understanding of the release mechanism of P during bacterial mediated redox reaction in wetland soils undergoing pulsing sediment deposition.