Temporal and spatial patterns of internal phosphorus recycling in a South Florida (USA) stormwater treatment area.

Large constructed wetlands, known as stormwater treatment areas (STAs), have been deployed to remove phosphorus (P) in drainage waters before discharge into the Everglades in South Florida, USA. Their P removal performance depends on internal P cycling under typically hydrated, but with occasionally desiccated, conditions. We examined the spatial and temporal P removal capacity under different hydrologic conditions along a STA flow path. While inflow soils are P enriched, the outflow region of the wetland contained P-unsaturated soils with minimal net recycling of bound soil P to the water column as plant-available P. The outflow-region soils were characterized by low porewater soluble reactive P (SRP) (≤40 μg L) and high total sulfide (TS) (2-9 mg L) concentrations, and total ammoniacal nitrogen (TAN) and SRP flux rates that averaged 1.51 and 0.002 mg m d, respectively. Pronounced increases in porewater and surface-water concentrations of SRP, dissolved organic P (DOP), and TAN were observed immediately after rehydration of the cell after an extended drought. Elevated total P concentrations persisted at the outfall of the cell for several months thereafter, resulting in an annual outflow total P concentration nearly threefold higher than the long-term mean. Relative to processes that can occur during extended periods of inundation, such as sulfate-enhanced P release from organic matter mineralization or iron sulfide formation, aerobic oxidation of organic matter during prolonged dryout periods is a more significant biogeochemical process in compromising soil P retention in STAs.