Phosphorus release from the drying and reflooding of diverse shallow sediments

Phosphorus (P) retention is an important ecosystem service provided by sediments and soils. However, when shallow aquatic sediments and poorly drained soils dry and re-flood, they can be a source, rather than a sink, of P. Using experimental drying and re-flooding in the laboratory, we assessed the resultant sediment–water P exchange in a biogeochemically diverse set of sediments from 16 sites in Michigan. The direction and magnitude of P exchange to pore waters and surface waters upon re-flooding varied among sediments. Different sediment properties were related to P release to pore water than to P release to overlying surface water, suggesting that different processes control two phases of sediment P release: mobilization from solid to dissolved forms in the sediment pore water; and movement of dissolved P from pore water into overlying surface water. We observed especially high P release in dried and re-flooded sediments with high amounts of loosely sorbed phosphate, suggesting that drained sediments with a legacy of high P loads will be most likely to release P and experience internal eutrophication when re-flooded. The differential responses of sediments suggest that aquatic ecosystem restoration and management for nutrient removal must be evaluated with site-specific knowledge of sediment and soil biogeochemistry.