Constructed wetlands have the potential to be used for treatment of nitrogen-rich livestock wastewater but rather limited for removal of phosphorus (P). Our objectives were to evaluate the soluble P (SP) distribution in surface-pore water to determine P treatment limiting factors of two surface-flow constructed wetlands systems treating swine lagoon wastewater. Wetland system 1 was planted to rush and bulrushes and wetland system 2 was planted to bur-reed and cattails. For five years, P was applied at increasing rates of up to 6.0 kg/ha/d but P mass removal was less than 50% effective at rates higher than 4.0 kg/ha/d. In the sixth year, interstitial soil water and overlying water column profiles were sampled for SP analysis at four distances (9, 24, 50, and 70 m) from the inlet using soil pore water equilibrators. For each pore water profile in both systems, we found that SP levels were highest at or near (1-3 cm) the soil-water interface indicating that SP was moving from the soil pore water into the water column. The assumed controlling factors for the release of SP at the wetland soil-water interface were loading rates, soil pH, sorption capacity and reduced conditions. Prevalent soil reduction and declining sorption capacity may explain both, SP released through the soil-water interface into overlying water column and subsequent decline of P removal efficiency.