Phosphorus retention and sorption by constructed wetland soils in Southeast Ireland

It may be necessary to use constructed wetlands as a land use practice to mitigate phosphorus (P) loss from agriculture in Ireland. The objectives of this study were to determine the ability of two constructed wetland site soils to retain and sorb P. Intact soil/water column studies were used to determine P release/retention rates during a 30-day incubation period. Soil columns flooded with distilled water released P during the first 2 days; however, soluble reactive P (SRP) concentrations in overlying floodwaters decreased thereafter. Soils with overlying floodwaters spiked at 5 and 15 mg SRP L−1 retained highest amounts of P (p<0.05) with retention at these concentrations controlled by SRP in overlying waters. Retention rates by soils ranged between 0.3 and 60.9 mg P m−2 d−1. Maximum P sorption capacity (Smax) was higher for wetland soils at Dunhill, Waterford (1464 mg P kg−1) in comparison to soils at Johnstown Castle, Wexford (618 mg P kg−1). Equilibrium P concentrations (EPC0) were low (in the μg SRP L−1 range), indicating a high capacity of these soils to sorb P. Phosphorus sorption parameters were significantly related to ammonium oxalate extractable aluminium (Al) and iron (Fe) content of soils.