Phosphorus Retention by Wetland Soils used for Treated Wastewater Disposal

Abstract

Abstract Wetlands function as buffers for nutrients loaded from terrestrial ecosystems through drainage and surface discharges. The objectives of our study were to (i) determine the P retention capacity of representative wetland soils being used for disposal of treated wastewater and (ii) relate P retention characteristics to selected physicochemical properties to evaluate likely mechanisms of P removal in the soils. Intact soil cores (0–40 cm) and bulk soil samples (0–15 cm) were collected from a system of natural and constructed wetlands currently being used for disposal of treated wastewater. Floodwater P concentrations of the intact soil cores were monitored over time to determine the rate of P removal. Batch experiments were conducted to determine maximum P retention capacity of the soils. Soil samples were analyzed for inorganic P pool sizes, and selected physicochemical properties. During a 21 d hydraulic retention time, the constructed wetlands (sandy, low organic matter soils) retained 52 to 66% of added P, as compared with 46 to 47% retained by the natural wetlands (high organic matter soils). The P retention maximum, as estimated using the Langmuir model, ranged from 196 to 1821 mg P kg −1 (aerobic incubations) and from 32 to 1415 mg P kg −1 (anaerobic incubations). The P sorption maximum for these soils could be predicted by batch equilibration with a single high P solution. Anaerobic conditions increased P solubility. Organic P pools and the Fe‐Al‐bound fraction seemed to control P chemistry in these natural and constructed wetlands.