PHOSPHORUS RETENTION BY WETLAND SOILS USED FOR TREATED WASTE-WATER DISPOSAL

Wetlands function as buffers for nutrients loaded from terrestrial eco systems through drainage and surface discharges. The objectives of our study were to (i) determine the P retention capacity of representativ e wetland soils being used for disposal of treated wastewater and (ii) relate P retention characteristics to selected physicochemical proper ties to evaluate likely mechanisms of P removal in the soils. Intact s oil cores (0-40 cm) and bulk soil samples (0-15 cm) were collected fro m a system of natural and constructed wetlands currently being used fo r disposal of treated wastewater. Floodwater P concentrations of the i ntact soil cores were monitored over time to determine the rate of P r emoval. Batch experiments were conducted to determine maximum P retent ion capacity of the soils. Soil samples were analyzed for inorganic P pool sizes, and selected physicochemical properties. During a 21 d hyd raulic retention time, the constructed wetlands (sandy, low organic ma tter 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 re tention maximum, as estimated using the Langmuir model, ranged from 19 6% 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 soluti on. Anaerobic conditions increased P solubility. Organic P pools and t he Fe-Al-bound fraction seemed to control P chemistry in these natural and constructed wetlands.