Wetlands and Aquatic Processes Phosphorus Retention by Wetland Soils used for Treated Wastewater Disposal

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 (U) 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. Flood water P concentrations of the intact soU 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 tune, 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 nunriinnm, as estimated using the Langmuir model, ranged from 196 to 1821 mg P kg* (aerobic incubations) and from 32 to 1415 mg P kg" (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. "VTt TETLANDS frequently are used to remove N and P V V from treated wastewater prior to release into receiving ground and surface waters (Kadlec, 1987). Elevated N and P concentrations in wastewater have been associated with pollution of surface waters and N, as NOs, is associated with pollution of groundwaters. Nitrification and denitrification effectively reduce floodwater N loads in Soil and Water Science, P.O. Box 110510, Univ. of Florida, Institute of Food and Agricultural Science, Gainesville, FL 32611. Contribution of the Florida Agric. Exp. Stn. Journal Series no. R-03108. Received 12 Apr. 1993. "Corresponding author. Published in J. Environ. Qual. 23:370-377 (1994). treated wastewater applied to wetland systems (Gale et al., 1993a, b). Several studies (Boyt et al., 1977; Fetter et al., 1978; Schwartz, 1989) suggest that freshwater wetlands, swamps, marshes, and flooded soil systems can also reduce P levels of nutrient laden waters. Richardson (1985) cautioned, however, that wetland soils can function as either source or sink for P to the overlying floodwater moving through the wetland. Physical, chemical, and biological processes functioning hi overlying water and underlying sediments regulate P dynamics in wetlands. A significant portion of floodwater and porewater P can be removed through uptake by macrophytes and algae (Kloptek, 1975; Toth, 1972; Syers et al., 1973). Kadlec (1989) states, however, that litter and sediments are the key components of a wetland system in the regulation of nutrient cycling. Patrick and Khalid (1974) found that under anaerobic conditions, soils released P to solutions low in soluble P and sorbed P from solutions containing high concentrations of soluble P. Reactions of P that occur hi flooded soils include reduction of FeP, dissolution of occluded P, hydrolysis of Feand Al-bound P hi acid soils, increased mineralization of organic P hi acid soils, greater P diffusion, and increased solubility of Ca-P in calcareous soils (Sanchez, 1976). The amount of soluble P released into overlying waters of wetlands depends on the capacity of the soil to desorb or adsorb P from solution, mineralization of organic P, and diffusion of P from sediment to overlying waters (Nichols, 1983). These processes help determine whether the P concentration hi the interstitial and overlying water is adequate for the nutritional requirements of plants and aquatic organisms, and whether these systems can be used Abbreviations: BOD, biochemical oxygen demand; SRP, soluble reactive phosphorus; IR, infrared; AAS, atomic adsorption spectrophotometry; ANOVA, analysis of variance; HRT, hydraulic retention time; EPC, equilibrium phosphorus concentration. GALE ET AL.: PHOSPHORUS RETENTION BY WETLAND SOILS 3’71 to remove P from the overlying waters. Inorganic P exchange rates depend on the capacity of the sediment to adsorb and desorb P (Reddy, 1983a). Mineralization organic P depends upon the amount of P containing organic matter and the rate of phosphatase enzyme production (Golterman, 1984). Soil factors affecting P retention include: amount and type of clay, amount of Fe and A1 oxides and Ca compounds, and pH (Froelich, 1988; Goiterman, 1984; Khalid et al., 1977; Richardson, 1985). The city of Orlando, FL, currently uses a series of constructed and natural wetlands for the disposal of treated wastewater. The constructed wetlands consist of sandy, low organic matter soils, whereas the natural wetlands consist of high organic matter soils. The treated wastewater alternately flows through constructed and natural wetlands at the site. Our objectives were to determine the rate of soilmediated P removal and to estimate the potential capacity for P removal by soils indigenous to both the constructed and natural wetlands. These data were compared to soil physicochemical properties to determine the mechanisms most likely responsible for P removal in these soils. MATERIALS AND METHODS