Wetlands and Aquatic Processes Influence of Flooding on Phosphorus Mobility in Manure-Impacted Soil H. K. Pant,* V. D. Nair, K. R. Reddy, D. A. Graetz, and R. R. Villapando ABSTRACT of the major sources of P loading to Lake Okeechobee. The soils in the basin are primarily Spodosols; the A Agricultural lands are often used for constructing stormwater treat- and E horizons of the soil profile are sandy and have ment areas (STAs) to abate nutrient loading to adjacent aquatic low P sorption capacities, whereas Bh and Bw horizons systems. Flooding agricultural lands to create STAs could stimulate have greater P sorption capacities (Mansell et al., 1991) a significant release of phosphorus (P) from soil to the water column. To assess the suitability of agricultural lands, specifically those im- due to greater Al- and Fe-oxide contents (Nair et al., pacted by animal operations, for the construction of STAs, soils from 1998). Although a constructed wetland can be a cost- different components of the New Palm–Newcomer dairies (Nubbin effective method for removing soluble P from runoff Slough Basin, Okeechobee, Florida, USA) were collected by horizon waters, the ambient storage of P in soils, as well as other and their P retention and release capacities estimated. In general, P physico–chemical characteristics of the soils (e.g., pH, released from A-horizon soil under flooded (anaerobic) conditions Ca, Mg, Fe, and Al contents) are critical for selecting was greater than under drained (aerobic) conditions due to redox an STA construction site. Studies of P sorption charac- effect on iron (Fe) and consequent P releases. However, the P released teristics and release potential of soils of any proposed from Bh-horizon soil was greater under aerobic conditions than under site should indicate whether the STAs would effectively anaerobic conditions, possibly due to excessive aluminum (Al) content sequester P. The objectives of this study were to (i) in the horizon. Double acid–extractable calcium (Ca), magnesium determine the P sorption capacity of soils collected from (Mg), Al, and P explained 87% of the variability in P release under different horizons and (ii) estimate the P releasing po- aerobic conditions, and 80% of that under anaerobic conditions. The tential of the soils to the water column. P release maxima indicated a high solubility of P in A-horizon soil from both active and abandoned dairies (13 and 8% of the total P, respectively), suggesting that these soils could function as potential MATERIALS AND METHODS sources of P to the overlying water column when used in STA construc- tion. Preestablishment of vegetative communities or chemical amend- Site Description ment, however, could ameliorate high P flux from soil to the water Construction of the Nubbin Slough STA is proposed as a column. part of the Lake Okeechobee Water Retention/Phosphorus Removal Critical Restoration Project. The project is designed to increase regional water storage north of Lake Okeechobee L and areas used for animal operations are known to by on-site wetland restoration and water retention, and to accumulate P in soils. Both surface and subsurface reduce P in surface runoff. The site is located in Newcommer discharges from these areas have been detrimental for and New Palm dairies, which are 2.1 km north of Lake Okee- protecting and restoring adjacent water bodies. Con- chobee and 10.5 km southeast of the town of Okeechobee. The site occupies an area of approximately 4.49 km 2 (Fig. 1). structed wetlands and retention reservoirs are consid- The site has an abandoned–intensive component, referring to ered low-cost alternatives for treating wastewater from land that had previously been under dairy production for 34 various sources, including agricultural runoff. A signifi- yr, but had been used for pastures or forage production for cant portion of the water-column P can be removed by the last 8 yr. The site also has an active–intensive component, biological (Newbold et al., 1983; Greenway and Wool- referring to areas with active dairies for the last 20 yr, and a ley, 1999; Reddy et al., 1999) and chemical processes native component, referring to nonimpacted or forested areas. both in natural and constructed wetlands (Diaz et al., The abandoned–intensive, active–intensive, and native com- 1994; Pant et al., 2001). In many watersheds, land areas ponents occupy 13, 18, and 7% of the total land area of the previously used for agricultural operations are being proposed STA, respectively, while the forage–pasture, grazing converted to wetlands. Flooding these areas can stimu- field, and spray field components account for 31, 15, and 16% late the dissolution of stored P and increase the P con- of the total, respectively. The adjacent land use of the pro- posed STA site includes intensive agriculture, wetland and centrations in the water column (Pant and Reddy, 2000). upland forests, and urban and reclaimed lands. Soils from A, Phosphorus runoff from dairies and beef cattle E, and Bh horizons of representative locations (from three to ranches in the Okeechobee Basin, Florida, USA is one nine locations within each component) of the Newcomer–New Palm dairy site were sampled for P retention and release H.K. Pant, V.D. Nair, K.R. Reddy, and D.A. Graetz, University of studies. An auger was used to get samples from different Florida, Soil and Water Science Department, Institute of Food and depths, and the horizon was identified by visual difference in Agricultural Sciences, P.O. Box 110510, Gainesville, FL 32611-0510. the soil layer. The average depths were 10, 20, and 11 cm for R.R. Villapando, The South Florida Water Management District, Lake Okeechobee Division, 3301 Gun Club Road, West Palm Beach, FL 33406. Received 15 Oct. 2001. Corresponding author (hari@mail. Abbreviations: DA-, double acid–extractable; SRP, soluble reactive ifas.ufl.edu). phosphorus; STA, stormwater treatment area; TP, total phosphorus; WSP, water-soluble phosphorus. Published in J. Environ. Qual. 31:1399–1405 (2002). 1399 Published July, 2002