Forms of Soil Phosphorus in Selected Hydrologic Units of the Florida Everglades

The Florida Everglades wetland ecosystem is subject to changes in hydroperiod and nutrient loading, resulting in soil P enrichment and changes in vegetation communities. The objectives of this study were to: (i) quantify the forms of inorganic and organic P in soils from four hydrologic units of the Everglades, and (ii) develop empirical relationships among various soil P forms. Soil samples from selected hydrologic units, including the Water Conservation Areas (WCAs) and the Holey Land Wildlife Management Area (HWMA), were obtained at various locations along transects perpendicular to each nutrient input source, while selected field sites were sampled in the Everglades Agricultural Area (EAA). Spatial distribution of total P in the surface 0to 10-cm soil depth showed distinct gradients in the WCAs and HWMA soils, with high total P in soils closer to sources (canals and inflow structures) than in interior, unimpacted areas. Soil ash content and bulk density were also altered as a result of soil subsidence (for EAA soils), hydrology, and nutrient loading (for the WCAs and the HWMA soils). Influence of P loading was primarily confined to the top 30-cin soil depth, with about one-third of the P stored in the inorganic pool (primarily as Caand Mg-bound P), and the remainder present as organic P. Inorganic P content was higher in surface soils and decreased with depth. Soil P enrichment indicated that for approximately 5 km from the inflow structures or canals, soils have been impacted by nutrient loading. Empirical relationships developed in this study should be useful for estimating soil P forms at the landscape level, using total P data available for a large number of sites throughout the Everglades region. M AND REACTIVITY OF P in wetlands under variable hydrologic conditions are controlled by the chemical composition of P in soil and water, relative sizes of various P pools in the soil, interactions of soluble fractions with solid phases, and decomposition of soil organic matter. Phosphorus is present in both organic and inorganic forms, with organic forms present as the dominant pool in many wetlands. Forms of inorganic P (Pi) in soils are usually determined by sequential extractions with acid and alkaline reagents, as proposed by Chang and Jackson (1957) and later modified by others for soils and sediments (Psenner et al., 1988; Ruttenburg, 1992; Olila et al., 1994). A modification of this scheme has been adopted for wetland soils (Quails and Richardson, 1995; Reddy et al., 1995). These schemes typically identify P in the following groups: (i) labile P; loosely adsorbed; (ii) P; associated with Fe and Al; (iii) Pi associated with Ca and Mg; (iv) alkali-extractable organic P (fulvicand humic-bound P); and (v) residual organic P. Forms of organic P (P0) have also been distinSoil and Water Science Dep., Univ. of Florida, 106 Newell Hall, P.O. Box 110510, Gainesville, FL 32611-0510. Florida Agricultural Experimental Station Journal Series no. R-05940. Contribution from the Univ. of Florida Wetland Biogeochemistry Lab. Received 2 Dec. 1996. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 62:1134-1147 (1998). guished using acid and alkaline extractions of soils (Sommers et al., 1972; Ivanoff et al., 1998). The Florida Everglades wetland ecosystem is subject to changes in hydroperiod as a result of alterations in flow patterns and seasonally controlled water inputs. Introduction of nutrient-enriched water and alterations in the hydrologic regime have resulted in changes of vegetation communities (Davis, 1991) and in soil P enrichment (DeBusk et al., 1994; Newman et al., 1997). The objectives of this study were to: (i) quantify the forms of Pi and P0 in soils from four hydrologic units of the Everglades, and (ii) develop empirical relationships among various soil P forms. Our results are part of a larger project designed to determine soil processes regulating P retention in subtropical wetlands using the Florida Everglades as a case study (Koch and Reddy, 1992; Reddy et al., 1993; DeBusk et al., 1994; KochRose et al., 1994; Newman et al., 1997). MATERIALS AND METHODS