Long-Term Nutrient Accumulation Rates in the Everglades

Anthropogenic nutrient inputs to the northern Everglades of Florida during the last three decades have resulted in alteration of vegetation and soil nutrient storage. Due to the nutrient-limited status of this ecosystem, increased loading may have altered the capacity for long-term nutrient accumulation. Our study was conducted to determine the potential long-term nutrient accumulation rates for this ecosystem along a gradient of nutrient loading. Accumulation rates were calculated using the vertical peat accretion rates, as determined by Cs dating, and nutrient concentration profiles. Intact soil cores were obtained along a 15-km transect and evaluated as a function of distance from the inflow structure. Soil cores were sectioned into 1-cmdepth increments and analyzed for '-"Cs, P, N, C, and selected cations. Vertical accretion rates of peat decreased logarithmically with distance from the inflow, with rates of 1.1 cm yr~' at 0.3 km from the inflow to about 0.25 cm yr' in unimpacted sawgrass (Cladium jamaicense Crantz)-dominated areas. Phosphorus, N, and C accumulation rates in soil and floodwater total P concentrations also showed similar relationships. The P accumulation rates ranged from 0.54 to 1.14 g P myr~' in cattail (Typha spp.)-dominated areas, and 0.11 to 0.25 g P myr~' in sawgrass-dominated areas. The C/P and N/P accumulation ratios increased with distance from the inflow, suggesting that a greater proportion of P accumulated in the system, compared with C and N. Similar P retention coefficients were obtained when calculated using either changes in surface water total P concentration, or the long-term P accretion rates. These findings suggest that P was either directly adsorbed by soil or precipitated with Ca in the water column and deposited on the soil surface. This hypothesis was further supported by a highly significant correlation between P and Ca accretion rates, suggesting that Ca-bound P controls equilibrium concentrations in this ecosystem. L NUTRIENT ACCUMULATION in wetland ecosystems is determined by the balance between inputs and outputs. Nutrients in wetlands undergo several biogeochemical transformations, some resulting in the loss of certain nutrients as gaseous end products or through leaching and discharge to outflow, while K.R. Reddy and W.F. DeBusk, Soil and Water Science Dep., Institute of Food and Agricultural Sciences, 106 Newell Hall, Univ. of Florida, Gainesville, FL 32611; R.D. DeLaune, Laboratory for Wetland Soils and Sediments, Louisiana State Univ., Baton Rouge, LA 70803; and M.S. Koch, Dep. of Everglades Systems Research, South Florida Water Management District, P.O. Box 24680, West Palm Beach, FL 33416. Florida Agric. Exp. Stn. Journal Series no. R-02740. Received 13 May 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:1147-1155 (1993). others result in nutrient accumulation within the ecosystem. Nutrient accumulation can occur through sedimentation or organic matter accumulation. In peatdominated wetlands, a major portion of the nutrients is stored in live and detrital plant tissue, microbial biomass, and stabilized soil organic matter. Nutrients stored in vegetation and microbial biomass can be readily released through natural die-off and decomposition (Davis, 1991). In herbaceous wetlands, nutrient storage in vegetation is usually short term (Reddy and DeBusk, 1987), while in forested wetlands incorporation of nutrients into woody tissue of trees can result in long-term storage (Richardson and Davis, 1987). As C and N are cycled through a wetland, a portion can be lost as gaseous end products. For example, organic C is converted to CO2 and CH4, and is lost from the system. This process is influenced by the hydrologic regime of the system, with frequent wet and dry cycles increasing decomposition rates and loss of C (Reddy and Patrick, 1975). In the EAA, oxidation of organic matter under drained conditions accounted for soil loss of about 3 cm yr (Snyder et al., 1978). However, CO2 fixation by vegetation and accumulation of detrital material in many wetlands usually offsets decomposition, resulting in a net C accumulation. Similarly, organic N is mineralized to NH4-N, which is subsequently lost through nitrification-denitrification and NH3 volatilization reactions (Reddy and Patrick, 1984). However, P released during decomposition is usually retained by the wetland through sorption and precipitation reactions (HowardWilliams, 1985). Nutrients added to a wetland are rapidly incorporated into living and detrital plant material, and eventually incorporated into soil organic matter (Puriveth, 1980; Day, 1982; Davis and van der Valk, 1983; DeBusk and Reddy, 1987). Long-term nutrient retention by soil organic matter is affected by environmental factors such as temperature, hydroperiod and fire. Nutrient accumulation rates have been estimated for many wetland ecosystems using Cs as a marker (DeLaune et al., 1978; Hatton et al., 1983; Kadlec and Robbins, 1984; Patrick and DeLaune, 1990). Peat Abbreviations: EAA, Everglades Agricultural Area; WCA, Water Conservation Area; ENP, Everglades National Park; SRP, soluble reactive P; TKN, total Kjeldahl N. 1148 SOIL SCI. SOC. AM. J., VOL. 57, JULY-AUGUST 1993