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-137 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-cm-depth increments and analyzed for Cs-137, 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-1 at 0.3 km from the inflow to about 0.25 cm yr-1 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 m-2 yr-1 in cattail (Typha spp.)-dominated areas, and 0.11 to 0.25 g P m-2 yr-1 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.