Distribution of soil and plant nutrients along a trophic gradient in the Florida Everglades.

Historically, atmospheric precipitation has been the primary source of N and P to the Florida Everglades. Alterations to the natural hydrology, surface water runoff from agricultural lands, and controlled releases from Lake Okeechobee have increased nutrient loading to the Everglades. A nutrient front encompassing approximately 8000 ha has developed in a northern Everglades marsh, Water Conservation Area 2A (WCA-2A; 44 684 ha), during the last three decades from surface water P and N loading, in addition to atmospheric inputs. Soil cores (0-60 cm) and plant tissue were collected from sawgrass, Cladium jamaicense Crantz, and cattail, Typha domingensis Pers. , stands at a distance of 1.6, 5.6, and 9.3 km south of major surface water inflows in WCA-2A: Site N (northern), Site C (central), and Site S (southern), respectively. Although N loading was approximately 10-fold greater at Site N compared with Sites C and S, no significant difference in total N was found between sites at any soil depth. In contrast, P accumulated threefold in soils at Site N compared with Site S (P < 0.05). Organic P accounted for approximately 75% of the total P. Acid-extractable inorganic P (HCI-P,), as an indicator of Ca-bound P, accounted for 80% of the inorganic P and was significantly correlated to dissolved P concentrations of the soil pore water (r = 0.89). Alkali-extractable inorganic P (NaOH-P,), as an indicator of the Feand Al-bound P, comprised 20% of the total inorganic P. High pH values (>8.0) were measured from pore water associated with benthic algal mats. Interstitial P concentrations were 2 to 3 orders of magnitude higher at Site N (>1000 M-g L~') than at Site S (<4 fig L~') and plant tissue N/P ratios at Site N and C were lower, 11:1 compared with 40:1 at Site S. These data suggest P may be an important nutrient limiting primary productivity in the Everglades and that Ca-P precipitation, catalyzed by algal photosynthesis, may be an important mechanism for soil P assimilation. T FLORIDA EVERGLADES historically comprised approximately 1 million ha (Davis, 1943), and today represents the largest and most important freshwater subtropical peatland in North America. This ecosystem provides habitat for a high diversity of species including large populations of wading birds, and several threatened and endangered species: wood storks, snail kites, bald eagles, Florida panthers, and American crocodiles. The Everglades is unique when compared with other extensive wetland systems of the southern USA due M.S. Koch, South Florida Water Management District, Division of Everglades Systems Research, P.O. Box 24680, West Palm Beach, FL 33416; and K.R. Reddy, Soil and Water Science Dep. , Institute of Food and Agricultural Sciences, Univ. of Florida, Gainesville, FL 32611. Joint contribution of the South Florida Water Management District and the Univ. of Florida. Florida Agric. Exp. Stn. Journal Series no. R-02208. Received 23 July 1991. "Corresponding author. Published in Soil Sci. Soc. Am. J. 56:1492-1499 (1992). to its evolution from the accumulation of organic matter within a limestone depression (Davis, 1943; Gleason et al., 1984). This hydrological setting led to a lack of nutrient inputs from mineral sediment deposition, except for infrequent sheet flooding in the upper glades from Lake Okeechobee (Parker, 1974) and marine inputs in the southern regions. Thus, historically, the major source of nutrient loading to the Everglades has been in the form of atmospheric precipitation (Parker, 1974; Waller, 1975). Low nutrient content, particularly P, in the Everglades peats support this hypothesis (Parsons, 1977; Gleason et al., 1984). Nutrient limitation has been implicated as a primary factor, in addition to hydrologic conditions, controlling the persistence of the endemic Everglades flora (Steward and Ornes, 1975; Davis, 1991). Another important ecological factor, fire, has been reported to contribute to the internal recycling of limited nutrients (Steward and Ornes, 1975) and influences vegetation community structure (Forthman, 1973; Hofstetter, 1984), an ecological strategy known to sustain high productivity of an otherwise nutrient-limited ecosystem. Peatlands that have evolved from organic matter accumulation driven primarily by atmospheric precipitation (ombrotrophic) are characteristically nutrient poor (Pollet, 1972; Moore and Bellamy, 1974). The breadth of our understanding of nutrient cycling in these ombrotrophic peatlands is primarily based on studies from temperate regions (Pollet, 1972; Moore and Bellamy, 1974; Richardson et al., 1978). The subtropical Everglades differ from temperate peatlands in several aspects that significantly affect nutrient cycling and ecosystem productivity: (i) the majority of areas are neutral or alkaline due to peat accumulation over a limestone bedrock, with the exception of the Loxahatchee Wildlife Refuge (WCA1) in the northern Everglades, which is characterized as acidic (Swift and Nicholas, 1987); (ii) temperatures are mild due to the subtropical environment, which would support higher rates of microbial activity; and (iii) soils are subject to periodic oxidation due to droughts and fires. There have been very few investigations on nutrient cycling in the Florida Everglades; however, available data support the hypothesis that the Everglades are nutrient limited (Steward and Ornes 1975; Swift and Nicholas, 1987; Davis, 1991). Our objectives were to determine (i) the inorganic and organic pools of P and N in the northern Everglades peats (WCA-2A), (ii) the N and P storage in Abbreviations: P,, inorganic P; P0, organic P; WCA, Water Conservation Area; ENP, Everglades National Park.