Characteristics and Source of Soil Phosphorus in Tree Islands of the Florida Everglades.

The Florida Everglades is a phosphorus (P) limited, freshwater wetland ecosystem.  Soil P in tree islands within the Everglades has been reported to greatly exceed concentrations of the surrounding marsh.  Tree island ecosystems in the Everglades offer a unique opportunity to study soil P characteristics because the wide range (0.02 – 10%) of soil P concentrations reported in tree islands suggests heterogeneous accumulation mechanisms among islands. Characterization of soil P was approached by conducting sequential chemical P extraction, elemental analysis, determination of carbonate and non-carbonate Ca (ncCa), and non-carbon soil material (NCM) of surface soil (0-10 cm) from tree islands (n=26) in the Everglades.  Sequential selective dissolution of P revealed acid extractable P (HCl-P) is the primary form present in tree island soil. The relationship among HCl-P and carbonate Ca was not significant.  The positive correlation of HCl-P with NCM (r = 0.98, p < 0.0001) and ncCa ( r = 0.57, p < 0.01), suggest ncCa phosphates are the dominant form of soil P, rather than P associated with CaCO₃.  Soil ncCa:P molar ratio was negatively correlated with TP concentration (r = -0.95, p < 0.001) and approaches the theoretical ratio for hydroxyapatite with increasing P.  Apatite was identified by XRD in soils with elevated P concentration (i.e. < 6% TP). These soils also contained bone fragments.  Presence of apatite and bone fragments in tree island soils suggests deposition of biogenic Ca-P mineral material that may influence soil P biogeochemistry.