Phosphorus Sequestration in Soil Aggregates after Long-Term Cropping.

Cultivated soils in the Everglades are being converted to their historic use as pastures or seasonally-flooded prairies as parts of restoration efforts, but long-term cultivation may have altered soil P distribution and availability which may pose eutrophication hazards upon change in land use. The objectives of this study were to determine the distribution of P in soil chemical and physical fractions for contrasting long-term land management practices. The distribution of P in labile, Fe-Al bound, Ca bound, humic-fulvic acid, and residual pools in five aggregate-size fractions were measured for fields under sugarcane (Saccharum sp.) cropping for 50 yr and perennial pasture for 100 years. Both land uses were characterized by a high degree of macroaggregation, as aggregates >0.25 mm contained 76 and 83% of the total soil under cultivation and pasture, respectively. Cultivated soils sequestered a total of 107 kg ha-1 more P than pasture at 0-15 cm. The distribution of P in chemical fractions significantly varied between land uses as cultivation increased P sequestration in Ca-bound fractions more for sugarcane (244 kg P ha-1) than pasture (65 kg P ha-1). Pasture sequestered more P in organic pools, as storage in humic-fulvic acid and residual fractions were 26 and 25%, respectively, higher than cultivated soil. Labile P was 100% higher in pasture than cultivated soil, but Fe-Al bound P storage did not differ between land uses. Aggregation increased P sequestration in humic-fulvic acid and residual fractions, and P storage in organic pools increased with increasing aggregate size. In contrast, cultivation decreased aggregation and increased P accumulation in inorganic fractions. Long-term cultivation altered the distribution of soil P from organic to inorganic pools. The P stored in inorganic pools is stable under current land use, but may be unstable and pose eutrophication hazards upon onset of future land use change to the seasonally-flooded prairie ecosystem.