Fate of phosphorus in Florida spodosols contaminated with cattle manure

Phosphorus loading from dairies and beef ranches in the Lake Okeechobee watershed and the subsequent movement of the P into the drainage waters is a major factor influencing the eutrophication of Lake Okeechobee. The soils of this area are mainly Spodosols with the watertable lying between surface and spodic horizons for extended periods each year. In this study, the quantity of total P (TP) within the soil profile (A, E, Ph and Bw horizons) of dairies and beef ranches in the Lake Okeechobee basin was determined to evaluate the magnitude of P loading in these soils. The effect of cattle density was evident in TP concentrations throughout the soil profile. In the A horizon, mean TP concentrations were 1680, 165, and 34 kg P ha-l for high, low, and nonimpacted areas, respectively. The same trend, although at lower concentrations, was evident in the E, Bh, and Bw horizons. The quantity of P considered to be potentially mobile under leaching conditions (water-soluble P, Mehlich I extractable or NH4Cl extractable), also followed similar trends as the TP concentrations. Based on chemical fractionation data, nearly 80% of TP in the A horizon of the highly impacted soils may be considered leachable. We calculated that about 4000 kg P ha(-1) would be available for leaching in the soil profile of the high intensity areas immediately adjacent to the dairy barns. This ''labile P'' appears to be solubilized slowly over a long period of time (likely several years). There seems to be no natural mechanism whereby the P is stabilized through the formation of minerals, and even if such processes do take place, a vast amount of P still remains in a form in which it is readily transported along with the drainage water. The A and E horizons had poor P retention capacities while the retention capacity of the Bh horizon varied with the soil type, Myakka greater than or equal to Immokalee > Pomello. Due to the low P-retention capacity in the upper horizons of these soils, there is potential for significant subsurface lateral P transport.