STABILITY OF PHOSPHORUS FORMS IN DAIRY-IMPACTED SOILS UNDER SIMULATEDLEACHING

Dairy manure has been linked to elevated phosphorus (P) levels in surf ace waters of Lake Okeechobee basin. Prevalent soils of the basin (Aqu ods) retain little P in sandy, quartz-dominated upper horizons, and la teral P transport is favored by high water tables. The stability of ma nure-derived P forms is thus an important factor in the Okeechobee bas in and other regions where soil and hydrologic factors are unfavorable for P retention. The objectives of this study were to determine (1) t he forms of P in four dairy impacted soils and one associated stream s ediment, (2) the forms lost most readily following disturbance and sim ulated leaching, and (3) the effect of residence time prior to a leach ing event on the relative amount of P released. Samples included four Ap horizons from holding areas and a stream sediment from one of the d airy sites. All samples (500 g) were packed in columns and leached wit h synthetic rainwater at 0.4 ml/min for 6 h weekly. Concentrations of P, Ca, Mg, Al, and Fe in the leachate, along with pH and electrical co nductivity (EC) were determined. The form of solution P for selected l eachates was also evaluated using P-31 NMR spectroscopy. Samples befor e and after leaching were subjected to P fractionation. Results of P-f ractionation, solution speciation, and P-31 NMR indicated that P lost from surface horizons during leaching was dominantly in inorganic form s associated with Ca and Mg. Leachates of soil horizons contained much higher levels of P than did those of the sediment, despite comparable total-P levels. Recalcitrance of the sediment P probably relates to i ts dominant form, determined in a related study to be a poorly-crystal line apatite-like mineral; soil horizons contained no detectable cryst alline P. However, speciation modelling indicated that leachates were supersaturated with respect to hydroxyapatite, and in most cases to me tastable Ca-P forms. Lack of crystalline Ca-P could be related to kine tics, or to a poisoning effect of components such as Mg, Si and organi c acids in the dairy soil system. Elimination of the ''barrier'' to Ca -P crystallization could reduce the leaching of P from dairy systems.