Stability of phosphorus forms in dairy-impacted soils under simulated leaching

Dairy manure has been linked to elevated phosphorus (P) levels in surface waters of Lake Okeechobee basin. Prevalent soils of the basin (Aquods) retain little P in sandy, quartz-dominated upper horizons, and lateral P transport is favored by high water tables. The stability of manure-derived P forms is thus an important factor in the Okeechobee basin and other regions where soil and hydrologic factors are unfavorable for P retention. The objectives of this study were to determine (1) the forms of P in four dairy impacted soils and one associated stream sediment, (2) the forms lost most readily following disturbance and simulated leaching, and (3) the effect of residence time prior to a leaching event on the relative amount of P released. Samples included four Ap horizons from holding areas and a stream sediment from one of the dairy sites. All samples (500 g) were packed in columns and leached with synthetic rainwater at 0.4 ml /min for 6 h weekly. Concentrations of P, Ca, Mg, AI, and Fe in the leachate, along with pH and electrical conductivity (EC) were determined. The form of solution P for selected leachates was also evaluated using 3tp NMR spectroscopy. Samples before and after leaching were subjected to P fractionation. Results of P-fractionation, solution speciation, and 3tp NMR indicated that P lost from surface horizons during leaching was dominantly in inorganic forms 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 its dominant form, determined in a related study to be a poorly-crystalline apatite-like mineral; soil horizons contained no detectable crystalline P. However, speciaPaper presented at the workshop on Phosphorus Behavior in the Okeechobee Basin, sponsored by the south Florida Water Management District and the University of Florida, Institute of Food and Agricultural Sciences. " Corresponding author. 0925-8574/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0925-8574(95)00047-X 210 H.D. Wang et al . / Ecological Engineering 5 ~1995) 209-227 tion modelling indicated that leachates were supersaturated with respect to hydroxyapatite, and in most cases to metastable Ca-P forms. Lack of crystalline Ca-P could be related to kinetics, or to a poisoning effect of components such as Mg, Si and organic acids in the dairy soil system. Elimination of the "barrier" to Ca-P crystallization could reduce the leaching of P from dairy systems.