Water table effects on phosphorus reactivity and mobility in a dairy manure-impacted spodosol

Abstract

Many Florida soils present a unique situation for management of P in animal wastes due to fluctuating water table and predominantly sandy soils with low P retention capacity. These conditions may lead to significant P leaching, which can contaminate both surface and groundwaters. A column study was conducted to determine the fate of dairy manure-derived P under high and low water table conditions. Intact soil cores from low-impact (forage) and high-impact (intensive) areas of Dry Lake Dairy near Okeechobee, FL, were used in this study. Soluble reactive P (SRP) concentrations in the soil columns were monitored periodically to assess P leaching. The forms and distribution of P in the columns were determined to examine the nature and extent of P transformations in the soil. Maintaining the water table above (flooded) or below (drained) the Bh horizon resulted in substantial movement of P in the soil columns. High potential for surface water contamination with P was associated with the spodosol from heavily manure-impacted areas. Assuming a bulk density of 1.33 g cm −3 and an average thickness of 20 cm, with 61% of the total P in the labile pool, the intensive component would have over 3000 kg P ha −1 in the surface horizon that could potentially be available for transport. The intensive soil columns maintained high porewater SRP concentrations in the A and E horizons during the entire course of the study. This suggests low P sorption in the overlying horizons and high potential for lateral movement of P above the Bh horizon. Most of the labile P (NH 4Cl-P) that leached into the Bh horizon was transformed into Al/Fe-P, which accounted for about 65% of the total P in the Bh horizon of soil from the intensive component. The Bh horizon under flooded and drained conditions provided storage for P but at a limited capacity.