Kinetics of phosphorus release from manure-amended alkaline soil

The release and transport of phosphate from manure-amended soils is detrimental to surface water because it leads to eutrophication. However, reaction rates and mechanisms of P release from manure-amended alkaline soils have not been completely characterized. The objectives of the present study were to determine the rate of P release from a manure-amended soil and to apply solubility models to predict the mineralogical phases that control soil-solution P concentrations. Phosphate release kinetics were measured. on a soil that had received solid-dairy manure applications for more than a decade. To measure total P that would be released under leaching conditions, we conducted experiments in which we replenished the solution continuously until the P concentration reached a steady state (sequential desorption experiment). Measured P release kinetics indicate that 80% of the soluble P was released within 24 h, followed by a slow release that continued for up to 504 h. We applied several models, including a reversible first-order model, the Elovich model, and a modified version of the Elovich model that was empirically correlated to soil organic matter and percent clay, to describe our experimental data. The data were best fit with the Elovich model. Total P released from the manure-amended soil in the sequential desorption experiments was 29% from the surface soil (0-10 cm) and 8% from the subsurface soil (45-65 cm). Results from this study suggest that P release from manure-amended soils is controlled by rate-limited dissolution of meta-stable Ca-P mineral phases.