Phosphorus transformations under reduction in long-term manured soils

The widespread recycling of animal wastes as fertilizers can severely affect the phosphorus cycle, especially in soils that undergo periodical flooding and anaerobiosis. The aim of this study was to investigate the effect that reducing conditions have on the release and forms of P in soils that have received different doses (0, 60 and 120 Mg/ha) of liquid hog manure for 15 years. Samples of the topsoil (0-20 cm) and of subsoil (20-40 cm) were placed under static reducing condition. At the onset of the reduction, P release to the solution reached a maximum after 4 weeks of submersion then levelled off, presumably due to readsorption by the soil matrix. When an anionic resin was added to act as a P-sink the amount of P released nearly doubled for most samples. The soils that had received the highest dose of manure released more P than the others. Draining the soil solution at intervals after reduction removed up to 683 mu g P/kg of the soil with the highest manure dose. The forms of P measured after the experiments revealed a shift of the resilient forms towards more labile, bioavailable forms thus increasing the potential for P loss.