Organic phosphorus in solutions and leachates from soils treated with animal slurries

A substantial part of the total phosphorus (P) in soil solution and leachates can be present as dissolved organic phosphorus (DOP). The DOP may be more mobile than inorganic orthophosphate and thus it can be an important P source for surface water eutrophication. This paper describes a series of four experiments that investigated the effects of animal waste application to sandy soil on DOP leaching. The first experiment examined the effect of storing pig slurry on DOP fractionation, using gel permeation chromatography. Experiment two and three examined the immediate and long-term effects of animal waste application to soil in laboratory columns and outdoor maize-grown lysimeters, respectively. In the fourth experiment concentrations of DOP and total P (TP) in soil solutions at various depths were determined in a long-term field experiment. Results indicate that DOP in pig slurry is mainly found in high molecular weight (MW) compounds (Exp. 1). From the soil column more than 90%, and from the lysimeters more than 70% of TP leached as DOP (Exp. 2 and 3). In the lysimeter experiment (Exp. 3), leaching of TP mainly occurred in periods of low CI and NO3 concentrations, induced by high leaching rates. This may indicate that P transport in these periods was mediated by DOC or other colloidal particles. The field experiment (Exp. 4), showed that DOP as a percentage of TP in the soil solution increased from about 10% in the topsoil to more than 70% at 70 to 80 cm depth, which was mainly due to a decrease in MRP. Thus, DOP is an important form of P in leachates and in soil solutions in the subsoil, and characterization of this DOP can improve our understanding of P leaching from soils.