ORGANIC PHOSPHORUS IN SOLUTIONS AND LEACHATES FROM SOILS TREATED WITHANIMAL SLURRIES

A substantial part of the total phosphorus (P) in soil solution and le achates 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 descr ibes a series of four experiments that investigated the effects of ani mal waste application to sandy soil on DOP leaching. The first experim ent examined the effect of storing pig slurry on DOP fractionation, us ing gel permeation chromatography. Experiment two and three examined t he immediate and long-term effects of animal waste application to soil in laboratory columns and outdoor maize-grown lysimeters, respectivel y. 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 m ore than 90%, and from the lysimeters more than 70% of TP leached as D OP (Exp. 2 and 3). In the lysimeter experiment (Exp. 3), leaching of T P mainly occurred in periods of low CI and NO3 concentrations, induced by high leaching rates. This may indicate that P transport in these p eriods was mediated by DOC or other colloidal particles. The field exp eriment (Exp. 4), showed that DOP as a percentage of TP in the soil so lution 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 i s an important form of P in leachates and in soil solutions in the sub soil, and characterization of this DOP can improve our understanding o f P leaching from soils.