Mode of transport of surface-applied phosphorus-33 through a clay and sandy soil

Phosphorus (P) is the limiting nutrient for primary production in most freshwater ecosystems. The magnitude of P leaching from agricultural soils is therefore critical. Preferential flow has been proposed as a major cause for high P losses in structured clay soils. Undisturbed soil of two texturally different soils, that is, a clay soil in which preferential now was expected to be the main mode of mater transport and a sandy soil where piston flow is the dominant process, were used in this study, Use of labeled P made it possible to determine the origin of leached P. An equivalent of 100 kg P ha(-1), labeled with (33)p, was added to the soil surface of each lysimeter, Water equivalents to 100 mm were added on five occasions with 7 d between each watering event. Ponded flow conditions were established during periods when water was added, to trigger preferential flow behavior. Phosphorus leaching loads from clay columns were much higher than P loads from sand columns, The average P leaching load for the five clay columns was 4.0 kg ha(-1), compared to only 56 g ha(-1) for the three sand columns. The main part of leached P was in dissolved (PO4-P) form. The recently added P prevailed in leachate from the clay soil indicating rapid transport of added P from the soil surface through the profile via macropores.