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 flow was expected to be the main mode of water 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 (PO 4 -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.