Simulation of phosphorus losses from lysimeters

Because of water resources eutrophication and the need for water protection strategies, the estimation of diffuse phosphorus (P) leaching losses from agricultural soils has become an important issue. The objective of this study was to numerically depict P leaching and transformation processes to clarify the dominant factors controlling the P dynamics in soils and to investigate interactions between P and carbon (C) and nitrogen (N) cycles. We expanded the existing WASMOD modeling package with a P subroutine and tested model performance on an experimental data set from a 12 y lysimeter study. The soil water regime, expressed in terms of yearly variation and the 12 y sum of seepage, was accurately depicted for the investigated soils under grassland. The experimentally observed average yearly P leaching losses varied between 0.03 kg ha–1 for a sandy loam and 1.4 kg ha–1 for a sand. The pronounced, texture-related differences in P leaching were generally reflected by the model, although an immobile water fraction needed to be introduced to obtain satisfactorymodeling results. This demonstrated the strong effect of the flux field heterogeneity on P export. Phosphorus-sorption parameters were found to be more important for P leaching than the extent of the various P pools in the soils. Therefore, it is concluded that in P assessment studies, the sorption characteristics should be determined on an experimental and site-specific basis for top- and subsoils. Model sensitivity analysis revealed that the consideration of the P pools and their interrelation with the C and N cycles in the soil allows a differentiated analysis of the sorption-independent P dynamics. The WASMOD model is useful for the development of future agriculturemanagement strategies to reduce P leaching losses, because its requirement for input data is relatively low.