Transport of phosphate from soil to surface waters by preferential flow

Enrichment of lakes with soluble reactive phosphorus (SRP) leads to their deterioration as ecosystems, recreation areas, and drinking water reservoirs. In many cases, fertilized soils are their most important P source. Most studies dealing with P losses from soils to surface waters concentrate on erosion and surface runoff. Leaching is mostly considered to be of minor importance. On the basis of an in-situ sprinkling experiment with dye and bromide as tracers and of observations of the dynamics of SRP concentration and water discharge at the watershed scale, we identify soil macropores and artificial drainage systems as the most important pathways for the vertical and lateral transport of SRP from P-enriched soil surfaces to surface waters. A conceptional model explains why in drainage systems flow rate and SRP concentration are positively related. We estimate that more than half of the yearly SRP load is leached from the soil, and thus conclude that counter to the conventional wisdom, in the investigated watershed, leaching and not surface runoff is the most important mechanism for P transfer from soils to surface waters. It should be tested to see whether this conclusion can be generalized and also hold true for other watersheds with artificially drained, P-enriched soils with a low matrix permeability.