Preferential Transport of Phosphorus in Drained Grassland Soils

Phosphorus is the limiting factor for primary production in most freshwater ecosystems. In many areas, diffuse P losses from intensively cultivated land cause severe eutrophication of surface waters. We investigated the P export from two drainage systems under intensively used grassland in a catchment of the Swiss Plateau. Flow rate and nutrient concentrations were measured with a high temporal resolution during discharge events. During most flow peaks, P concentrations strongly increased with increasing flow rates. Concentrations of soluble-reactive P (SRP) reached up to 155 μmol L -1 . Phosphorus was mainly transported as soluble-reactive and particulate P. Organic P compounds, as well as P associated with colloids between 0.05 and 0.45 μm in effective diameter, were of minor importance. Estimated P loads from the drainage systems were 227 g SRP ha -1 within a period of 2.5 mo at site I and 1290 g ha -1 during 6 mo at site II. Estimation uncertainty was large (±21 and ±36% for the two sites, respectively) due to the weak correlation between discharge and concentration for all data from a given site. Water-extractable P in the soil was concentrated in the uppermost layer of the profiles or, for short periods after spreading of manure, deposited on the vegetation. The discharge-concentration relationship indicated that P was transported through preferential flow paths extending from close to the surface to the drains. Sprinkling experiments with a blue dye confirmed this conclusion. At one site, we observed preferential flow in a downhill direction within the saturated zone.