PREFERENTIAL TRANSPORT OF PHOSPHORUS IN DRAINED GRASSLAND SOILS

Phosphorus is the limiting factor for primary production in most fresh water ecosystems. In many areas, diffuse P losses from intensively cul tivated land cause severe eutrophication of surface waters. We investi gated the P export from two drainage systems under intensively used gr assland in a catchment of the Swiss Plateau. Flow rate and nutrient co ncentrations were measured with a high temporal resolution during disc harge events. During most flow peaks, P concentrations strongly increa sed with increasing now rates. Concentrations of soluble-reactive P (S RP) reached up to 155 mu mol 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 mu m in effective diam eter, were of minor importance. Estimated P loads from the drainage sy stems were 227 g SRP ha(-1) within a period of 2.5 mo at site I and 12 90 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 corr elation between discharge and concentration for all data from a given site. Water-extractable P in the soil was concentrated in the uppermos t layer of the profiles or, for short periods after spreading of manur e, deposited on the vegetation. The discharge-concentration relationsh ip indicated that P was transported through preferential now paths ext ending from close to the surface to the drains. Sprinkling experiments with a blue dye confirmed this conclusion. At one site, we observed p referential flow in a downhill direction within the saturated zone.