PHOSPHORUS LEACHING FROM SOILS CONTAINING DIFFERENT PHOSPHORUS CONCENTRATIONS IN THE BROADBALK EXPERIMENT

Total P (TP), total particulate P (TPP), dissolved reactive P (DRP), a nd dissolved organic P (DOP) were determined in waters from pipe-drain s (at 65-cm depth) from the Broadbalk Experiment at Rothamsted. Soils that have received either no P, P in farmyard manure (about 40 kg P ha (-1)) or superphosphate (up to 35 kg P ha(-1)) annually for >150 yr, n ow contain 0.5 M NaHCO3-extractable P concentrations (Olsen-P) in the plow layer (0- to 23-cm depth) between 5 and 100 mg kg(-1) soil. Our a im was to determine if significant quantities of P could be detected i n the drainage water and their relationship to soil P concentrations. On five occasions between October 1992 and January 1994, both TP and D RP from plots receiving superphosphate frequently exceeded 1 mg L(-1) and were high compared with literature data. Ranging between 66 and 86 % of TP, DRP was the largest fraction in drainage water. It remained l ow (<0.15 mg L(-1)) from plots below 60 mg Olsen-P kg(-1). There was t hen a rapid increase in DRP up to the maximum Olsen-P concentration. A simple linear Split-Line Model described this relationship very well for all drainage events. This implies that up to 60 mg Olsen-P kg(-1) soil (the change point), P was retained strongly in the plow layer. Ab ove this, P losses in the drainage water were much more closely relate d to Olsen-P than commonly suggested. The mechanisms could either be p referential how or rapid transport of P in forms less susceptible to s orption but finally measured as DRP. The results suggest enhanced P lo sses through subsurface runoff on heavy soils, once a certain plow lay er concentration of Olsen-P is exceeded.