Phosphorus leaching from soils containing different phosphorus concentrations in the Broadbalk Experiment

Total P (TP), total particulate P (TPP), dissolved reactive P (DRP), and dissolved organic P (DOP) were determined in waters from pipe-drains (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, now 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 aim was to determine if significant quantities of P could be detected in the drainage water and their relationship to soil P concentrations. On five occasions between October 1992 and January 1994, both TP and DRP 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 low (<0.15 mg L(-1)) from plots below 60 mg Olsen-P kg(-1). There was then 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. Above this, P losses in the drainage water were much more closely related to Olsen-P than commonly suggested. The mechanisms could either be preferential how or rapid transport of P in forms less susceptible to sorption but finally measured as DRP. The results suggest enhanced P losses through subsurface runoff on heavy soils, once a certain plow layer concentration of Olsen-P is exceeded.