Increased predicted losses of phosphorus to surface waters from soils with high Olsen‐P concentrations

Diffuse soluble reactive P (SRP) & total P (TP) loads from over 50 major river catchments in Northern Ireland were predicted using an export coefficient modelling approach. Phosphorus export coefficients for each CORINE land cover class, derived from satellite imagery, allowed the prediction of P loads from a breakdown of the CORINE land cover classes by catchment using a GIS. This approach was validated using observed P loads calculated from flow and concentration data. Mean measured Olsen‐P concentrations in the soil A‐horizon were also determined on a catchment basis. Plots of P loads to the watercourse versus Olsen‐P concentrations in the soil showed a breakpoint around 22 mg Olsen‐ P l −1 for both SRP & TP data. Below Olsen‐P concentrations of 22 mg l −1 , SRP & TP losses were essentially independent of Olsen‐P at 0.28 and 0.63 kg P ha −1 yr −1 , respectively. Above Olsen‐P concentrations of 22 mg l −1 , there was considerable spread in the P loss data. Nevertheless, significant upward trends in SRP and TP losses to watercourses were detected with increasing Olsen‐P at a rate of approximately 0.5 and 1.0 kg P ha −1 yr −1 , for SRP and TP respectively, for each 10 mg l −1 increase in Olsen‐P.