Characterising phosphorus loss in surface and subsurface hydrological pathways

The magnitude and composition of the phosphorus (P) load transported in surface and subsurface hydrological pathways from a grassland catchment depends on the discharge capacity of the flow route and the frequency with which the pathway operates. Surface runoff is an important pathway for P loss, but this pathway is spatially limited and temporarily confined to high magnitude, high intensity rainfall events. High P concentrations (mean 1.1 mg TP 1-1) were recorded, with most P transported in the dissolved fraction. Preferential flow pathways, particularly soil macropore- s and field drains, are important contributors to the overall P load; most P is transported in the particulate fraction and associated with organic or colloidal P forms. High P concentrations (mean: 1.2 mg TP l-1) were recorded in macropore flow in the upper 0-15 cm of a grassland soil, and generally declined with increasing soil depth. On average, P concentrations in drainflow were over six times greater in stormflow compared to baseflow. Stormflow P losses in drainflow were predominantly in the particulate fraction; significant correlation (P < 0.01) was recorded with suspended sediment concentrations in drainflow. Phosphorus concentrations in groundwater were low ( < 0.2 mg TP l-1 at 150 cm), although this pathway may contribute to stream flow for the majority of the year.