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 ly1. were recorded, with most P transported in the dissolved fraction. Preferential flow pathways, particularly soil macropores 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 ly1. 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 ly1 at 150 cm., although this pathway may contribute to stream flow for the majority of the year.