Phosphorus transport from diffuse agricultural sources: shallow subsurface pathways in grassland soils

Phosphorus (P) can be transferred from grassland soils along a number of surface and subsurface hydrological pathways. Currently, our understanding of subsurface transport mechanisms for P is limited. This research examines P transport in shallow subsurface pathways (matrix and macropore flow) under controlled laboratory conditions, and under true field conditions. Simulated rainfall experiments were performed on triplicate soil cores (0.01 m(3)) extracted in summer and winter from an intensively managed grazed grassland field. Matrix and macropore flow samples were collected in situ in this field. Despite the differences in sampling approach, the forms and amounts of P transported in matrix and macropore flow exhibited distinct similarities. In summer matrix flow, for example, total P (TP) concentrations ranged from 47-185 mu g l(-1) in the field and from 76-163 mu g l(-1) in leachate from cores, with soluble P constituting 65% and 61% of TP, respectively. These findings further current understanding of field-scale P transport processes.