Phosphorus in runoff from a fertile dairy pasture

Losses of phosphorus (P) from grazed pastures in runoff can significantly reduce the water quality in streams draining agricultural catchments. Many preventative strategies depend for their success on the association of the P with large soil particles (>0.45 mu m). We investigated the forms of P in runoff from a dairy pasture of high fertility. A 3.6-ha paddock at Darnum in West Gippsland, Victoria, was set up to measure runoff volume and rainfall, and to sample runoff water. Runoff from the site was restricted to 3 months in late winter and spring. Total runoff for 1994 of 660 m(3) had an average total P concentration of 5.2 mg/L. Runoff from a single storm system over 8 days in early November accounted for 56% of the total runoff volume and 69% of the P lost. Of the total P in the runoff, 93% passed through a filter with 0.45-mu m pores and 91% was reactive in an ascorbic acid-molybdate medium, without digestion; 89% of the P was both reactive in the ascorbic acid-molybdate medium and in the <0.45-mu m fraction. There was no apparent relationship (P > 0.05) between P concentration and soil cover. In the Darnum catchment, the major runoff events occurred when the soil was saturated. The low settling velocities of materials <0.45 mu m and the amount of P moving in larger storms would suggest that buffer strips and riparian zones are unlikely to decrease P losses substantially from dairy pastures in this area. Further, since erosion is unlikely to be the major process by which P is lost, control measures which simply limit erosion will be similarly ineffective at preventing P losses under the conditions that prevailed in this study.