Potential phosphorus and sediment loads from sources within a dairy farmed catchment

Phosphorus (P) losses from intensively farmed dairy pastures can impair surface water quality. One of the first steps in mitigating this loss is to determine where in a field the potential for P loss is greatest. This study compared P export in overland flow from grazed pasture with areas that receive elevated P inputs and stock traffic (e.g. gateway, water trough, stream crossing and cattle lane). Intact soil blocks were removed, simulated rainfall applied and overland flow analysed for P fractions and suspended sediment (SS). Soil bulk density, hydraulic conductivity, porosity, Olsen P and water soluble P were also measured. P loss from the sites was in the order: trough > crossing > gateway > pasture. Total P losses from the trough averaged 4.20 mg P/m2 while the pasture exported 0.78 mg P/m2. In addition, runoff from lane soil was measured with total P averaging 5.98 mg P/m2, however the method used was different from the other soils. Using stepwise linear regression, Olsen P or H2O-P, % bare ground and % saturation were the most commonly occurring variables to predict P loss among the sites. This suggests that locating and minimizing the size of these areas in fields has the potential to significantly decrease P loss to surface waters.