The concentration of phosphorus (P) in runoff from dairy pastures has been shown in numerous studies in SE Australia to be undesirably high. Often systematic concentrations of P are in the range 1-5 mg/L, composed predominantly (e.g. >90%) of dissolved (i.e. <0.45 micron) forms of P. Although it is possible to reduce incidental losses of P (e.g. by not applying fertiliser immediately prior to an expected runoff event), reducing the systematic losses of P is highly desirable. The systematic concentration of P is the result of mobilisation of P primarily from the soil P pool. Because P is highly stratified in permanent pasture systems, i.e. large concentrations of P exist at the very surface (e.g. 0-1 cm), de-stratification (i.e. mixing of the topsoil by means such as cultivation) offers a method to reduce the concentration of soluble P at the soil-runoff interface. It is hypothesised that reductions in soluble P in surface soil will lead to reductions in P exported in runoff. Furthermore, because runoff containing P is typically only produced in the lower areas of the landscape, decreasing the soluble soil P pool in these areas may result in a large decrease in runoff P concentration whilst importantly minimising the impacts on pasture and farm management. A combination of laboratory studies, rainfall simulations and a catchment hydrological-P export model is being used to investigate the effect of soil de-stratification on P export in runoff from sub-catchments.

Soil samples of a wide range of fertility levels (Olsen P:23-132 mg/kg) representing the top 1 cm of soil (controls) and their corresponding (0-10 cm) increments were incubated in the laboratory at 60% field capacity (20^oC) for 30 days to simulate de-stratification. Soil soluble P (10 mM CaCl₂, 1:5 soil:solution, 30 mins) was then measured on the control and de-stratified soil. De-stratification reduced soluble P in the soil (P<0.001) by on average 82% (Figure 1a). This data was then used in a soil P-runoff P model derived using rainfall simulation to predict the effect of de-stratification on runoff P concentration (Figure 1b).

Figure 1. The effect of de-stratification on a) soil P and b) the modelled effect of de-stratification on runoff [P]; undisturbed soil (◊) and a de-stratified soil (♦).

The effect of de-stratification is currently being further investigated using a combination of further rainfall simulations, combined with modelling of the impact on P export at the paddock and sub-catchment scale using a hydrological model (TOPMODEL) coupled with a runoff P model.