Pathways and processes of phosphorus loss from sheep pastures

Sheep producers in Victoria are applying more phosphorus (P) fertiliser and increasing stocking rates to increase production. At the same time, there is increasing awareness amongst research, community and producer groups that P-rich runoff water from agricultural land can contribute to the growth of undesirable algal blooms in surface waters. The loads, concentrations and forms of P in surface and subsurface hydrological flow pathways were estimated or measured directly on high and low P fertility hillslope plots in south-west Victoria to assess how this practice change could affect the P status of runoff and drainage water. Smallplot rainfall simulator studies were also conducted to investigate pasture management treatmenteffects. The spatial and temporal distribution of P loss from hillslope pastures, and the processes of P mobilisation in runoff identified in this study were used to identify appropriate management practises to help minimise P losses in runoff. Runoff P concentrations were then related to properties of pasture soils and runoff at a range of locations across Victoria to identify whether simple models and/or soil P tests could be used to predict P concentrations in runoff.Results from the study suggested that P concentrations in runoff from both low (8 kg P applied/ha) and high (25-40 kg P applied/ha) P fertility sheep pastures were unacceptably high relative to targets set for stream water quality in Victoria (annual mean concentrations >0.04 mg P/L), and that the dissolved reactive P fraction is the P fraction in runoff that increases the most as pasture P fertility increases. The volume of runoff was the primary factor influencing total P loads. Runoff was generated mainly by saturation excess flow during winter and spring fromwaterlogged soils that occupied <25% of the hillslope area. Subsurface hydrological pathways were only more important than surface pathways for P movement in dry years.Key management practises that are therefore recommended to minimise P losses from pastures in Victoria, are to avoid applying high rates of P fertiliser to landscape areas thatbecome seasonally waterlogged, to consider retiring these areas from production and to maintain groundcover year-round to minimise erosion. An empirical model that uses a linear dissociation constant to describe the partitioning of P between runoff sediment and solution phases was reasonably well suited to predicting annual average DP concentrations in runoff from sheep pastures. The results also suggested, however, that factors such as runoff volume and fertiliser management practices (eg timing of application) should be considered in predictive models. The model was analogous to simplerelationships developed between runoff P concentrations and soil test P. For the soils examined, which had a limited range of P sorption capacity (Pmax 862-1374 mg/kg), the POlsen soil test was a better predictor of runoff P concentrations than water extractable P or the degree of soil P saturation. Relationships between soil P status and hillslope runoff water quality need to be investigated for a wider range of soils, agricultural practices and catchments in Australia if environmentally critical soil P thresholds are to be set.