Carbon and phosphorus losses from dairy pasture in South Australia

Runoff (overland flow and A/B horizon interflow) was measured from 2 grazed dairy pastures at Flaxley, South Australia, from 1996 to 1998. Runoff ranged from 0.4% to 10% of annual rainfall and >90% of this was overland flow. Phosphorus and carbon were measured in runoff. As much as 2.3 kg/ha of phosphorus and 10.7 kg/ha of total dissolved carbon were lost from the subcatchments in the wettest year. Over the study period, 98% of total phosphorus and 86% of total dissolved carbon were lost in overland flow. Around 45% of phosphorus was dissolved and 69% of total dissolved carbon was dissolved organic carbon. The proportion of phosphorus present in the particulate form decreased during each runoff season, and was highest in the wettest year. There was no consistent trend in the proportion of total dissolved carbon present as dissolved organic carbon because the factors found to affect dissolved organic carbon loss were different from those affecting dissolved inorganic carbon loss. Predictive relationships based on factors such as the time of year when the storm occurred and runoff volume have been developed from the 3 years of data and they explain a high proportion of variability of phosphorus and carbon loads.