Phosphorus uptake and release in surface drains

Irrigated dairy farms in Australia contain extensive surface drainage networks that transport water and nutrients off the property into community drains, rivers and lake systems. These drainage networks are an important hydro-chemical link between the paddock, farm and catchment scales. This paper presents a preliminary investigation into phosphorus transfer in two drains from an irrigated dairy farm in southeastern Australia. The effects of drain condition (bare earth vs. grazed pasture) and management (weed growth and herbicide application) are investigated over four drainage events. The results show that farm drains (160 m long) significantly alter the phosphorus concentrations in drainage water. The changes in phosphorus concentration and to a lesser extent phosphorus form appear to be affected by the condition as well as the management of the drain. For example the wetting front concentrations increased by 4.4 mg P/l down a pasture-based drain and decreased by 1.2 mg P/l down an earthen drain. Understanding the transport of phosphorus between the paddock and farm boundary appears to be an important step in developing strategies to minimise the potential export of phosphorus from irrigated dairy farms.