Forms and mechanisms for phosphorus leaching in an alluvial soil.

The fresh water quality of New Zealand is considered very clean by international standards however in dense urban and rural areas the water quality has degraded. Economic incentives of a high milk solid price has led to an increase in dairy herds and milking platforms in many regions. The conversion of lowland dry stock pastures into dairy units has been widespread in some of the most vulnerable regions prone to leaching. Many of these areas contain glacial outwash fans and till deposits of gravels and sands covered by thin soil layers, which are characterised as having low Anion storage capacity (ASC) and a high potential to leach phosphorus (P) under irrigation. Leaching studies utilising monolithic soil lysimeters and lab based leaching experiments were undertaken on soils taken from a recently established dairy unit, that occupies part of the Manuherikia River flood plain within the Alexandra basin of Otago, New Zealand. The research objective was to study the potential for P to move down the vadose zone and into groundwater from common P applications under irrigated and grazed dairy pastures. This was addressed by firstly examining the bioavailability of different organic P forms leached in these gravels, secondly, to determine if low ASC gravel soils coincide with enhanced vertical P loss and thirdly, estimating how long intensive farming of these areas would take to enrich groundwater P. Results indicate that although there is attenuation of P occurring within the root zone, the low ASC of the soil leads to significant loss of P towards the vadose zone. A median value of 0.273 mg/l filterable reactive P (FRP) at 1m depth was determined in the receiving gravels of the study area, which exceeds the ANZECC 2000 guidelines trigger value of 0.010 mg/l FRP for lowland rivers. There is also evidence that the majority of loss occurs in the form of preferential flow. This is also enhanced by dung, due to the inhibition of P sorption sites by organic compounds in dung and increases the incidence of labile organic forms of P moving downward towards ground water. There is a strong argument that current concentrations of P potentially reaching ground water underlying these soils, pose a risk to surface water qualit