Phosphorus source areas in a dairy catchment in Otago, New Zealand

Abstract

It is important to recognise source areas of phosphorus (P) in agricultural catchments and to understand how they contribute to catchment losses of P in order to effectively target mitigation strategies to decrease losses to surface waters. In a small dairy catchment (4.1 ha), soil physical properties and overland flow from pasture, a laneway, and around a watering trough were measured, together with subsurface flows from pasture and catchment discharge. Soil measured around the trough and in the laneway was found to be enriched in Olsen P (56 and 201 mg P/kg, respectively) compared with the pasture (24 mg P/kg), as well as having a greater bulk density resulting from more frequent use by animals. Dissolved P losses from lane and trough plots were greatly enhanced via dung. At the catchment scale, sources and transport processes resulted in losses mainly in the particulate P form (0.21 mg/L), while dissolved reactive P (DRP) concentrations were enriched during storm events (0.08 mg/L). Subsurface flow was found to be an important contributor of discharge and likely P losses, and this warrants further investigation. The scaling up of overland-flow plot data suggested that the laneway contributed up to 89% of the DRP load when surface overland flow was likely. This represents a substantial source of P loss on dairy farms. Additionally, the variation of sources and transport processes with season adds another aspect to the critical source area concept, and suggests that given the loss during summer and high algal availability of dissolved P, mitigation strategies should target decreasing dissolved P loss from the laneway.