Potential phosphorus losses from grassland soils irrigated with dairy factory wastewater

Applying phosphorus (P)-rich wastewater to land can significantly enrich P in topsoil and consequently increase the risk of P losses in surface runoff and leaching. We collected 654 samples (0-7.5, 7.5-15, and 15-30 cm depths) of wastewater-irrigated and non-irrigated soils at seven sites across New Zealand to assess the potential P mobility in irrigated soils. The risk of P loss to surface runoff was tested by water extractable P (WEP) whereas the risk of P loss by leaching was tested by calcium chloride extractable P (CaCl2-P). These environmental P tests are rarely analysed; hence, a published relationship was used to determine if the risk of P losses could be predicted using anion storage capacity (ASC) and Olsen P, two agronomic tests used in New Zealand. The magnitude of WEP and CaCl2-P could be predicted using the published relationship, but only in non-irrigated soils. Additionally, the mixed model analysis that compared soils and irrigated treatments showed that wastewater-irrigated soils had a greater pH, WEP and CaCl2-P concentration to 30 cm depth. However, in both irrigated and non-irrigated soils a change point or threshold in ASC was found that could be used to show an increased likelihood of enhanced P leaching (as CaCl2-P) to deeper layers. In addition, the WEP and CaCl2-P thresholds, can be used to signal the need for management changes to decrease the risk of P losses to surface water and groundwater.