Effect of variable soil phosphorus on phosphorus concentrations in simulated surface runoff under intensive dairy pastures.

摘要

Intensive dairy operations in Australia regularly apply P fertiliser to maintain productive pasture species. However, extractable soil test P (STP) concentrations in this industry commonly exceed those required to maximise pasture production, a situation which can increase the risk of P loss to surrounding waterways. The current study examined relationships between STP (Olsen P and CaCl₂ P) and surface runoff P concentrations from a red silty loam (Ferrosol), commonly used for pasture production in south-eastern Australia. Soil was mixed and re-packed into soil trays and a rainfall simulator was used to generate surface runoff. A wide range of soil Olsen P concentrations (0-20 mm, 15-724 mg/kg; 0-100 mm, 9-166 mg/kg) was created by surface-applying a range of P fertiliser rates 8 months before the rainfall simulations. A comparison of the 2 STP methods suggests that Australian soils have higher labile P concentrations for given Olsen P concentrations compared with those measured internationally, suggesting a greater likelihood of P loss in runoff. Furthermore, significant curvilinear relationships between STP and dissolved reactive P (DRP<0.45 micro m) in surface runoff for both Olsen P depths (0-20 mm, r2=0.94; 0-100 mm, r2=0.91; P<0.01) were determined, as well as significant linear relationships between DRP and both CaCl₂ depths (0-20 mm, r2=0.83; 0-100 mm, r2=0.92; P<0.01). This confirmed that the concentrations of P in surface runoff increased with increasing STP, providing further evidence of an urgent need to reduce excessive STP concentrations, to reduce the risk of P loss to the environment.