Farm gate nutrient balances in south west Western Australia – an eco-efficiency indicator?

The ecology of estuaries on the south coast of Western Australia has been disrupted because of increased nutrient and sediment discharge from predominantly rural catchments. To tackle this problem at its source, it is important to evaluate nutrient balances and flows so that opportunities for management intervention can be better targeted. The nutrient studied in detail here is phosphorus (P) although this approach should be valid for other nutrients and elements. Specific data on nutrient ‘eco-efficiency’ such as the levels of P export in products, and the P inputs that give rise, are often limited. The case studies reported here developed P balances for a broad range of land uses in the Peel-Harvey, Torbay and Geographe Bay catchments in south west Western Australia, to provide P data for associated nutrient BMP modelling work, and to gain insight into P use efficiency and potential application of P balance data as an eco-efficiency indicator. The studies indicate that inputs greatly exceed outputs in products removed in most instances, leading to nutrient surplus and accumulation. In many cases P surplus was almost the same as P input, and given that many of the surface soils in these regions have exhausted their capacity to act as P sinks, the resultant P-use efficiency indicators reflect the inherent inefficiencies of the plant animal system. A range of nutrient balance and use efficiency signatures have emerged for different land uses, describing the inherent P use efficiency of each. Efficiencies varied within land uses, possibly due to management practices, and environmental situation. The P use efficiency provides an indicator of P surplus, but estimated levels of P surplus and loss to the environment are most strongly correlated with farm P inputs. The selection and prioritisation of management practices designed to reduce P loss needs to consider how each management action addresses the issue of “P balance”