Phosphorus Sinks to Phosphorus Sources: The Impact of Legacy P and Conservation Practices.

Characterization of the sinks and stores of phosphorus (P) within a watershed and assessment of the effectiveness of mitigation strategies in dealing with legacy P sources, is essential to predict the expected outcome of restoration schemes. Many current conservation practices are designed to limit the transport of P by trapping P in the landscape, effectively building up hotspots of legacy P stores. This presentation will provide an overview of the possible stores of legacy P within the landscape, residence times and mobilization mechanisms. We assess the unintended consequences of some conservation measures that provide short-term sinks for P reducing P loss, but may transition to sources in the long term. Examples, will be given, which show that once P has accumulated at the surface of no-till soils, in vegetative buffers and riparian zones (designed as filters for particulates), and in fluvial sediments, a slow release of P can occur over time periods of 1 year to decades. We discuss the speciation of P within these P stores and the potential mechanisms for mobilization, highlighting the importance of organic soil P and the microbial biomass for P immobilization and release. We conclude by discussing the future prospects for decreasing P loss from agriculture in light of legacy P stores and predicted changes in climate and outline six key considerations for the design and selection of appropriate conservation practices; (1) pollutant form, (2) transport pathway, (3) success indicators, (4) climatic influences, (5) conservation implementation and (6) conservation compliance.