of Environmental Quality CELEBRATING THE 350 TH ANNIVERSARY OF DISCOVERING PHOSPHORUS — FOR BETTER OR WORSE

Since its discovery in 1669, phosphorus (P) in the form of fertilizer has become an essential input for many agroecosystems. By introducing a concentrated P source, fertilizers increase short-term P export potential soon after their application and longer-term export potential by increasing soil fertility (legacy P). The 4R concept was developed to help mitigate P exports from the fertilizers that sustain agricultural productivity. This review investigates the factors affecting P exports soon after the application of mineral fertilizers to pasture-based grazing systems and studies quantifying its potential impact in different systems, with an emphasis on Australasia. Initially, P fertilizers and reactions that might affect their short-term P export potential are reviewed, along with P transport pathways, the forms of P exported from grazing systems, factors affecting P mobilization into water, and studies demonstrating the possible short-term effects of fertilizer application on P exports. Using that foundation, we review studies quantifying the short-term impact of fertilizer application in different regions; they show that under poor management, recently applied fertilizer can contribute a considerable proportion (30–80%) of total farm P exports in drainage, but when fertilizer is well-managed, that figure is expected to be <10%. We then use three model systems of varying hydrology that are common to Australasia to demonstrate the principles for selecting fertilizers that are likely to minimize P exports soon after their application. Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures D. M. Nash,* R. W. McDowell, L. M. Condron, and M. J. McLaughlin Phosphorus (P) is the sixth most abundant element in living organisms. Since its discovery in 1669, P has become an essential input for many agroecosystems (Lun et al., 2018; Sharpley et al., 2018), and P amendments have facilitated our transition to modern urbanized societies. However, the use of P amendments in agriculture has not been without its challenges. The application of amendments and accumulation of P in agricultural soils (Stutter et al., 2015; Tian et al., 2017) have increased the risk of P in drainage adversely affecting downstream water resources (ANZECC and ARMCANZ, 2000; Brogan et al., 2001; Cooke et al., 1993; Dodds and Smith, 2016; European Environment Agency, 1998; USEPA, 1996). In both terrestrial and aquatic systems, organisms extract P in the form of orthophosphate anions (H2PO4 /HPO4 2-) from water. The total P (TP) concentration needed in soil water for optimum agricultural production is often cited as >0.2 mg L−1 (Pierzynski et al., 2005). However, a soil water P concentration of ~0.9 mg L−1 was required for maximum pasture productivity in New Zealand field trials (Wheeler and Edmeades, 1995) and concentrations >>1 mg L−1 have been noted in Australia (Nash et al., 2007; Toifl et al., 2000; Watkins et al., 2012). For freshwater systems, TP concentrations designed to minimize the potential for adverse algal (viz. periphyton) growth are <0.009 to 0.07 mg L−1, varying by region or jurisdiction (ANZECC and ARMCANZ, 2018; CCME, 2004; EPA, 2003; Miltner, 2010; Smith and Tran, 2010; Stevenson et al., 2008). Given that some P is always present in soil water, it follows that transfers from land to water (i.e., P exports) are an inevitable consequence of drainage from terrestrial systems (Haygarth et al., 2005) and streams draining agricultural catchments often suffer “impaired” water quality In Australasia (i.e., Australia and New Zealand), intensive pasture-based grazing is often the primary land use in catchments with impaired water quality (Dairy Australia, 2019; McDowell et al., 2009a; Meat and Livestock Australia, 2017; Abbreviations: DAP, di-ammonium phosphate; DRP, dissolved reactive phosphorus; MAP, mono-ammonium phosphate; MCP, monocalcium phosphate; PR, phosphate rock; SSP, single superphosphate; TDP, total dissolved phosphorus; TP, total phosphorus; TSP, triple superphosphate. D.M. Nash, School of Agriculture and Food, Faculty of Veterinary and Agricultural Sciences, The Univ. of Melbourne, Parkville, VIC 3010 Australia; D.M. Nash, Soil and Allied Services Pty. Ltd., 1500 Warragul Rd., Strzelecki, VIC 3950 Australia; R.W. McDowell, AgResearch, Lincoln Science Centre, Private Bag 4749, Christchurch 8140 New Zealand; R.W. McDowell and L.M. Condron, Faculty of Agriculture and Life Sciences, P.O. Box 85084, Lincoln Univ., Christchurch 7647 New Zealand; M.J. McLaughlin, School of Agriculture Food and Wine, Waite Research Institute, The Univ. of Adelaide, PMB 1, Glen Osmond, SA 5064 Australia. Assigned to Associate Editor Philip Haygarth. © 2019 The Author(s). Re-use requires permission from the publisher.