br Introduction Phosphorus P is
Introduction Phosphorus (P) is an important nutrient for plant growth; however, in many regions of the world soils are low in plant-available P (Holford, 1997). Therefore, P fertilisers play an important role in agriculture. Most P fertilisers in current use are derived from phosphate rock; however, easily mined deposits of phosphate rock are becoming scarce (Cordell and White, 2011). It is therefore imperative that alternative sources of fertiliser P are found to ensure the sustainability of food production systems. Organic amendments (OA) such as manures, composts, and plant residues have long been used to provide nutrients to crops (Quilty and Cattle, 2011). Moreover, utilising OA can recycle large amounts of P that could otherwise cause environmental problems. However, OA can vary considerably in the amount of P they contain (Sharpley and Moyer, 2000). While many studies have found that OA treatments provide plants with more P than unfertilised controls (e.g. Waldrip et al., 2011, Requejo and Eichler-Löbermann, 2014, Duong et al., 2012), most studies do not have appropriate comparisons among OA or with mineral fertilisers. For example, studies often include P over-application or application of P and N at varying rates, which make it difficult to interpret P results. Moreover, studies rarely examined the chemical nature of the P in OA (with Peirce et al., 2013 as one good exception), which is necessary to better understand plant P uptake from OA. Studies which link plant P uptake from OA with chemical properties of OA and soil chemical and biological processes are needed if OA are to be accurately used in agriculture. The carbon (C):P ratio of OA is often used as an indicator of fertiliser quality (Takeda et al., 2009) and therefore plant P uptake from OA. If C:P is high, more C is added to soil to reach the same level of P addition than for OA with a low C:P. Carbon is often the factor limiting microbial dabigatran etexilate mesylate mg in soil. Therefore, if large amounts of C are added with OA there is a rapid increase in microbial biomass and microbial demand for P (Malik et al., 2013, Ros et al., 2006, Takeda et al., 2009). Soil microorganisms immobilise P when C:P ratios of OA are higher than 20 (Malik et al., 2013, Takeda et al., 2009), reducing the amount of P available for plant uptake. However, this has not been well investigated for OA with a C:P below 20. Moreover, the chemical nature of the C in OA might also be important. For example, if OA contain stable C forms that cannot be readily decomposed, then OA might not stimulate microbial growth. Moreover, if immobilisation does not occur, then the forms of P present in the OA may be a better indicator of fertiliser quality. Different species of P behave in different ways when added to soil, some contributing more than others to the plant-available P pool. More than 60% of the P in OA is commonly found to be orthophosphate (Sharpley and Moyer, 2000). If this orthophosphate is soluble it can readily leach out of amendments and become available to plants or be sorbed to soil particles (Alamgir and Marschner, 2013, Malik et al., 2013). Conversely, insoluble orthophosphate needs to be solubilised before it is available to plants. Both soil microbes and plant root exudates can play a large role in solubilising P in soil (Richardson et al., 2011). In addition to orthophosphate, OA contain organic P species. Phospholipids and nucleic acids can be rapidly mineralised by soil microbes (Harrison, 1982, Islam and Ahmed, 1973) and therefore can quickly contribute to the plant-available pool of P. Phytate, which is present in seeds (Nelson et al., 1968, Noack et al., 2012) and therefore in manures of livestock fed on grains (Toor et al., 2005), has been considered to be stable in soils (He et al., 2006, Celi et al., 1999). However, there is emerging evidence that suggests otherwise (Doolette et al., 2010, Hill and Richardson, 2007, Peirce et al., 2013). When added to a calcareous soil, Doolette et al. (2010) report the disappearance of phytate coupled with an increase in orthophosphate P and α- and β-glycerophosphate over 13 weeks, suggesting microbial degradation of phytate. Therefore, soil microbial activity can have a large influence on soil P transformations and plant P uptake.