# Phosphorus Distribution in Soils from Australian Dairy and Beef Rearing Pastoral Systems
## 1 Introduction  
Phosphorus (P) exists in soils in both inorganic and organic forms [1; 2]. Many agricultural areas have relatively high soil total P concentrations compared to background environmental levels [3]. However, these high concentrations are often sub-optimal for agricultural practices, as most of the soil P is not readily available to plants because it is bound in insoluble inorganic and organic forms, such as calcium, iron and aluminum phosphate, inositol hexakisphosphate, and phytate. The deficit of available P is often treated through the addition of inorganic P fertilisers. A proportion of this is also converted into insoluble forms [4; 5], which accumulate, raising overall soil P concentrations. The organic P content of soil depends on a number of factors, including soil drainage, soil pH, the inorganic content of parent material and cultivation [6]. As a consequence, the organic P fraction of soils varies significantly, generally representing between 20% and 80% of total soil P [6; 7; 8]. Pasturesoils tend to contain a higher proportion of organic P than cultivated soils [9]. Some of the \"available\" organic P forms play important role in biological cycling of P in soils, however the dynamics of organic P are not completely understood and require further investigation [10; 11]. Organic P determinations in soil can be difficult due to extraction, separation and detection problems [12]. First identified in 1940 [13; 14], phytate (C6H6 Ca6O24P6 as a calcium salt) has been considered the abundant organic P form [6; 11; 15], representing up to 50% of all soil organic P [6; 16; 17]. However, an investigation by [PERSON] and [PERSON] [18] involving phytate spiking, has indicated that this organic P may have been previously overestimated due to misassignment of resonances in \({}^{31}\)P NMR spectra. Their study showed that phytate contributed <5% and <3% of organic P and total P, respectively, in the four Australian grazing soils analysed. Many methods have been used to extract organic P, including base extraction, chromatographic isolation and acid or base hydrolysis [18; 19; 20; 21; 22]. More recently, phosphorus-31 (\({}^{31}\)P) nuclear magnetic resonance (NMR) spectroscopy has been adopted as it removes many limiting factors of these chromatographic methods [12]. Phosphorus is particularly amenable to NMR analysis because NMR-active \({}^{31}\)P is the only P isotope found naturally. The main advantage of NMR is that it has the potential to be able to detect and quantify all P species simultaneously [23]. First used for soil characterisation in 1980 [24], \({}^{31}\)P NMR spectroscopy has been used for both qualitative and quantitative estimates of both organic and inorganic soil P compounds [2; 25]. The use of \({}^{31}\)P NMR spectroscopy enables quantification of many phosphate compounds, including inorganic orthophosphate (PO4\({}^{3-}\)), polyphosphate (P\({}_{n}\)O\({}_{3n+1}\)\({}^{(n+2)-}\))and pyrophosphate (P\({}_{2}\)O\({}_{7}\)\({}^{4-}\)), organic orthophosphate monoesters (RPO4\({}^{2-}\)) and diesters (R\({}_{2}\)PO4\({}^{-}\)), as well as some specific organic P compounds [8; 11; 25; 26]. Of the organic compounds, monoesters are usually the principal form found in soil [6]. Monoester phosphates combine with metal ions, like Fe, to produce moderately labile insoluble complexes [16; 27; 28]. However, it has recently been reported that monoesters such as Inositolhexapphosphate isomers are often dominating and they do not produce moderately labile insoluble complexes, but are strongly bound to the soil solid phase [29]. Organic diesters are found in relatively small concentrations compared to monoesters. Diester phosphates are considered more labile than monoester phosphate and are generally more accessible to attack by soil microbes, which in combination contributes to lower concentrations in soil [27; 28].