Total unreactive phosphorus

For these reasons, numerous attempts have been made to identify and characterize DOP, but with little success because it is usually present in very low concentrations. Typical values in lake waters range from 5 to 100 xg of P/L in oligotrophic to eutrophic systems. Colorimetric methods have been used extensively to detect and differentiate between soluble reactive phosphorus (SRP) and soluble unreactive phosphorus (SUP) at concentrations as low as 10 xg of P/L (I). SRP is generally considered to consist of only orthophosphate compounds, whereas SUP is composed of all other phosphorus species, primarily organic phosphorus compounds. The sum of SRP and SUP is equal to the total soluble phosphorus (TSP). These methods were used to study the dynamics of bulk phosphorus fractionation between the sediments, suspended particulate matter, the biota, and the dissolved fraction (2). Despite these studies, very little is known regarding the identity and characteristics of the DOP in the hydrosphere. 

The total abundance of each condensate is limited by the abundance of the least abundant element in the condensate. For example, the mineral schreiber-site Fe3P forms by reaction of P-bearing gases with Fe metal at about 1300 K and 10 4 bar total pressure. Phosphorus has an atomic abundance of 8,373 atoms, which is about 1% of the atomic abundance of iron. There are 3 Fe atoms in each molecule of schreibersite. Thus Fe3P formation consumes only 3% of the total iron abundance, while removing all phosphorus from the gas phase. Likewise, troilite formation removes all sulfur from the gas because its abundance is only 53% of that of iron, while unreacted Fe metal remains present at lower temperatures until it is consumed by formation of Fe-bearing oxides and silicates. 

The organic phosphorus content of solutions is generally determined by the difference between total phosphorus and molybdate-reactive phosphorus. As this fraction may also contain condensed phosphates (Ron Vaz etal., 1993), the operational term molybdate-unreactive phosphorus is used throughout the chapter, rather than organic phosphorus, which is used in a generic sense. 

The second reason for acid-digestion is the determination of the total soil elemental content of, e.g. potassium, phosphorus or trace elements. This is seldom done for potassium in normal soil samples, mainly because the total K in soils is of no value as an index to the availability of K to plants, nor is it always of value in tracing the movement or accumulation of applied fertilizer K (Pratt, 1965). The unreactive soil phosphorus is obtained by subtracting the naturally leached reactive phosphorus from the total phosphorus, and a method for determining the latter by extraction with sulphuric acid and potassium persulphate is cited by Turner and FHaygarth (2000). They analysed... 

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