Forms of organic phosphorus in water, soils, and sediments

The mineralization of labile forms of organic P (OP) is a critical link for internal P cycling and, to a large degree, may determine the productivity of an ecosystem. However, OP is typically identified simply as the difference between total P (TP) and inorganic P (IP). This chapter provides an overview of techniques available to identify and quantify different forms of OP, and summarizes results from Florida ecosystems, emphasizing wetland and aquatic systems. The ability of OP compounds to act as either a sink or a source for bioavailable P is dependent on their molecular structure. Therefore, the first task in examining the role of OP in internal cycling is to identify its different forms. Organic P identification can be separated into groups based on biological availability, chemical fractionation, and physicochemical separation. Bioavailable P has been estimated using both direct (uptake of radiotracers) and indirect measures such as resins, chemical extraction of P, and enzymatic hydrolysis. Conventional characterization of OP forms in soils and sediments is through chemical fractionation in which alkali media are used to extract different fractions of OP based on the mechanism by which the OP fraction is incorporated in the other soil or sediment components. These operationally defined characterizations can be used to monitor turnover rates of labile and nonlabile forms of OP and to make broad comparisons among different regions. More direct quantification of OP forms can be obtained by considering physicochemical characteristics at the molecular level. For example, obtaining P-31-nuclear magnetic resonance (NMR) spectra on alkaline extracts is the most commonly used technique for the quantification of broad classes of OF. The diagnostic property in P-31-NMR spectroscopy is the chemical shift of the P nuclei. Other methods for direct identification of OP compounds include enzymatic hydrolysis and physical separation based on molecular weight using chromatography. The separation and identification of OP compounds is an expensive and time-consuming process and, to date, <50% of P forms are thought to have been identified. This paper discusses the advantages and disadvantages of the different techniques and the data they produce to highlight existing knowledge and identify future areas of research.