Phosphorus is one of the most important elements on Earth. It participates in, influences, or controls many of the biogeochemical processes occurring in the biosphere. Feedbacks between the P and other global chemical cycles have been suggested to control many basic characteristics of the biosphere such as the oxygen content of the atmosphere. To understand the interaction between P and other biogeochemical processes and elemental distributions, it is necessary to understand the distribution of P on the Earth's surface and the processes that control its distribution. The strategy of this chapter, therefore, is to (1) discuss the chemical forms in which P is present in the environment; (2) describe the processes that control its distribution in terrestrial, aquatic, and oceanic systems; and (3) define the major P reservoirs on the Earth's surface and the rate at which P is exchanged between these reservoirs. Because all of these subjects must be addressed within this single chapter, the discussion is somewhat superficial and intended to expose the reader to the individual topics rather than to provide a thorough discussion of each. References in each section provide more detailed presentations of the individual topics.
The global occurrence of P differs from that of the other major biogeochemical elements, C, N, S, O, and H in several very important aspects. First, while gaseous forms of P can be produced in the laboratory, none have ever been found in significant quantities in the natural environment. Thus, although some P is transported within the atmosphere on dust particles and dissolved in rain and cloud droplets, the atmosphere generally plays a minor role in the global P cycle. It should be noted, however, that at certain locations this small atmospheric source of P could be important. An example is the surface waters in the central gyres of the oceans where extremely low standing stocks of P are observed and the transport of P from other potential sources is very slow.
The second significant difference between P and the other major biogeochemical elements is that oxidation-reduction reactions play a very minor role in controlling the reactivity and distribution of P in the natural environment. While several oxidation states for P are chemically possible, these forms are generally restricted to controlled laboratory settings. In natural systems, therefore, P is almost exclusively present in the +V oxidation state where it is found as phosphate (PO|~), a tetrahedral oxy-anion. Nearly all dissolved and particulate forms of P are combined, complexed or slightly modified forms of this ion. In general, the biogeochemical cycle of P is synonymous with that of phosphate.
Finally, P also differs from other elements in that it is overwhelmingly dominated by a single, stable isotopic form containing 15 protons and 16 neutrons. There are only two naturally occurring radioactive forms of P: 32P and 33P, which are produced in the atmosphere by nuclear reactions with argon. A small amount of 32P is
Copyright r 2000 Academic Press Limited All rights of reproduction in any form reserved also contributed by 32Si decay. Because these isotopes have extremely short half-lives (32P half-life, 14.3 days; 33P half-life, 25.3 days), their activities in the environment are always very low and account for a minute portion of the total P in the Earth. Nevertheless, modern analytical capabilities have made the study of these isotopes possible, providing new insights in aquatic biogenic processes (Waser et al., 1996; Lai and Lee, 1988; Benitez-Nelson and Buessler, 1999).
14.1.1 Dissolved Inorganic Forms of Phosphorus
Phosphate, POl~, is the fully dissociated anion of triprotic phosphoric acid, H3P04:
The dissociation constants for these equilibria for freshwater and seawater are listed in Table 14-1. The proportion of the individual pro-tonated species in distilled water and seawater over the pH range of 2 to 10 is shown in Fig. 14-1. At the pH of freshwater systems (very roughly 6-7), H2P04~ is the dominant phosphate species. The high ionic strength of seawater and the presence of cations such as Ca2+, Mg2+, and Na+, which form ion pairs with the P04~ species, significantly alter the dissociation of phosphoric acid. In seawater at a pH of 8, HP04 dominates. The importance of ion pairs on the PO4 activity in seawater is further
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