n2o rock derived, which severely restricts its input rate and makes it depletable (Walker and Syers, 1976). In highly weathered soils, where P is no longer supplied from the parent material, dust deposition may represent the majority of P inputs (Vitousek, 2004). The soil P cycle has been represented in several forms, depending on the particular emphasis used by the author (e.g., soil solution P, soil-plant relations, or microbially mediated P transformations). In Fig. 15.3, soil solution P is placed in the central position, and the P cycle is divided into two subcycles: a biological one in which pools are defined in terms of biological constituents or stability and transfers are primarily microbially mediated, and a geochemical subcycle in which pools are defined in terms of chemical or mineralogical composition and transfers are primarily abiotic. While soil P is not particularly mobile, some losses from soil in dissolved organic form have been observed in highly weathered soils and were found to be of a magnitude similar to that of inputs (Vitousek, 2004). Unlike C and N, phosphorus does not show large biologically mediated fluxes to and from the atmosphere, nor does it serve as a primary energy source for microbial oxidations. Nevertheless, soil organisms are intimately involved in the cycling of soil P. They participate in the solubilization of inorganic P and in the mineralization of organic P. They are also important in the immobilization of available soil P.

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