The Global Phosphorus Cycle

The global P cyde proposed by Lerman et al. (1975) is presented in Fig. 14-7. This representation has been modified slightly to include the atmosphere (Graham, 1977). For clarity, the mass of P estimated to be contained in each of these reservoirs is listed in Table 14-3 along with the method by which it was calculated. 

The reservoir representing the land (2) is defined as the amount of P contained in the upper 60 cm of the soil. This rather narrow definition of the land reservoir is made because it is through the upper portions of the soil system that the major interactions with the other P reservoirs occur. Specifically, most plants receive their nutritive P needs from the upper soil horizons and the return of P to the soil system by the decomposition of plant matter is also concentrated in this upper soil zone. Similarly, the major interactions with the atmosphere, ground-waters, and rivers occur near the soil surface. And, finally, phosphate in the form of fertilizer is applied directly to the soil surface. Thus, in attempting to represent the land and its interaction with other reservoirs, the surface soil horizon most directly interacts with all components and best represents the d)mamical nature of this reservoir. Phosphorus in soils deeper than 60 cm and in crusted rocks is included in the sediment reservoir (1). This reservoir accounts for all of the particulate P that exchanges with the other reservoirs only on very long time-scales. 

Land 2.00 X 10 Computed from land area of 133 x 10 km, assumed soil thickness of 60 cm, densityof2.5g/cm andameanPcontentof 0.1% (Taylor, 1964) 

Land biota 3.00 X 10 Computed from an estimate of the N in land biota (12 x lo tonsN Delwiche, 1970) and a mean P N atomic ratio in land plants (1.8 16 Deevey, 1970) 

Oceanic biota 1.38 X 10 Computed from N in ocean biota (1 x lO tons N Vaccaro, 1965) and a mean P N atomic ratio of 1 16 (Redfieldef al., 1963) 

The main reservoirs and exchange pathways of the global P cycle are schematically presented in Fig. 14-7. This representation is primarily taken from Lerman et al. (1975) and modified to include atmospheric transfers. The mass of P in each reservoir and rates of exchange are taken from Mackenzie et al. (1993) and Follmi (1996). 

In choosing these reservoirs to describe the P cycle, compromises were made to maintain a general focus and global scale and yet avoid being too general and hence lose information about important transfers and reservoirs. The following is a brief discussion of the rationale behind the choice of the reservoir definitions and their estimates. For the purpose of discussion, the reservoirs have been numbered as presented in Lerman et al. (1975) with the addition of the atmosphere (reservoir 8). The total P content of each reservoir and comments concerning the estimate are provided in Table 14-3. 

The land biota reservoir (3) represents the phosphorus contained within all living terrestrial organisms. The dominant contributors are forest ecosystems with aquatic systems contributing only a minor amount. Phosphorus contained in dead and decaying organic materials is not included in this reservoir. It is important to note that although society most directly influences and interacts with the P in lakes and rivers, these reservoirs contain little P relative to soil and land biota and are not included in this representation of the global cycle. 

The ocean system is separated into three major reservoirs that best represent the dominant pools and pathways of P transport within the ocean. The surface ocean reservoir (5) is defined as the upper 300 m of the oceanic water column. As discussed in an earlier section and displayed in Fig. 14-6, the surface layer roughly corresponds to the surface mixed layer where all 

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Fig. 14-7 The global phosphorus cycle. Values shown are Tmol and Tmol/yr for reservoirs and fluxes, respectively. (T = 10 ).

The elements involved in flux in a model of the global phosphorus cycle are presented in Figure 4.1 and Table 4.4, according to which the balance system of equations will be ... 

Compton, J., Mallinson, D., Glenn, C.R., Filippelli, G, Follmi, K., Shields, G, and Zanin, Y. (2000) Variations in the global phosphorus cycle. In Authigenesis from Global to Microbial to Microbial (Glenn, C.R., Prevot-Lucas, L, and Lucas, J., eds.), pp. 21-33, Spec. Publ-SEPM, Vol. 66, Tulsa, OK. 

Table 1 Major reservoirs active in the global phosphorus cycle and associated residence times. 

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