Global cycle

The global movement of phosphorus occurs through three interconnected cycles the main compartments, and their P contents and annual fluxes based on data of Pierrou are given in Fig. 1. 

FIGURE 1. Global P compartments and annual fluxes between them (Mt P). t Mined for fertiliser use. From Emsley Reproduced by permission of Springer-Verlag. 

The primary cycle is inorganic, in which phosphate rock formation, mainly from diagenesis of P-enriched sediments in marine and fresh waters, takes place over very long periods of time (ca. 10 -10 y M.  

The much faster land-based organic cycle is sustained by application of this rock phosphate to soils as fertiliser, as well as contributions from natural weathering. 

According to Pierrou, about 13 Mt P reach the soils of the world annually by each of these pathways, while a similar amount (ca. 12 Mt) is lost through soil erosion and leaching. Plants have been estimated to remove somewhere between 178 and 240 Mt P, while biological returns amount to about 100 Mt annually. An even faster water-based P cycle, with turnover times measured in months rather than years as for the land-based cycle, is essentially closed, with the very small quantity of P entering the oceans in rivers being approximately balanced by that removed in sediments.  

---

Human interaction with the global cycle is most evident in the movement of the element carbon. The burning of biomass, coal, oil, and natural gas to generate heat and electricity has released carbon to the atmosphere and oceans in the forms of CO2 and carbonate. Because of the relatively slow... 

A TBma,) explicit functions of the available concentrations of the other nutrients. This approach allows for a pronounced interdependence between the fluxes of the different nutrients but it does not ensure that the Redfield ratios are maintained. In the second approach the contents of the nutrients in the biota reservoir are forced to remain close to the Redfield ratios. This method was used by Mackenzie et al. (1993) in their study of the global cycles of C, N, P, and S and their interactions. They were able to demonstrate how a human perturbation in one of these element cycles could influence the cycles of the other elements. 

The carbonate system plays a pivotal role in most global cycles. For example, gas exchange of CO2 is the exchange mechanism between the ocean and atmosphere. In the deep sea, the concentration of COi ion determines the depth at which CaCOs is preserved in marine sediments. 

Soderlund, R. and Svensson, B. H. (1976). The global nitrogen cycle. In "Nitrogen, Phosphorus and Sulfur - Global Cycles" (B. H. Svensson and R. Soderlund, eds), Ecol. Bull. No. 22, pp. 23-73, SCOPE. Swedish Natural Science Research Council, Stockholm. 

Hypothetical problem for chemists consider the global cycle of selenium which has many chemical similarities to sulfur. Construct a box diagram for the global selenium cycle based on known similarities and differences of Se and S. 

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. 

Lerman et al. (1975) considered several cases in which mankind s activities perturbed the natural cycle. If we assume that all mined P is supplied to the land as fertilizer and that all of this P is incorporated into land biota, the mass of the land biota will increase by 20%. This amount is small relative to the P stored in the land reservoir. Since P incorporated into land biota must first decompose and be returned to the land reservoir before being transported further, there is essentially no change in the other reservoirs. Thus, although such inputs would significantly alter the freshwater-terrestrial ecosystem locally where the P release is concentrated, the global cycle would be essentially unaffected. 

In this final section, the global cycles of two metals, mercury and copper, are reviewed. These metals were chosen because their geochemical cycles have been studied extensively, and their chemical reactions exemplify the full gamut of reactions described earlier. In addition, the chemical forms of the two metals are sufficiently different from one another that they behave differently with respect to dominant... 

Fig. 15-16 The (a) present day and (b) pre-industrial global cycles for mercury. Units are 10 g Hg (burdens) and 10 g Hg/yr (fluxes). Hgp refers to mercury in particles. Redrawn from Mason et al, 1994.

Lantzy, R. J. and Mackenzie, F. T. (1979). Atmospheric trace metals global cycles and assessment of man s impact. Geochim. Cosmochim. Acta 43,511-525. 

Nriagu, J. O. The Global Cycle of Nickel John Wiley and Sons New York, 1980. 

Chilvers D.C., Peterson P.J. Global cycling of arsenic. In Lead, Mercury and Arsenic in the Environment, T.C. Hutchinson, K.M. Meema, eds. New York John Wiley. 1987. 

The Role of the Ocean in Global Cycling of Persistent Organic Contaminants... 

Modified from Nriagu, J.O. 1980b. Global cycle and properties of nickel. Pages 1-26 in J.O. Nriagu (ed.). Nickel In the Environment. John Wiley, NY. 

Human activity has an enormous influence on the global cycling of nutrients, especially on the movement of nutrients to estuaries and other coastal waters. For phosphorus, global fluxes are dominated by the essentially one way flow of phosphorus carried in eroded materials and wastewater from the land to the oceans, where it is ultimately buried in ocean sediments. The size of this flux is currently estimated at 22 x 106 tons per year. Prior to increased human agricultural and industrial activity,... 

Morse, J. W., and F. J. Mackenzie (1990), Geochemistry of Sedimentary Carbonates, Elsevier, Amsterdam. (Comprehensive treatment of carbonate geochemistry, covering the range from electrolyte chemistry of carbon-containing waters to the global cycles of carbon.)... 

Wollast, R and F. T. Mackenzie (1983), "Global Cycle of Silica", in S. R. Aston, Ed., Silicon Geochemistry and Biogeochemistry, Academic Press, New York, 39-76. 

Meijer, S.N. Ockenden, W.A. Steinnes, E. Corrigan, B.P. Jones, K.C. 2003, Spatial and temporal trends of POPs in Norwegian and UK background air Implications for global cycling. Environ. Sci. Technol. 37 454-461. 

---