Inorganic Cycle

Phosphorus circulates through the environment in three natural cycles. The first of these is the inorganic cycle, which refers to phosphorus in the crust of the Earth. Through millions of years, phosphorus has moved slowly through the inorganic cycle, starting with the rocks which slowly weather to form soil, from which the phosphorus is gradually leached from the land into rivers and onward to the sea, where it eventually forms insoluble calcium phosphate and sinks to the seafloor as sediment. There it remains until it is converted to new, so-called sedimentary rocks as a result of geological pressure. On a timescale of hundreds of millions of years, these sediments are uplifted to form new dry land and the rocks are subject to weathering, completing the global cycle. In addition, some phosphorus can be transferred back from the ocean to the land by fish-eating birds whose droppings have built up sizable deposits of phosphate as guano on Pacific coastal regions and islands, and by ocean currents that convey phosphorus from the seawater to these regions. A simplified schematic of the global phosphorus cycle is presented in Figure 1.

The global cycle of phosphorus is unique among the cycles of the major biogeochemical elements in having no significant gaseous compounds. The biospheric phosphorus flows have no atmospheric link from ocean to land. A little phosphorus does get into the atmosphere as dust or sea spray, accounting for 4.3 million metric tons of phosphorus per year (MMT P yr-1) and 0.3 MMT P yr~\ respectively, but the amounts are several orders less important than other transfers in the global phosphorus cycle. The amount 4.6 MMT P yr-1 of atmospheric phosphorus deposition, being balanced by the phosphorus carried by the wind and the sea spray, cannot offset the endless drain of this element from the land due to erosion and river transportation. Fortunately, increased anthropogenic mobilization of the element has no direct atmospheric consequences.

Nearly all the phosphorus on land is originally derived from the weathering of calcium phosphate minerals, especially apatite [Ca5(PO4)3OH]. Around 13 MMT Pyr-1 of

Figure 1 The human-intensified global phosphorus cycles.

this is released to form soils each year. However, this amount cannot offset the annual losses of phosphorus from the land. Taking into account all four forms of phosphorus (dissolved and particulate, organic and inorganic), the total amount of annual phosphorus losses from the lithosphere into freshwaters is estimated at 18.7-31.4 MMT Pyr-1.

The uncertainty in the estimate is mainly due to a lack of knowledge on the biogeochemical processes of the particulate inorganic phosphorus (PIP), which constitutes the major component in the total loss. Not all of the eroding phosphorus can eventually reach the ocean. About 3.0 MMT Pyr-1 is carried away by wind into atmosphere, and at least 25% of that is redeposited on adjacent cropland and grassland or on more distant alluvia. Consequently, the amount of phosphorus transported by freshwaters into the ocean is probably in the range 12-21 MMT Pyr-1. This result agrees with the most likely value of 17-22 MMT Pyr-1 given by some previous estimates. The riverborne transport of phosphorus constitutes the main flux of the global phosphorus cycle. The loss, as a result of erosion, pollution, and fertilizer runoff, must be considerably higher than it was in prehuman times. It can be argued that the human-intensified phosphorus flux caused by wind and water erosion is at least 2 or even 3 times its prehistoric level.

cycle which transfers it from soil to plants, to animals, and back to soil again; and a water-based organic cycle which circulates it among the creatures living in rivers, lakes, and seas. The land-based cycle takes a year on average and the water-based cycle organic cycle only weeks. It is the amount of phosphorus in these two cycles that governs the biomass of living forms that land and sea can sustain.

The amount of phosphorus in the world's soils is roughly 90-200 x 103 MMT P according to various estimates. While the total phosphorus content of soils is large, only a small fraction is available to biota in most soils. This constitutes an available phosphorus pool containing 1805-3000 MMT P, most likely 2000-2600 MMT P. A larger amount, in the range 27-840 x 106 MMT P, can be found in the oceans. The seawater contains 80-120 x 103 MMT P and the rest is accumulated in sediments.

The ocean water loses phosphorus continually in a steady drizzle of detritus to the bottom, where it builds up in the sediments as insoluble calcium phosphate. Despite the geological remobilization, there is a net annual loss of millions of tons of phosphate a year from the marine biosphere. Thus the ocean sediments are by far the largest stock in the biogeochemical cycles of phosphorus.

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