Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.
The global phosphorus cycle includes four major processes:
:(i) tectonic uplift and exposure of phosphorus-bearing rocks such as apatite to surface weathering;
:(ii) physical erosion, and chemical and biological weathering of phosphorus-bearing rocks to provide dissolved and particulate phosphorus to soils, lakes and rivers;
:(iii) riverine and subsurface transportation of phosphorus to various lakes and run-off to the ocean;
:(iv) sedimentation of particulate phosphorus (e.g., phosphorus associated with organic matter and oxide/carbonate minerals) and eventually burial in marine sediments (this process can also occur in lakes and rivers).
In terrestrial systems, bioavailable P (‘reactive P’) mainly comes from weathering of phosphorus-containing rocks. The most abundant primary phosphorus-mineral in the crust is apatite, which can be dissolved by natural acids generated by soil microbes and fungi, or by other chemical weathering reactions and physical erosion. The dissolved phosphorus is bioavailable to terrestrial organisms and plants and is returned to the soil after their decay. Phosphorus retention by soil minerals (e.g., adsorption onto iron and aluminum oxyhydroxides in acidic soils and precipitation onto calcite in neutral-to-calcareous soils) is usually viewed as the most important process in controlling terrestrial P-bioavailability in the mineral soil. This process can lead to the low level of dissolved phosphorus concentrations in soil solution. Various physiological strategies are used by plants and microorganisms for obtaining phosphorus from this low level of phosphorus concentration.
Soil phosphorus is usually transported to rivers and lakes and can then either be buried in lake sediments or transported to the ocean via river runoff. Atmospheric phosphorus deposition is another important marine phosphorus source to the ocean. In surface seawater, dissolved inorganic phosphorus, mainly orthophosphate (PO), is assimilated by phytoplankton and transformed into organic phosphorus compounds. Phytoplankton cell lysis releases cellular dissolved inorganic and organic phosphorus to the surrounding environment. Some of the organic phosphorus compounds can be hydrolyzed by enzymes synthesized by bacteria and phytoplankton and subsequently assimilated. The vast majority of phosphorus is remineralized within the water column, and approximately 1% of associated phosphorus carried to the deep sea by the falling particles is removed from the ocean reservoir by burial in sediments. A series of diagenetic processes act to enrich sediment pore water phosphorus concentrations, resulting in an appreciable benthic return flux of phosphorus to overlying bottom waters. These processes include
: (i) microbial respiration of organic matter in sediments,
: (ii) microbial reduction and dissolution of iron and manganese (oxyhydr)oxides with subsequent release of associated phosphorus, which connects the phosphorus cycle to the iron cycle, and
: (iii) abiotic reduction of iron (oxyhydr)oxides by hydrogen sulfide and liberation of iron-associated phosphorus.
Additionally,
: (iv) phosphate associated with calcium carbonate and
: (v) transformation of iron oxide-bound phosphorus to vivianite play critical roles in phosphorus burial in marine sediments.
These processes are similar to phosphorus cycling in lakes and rivers.
Although orthophosphate (PO), the dominant inorganic P species in nature, is oxidation state (P5+), certain microorganisms can use phosphonate and phosphite (P3+ oxidation state) as a P source by oxidizing it to orthophosphate. Recently, rapid production and release of reduced phosphorus compounds has provided new clues about the role of reduced P as a missing link in oceanic phosphorus.