Phosphorus Cycle

Phosphorus (P) is an essential element for all life forms on Earth. Being a nonsubstitutable constituent of nucleoside phosphates (e.g., ADP [adenosine diphosphate], ATP [adenosine triphos-phate]) and nucleic acids (e.g., DNA [deoxyribo-nucleic acid], RNA [ribonucleic acid]), phosphorus plays a vital role in biochemical reactions involving energy transfer and transcription of genetic information in living organisms. Moreover, phosphorus has a generally low bioavailability and is a nutrient that commonly limits the productivity of terrestrial, freshwater, and marine ecosystems. In contrast with carbon, nitrogen, and sulfur, which can exist in several oxidation states, phosphorus does not take part in reduction-oxidation reactions and is predominantly present in the +5 oxidation state as phosphate (PO4-3). On a [Page 2170]centennial timescale, global transfer of phosphorus occurs unidirectionally from the terrestrial system via river transport to the oceanic system. This entry provides an overview of the major phosphorus reservoirs and the rate of phosphorus exchange between these reservoirs in the global phosphorus cycle.

Containing only 0.03 Tg P (teragram of phosphorus), the atmosphere represents a very minor phosphorus reservoir globally. Compared with other major biogeochemical cycles, the cycling of phosphorus is unique in lacking a stable gaseous component. The majority of atmospheric phosphorus is adsorbed on particulate matter emitted from high-temperature fossil fuel combustion and sea spray. These particulate phosphori have a short residence time of approximately 3 days and are deposited rapidly on terrestrial land (3.1 Tg P/yr. [per year]) and surface ocean (0.6–1.6 Tg P/yr.). The global flux of phosphine gas (PH3, <0.04 Tg P/yr.) is not significant because of the lack of a favorable environment for microbial production and the great ease of PH3 oxidation by air.

Phosphorus cannot be sequestered from the atmosphere by plants and microbes, unlike carbon and nitrogen. Apatite, with calcium and phosphate ions linked together in a hexagonal crystal structure, is the most abundant phosphorus mineral in the Earth's crust. The weathering of apatite rocks is the sole major source of phosphorus in the global phosphorus cycle, releasing 20 Tg P annually to the soil. Global soils in the top 60 cm (centimeters) store a total of 2 × 105 Tg P, but a large proportion of phosphorus is adsorbed on iron and aluminum oxides and is thus not readily available for biota. Although the available phosphorus concentration in soil is low, internal recycling of phosphorus helps sustain the growth of terrestrial organisms, which have a phosphorus reservoir size of 2,600 to 3,000 Tg. Assimilation of phosphorus by land biota from soils roughly equals the return of phosphorus from biota to soil, with a flux of 63 to 200 Tg P/yr.

Rivers are a major conduit of phosphorus, connecting between the terrestrial and oceanic systems. About 90% of phosphorus transported by rivers (18–20 Tg P/yr.) is associated with the particulate fraction. Natural denudation and human-induced erosion of physically weathered materials account for one third and two thirds of the riverine particulate phosphorus flux, respectively. Most of the particle-bound phosphorus enters the ocean and settles on the shallow continental shelf quickly, without participating in the biological cycle. Dissolution of phosphorus-containing minerals is very low in the alkaline and strongly buffered oceans, but some of the phosphorus adsorbed on particle surfaces can be remo-bilized in seawater with high ionic strength. Rivers also carry 1.0 to 1.8 Tg P in dissolved reactive form to surface oceans annually, providing a major phosphorus source for uptake by marine biota.

...