Table of Contents
Introduction
Material cycle
Geotectonic cycle
Water cycle
Carbon cycle
Nitrogencycle
Sulfur cycle
Cycles of mineral elements
　Phosphorus cycle

『Figure 6. Phosphorus cycle due to human activity. (Source: Vaclav Smil, Global Material Cycles and Energy)（略）

Unlike C, N and S, phosphorus (P) does not form any long-lived atmospheric compounds and hence its global cycle is just a part of the grand, and slow, process of denudation and geotectonic uplift. But on a small scale the element is rapidly recycled between organic and inorganic forms in soils and water bodies. Human actions are now mobilizing annually more than four times as much P as did the natural processes during the preagricultural era. Phosphorus is absent in the polymers that make up bulk of the plant mass (cellulose, hemicellulose and lignin) as well as in proteins, but it is abundant (as hydroxyapatite) in bones and teeth. The element is indispensable in phosphodiester bonds linking mononucleotide units of RNA and DNA, and all life processes are energized by transformations of adenosine triphosphate. Phosphorus is one of the three macronutrients required by plants; it is particularly important for the growth of young tissues, flowering and seed formation. The element is also an essential micronutrient in humans.

The Earth's crust is the largest reservoir of the element, nearly all of it bound in apatites, calcium phosphates containing also iron, chlorine or OH group. Soluble phosphates are released by weathering, but they are rapidly transformed to insoluble compounds in soils. Consequently, plants must be able to absorb phosphorus from very dilute solutions and concentrate it hundred- to thousand-fold in order to meet their needs, and P released by decomposition of biomass must be rapidly reused. The element is even less available in fresh waters and in the ocean where its particulate forms sink into sediments and where soluble phosphates are rapidly recycled in order to support phytoplanktonic photosynthesis in surface waters. Only in the areas of upwelling are the surface layers enriched by the influx from deeper waters. Phosphorus in marine sediments can become available to terrestrial biota only after the tectonic uplift reexposes the minerals to denudation; the element's global cycle thus closes only after tens to hundreds of millions of years.

Terrestrial phytomass stores about 500 Mt P and plant growth assimilates up to 100 Mt P/year. Phosphates dissolved in rain and dry-deposited in particles amount to only about 3 Mt a year. Soils store about 40 Gt P, with no more than 15% of this total bound in organic matter. Natural denudation and precipitation transfer annually about 10 Mt of particulate and dissolved P from land to the ocean. Marine phytomass stores only some 75 Mt P but because of its rapid turnover it absorbs annually about 1 Gt P from surface water. Mixing between sediments and surface layer is effective only in very shallow waters, and near-surface concentration of the nutrient are high only in coastal areas receiving P-rich runoff.

Human intensification of biospheric P flows is due to accelerated erosion and runoff caused by large-scale conversion of natural ecosystems to arable land, settlements and transportation links; recycling of organic wastes to fields; releases of untreated, or insufficiently treated, urban and industrial wastes to streams and water bodies; applications of inorganic fertilizers; and combustion of biomass and fossil fuels. The global population of nearly 7 billion people now discharges every year more than 3 Mt P in its wastes. A large share of human waste produced in rural areas of low-income countries is deposited on land, but progressing urbanization puts a growing share of human waste into sewers and then into streams or water bodies. During the 1940s P-containing detergents became another major source of waterborne P. Neither the primary sedimentation of urban sewage (which removes only 5-10% of P), nor the use of trickling filters during the secondary treatment (removing 10-20% P) prevent eutrophication, undesirable enrichment of waters with the nutrient that most often limits the growth of phytoplankton.

Production of inorganic fertilizers began during the 1840s with the treatment of P-containing rocks with dilute sulfuric acid. The resulting ordinary superphosphate (OSP) contains 7-10% P, ten times as much as recycled P-rich manures. Discovery of huge phosphate deposits in Florida (1870s), Morocco (1910s) and Russia (1930s) laid foundations for the rapid post-WWII expansion of fertilizer industry. Global consumption of phosphatic fertilizers was about 16 Mt P/year in 2006. The top three producers (USA, China and Morocco) now account for about 2/3 of the global output. Crops receiving the highest applications are the US Corn Belt corn (around 60 kg P/ha), Japanese rice (over 40 kg P/ha) and Chinese and West European winter wheat (more than 30 kg P/ha).

Phosphorus applied in both organic and inorganic fertilizers is involved in complex reactions which transform a large part of soluble phosphates into much less soluble compounds. This process of P fixation has been known since 1850 and for more than 100 years it was seen as rapid, dominant and irreversible. This misconception resulted in decades of excessive use of P in cropping. Mounting evidence of relatively high efficiency with which crops use the nutrient had finally changed that wasteful practice during the closing decades of the 20th century. Leaching into ground waters and the runoff from fertilizers and manures and sewage discharges enrich surface waters with P. Because a single atom of P supports the production of as much phytomass as 16 atoms of N and 106 atoms of C, even relatively low additions of the nutrient can cause eutrophication of lakes, streams and shallow waters. Just 10 g P/L will support algal growth that will greatly reduce water's clarity, and concentrations above 50 g P/L cause deoxygenation of bottom waters during the decomposition of the accumulated biomass and may result in extensive summer fish kills.

By far the most effective measure to moderate the human impact on P cycle would be to reduce the intake of animal foods: because more than 60% of all P fertilizers are used on cereals, and because more than 60% of all grains in rich countries are used as animal feed the need for P fertilizers would decline appreciably. Good agronomic practices can reduce P applications and limit the post-application losses. Phosphorus in sewage can be effectively controlled either by the use of coagulating agents or by microbial processes. Lake eutrophication has been greatly reduced due to the elimination or restricted use of P-based detergents and increased P removal from sewage.』

　Calcium cycle
　Silicon cycle
Reasons for concern
Editor's note
Further reading