Wednesday, January 19, 2011


The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth.
The production of phosphine gas is allowed only in specialized, local conditions. Low phosphorus (chemical symbol, P) availability retards microbial growth, which has been shown in studies of soil microbialbiomass. Soil microorganisms act as sinks and sources of available P in the biogeochemical cycle. However, the major transfers in the global cycle of P are not driven by microbial reactions. Further studies need to be performed for integrating different processes and factors related to gross phosphorus mineralization and microbial phosphorus turnover in general.

Ecological function

Phosphorus is an essential nutrient for plants and animals in the form of ions. Phosphorus is a limiting nutrient for aquatic organisms. Phosphorus forms parts of important life sustaining molecules but are very common in the biosphere. Phosphorus does not enter the atmosphere, remaining mostly on land and in rock and soil minerals. Eighty percent of the phosphorus is used to make fertilizers and a type of phosphorus such as dilute phosphoric acid is used in soft drinks. Phosphates may be effective in such ways but also causes pollution issues in lakes and streams. Over enrichment of phosphate can lead to algae bloom, because of the excess nutrients. This causes more algae to grow, bacteria consumes the algae and causes more bacteria to grow in large amounts. They use all the oxygen in the water during cellular respiration, causing many fish to die.
Phosphorus normally occurs in nature as part of a phosphate ion, consisting of a phosphorus atom and some number of oxygen atoms, the most abundant form is orthophosphate. Most phosphates are found as salts in ocean sediments or in rocks. Over time, geologic processes can bring ocean sediments to land, and weathering will carry these phosphates to terrestrial habitats. Plants absorb phosphates from the soil, then bind the phosphate into organic compounds. The plants may then be consumed by hervibores who in turn may be consumed by carnivores. After death, the animal or plant decays, and the phosphates are returned to the soil. Runoff may carry them back to the ocean or they may be reincorporated into rock.

Biological Function

The primary biological importance of phosphates is as a component of nucleotides, which serve as energy storage within cells (ATP) or when linked together, form the nucleic acids DNA and RNA.The double helix of our DNA is only possible because of the phosphate ester bridge that binds the helix. Besides making biomolecules phosphorus is also found in bones, whose strength is derived from calcium phosphate in enamel of mammalian teeth; exoskeleton of insects and phospholipids (found in all biological membranes). It also functions as buffering agent in maintaining acid base homeostasis in the human body.

Process of the cycle

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.
Unlike other cycles of matter compounds, phosphorus cannot be found in air as a gas. This is because at normal temperature and circumstances, it is a solid in the form of red and white phosphorus. It usually cycles through water, soil and sediments. Phosphorus is typically the limiting nutrient found in streams, lakes and fresh water environments. As rocks and sediments gradually wear down, phosphate is released. In the atmosphere phosphorus is mainly small dust particles.

Phosphorus minerals

The availability of phosphorus in ecosystem is restricted by the rate of release of this element during weathering. The release of phosphorus from apatite dissolution is a key control on ecosystem productivity. The primary mineral with significant phosphorus content, apatite [Ca5(PO4)3OH] undergoes carbonation weathering releasing phosphorus contained different forms. This process decreases the total phosphorus in the system due to losses in runoff.
Little of thus released phosphorus is taken by biota (organic form) whereas, large proportion reacts with other soil minerals leading to precipitation in unavailable forms. The later stage of weathering and soil development. Available phosphorus is found in a biogeochemical cycle in the upper soil profile, while phosphorus found at lower depths is primarily involved in geochemical reactions with secondary minerals. Plant growth depends on the rapid root uptake of phosphorus released from dead organic matter in the biochemical cycle. Phosphorus is limited in supply for plant growth. 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.
Low-molecular-weight (LMW) organic acids are found in soils. They originate from the activities of various microorganisms in soils or may be exuded from the roots of living plants. Several of those organic acids are capable of forming stable organo-metal complexes with various metal ions found in soil solutions. As a result, these processes may lead to the release of inorganic phosphorus associated with aluminium, iron, and calcium in soil minerals. The production and release of oxalic acid by mycorrizhal fungi explain their importance in maintaining and supplying phosphorus to plant.
The availability of organic phosphorus to support microbial, plant and animal growth depends on the rate of their degradation to generate free phosphate. There are various enzymes such as phosphatases,nucleases and phytase involved for the degradation. Some of the abiotic pathways in the environment studied are hydrolytic reactions and photolytic reactions. Enzymatic hydrolysis of organic phosphorus is an essential step in the biogeochemical phosphorus cycle, including the phosphorus nutrition of plants and microorganisms and the transfer of organic phosphorus from soil to water bodies. Many organisms rely on the soil derived phosphorus for their phosphorus nutrition.

Human interference

Nutrients are important to the growth and survival of living organisms, and hence, are essential for development and maintenance of healthy ecosystems. However, excessive amounts of nutrients, particularly phosphorus and nitrogen, are detrimental to aquatic ecosystems. Natural eutrophication is a process by which lakes gradually age and become more productive and may take thousands of years to progress. Cultural or anthropogenic eutrophication, however, is water pollution caused by excessive plant nutrients, which results in excessive growth in algae population. Surface and subsurface runoff and erosion from high-P soils may be major contributing factors to fresh water eutrophication. The processes controlling soil P release to surface runoff and to subsurface flow are a complex interaction between the type of P input, soil type and management, and transport processes depending on hydrological conditions.
Repeated application of liquid hog manure in excess to crop needs can have detrimental effects on soil P status. In poorly drained soils or in areas where snowmelt can cause periodical waterlogging, Fe-reducing conditions can be attained in 7–10 days. This causes a sharp increase in P concentration in solution and P can be leached. In addition, reduction of the soil causes a shift in phosphorus from resilient to more labile forms. This could eventually increase the potential for P loss. This is of particular concern for the environmentally sound management of such areas, where disposal of agricultural wastes has already become a problem. It is suggested that the water regime of soils that are to be used for organic wastes disposal is taken into account in the preparation of waste management regulations.
Human interference in the phosphorus cycle occurs by overuse or careless use of phosphorus fertilizers. This results in increased amounts of phosphorus as pollutants in bodies of water resulting in euthropication. Eutrophication devastates water ecosystems.
Eutrophication will impact the global carbon cycle, but will probably do little to offset anthropogenic carbon emissions. Total excess input from 1600 to 3600 AD is 1860 Tg (teragrams) of phosphorus. Given that, in the marine environment, between 106 and 170 units of carbon are buried per unit of phosphorus one can predict that excess phosphorus would effectively bury 76,000 to 123,000 Tg carbon. In essence, this burial removes carbon from the atmosphere through the biological fixation of carbon dioxide during photosynthesis. The present annual rate of anthropogenic carbon addition to the atmosphere is 7900 Tg carbon, so the phosphorus eutrophication effect would only account for about 10-15 years of anthropogenic carbon emissions to the atmosphere over the next 2000 years (i.e. only 0.6% of total projected carbon emissions, if emissions stay constant).
Although the net effect as a carbon sequestration mechanism is minimal, the ecological impact of phosphorus fertilization to the ocean could be extreme. Given the other assaults on marine ecosystems, including warming, and acidification of surface ocean waters from higher carbon dioxide levels, it would be pure speculation to project how P eutrophication would affect ecosystem structure and distribution in the future. However, those who have witnessed local eutrophication in ditches, streams, ponds, and lakes can attest to the ecological devastation that excess nutrients and the proliferation of monocultures can cause in such isolated environments. The eutrophication of coastal and open-marine ecosystems would result in a grim future for ecological diversity.

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