Phosphorus cycle is defined as the biogeochemical cycle by which phosphorus is exchanged between the biosphere, lithosphere and hydrosphere components of the earth. Phosphorus occurs in nature as part of a phosphate ion, and the most abundant form in which phosphorus occurs in nature is orthophosphate (PO43-). Phosphorus can be found on earth in water, soil and sediments. 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 (Figure 1). Phosphorus can also be found as salts in ocean sediments or in rocks. Plants absorb phosphates from the soil; and when consumed by herbivorous animals, the phosphates are assimilated by herbivores and then by carnivores that consume herbivores. Phosphorus passes on through the food chain when the plants are consumed by other organisms.
Figure 1. Phosphorus cycle.
The phosphates are returned to the soil through the death and decomposition of plants and animals. Phosphates are important components of nucleotide molecules in living organisms such as the adenosine triphosphate (ATP) – which serve as energy storage within living cells. The bones of animals are also composed of phosphorus derived from calcium phosphates. Phosphorus is important for plant growth and development. Phosphorus is most commonly found in rock formations and ocean sediments as phosphate salts. Phosphate salts that are released from rocks through weathering usually dissolve in soil water and will be absorbed by plants. And because the quantities of phosphorus in the soil are generally small, it is often the limiting factor for plant growth. Thus phosphate fertilizers are often applied on farmlands to support plant growth.
Unlike other biogeochemical cycles, the phosphorus cycle cannot be found in the air in the gaseous state, and it is the slowest cycle. Thus the atmosphere does not play significant role in the movement of phosphorus across the earth. This is because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on the earth. In addition, phosphorus is usually liquid at normal temperatures and pressures. It is mainly cycling through water, soil and sediments. But in the atmosphere phosphorus can mainly be found as very small dust particles. Phosphorus compounds reside primarily in rocks; and phosphorus does not go through an atmospheric phase as aforementioned, but rather, phosphorus-laden rocks release phosphate (PO4–3) into the ecosystem following weathering and erosion. Phosphates not taken up by plants go into the sedimentary phase, where they are very chemically reactive with other minerals; and some of these reactions produce compounds that effectively remove phosphates from the active nutrient pool.
Phosphorus does not enter the atmosphere, remaining mostly on land and in rock and soil minerals. The availability of phosphorus in the ecosystem is restricted by the rate of release of this element during weathering. Phosphorus is important to the growth and survival of living organisms, and hence, it is very essential for development and maintenance of healthy ecosystems. However, some human activities such as mining the earth for phosphorus, cutting down of trees in the tropical rain forests and converting mined phosphorus compounds into fertilizers as well as their global transportation have greatly influenced the phosphorus cycle.
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