In this essay we will discuss about the phosphorus cycle with the help of a diagram.
Phosphorus is one of the nutrients of major importance to biological systems and is the constituent of energy rich compounds e.g. ADP, ATP, GTP. It is also found in plasma membrane, shells, bones and teeth. Phosphorus is indispensable for life, metabolism and required for encoding of the information in genes as it is also the component of nucleotides of nucleic acids.
Phosphorus cycle is an example of sedimentary cycle having its main reservoir not in atmosphere but the greatest reservoir of phosphate in the world is relatively insoluble ferric and calcium phosphate in rocks. Phosphorus is usually used in phosphate form. Phosphates are present in soil as rock phosphate, in combination with calcium, iron and aluminium.
The inorganic phosphorus is added to soil as a result of weathering of phosphate rocks by the action of dilute nitric acid formed during nitrification and is released by leaching, erosion and mining for agricultural use. It is estimated that about 2 million tonnes of phosphatic rock is lost to sea. In soil, there are two main supply pools to phosphorus, inorganic and insoluble organic.
Phosphate circulates in the abiotic environment in lithosphere as well as hydrosphere. The plants get phosphorus from soil especially as orthophosphate ions and is then transferred to consumers and decomposers as organic phosphate through the food chain. After death and decay of organisms phosphorus is recycled due to action of decomposers especially by phosphate -solubilising bacteria.
Some quantity of phosphorus is deposited in deep sediments in oceans and is brought to the cycle by upwelling. Only 60,000 tonnes (less than 0.5% of phosphorus discharged from the rivers) of elementary phosphate is added to soil mainly through sea birds and fish.
This shows that uplift of phosphorus is not extensive. It is so as most phosphate is lost by two physical processes—physical adsorption (controls the concentration of dissolved phosphorus in soils and lakes) and sedimentation (binding the phosphorus with cations like aluminium, calcium, iron, etc. to form insoluble compounds).
Phosphate fertilizers are added to soil to restore the fertility. Guano (excreta of marine birds) and soil obtained from lake or ocean bed contain plenty of phosphorus.
Once phosphorus becomes a part of soil water as phosphate or in dissolved state of water in any aquatic system, it re-enters the cycle through producers. In aquatic systems, phosphorus is taken from water by phytoplanktons, which are consumed by zooplanktons.
The zooplanktons excrete phosphorus into water. Phosphorus cycle is an imperfect cycle as the biological processes like teeth and bone formation, and excretion account for considerable losses of the phosphorus from the cycle. It also forms insoluble and unavailable salts with metals like A13+, Fe2+ and Ca2+. It shows one way flow (Phosphate rocks land ecosystem oceans ocean sediment).
Another important point about phosphorus cycle is that high concentration of phosphate in natural water causes eutrophication and pollution.