Phosphorus Cycle

Phosphorus is an essential nutrient to biological systems. Its requirement is mainly seen in nucleic acids, cell membrane, bones and teeth.

Importance of Phosphates

a) It is the main constituent of energy rich compounds like ADP (Adenosine diphosphate), ATP (Adenosine triphosphate), GTP (Guanine tri phosphate) etc.

b) Its a major constituent of cell membrane, nucleic acid, cellular energy transfer systems.

c) Essential for metabolic reactions releasing energy.

d) Required for encoding of the information in genes (as it is the component of nucleotides and nucleic acids).

Phosphorus has no gaseous phase and hence forms a part of sedimentary cycle.

Sources of Phosphate

1) Greatest reservoirs of phosphate are the insoluble ferric and calcium phosphates in rocks (rock phosphate) in combination with calcium, iron and aluminium. Slow process of weathering releases phosphates to the soil.

2) Phosphates are also added to the soil by man in the form of artificial fertilizers.

3) Some quantity of phosphorus is deposited in deep sediments in oceans and is brought to the cycle by upwelling (but not extensive).

Phosphate Utilisation

i) Considerable amount of phosphorus from the soil rock is washed into the sea by rains and floods (an estimated amount of 2 million tonnes of phosphotic rock is lost to sea) where sea weeds take up phosphorus and is then passed onto fishes and sea birds.

ii) Terrestrial plants absorb phosphorus as phosphate ions (orthophosphate ions) from the soil.

iii) Animals obtain phosphates by consuming plants as food (as organic phosphate through the food chain).

Phosphorus production

1) Some amount of phosphorus is returned to earth in the form of birds excreta - Guano deposits (excreta of marine birds) and dead fish (around 60,000 tonnes which is less than 0.5% of phosphorus discharged from rivers).

2) Death and decay of organisms and decomposition of organic matter by microorganisms releases the phosphates into the soil, making them available to plants once again.

3) Zooplanktons excrete phosphorus into water (negligible amount)

Once phosphorus becomes a part of the soil water as phosphate or in dissolved state in any aquatic system, it re-enters the cycle through producers.

When phosphates form compounds with metals like aluminium, iron or calcium, phosphorus become unavailable to plants (becomes sediment and are deposited in deep ocean floor), and is lost to the phosphorus cycle until chemically changed.

Phosphorus incorporated in bones and teeth also remain outside the natural cycle for a long time as the bones and teeth are resistant to decay.

Therefore, phosphorus cycle is an imperfect cycle and shows one way flow which can be represented as:

Phosphate rocks Land ecosystem Oceans Ocean sediment

Note:

High concentration of phosphates in natural water causes 'Eutrophication' and pollution.