Phosphorus Cycle

Phosphorus Biogeochemical Cycle

The phosphorus cycle could be described as a process in which elemental phosphorus moves through the different spheres of the biome, namely the lithosphere, hydrosphere and biosphere.   Phosphorus is essential for the growth of animals and plants and also for the microbes inhabiting the soil but is depleted over time. It is a critical component in the formation of the genetic material (DNA/ RNA) and various other components of an organism’s body such as the phospholipid bilayer in animals.  In this topic, we would discuss the P cycle and how the phosphorus cycle on land is carried out. 

Phosphorus cycle steps

There are three steps in the phosphorus cycle. These are 

  1. Weathering

  2. Absorption by plants and animals

  3. Return of phosphorous to the ecosystem

Weathering: Phosphorus in nature is mostly found in the form of phosphate ions. Phosphate compounds can be observed in sedimentary rocks and when the rocks weather (wear down over time) the compounded phosphorus slowly leeches into the soil and water. Mineral and volcanic dust also contains phosphorus which makes them important sources of the element. It should be noted that unlike other elements like carbon, nitrogen and sulphur, P doesn’t have a gaseous form in nature.

Absorption by plants and animals:  Phosphorus in the soil can be taken up by the plant and it is then transferred to animals when they feed on plants. There is very less amount of  ‘P’ in soil and that is why farmers use fertilisers which are rich in this element so plants can take it up. The rate of the P cycle is faster in plants and the animals than in rocks.

Return of phosphorus to the ecosystem: As plants and animals excrete wastes and die, phosphate compounds are taken up by detritivores or are returned to the soil. These compounds can also be carried off to rivers, lakes and oceans (the aquatic ecosystem). Here the aquatic organisms take up phosphorus. Additionally, a new sedimentary layer is formed from the excess phosphorus on the ocean floor. Over longer periods, sedimentary rocks with phosphorus may move to the ocean from land in a geological process known as uplift. The process is very slow and generally happens over 20,000- 100,000 years.

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Human impact on the phosphorus cycle

Humans have always had a critical impact on the P cycle due to many human activities. These are the use of fertilizer in agriculture, Phosphorus distribution of food products, and artificial eutrophication. 

Fertilizers generally contain more phosphorus and nitrogen and it adds to the phosphorus levels in the soil and is detrimental when these fertilisers are washed into local aquatic ecosystems. When phosphorus is added to water at a rate achieved by natural processes, it is known as natural eutrophication. A natural supply of P over time offers nutrients to aquatic ecosystems and helps in increasing the productivity of that ecosystem.

However, when excess phosphorus is drained into aquatic systems, it has harmful effects and has adverse effects on the system and can lead to excessive growth of algae due to the overabundance of nutrients. These algae die or form algae blooms, which are toxic to aquatic plants and animals. Hence, human activities directly harm aquatic ecosystems, when excess amounts of phosphorus are leached into any water body.

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FAQ (Frequently Asked Questions)

1. Why is phosphorus not available in free form in nature?

Elemental phosphorus is highly reactive and when exposed to air it goes through a chemical reaction with oxygen. That is why in nature phosphorus is found a phosphate (PO4-3). Phosphates contain one phosphorus atom that is bound to four oxygen atoms, and all oxygen atoms are bound to each other. The general form of phosphate that we see in nature is HPO4-2

The phosphate anion bonds with carbon, nitrogen, and hydrogen to form ATP, for example. Also with calcium (and occasionally hydrogen), it forms calcium phosphate which is a component of bones.

2. What is Eutrophication?

Eutrophication can be defined as a process of enrichment of water by nutrient salts that causes structural changes to the aquatic ecosystem. Examples of these changes are increased growth of algae and aquatic plants, general deterioration of the quality of water, depletion of fish species etc.

Eutrophication is harmful to the aquatic ecosystem as some algae produce toxic compounds and when they die, they are decomposed by microbes. During decomposition, a large amount of oxygen is used to break down algal bodies. The spike in oxygen usage lowers the levels of oxygen in water bodies which can lead to the death of many aquatic organisms by hypoxia (lack of oxygen). It can be controlled by reducing the usage of artificial fertilisers.

3. Why is most phosphorous not available to plants?

Most of the available P in soil is not available to plants. There are many reasons. 

  • Bacteria convert the available phosphate into organic which cannot be taken up by plants. 

  • Inorganic phosphorus available in the soil can be chemically bound to soil particles, which makes it unavailable for plants. 

  • Also, inorganic phosphorus compounds need to be soluble so they can be taken up by plants. If the soil’s pH is less than 4 or more than 8, the available phosphorus becomes tied up with other compounds which cannot be utilised by plants.