Is phosphorus important? That depends—do you like having DNA , cell membranes, or bones in you body? Phosphorus is an essential nutrient for living organisms.
It’s a key part of nucleic acids, like DNA and of the phospholipids that form our cell membranes. As calcium phosphate, it also makes up the supportive components of our bones.
In nature, phosphorus is often the limiting nutrient in other words, the nutrient that’s in shortest supply and puts a limit on growth and this is particularly true for aquatic, freshwater ecosystems.
What is Phosphorus Cycle?
The Phosphorus Cycle is the biogeochemical cycle that describes the transformation and translocation of phosphorus in soil, water, and living and dead organic material.The phosphorus cycle is slow compared to other biogeochemical cycles such as the water, carbon, and nitrogen cycles.
In nature, phosphorus is found mostly in the form of phosphate ions PO43-. Phosphorus additions to soil occur due to additions of inorganic and organic (manure) fertilizer and the degradation and decomposition of organic (plant and animal) material.
Export of P from soil occurs mainly through plant uptake. Phosphorus may also be exported from soil via surface runoff and erosion or subsurface loss through leaching.
Volcanic ash, aerosols, and mineral dust can also be significant phosphate sources, though phosphorus has no real gas phase, unlike other elements such as carbon, nitrogen, and sulfur.
Phosphate compounds in the soil can be taken up by plants and, from there, transferred to animals that eat the plants. When plants and animals excrete wastes or die, phosphates may be taken up by detritivores or returned to the soil.
Phosphorus-containing compounds may also be carried in surface runoff to rivers, lakes, and oceans, where they are taken up by aquatic organisms.
When phosphorus-containing compounds from the bodies or wastes of marine organisms sink to the floor of the ocean, they form new sedimentary layers. Over long periods of time, phosphorus-containing sedimentary rock may be moved from the ocean to the land by a geological process called uplift.
However, this process is very slow, and the average phosphate ion has an oceanic residence time time in the ocean of 20,000 to 100,000 years.
Steps of Phosphorus Cycle
Following are the important steps of phosphorus cycle:
- Weathering.
- Absorption by Plants.
- Absorption by Animals.
- Return to the Environment through Decomposition.
#1.Weathering.
Phosphorus is found in the rocks in abundance. That is why the phosphorus cycle starts in the earth’s crust. The phosphate salts are broken down from the rocks. These salts are washed away into the ground where they mix in the soil.
#2.Absorption by Plants.
The phosphate salts dissolved in water are absorbed by the plants. However, the amount of phosphorus present in the soil is very less. That is why the farmers apply phosphate fertilizers on agricultural land.
The aquatic plants absorb inorganic phosphorus from lower layers of water bodies. Since phosphate salts do not dissolve in water properly, they affect plant growth in aquatic ecosystems.
#3.Absorption by Animals.
The animals absorb phosphorus from the plants or by consuming plant-eating animals. The rate of the phosphorus cycle is faster in plants and animals when compared to rocks.
#4.Return to the Environment through Decomposition.
When the plants and animals die they are decomposed by microorganisms During this process, the organic form of phosphorus is converted into the inorganic form, which is recycled to soil and water.
Soil and water will end up in sediments and rocks, which will again release phosphorus by weathering. Thus, the phosphorus cycle starts over.
Most phosphorus is unavailable to plants
Since most of our phosphorus is locked up in sediments and rocks, it’s not available for plants to use. A lot of the phosphorus in soils is also not available to plants.
The availability of phosphorus in soil to plants depends of several reversible pathways:
Bacteria: Bacteria convert plant-available phosphate into organic forms that are then not available to plants. Although other bacteria make phosphate available by mineralisation, the contribution of this is small.
Adsorption: Inorganic (and available) phosphorus can be chemically bound (adsorbed) to soil particles, making it unavailable to plants. Desorption is the release of adsorbed phosphorus from its bound state into soil solution.
pH: Inorganic phosphorus compounds need to be soluble to be taken up by plants. This depends on the acidity (pH) of the soil. If soils are less than pH 4 or greater than pH 8, the phosphorus starts to become tied up with other compounds, making it less available to plants.
Many plant crops need more phosphorus than is dissolved in the soil to grow optimally. In addition, crops are usually harvested and removed – leaving no decaying vegetation to replace phosphorus. Therefore, farmers replenish the phosphorus ‘pool’ by adding fertilisers or effluent to replace the phosphorus taken up by plants.
Human Impact on Phosphorus Cycle
A number of human activities, use of fertilizers, artificial eutrophication, etc. has a great impact on the phosphorus cycle.
Humans have altered the P cycle in aquatic systems, directly, by mining P-rich rock, and indirectly, through the manipulation of other element cycles and the alteration of aquatic food webs. Aquatic ecologists are becoming increasingly aware of the importance of these indirect alterations to biogeochemical cycles.
For millennia, phosphorus was primarily brought into the environment through the weathering of phosphate containing rocks, which would replenish the phosphorus normally lost to the environment through processes such as runoff, albeit on a very slow and gradual time-scale.
Other human processes can have detrimental effects on the phosphorus cycle, such as the repeated application of liquid hog manure in excess to crops. The application of biosolids may also increase available phosphorus in soil.
The phosphorus fertilizers increase the level of phosphorus in the soil. Overuse of these fertilizers reduces the fertility of the soil and is also harmful to the microorganisms present in the soil. When these are washed away into the nearby water bodies, they are hazardous to aquatic life.
Phosphate fertilisers replenish soil phosphorus
Many farmers replenish phosphorus through the use of phosphate fertilisers. The phosphorus is obtained by mining deposits of rock phosphate. Locally produced sulfuric acid is used to convert the insoluble rock phosphate into a more soluble and usable form – a fertiliser product called superphosphate.
In New Zealand, superphosphate is made using rock imported mainly from Morocco.
Adjusting the pH of the soil for efficient plant uptake of phosphate should be done prior to fertilisation. For example, adding lime reduces soil acidity, which provides an environment where phosphate becomes more available to plants.
Water pollution by fertilisers
When fields are overfertilised (through commercial fertilisers or manure), phosphate not utilised by plants can be lost from the soil through leaching and water run-off. This phosphate ends up in waterways, lakes and estuaries. Excess phosphate causes excessive growth of plants in waterways, lakes and estuaries leading to eutrophication.
Steps are being taken in agriculture to reduce phosphate losses in order to maximise the efficiency of fertiliser and effluent applications.
Eutrophication and dead zones
Most fertilizers used in agriculture and on lawns and gardens contain both nitrogen and phosphorus, which may be carried to aquatic ecosystems in surface runoff. Fertilizer carried in runoff may cause excessive growth of algae or other microbes that were previously limited by nitrogen or phosphorus.
This phenomenon is called eutrophication. At least in some cases, phosphorus, not nitrogen, seems to be the main driver of eutrophication.
Why is eutrophication harmful? Some algae make water taste or smell bad or produce toxic compounds.
Also, when all of those algae die and are decomposed by microbes, large amounts of oxygen are used up as their bodies are broken down. This spike in oxygen usage can sharply lower dissolved oxygen levels in the water and may lead to death by anoxia lack of oxygen for other aquatic organisms, such as shellfish and finfish.
Regions of lakes and oceans that are depleted of oxygen due to a nutrient influx are called dead zones. The number of dead zones has increased for several years, and more than 400 of these zones existed in 2008.
One of the worst dead zones is off the coast of the United States in the Gulf of Mexico. Fertilizer runoff from the Mississippi River Basin created a dead zone of over 8,463 square miles.
As you can see in the figure below, dead zones are found in areas of high industrialization and population density around the world.
How can eutrophication be reduced or prevented? Fertilizers, phosphorus-containing detergents, and improperly disposed of sewage can all be sources of nitrogen and phosphorus that drive eutrophication.
Using less fertilizer, eliminating phosphorus-containing detergents, and ensuring that sewage does not enter waterways e.g., from a leaky septic system are all ways that individuals, companies, and governments can help reduce eutrophication.