Phosphorus Cycle Definition
The phosphorus cycle is a biogeochemical cycle that deals with the movement of phosphorus in the form of different compounds through the Earth’s various systems like the biosphere, hydrosphere, and lithosphere.
- Phosphorus is an important element for all living beings as it is the raw material for the nucleotides as well as acts as a mineral for growth.
- Unlike other biogeochemical cycles, the atmosphere doesn’t act as a reservoir for phosphorus in the phosphorus cycle as most of the phosphorus compounds involved in the cycle are in the solid form.
- The phosphorus cycle is composed of various chemical, biological and microbiological processes, all of which occur over a long period of time.
- The cycle consists of processes like weathering that take hundreds of years to complete; thus, the phosphorus cycle is considered one of the slowest biogeochemical cycles.
- Phosphorus is also one of the scarcest elements found in nature which makes it one of the limiting agents.
- Phosphorus compounds move through the biosphere, both terrestrial and oceanic, to maintain a balance in its concentration.
- However, much of the phosphorus compounds are present in the lithosphere, where they are present in rocks and sedimentary deposits.
- Because phosphorus is a highly reactive compound, it is mostly found in a combined state with other elements. Thus, acid-producing microorganisms are important to form soluble phosphate from the insoluble compounds.
- The concentration of phosphorus in different reservoirs changes throughout time with the phosphorus in the soil running off to oceans and the phosphorus in the ocean, resulting in sediments.
- The phosphorus cycle is considered a closed system with the phosphorus transfer between the biosphere and soil is much larger than the gains and losses across the entire system.
Figure: Phosphorus Cycle on land and phosphorus cycle in a wetland. Image Source: Wikipedia.
Phosphorus Cycle Steps
The mechanism of the phosphorus cycle is not as clearly understood as other biogeochemical cycles. However, it is known that the phosphorus cycle is a slow process, consisting of the following steps:
- Rocks are one of the primary sources and reservoirs of phosphorus, where they remain combined with other elements.
- The phosphorus in the soil then reaches the soil by the action of rain or by acid produced by different microorganisms.
- Different microorganisms like Actinomycetes, Pseudomonas, Bacillus, Aspergillus, Penicillium, etc. are involved in the solubilization of phosphorus in the soil so that it is available to plants and other animals.
- Besides, rain also causes solubilization of inorganic phosphorus so that it is transported to the oceans.
- Natural processes like volcanic activity and asteroid activity also aid in the release of phosphorus into the soil.
2. Phosphorus intake by plants
- The available phosphorus in the soil is then taken up by different living beings like plants and microorganisms.
- The amount of phosphorus available to plants in the soil is very less, and thus, fertilizers containing phosphorus are to be added to improve plant growth and soil fertility.
- The aquatic plants in oceans also absorb phosphorus from the lower layers of aquatic sediments.
- However, some phosphorus salt might not dissolve properly in water, and thus the availability of phosphorus in water might also be limited.
- Plants can either take up phosphorus directly from the soil, or the phosphorus can be made available to plants by different symbiotic microorganisms.
3. Movement of phosphorus in the food chain
- The phosphorus absorbed by the plants is used to form different organic compounds in the producers.
- The organic compounds then move through the food chain as consumers feed on producers, resulting in the movement of phosphorus from one form of life to another.
- The organic form of phosphorus also undergoes changes as it moves through the consumers.
- The phosphorus transferred to the consumers is used for the formation of biomolecules like the nucleotides and connective tissues like bones.
4. Return of phosphorus to the ecosystem
- The phosphorus present in living beings can be transferred back to the reservoir in the lithosphere by the action of decomposing microorganisms on the dead plants and animals.
- In this step, the organic forms of phosphorus are converted into their inorganic forms by the process of mineralization.
- Different decomposing or saprophytic microorganisms like fungi and bacteria are involved in this step to balance the concentration of phosphorus in the ecosystem.
- The phosphorus in the soil is also transported to the ocean during rainfall or by the running off of soil to the water bodies.
- The phosphorus in the ocean undergoes deposition, forming layers of sediments that will lead to the formation of rocks and the cycle continues.
Phosphorus Cycle Examples
Along with the primary phosphorus cycle, there are different parallel systems that are indirectly involved in the transport of phosphorus through different reservoirs, to maintain a balance. There are different reservoirs in different ecosystems, all of which can absorb, the release of transport phosphorus from one form to another.
- The largest phosphorus reservoir on the Earth is the minerals in the lithosphere, which is dominated by the oceanic and freshwater sediments.
- The initial source of phosphorus on Earth is obtained via chemical weathering and soil formation processes.
- These compounds are either formed from natural processes like volcanoes and earthquakes, which increases the phosphorus content on the land.
- Besides, during the cycling process, the phosphorus returns back to the land as the phosphorus-rich soil runoffs reach the oceans.
- These soil particles form layers of sediments underneath the water which eventually forms mineralized rocks.
- The terrestrial biosphere and the ocean phosphorus reservoir also play an essential role in the phosphorus cycle.
- Most of the phosphorus is not available to living beings as it is present in an insoluble form.
- The biosphere, however, acts as a sink where the excess phosphorus can be stored. Living organisms take up phosphorus to form different biomolecules that are imperative for their growth and functioning.
- The microorganisms found in the biosphere also play an essential role where they help in the solubilization of inorganic phosphorus to make it available to the producers.
- The biosphere acts as a middle man in the recycling of phosphorus back to the lithosphere.
Phosphorus Cycle Significance
- The phosphorus cycle is essential to balance the concentration of phosphorus on the Earth’s surface so as to create a hospitable environment on the planet.
- Phosphorus is one of the important elements for all living beings, and its movement through different systems helps to understand different biological factors and factors that influence them.
- The phosphorus cycle is also tied to the availability of other elements and compounds like nitrogen and sulfur as phosphorus exists in a combined form with other elements in nature.
- The phosphorus cycle enables the flow of energy in the form of ATP through the food chain. The phosphorus compounds carry chemical energy trapped from producers to consumers to decomposers.
- The decomposition or mineralization of phosphorus is one of the processes involved in the natural waste removal system.
- The phosphorus cycle is responsible for increasing the availability of phosphorus in the soil for plant growth and soil fertility.
- Understanding the mechanism of the phosphorus cycle helps to understand the physiology of different microorganisms involved in the process.
Human impacts on the Phosphorus Cycle
- The use of artificial fertilizers in the soil affects the phosphorus levels in the soil, which in turn affects the overall phosphorus cycle.
- The most important anthropogenic change to the phosphorus cycle is a massive transfer from the vast and unavailable reserve pool to biologically available forms on land.
- The mining of phosphate-rich rocks also changes the availability of phosphorus in different ecosystems.
- The increase in the concentration of phosphorus in the oceans as a result of soil runoffs causes an increase in the formation of algal blooms, which then decreases the oxygen availability of the ocean.
References and Sources
- Yuan, Z., Jiang, S., Sheng, H., Liu, X., Hua, H., Liu, X., & Zhang, Y. (2018). Human Perturbation of the Global Phosphorus Cycle: Changes and Consequences. Environmental Science and Technology. 52(5), 2438–2450.DOI:10.1021/acs.est.7b03910
- Reinhard, C. T., Planavsky, N. J., Gill, B. C., Ozaki, K., Robbins, L. J., Lyons, T. W, Konhauser, K. O. (2016). Evolution of the global phosphorus cycle. Nature, 541(7637), 386–389.DOI:10.1038/nature20772
- Gachter, R., and Meyer, J. S. (1993). The role of microorganisms in mobilization and fixation of phosphorus in sediments. Hydrobiologia. 253(1-3), 103–121.DOI: 10.1007/bf00050731.
- Richardson, A. E., and Simpson, R. J. (2011). Soil Microorganisms Mediating Phosphorus Availability. Plant Physiology, 156(3), 989–996.DOI:10.1104/pp.111.175448
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