Phosphorus Cycle

Phosphorus is an essential element for life as it is the basic material for such things as DNA, teeth, bones, and shells. Phosphorus is constantly exchanged between the lithosphere, the hydrosphere, and living organisms. However, unlike the carbon and nitrogen cycles, it is a sediment cycle — phosphorus has practically no gaseous components and, therefore, does not involve atmospheric processes. Most of the phosphorus comes from the erosion of sedimentary rocks, which release small quantities in the form of dissolved phosphates that can be directly assimilated by plants.

The main processes that take place during the phosphorus cycle are as follows.

In nature, phosphorus is mostly found in the rocks of the lithosphere. Because of the effect of rain and wind, a small amount of phosphorus escapes from the rocks, usually in the form of phosphates.

Phosphates enter food chains through plants. Being necessary for their growth, phosphates are absorbed by them. Herbivores then ingest the phosphates by eating plants. The food chain ends when carnivores eat herbivores or other animals.

Phosphates ingested by animals return to the soil as fecal matter and urine. The degradation of dead animals and plants by decomposers also releases phosphates. Thus, the phosphates return to the soil and the cycle can then start again.

The phosphate cycle also takes place in the hydrosphere. Phosphates from rocks or excreted by animals and decomposers enter the oceans. Part of it helps plankton to grow, while the other part sinks to the bottom of bodies of water and mixes with sediments. Very slowly, over millions of years, these sediments form rocks and the phosphorus returns to its original form (see the arrow to the right of the image above).

The natural phosphorus cycle is destabilized by human activities, mainly due to additional inputs of phosphorus. Human activities, such as the spreading of large quantities of fertilizers rich in phosphates on agricultural land and the discharge of phosphate soap residues in residential and industrial wastewater, result in an imbalance in the biogeochemical phosphorus cycle. An excess of phosphorus in rivers, lakes, and coastal marine waters accelerates the growth of algae, thus promoting the process of eutrophication in aquatic environments.