xxxvii. Phosphorus Cycle

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xxxvii. Phosphorus Cycle

Phosphorus is an essential nutrient found in the macromolecules of humans and other organisms, including DNA and the phospholipids that form our cell membranes. Calcium phosphate also makes up the supportive components of our bones.

Elemental phosphorus exists in two major forms, white phosphorus, and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Earth.

Phosphorus plays a central role in aquatic ecosystems and is often the limiting nutrient thus limiting growth, in aquatic ecosystems. Another impact it has on the aquatic ecosystem is that excessive phosphorus may result in eutrophication. Phosphorus-containing compounds may also be carried in surface runoff to rivers, lakes, and oceans, where they get into the water bodies and aquatic organisms.

The main storage for phosphorus is in the earth’s crust and is found as phosphates in the land.

Phosphate compounds in the soil are taken up by plants and, transferred to animals that eat those plants. When plants and animals excrete wastes or die, phosphates are taken up by detritivores or returned to the soil.

In the ocean, phosphorus accumulates on the continental shelves in the form of insoluble deposits. When phosphorus-containing compounds from the bodies or wastes of marine organisms sink to the floor of the ocean, they form new sedimentary layers. After a long time, phosphorus-containing sedimentary rock may be moved from the ocean to the land by a geological process called uplift.

The Phosphorus cycle

The phosphorus cycle is the sedimentary biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere.

Phosphorus is found in large amounts as a mineral in phosphate rocks and enters the cycle from weathering and erosion. The crustal plates rise from the seafloor to expose the phosphates after millions of years.

The global phosphorus cycle includes four major processes:

Tectonic uplift and exposure of phosphorus-bearing rocks such as apatite to surface weathering.
Physical erosion along with chemical and biological weathering of phosphorus-bearing rocks to provide dissolved and particulate phosphorus to soils, lakes, and rivers.
Riverine and subsurface transportation of phosphorus to various lakes and run-off to the ocean.
Sedimentation of particulate phosphorus (e.g., phosphorus associated with organic matter and oxide/carbonate minerals) and eventually burial in marine sediments (this process can also occur in lakes and rivers).

Phosphates move quickly through plants and animals, but the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.

The cycle can also be categorized into four simpler steps-

Weathering
Absorption by plants
Absorption by animals
Decomposition and return to the environment

Anthropogenic impact on the cycle

Human activities like the use of fertilizers, artificial eutrophication impact the phosphorus cycle greatly.

The fertilizers increase the phosphorus levels in the soil than normal, which leads to soil losing its fertility.
It is harmful to the soil microorganisms and the runoff is also hazardous to aquatic life.
The amount of phosphorus waged away during the farm-to-table process leads to severe eutrophication, hence causing algal bloom and loss of aquatic life.