⏳ Estimated Reading Time: 6 - 8 minutes

Explain the steps and reservoir interactions in the phosphorous cycle.

The Cycling of Phosphorous

an illustration of a phosphorous atom, showing 15 electrons circling a central nucleus — ***Phosphorous has 15 protons, neutrons, and electrons.***

Phosphorous is an important element of living things.

You may not think that phosphorous is important to living things because you don’t really hear a lot about the importance of phosphorous to living organisms like you do with carbon and water, but phosphorous is found in DNA, RNA, and ATP - essential compounds for all living things.

Review the interactive below to see some examples of phosphorous in living organisms:

The law of conservation of matter states that matter can change form, but it is never destroyed. Thus, the amount of phosphorous on Earth is the same now as it was millions of years ago. So, the phosphorous needs to cycle through the environment and living things so it can be recycled for further use. Otherwise, we’d run out of phosphorous eventually.

Essential Knowledge

The phosphorous cycle is the movement of atoms and molecules containing the element phosphorous between sources and sinks.

Biogeochemical cycles are essential for life and each cycle demonstrates the conservation of matter.

The Phosphorous Cycle is Different

a snow-covered mountain — ***Phosphorous is stored in rocks, soil, and ocean sediments. It is NOT stored in the atmosphere.***

The phosphorous cycle differs from all the other biogeochemical cycles primarily in two ways. First, phosphorous is not an abundant nutrient in the environment, so it can often be the limiting factor in environments.

Second, phosphorous has no atmospheric component. This limits the supply of phosphorous in the environment.

If there is no atmospheric component of phosphorous, where is all the phosphorous stored? There are three primary phosphorous sinks on Earth: rocks, soil, and oceanic sediments.

Essential Knowledge

There is no atmospheric component in the phosphorous cycle, and the limitations this imposes on the return of phosphorous from the ocean to land make phosphorous naturally scarce in aquatic and many terrestrial ecosystems.

AP Exam Tip

You will need to know that the phosphorous cycle has no atmospheric component, making phosphorous the limiting nutrient in most undisturbed ecosystems.

The (Slow) Movement of Phosphorous

a black and white picture of a rocky mountain — ***Phosphorous moves SLOWLY through the environment by rock weathering.***

Phosphorus is released from rocks or sediment during the process of weathering, which can make up soil and then runoff into waterways or blow as dust throughout the ecosystem. Soil can also be released into the soil as an inorganic phosphate during decomposition.

Once phosphorous makes it into the soil, plants can absorb this phosphorous through their roots and incorporate it into their tissues.

From there, phosphorous can move through the food web and the ecosystem. Consumers eat plants and/or other consumers and use the phosphates found in their food to make bones, teeth, DNA, ATP, etc.

Phosphorous is released from consumers as waste while they are alive and then, as decomposers break down the previously living organisms, phosphorous in the tissues is released back into the environment.

a river surrounded by green trees in the foreground and a field in the mid-ground, and more trees in the background — ***Phosphorous dissolves in rivers and oceans and is used by marine life.***

Once the phosphorous is released back into the environment, producers can absorb it from the soil.

In most environments, phosphorous is considered a limiting nutrient. This is important!

Essential Knowledge

In undisturbed ecosystems, phosphorous is the limiting factor in biological systems.

Human Impacts

runoff — ***Fertilizers with phosphorous and nitrogen pollute water bodies.***

Most fertilizers have nitrogen and phosphorous in them because these nutrients tend to be limiting in ecosystems. When fertilizers are added to crops, agricultural productivity is increased. This increases the rate of the phosphorous cycle, which is usually a very slow process.

Nitrogen- and phosphorous-rich water can also percolate into the groundwater, increasing the concentration of these nutrients in groundwater.

The addition of phosphorous to aquatic environments can be problematic. Phosphorous is also limiting in these environments. You might think it would be a good thing to add a nutrient that is limiting to the environment because it stimulates growth. However, these ecosystems are adapted to phosphorous as a limiting nutrient.

When algae and cyanobacteria grow unchecked, their populations get larger than what is expected for the environment.

coastal waters with a large green algal bloom caused by runoff from the land — ***Too much of a good thing can be too much of a good thing. Too much phosphorous causes algae and cyanobacteria blooms.***

You may have seen a pond that is located near lawns or agricultural areas. These ponds are green because fertilizer from the lawns and/or agricultural areas has run off into the pond, increasing the amount of nitrogen and phosphorous in the water, and causing an explosion in the algal and cyanobacteria populations.

When the algal and cyanobacteria populations experience explosive growth in these waters, they frequently cover the surface of the pond.

When the algal and cyanobacteria populations experience explosive growth in these waters, they frequently cover the surface of the pond. This allows the floating algae to get the sunlight they need, but they also block sunlight from reaching the bottom of the waterway, pond, or lake. This means that photosynthetic organisms attached to the bottom die because they can no longer photosynthesize when the sunlight is blocked. Once these plants die, they are decomposed by decomposers. The process of decomposition uses oxygen in the water, reducing the amount of oxygen in the waterway, because the plants are not producing more oxygen (they died!). The lack of oxygen causes more organisms in the water to die, which increases the amount of decomposition and oxygen usage, which further reduces the amount of oxygen available in the water. This process continues until there is no more oxygen in the water.

Once an area no longer has oxygen, it is called a dead zone.

The entire process, from explosive algal growth to dead zone, is called cultural eutrophication. We will discuss eutrophication in more depth when we discuss aquatic pollution.

While phosphorous is not a nutrient we think about and doesn’t seem as important as carbon or water, it is critically important to ecosystems. Too little phosphorous can halt the growth of living organisms and too much phosphorous can cause total ecosystem collapse.

Essential Knowledge

The increase in nutrients in eutrophic aquatic environments causes an algal bloom.

When the algal bloom dies, microbes digest the algae, along with the oxygen in the water, leading to a decrease in the dissolved oxygen levels in the water.

Eutrophication occurs when a body of water is enriched in nutrients.

AP Exam Tip

You should know the ways human activity affects the phosphorous cycle.

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