The slow cycling of phosphorus through the biosphere. How phosphorus-containing fertilizers can cause aquatic dead zones.
- Phosphorus is an essential nutrient found in the macromolecules of humans and other organisms, including
- The phosphorus cycle is slow. Most phosphorus in nature exists in the form of phosphate ion—
- Phosphorus is often the limiting nutrient, or nutrient that is most scarce and thus limits growth, in aquatic ecosystems.
- When nitrogen and phosphorus from fertilizer are carried in runoff to lakes and oceans, they can cause eutrophication, the overgrowth of algae. The algae may deplete oxygen from the water and create a dead zone.
Is phosphorus important? That depends—do you like having
, cell membranes, or bones in you body? Hint: The answer is probably yes!
Phosphorus is an essential nutrient for living organisms. It’s a key part of nucleic acids, like
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.
Natural cycling of phosphorus
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—
. Phosphate compounds are found in sedimentary rocks, and as the rocks weather—wear down over long time periods—the phosphorus they contain slowly leaches into surface water and soils. 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.
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.
Want to join the conversation?
- phosphorous cycle is perfect or imperfect cycle ?(8 votes)
- It is an imperfect cycle. This is because not all phosphorous can be retained in the cycle no matter how hard you try. Even without human interference, some phosphorous is going to escape from the cycle.(11 votes)
- is there a way that we block the cycle?
example : after using phosphorous for fertilizer, we make irrigation canal for fertilizer residu to transport and we harfest the residu to make fertilizer again?
is that possible?(7 votes)
- Do you mean to recycle? Sounds nice to me. Not sure what agriculture engineers can say about it. We should ask them :D(7 votes)
- what happens if there is too much nitrogen or phosphorus in an environment?(5 votes)
- If too much nitrogen or phosphorus is present in an environment, plant growth is greatly increased. This is due to the fact that both nitrogen and phosphorus are limiting nutrients. While this abundance of nutrients may be fine in your backyard, it can potentially be very harmful. For example, when too much nitrogen and phosphorus are present in a marine biome, eutrophication takes place. This leads to dead zones.(6 votes)
- If it takes such a long time for Phosphorous to go back to the land from the ocean, and it sounds like surface runoff carries phosphorous to the ocean much quicker, will we eventually run out of Phosphorus?(4 votes)
- Well, I do think it is possible, mostly because most of the industrial phosphorous is obtained from rocks so of course it is necessary for millions of years to pass before new rocks are formed, it is a limited nutrient by all means. But I don't think we are going to be able to see such a thing happening in quite a long time. I think it is by far easier that all our continental water reservoirs become euthrophicated before that.(4 votes)
- How can phosphorus escape rocks?(3 votes)
- Phosphorus can escape rocks when natural processes like wind and weathering chip away phosphorus in a soluble form, where it is then ingested by animals(4 votes)
- what is the time frame of the phosphorus cycle?(3 votes)
- What is the biological importance of the phosphorus cycle?(2 votes)
- does phosphate return to the soil and does phosphorus get decomposed by decomposers first?(2 votes)
- Phosphate is not back in its chemical free form but via organism since it gets incorporated in living organisms therefore once they decay you have phosphorus in the soil.(2 votes)
- What are the biotic and abiotic factors in this cycle?(2 votes)
- Biotic factors are the living things in an ecosystem so they do not recycle. Abiotic factors are the different physical and chemical components available like temperature, air, water, minerals, rocks,and PH(2 votes)