The Phosphorus Cycle: The Slow Pathway

Phosphorus is essential for plant and animal growth, as well as the health of microorganisms in the soil. We plot the 4 main steps of the phosphorus cycle on land and sea. We also look at how the use of phosphorus-based fertilizers and other agro-chemicals is causing ecological problems, such as eutrophication of freshwater systems, as well as hypoxic zones in estuarine and coastal areas.
Photo: Agronom (CC BY-SA 3.0)

The phosphorus cycle is another of the biogeochemical cycles which circulate essential chemicals through the various compartments of Planet Earth, in order to support life. Other pathways include the nitrogen cycle, the carbon cycle and the sulfur cycle, among others.

Unlike some of the other biogeochemical pathways, the phosphorus cycle is a slow cycle. This is because very little phosphorus circulates in the atmosphere owing to the fact that it’s not normally a gas. The phosphorus found in the atmosphere is likely to be a small amount of phosphoric acid (H3PO4), used in the manufacture of fertilizers and other chemical processes, and sometimes associated with acid rain.

The only other sources of phosphate in the atmosphere include volcanic ash, aerosols, and mineral dust sucked upwards from the Sahara Desert and other locations. For example, biomes like the Amazon Rainforest benefit substantially from phosphate dust blown across the Atlantic from Africa.

Because most phosphorus doesn’t circulate from land to air, or vice versa, most phosphorus ends up in sedimentary rock, and only reappears via tectonic uplift over geological time scales. This makes the phosphorus cycle an extremely slow cycle. 1 And because phosphorus takes so long to re-emerge from rock, the soil naturally becomes more and more depleted of phosphorus over time. 2 3

In a nutshell, the chemical element phosphorus circulates through the lithosphere (rocks), pedosphere (soils), biosphere (plants, animals), and hydrosphere (oceans).

Why is Phosphorus Important to Life?

Phosphorus is essential for plant and animal growth, as well as the health of microorganisms in the soil. Its biological value stems from its importance as a component of nucleotides – molecules that serve as the basic building blocks of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Phosphates are also an essential component of ATP (adenosine triphosphate) – the molecule that stores the energy we need to do just about everything we do: from building proteins, to muscle contraction and nerve impulse propagation. ATP powers all the physiological processes that require energy. 4

In the human body, 80 percent of phosphorus is found in teeth and bones, and it’s also a vital component of phospholipid membranes of cells.

The Main Steps of the Phosphorus Cycle

The phosphorus cycle involves four main steps, as follows:

1. Weathering from Rocks into Soils

The main source of phosphorus is sedimentary rock (that is, rock formed over millions of years from compacted sediments). Thus, the first stage of the phosphorus cycle involves the chemical weathering of phosphorus from exposed surfaces of sedimentary phosphorus-bearing rocks such as apatite. Rain and other sources of precipitation, cause the phosphorus to be washed into the soil. See also: What is the Water Cycle?

2. Absorption by Plants and Animals

Once the inorganic phosphorus arrives in the soil, plants as well as microorganisms like fungi and bacteria, are able to absorb it and grow. If the phosphorus gets washed away into the local freshwater systems, aquatic plants will absorb it from the water and they too will grow. Animals absorb phosphorus from eating plants and drinking water.

3. Decomposition of Organic Remains

When plants and animals die, their organic remains are broken down by bacteria, fungi and other decomposers, which leads to inorganic phosphorus being returned to the soil or (in the case of aquatic environments) the water, for re-use by other plants and living beings. This is a cycle within the phosphorus cycle. See also: Why is the Soil So Important to the Planet?

Other processes, however, tend to deplete the soil of usable phosphorus by hindering its reabsorption. For example, other types of bacteria convert plant-available inorganic phosphate into organic forms that are not usable for plants.

In addition, inorganic phosphorus can become chemically bound (attached) to particles of soil, which also makes it unavailable to plants. Also, plant take-up of inorganic phosphorus is regulated by the acidity (pH) of the soil. If soils have a pH of less than 4 or more than 8, the phosphorus starts to become bound up with other compounds, making it less available to vegetation or aquatic plants. (One reason why farmers add lime to the soil to reduce soil acidity and make phosphate more available to plants.)

Finally, crops are often harvested in such a way that little vegetation is left to decay in the fields. This means that there is less and less phosphorus for microorganisms to break down and recycle. Which is why farmers resort to synthetic phosphorus-based fertilizers to replenish the phosphorus content of the soil.

4. Transported To The Ocean

Over time, phosphorus in the soil ends up in lakes, rivers, and other freshwater bodies, from where it eventually drains into the oceans. Once there, it settles on the sea bed where it is slowly incorporated into sediments over a period of 20,000 to 100,000 years.

If it remains within a closed freshwater lake with no access to the ocean, it will form sediments on the lake bottom. 5 Over millions of years, these phosphorus-rich sediments are subjected to intense heat and compaction, and gradually become lithified (turned into rock). Once it becomes converted into rock, tectonic processes take over, and (over geological time) force it to the surface, allowing the chemical weathering process in step 1 to start over. 6

Effect of Human Action on the Phosphorus Cycle

Humans have had a major impact on the phosphorus cycle due to a variety of activities, including the use of phosphorus- or nitrogen-based agrochemicals, like fertilizers. (As much as 80 percent of all mined phosphorus is used to make fertilizers.)

The increased use (abuse) of such synthetic fertilizers leads inevitably to significant leakage of phosphorus into the environment. For example, it can leach into groundwater systems as a result of rain and irrigation. Or it can be lost due to surface runoff during periods of heavy rain, when the soil is already saturated. Phosphorus in the soil can become dissolved in the water and get washed away in the surface run-off.

In both cases, phosphorus leaks into the local ecosystems, causing pollution, loss of biodiversity, and eutrophication of freshwater aquatic systems. Eutrophication – the excessive growth of phytoplankton whose decomposition after death causes a major loss of oxygen in the water – can be harmful to the biodiversity of the local ecosystem, and may lead to death of many aquatic species by hypoxia. 7 (Note: Excess phosphorus causes eutrophication of freshwater ecosystems, while excess nitrogen results in eutrophication in saltwater estuaries and coastal habitats, upsetting the marine food web over a wide area.) See also: What is the Oxygen Cycle?

Eutrophication: When rivers turn green
Eutrophication in the Potomac River, Washington, D.C. The algae blooms occur where lakes and rivers have high concentrations of nutrients – in particular, nitrogen and phosphorus. Some lakes and rivers have naturally high nutrient concentrations but in places like the Potomac River, man-made pollution from waste water is the main cause. Photo: Alexandr Trubetskoy/CC BY-SA 3.0

How can eutrophication be prevented? Basically, by managing the use of phosphorus and nitrogen fertilizers, and making sure that human sewage or livestock effluent does not enter waterways from leaky septic tank systems, or inefficient water treatment plants. 8

Effects of Climate Change on Phosphorus Cycle

The full impact of global warming on the chemical composition of soil remains unclear. However, studies show that plant-available inorganic phosphorus in the soil, decreases significantly as annual temperature increases. 9 So as rising temperatures become more widespread, plants will find less phosphorus in the soil to support their growth.

RELATED

Is more CO2 good for plants?
7 Effects of Climate Change on Plants

This will undoubtedly lead to more phosphate fertilizers and more polluting run-off. Scientists now believe, for example, that major ocean anoxic events in the past are linked to the global input of nutrients into the oceans. Consequently, the deliberate mobilization of phosphorus for agricultural purposes could be an important factor in future events of this kind. 10

To understand more about the timeline of our planet and its slow phosphorus cycle, see: History of Earth in One Year (Cosmic Calendar).

References

  1. Phosphate mineral reactivity: from global cycles to sustainable development“. Oelkers EH, et al; (February 2008). Mineralogical Magazine. 72 (1): 337–40. []
  2. Understanding ecosystem retrogression“. Peltzer DA, et al. (November 2010). Ecological Monographs. 80 (4): 509–29. []
  3. See also: Gabriel M. Filippelli. 2017. Encyclopedia of Paleoclimatology and Ancient Environments. []
  4. Adenosine Triphosphate[]
  5. “Phosphorus Cycle.” The Environmental Literacy Council. []
  6. The Global Phosphorus Cycle“. Ruttenberg KC (2014). Treatise on Geochemistry. Elsevier. pp. 499–558. []
  7. National Aquatic Resource Surveys: Indicators: Phosphorus. []
  8. Ecology. Controlling eutrophication: nitrogen and phosphorus“. Conley DJ, et al; (February 2009). Science. 323 (5917): 1014–5. []
  9. Effects of Climate on Soil Phosphorus Cycle and Availability in Natural Terrestrial Ecosystems.” Enqing Hou, et al; Glob Chang Biol. August 2018; 24(8): pp.3344-3356. []
  10. Ocean deoxygenation, the global phosphorus cycle and the possibility of human-caused large-scale ocean anoxia.” Andrew J. Watson, Timothy M. Lenton, Benjamin J. W. Mills. Royal Society. August 2017. []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email