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My Resources » Environmental Science » Cycle Articles & Questions » Phosphorus and sulfur cycle articles

Phosphorus and sulfur cycle articles

Phosphorus Cycle

Phosphorus is an important element for all forms of life. As phosphate (PO4), it makes up an important part of the structural framework that holds DNA and RNA together. Phosphates are also a critical component of ATP – the cellular energy carrier – as they serve as an energy ‘release' for organisms to use in building proteins or contacting muscles. Like calcium, phosphorus is important to vertebrates; in the human body, 80% of phosphorous is found in teeth and bones.

The phosphorus cycle differs from the other major biogeochemical cycles in that it does not include a gas phase; although small amounts of phosphoric acid (H3PO4) may make their way into the atmosphere, contributing – in some cases – to acid rain. The water, carbon, nitrogen and sulfur cycles all include at least one phase in which the element is in its gaseous state. Very little phosphorus circulates in the atmosphere because at Earth's normal temperatures and pressures, phosphorus and its various compounds are not gases. The largest reservoir of phosphorus is in sedimentary rock.

It is in these rocks where the phosphorus cycle begins. When it rains, phosphates are removed from the rocks (via weathering) and are distributed throughout both soils and water. Plants take up the phosphate ions from the soil. The phosphates then moves from plants to animals when herbivores eat plants and carnivores eat plants or herbivores. The phosphates absorbed by animal tissue through consumption eventually returns to the soil through the excretion of urine and feces, as well as from the final decomposition of plants and animals after death.

The same process occurs within the aquatic ecosystem. Phosphorus is not highly soluble, binding tightly to molecules in soil, therefore it mostly reaches waters by traveling with runoff soil particles. Phosphates also enter waterways through fertilizer runoff, sewage seepage, natural mineral deposits, and wastes from other industrial processes. These phosphates tend to settle on ocean floors and lake bottoms. As sediments are stirred up, phosphates may reenter the phosphorus cycle, but they are more commonly made available to aquatic organisms by being exposed through erosion. Water plants take up the waterborne phosphate which then travels up through successive stages of the aquatic food chain.

While obviously beneficial for many biological processes, in surface waters an excessive concentration of phosphorus is considered a pollutant. Phosphate stimulates the growth of plankton and plants, favoring weedy species over others. Excess growth of these plants tend to consume large amounts of dissolved oxygen, potentially suffocating fish and other marine animals, while also blocking available sunlight to bottom dwelling species. This is known as eutrophication.

Humans can alter the phosphorus cycle in many ways, including in the cutting of tropical rain forests and through the use of agricultural fertilizers. Rainforest ecosystems are supported primarily through the recycling of nutrients, with little or no nutrient reserves in their soils. As the forest is cut and/or burned, nutrients originally stored in plants and rocks are quickly washed away by heavy rains, causing the land to become unproductive. Agricultural runoff provides much of the phosphate found in waterways. Crops often cannot absorb all of the fertilizer in the soils, causing excess fertilizer runoff and increasing phosphate levels in rivers and other bodies of water. At one time the use of laundry detergents contributed to significant concentrations of phosphates in rivers, lakes, and streams, but most detergents no longer include phosphorus as an ingredient.


Sulfur Cycle

Sulfur is mainly found on Earth as sulfates in rocks or as free sulfur. The largest deposits of sulfur in the United States are in Louisiana and Texas. Sulfur also occurs in combination with several metals such as lead and mercury, as PbS and HgS. Sulfur appears as the yellow aspects of soil in many regions.

Source: Uni-bremen.

Sulfur was mined early in the form of the yellow element and used for gunpowder and fireworks. While bacteria digest plant matter, they emit H2S, hydrogen sulfide, a gas that has the "rotten egg" smell characteristic of swamps and sewage. Sulfur is an essential element of biological molecules in small quantities.

Sulfur and its compounds are important elements of industrial processes. Sulfur dioxide (SO2) is a bleaching agent and is used to bleach wood pulp for paper and fiber for various textiles such as wool, silk, or linen. SO2 is a colorless gas that creates a choking sensation when breathed. It kills molds and bacteria. It is also used to preserve dry fruits, like apples, apricots, and figs, and to clean out vats used for preparing fermented foods such as cheese and wine.

Sulfuric acid, H2SO4, is a very widely used chemical. Over 30 million tonnes of sulfuric acid are produced every year in the U.S. alone. The acid has a very strong affinity for water. It absorbs water and is used in various industrial processes as a dehydrating agent. The acid in the automobile battery is H2SO4. It is used for "pickling" steel, that is, to remove the oxide coating from the steel surface before it is coated with tin or electroplated with zinc.

Sulfur is also a biologically important atom. Although only small amounts of sulfur are necessary for biological systems, disulfide bridges form a critical function in giving biological important molecules specific shapes and properties. (See Biological Systems.)

Sulfur is released into the atmosphere through the burning of fossil fuels --especially high sulfur coal--and is a primary constituent of acid rain. Sulfuric acid (H2SO4) is the primary constituent of acid rain (see Atmospheric System) in about all regions other than California. Sulfur dioxide and carbonyl sulfide (COS) occur in small quantities in the atmosphere; but due to its high reactivity, sulfur is quickly deposited as compound (sulfates) on land and other surfaces.


Figure S1: The Sulfur Cycle.

Figure S1 shows the biogeochemical cycle of sulfur. As in the case of nitrogen, the figure shows the large quantities. Local activities such as coal burning can release large amounts in a small area. Sulfur compounds can also be transported from the higher altitudes from tall "smoke stacks" and contribute to acid rain far from the sources.

Ms. Rebecca Chunn
Glynn County Schools
1001 Mansfield St.
Brunswick, GA 31520