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The teacher's online companion to Science World, providing your middle school and high school students with science news and rich informational texts that connect STEM to the Common Core

In the water cycle, water circulates from Earth to the atmosphere and back again.
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Merkushev Vasiliy/Shutterstock
Nutrient Cycles
From Grolier's Multimedia Encyclopedia
The carbon cycle flows between the atmosphere, land, and ocean.
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Copyright, University Corporation for Atmospheric Research 
During the nitrogen cycle, nitrogen in the atmosphere or soil can go through chemical and biological changes, combine into living and non-living material, and return back to the soil or air.
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Copyright, University Corporation for Atmospheric Research 

Nutrient cycle is the term used to describe the natural processes that keep all the elements needed to support life circulating within the ecosphere.

The Earth is composed of 92 naturally occurring elements, of which about 20 are used in measurable amounts by most living organisms. About 70 are used in small quantities by microorganisms in trapping energy, forming protoplasm and enzymes, and carrying out life activities. The availability of these elements in forms that plants and animals can use is absolutely essential for the survival of life on Earth. Although the elements are constantly recycled, energy is not. Thus a constant inflow of solar energy essentially keeps life going.

The water cycle, or hydrologic cycle, underlies all the other nutrient cycles, since water is essential to all but a few of the simplest life forms. It is also the main mode of transport of many other nutrients and the agent that leaches nutrients from rocks and soils. Two other basic cycles, the carbon cycle and nitrogen cycle, are discussed separately as well as below.

CARBON

The carbon cycle clearly illustrates the complexity of relationships involved in the processes of recycling. Green plants have the unique ability to trap solar energy and to combine the inorganic substances carbon dioxide and water to make glucose and other carbohydrates through the process of photosynthesis, at the same time releasing surplus oxygen to the atmosphere. They also, immediately or later (or both), oxidize some of the foods they have produced to obtain the trapped solar energy. Even so, they consistently produce more food than they oxidize and would ultimately have all the available carbon dioxide tied up if it were not for animals that feed on plants and return carbon dioxide to the atmosphere through respiration, excretion, fermentation, and decay (by bacterial action). This same series of relationships is part of the water cycle, the nitrogen cycle, and several mineral cycles.

At each stage oxidation, energy release, and a great deal of other chemical activity has taken place, including the breaking down of proteins into amino acids and the forming of new proteins; the building of skeletal parts; the forming and storage of fats, as each living thing organizes the nutrients according to its genetic code; and the elimination of unusable material. Frequently bacteria and other agents of decomposition play the most important role in keeping nutrient cycles functioning.

NITROGEN

Animals obtain the nitrogen needed for their body tissues by feeding on animal and vegetable proteins. Plants synthesize protein from inorganic compounds from the soil, or from free nitrogen in the air, with the aid of particular soil bacteria. In the last stage, decomposition by bacteria releases the amino group from protein as ammonia.

SULFUR

Sulfur is derived from volcanic gases and the weathering of iron-containing sulfides in igneous rocks. In the sulfur cycle, anaerobic bacteria in the soil use sulfur in the same way that denitrifying bacteria use nitrogen. Other sulfur bacteria contain purple or green pigments that operate much as chlorophyll does and that enable the bacteria to break down carbon dioxide and hydrogen sulfide, using the Sun's energy to produce hydrocarbons.

PHOSPHORUS

Phosphorus is basic to all life because it is a major component of deoxyribonucleic acid (DNA), the chemical that makes up the nucleus of all cells and controls reproduction at the levels of both the cell and the organism. Phosphates are taken up by the roots of green plants and used in organic synthesis. They are then passed to animals through food chains and, ultimately are released to soil through bacterial and fungal decomposition after the death of an animal.

Wasting phosphorus has more-serious implications than the eutrophication of lakes. First, the supplies of usable phosphorus are relatively small compared to the demands of the ecosystem. Phosphorus running off the land or dumped into a river moves through waterways to the ocean, where some of it, along with other nutrients, is deposited in deep waters and becomes unavailable to plants and, therefore, to animals. In addition, phosphorus is one of the slower recyclers because large reserves exist in bones, teeth, and mollusk shells, which are not readily decomposed.

OTHER ELEMENTS

In addition, iron, calcium, potassium, magnesium, cobalt, cadmium, copper, manganese, zinc, molybdenum, boron, sodium, chlorine, iodine, and other less commonly used elements are all essential to many forms of life. Several, such as iron and calcium, have complicated biogeochemical patterns that may strongly affect the availability of other elements. Several cycles are little known.

IMPORTANCE

The far-reaching nature of nutrient cycles has become obvious in recent years with the discovery of DDT in the tissues of Antarctic penguins. The penguins obtained the DDT by eating shrimp that fed on oceanic plankton that, in turn, had taken up the pesticide when it washed off the land. A similar sobering indicator appeared at the other end of the globe when significant amounts of strontium-90 from atomic explosions were transported by air currents, deposited in Arctic soil, taken up by lichens, eaten by caribou, and ultimately found in the tissues of human inhabitants of the region.

In fact, nutrient cycles point out human vulnerability; as life continues because of the interwoven and interdependent relationships of green plants and animals, supplemented by the activities of bacteria, with the nutrients constantly being transported within the ecosystem and within organisms by the action of water, anything that disrupts or interrupts these cycles has far-reaching and potentially lethal effects.

Helen Ross Russell