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The Nutrient Cycle
 
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The Nutrients

Nutrients are the chemical elements essential for growth. Unlike energy which flows through a food chain, these are cycled within the food chain.

Carbon, nitrogen, oxygen, hydrogen, potassium, calcium and phosphorous are all elements essential for plant growth. These nutrients are both gases and minerals. They move in cycles between the physical environment and living things. The nutrients are used for growth and are converted to tissue. All organisms die and decompose, releasing the nutrients to be used again.

Gases drawn from the atmosphere, such as carbon, oxygen and nitrogen, and eventually returned to the atmosphere. Minerals drawn from the earth are eventually returned there eg sulphur and potassium are described as sedimentary cycles.

Nutrients can be introduced into an ecosystem by visiting birds and animals or by the addition of fertilizer. They can also be lost as a result of water runoff or changes such as deforestation.



The Carbon Cycle

The carbon cycle is of primary concern and linked to energy use and global climate change. All green plants fix carbon and oxygen in the form of carbon dioxide from the atmosphere, using the energy from the sun in the process known as photosynthesis.

Plants use carbon and oxygen to build carbohydrates eg glucose Some is converted into starch and cellulose and other compounds that constitute the plant. When an animal eats a plant the organic material is digested and the stored energy incorporated into the tissues of the animal. Carbon atoms of the plant become atoms of the animal, and energy passes up the food chain. Plants, like animals also breathe (respiration) and CO2 is passed back into the air cycle.

This type of cycling applies to nearly all the elements of the planet. No new matter is created but is repeatedly rearranged. The trapping of energy along with the uptake of nutrients are the basic activities of all living things.

The resultant chemical energy is used in two ways:
  1. Maintenance, growth and reproduction (living)
  2. Stored energy for future use.
In plants, this is the STORED DRY MASS. The rate at which this is accumulated is the NET PRIMARY PRODUCTION of the system in which the plant lives, known as an ecosystem.

When plant material dies it is decomposed by organisms in the soil ,the energy from the plant is converted to be used by the organisms which respire releasing CO2 back into the atmosphere. Other plants may be entombed in boggy areas and be compressed to form peat. The energy here is stored in the peat, which ultimately becomes encased in the earth's crust to form coal. On combustion the coal releases the energy in a series of chemical changes to produce heat and release the trapped carbon.

The cycle becomes out of balance as this energy store of fossil fuels is used up at a greater rate than it is produced,. The amount of heat generated and the resultant carbon released do not balance with the amount of radiation from the sun reaching the earth and the amount of uptake of carbon dioxide by plants to make new plant growth, ultimately to decay or form fossil fuel store.



The Nitrogen Cycle

As with the other nutrients, so to is there a cycle for nitrogen.

Four fifths of the atmosphere consists of oxides of nitrogen. This is virtually useless in this form, as most plants cannot take in that form.

Instead the nitrogen must be "fixed", meaning the nitrogen oxide gas is transformed into other chemicals like ammonia or nitrates in the soil. This is carried out by "nitrogen fixing" bacteria, such as azotobacter. Some countries used to use azotobacter like a fertiliser.

One of the main groups of fertilisers are nitrogen based, as they can supply the much need nitrogen to green plants. Otherwise, plants rely on the bacteria. Some plants grow in association with the bacteria. The most famous are leguminous plants such as peas and beans. This is why these plants used to be essential in crop rotation schemes, as they were seen to "put" nitrogen into the soil.

The nitrogen compounds in the soil are taken up by plants and there used to make amino acids which are the building blocks of proteins, the essential compounds of life. Nitrogen can be passed from organism to organism as amino acids.

When organisms die the nitrogen is broken back down to basic nitrogen compounds, which can be reabsorbed by plants or animals.

The balance of the nitrogen cycle is severely challenged when too much external nitrogen fertiliser is introduced from other ecosystems, causing eutrophication.



Summary

In some parts of the world there is an excess of nitrogen, while in others a shortage.

Rain forests contain more stored dry mass (energy) than other systems. As these forests are destroyed, huge quantities of CO2, on combustion of the wood, are released into the atmosphere. The very plants that are needed to assimilate this into their mass are destroyed so creating one cycle we don't want to see - a vicious cycle.


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2002 Edition