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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:
- Maintenance, growth and reproduction (living)
- 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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