Biogeochemical cycles

nitrogen, carbon and phosphorus cycles

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Biogeochemical cycles
These involve the linked biochemical and physical processes that move elements that are
important to living organism.
Living organisms are made of many elements. Hydrogen, oxygen, carbon, nitrogen, sulphur
and phosphorus are especially important, but many others are also needed. For some of
these, there are not enough atoms freely available in the environment for each one to be
used only once. The processes that use and move these elements join up to produce cycles
that ensure future availability and the continuation of life.
Human activities can change the rates of movement of these elements between reservoirs,
which can cause some reservoirs to build up while others become depleted. An
accumulation may cause pollution while a shortage may reduce the availability for humans
and other living organisms.
All biogeochemical cycles are unique because each element has unique properties and
living organisms use them in different ways. Most of the processes are driven by energy from
the sun.
The natural rates of movement of an element in and out of a reservoir will be the same as
the amount in the reservoir is likely to be in a state of balance. The rate of movement and
amount in the reservoir will determine the average `residence time' of atoms in the reservoir.
An understanding of these cycles aids the management of nutrient supply systems and the
development of more sustainable human activities such as:
Management of agricultural nutrients
Prevention of eutrophication
Predicting global climate change
The Carbon Cycle
A simplified diagram of the cycle-

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Carbon is the basis of life because it is the only element that can form long chains of atoms
and therefore produce a huge variety of different compounds, in combination with
elements such as hydrogen, oxygen, nitrogen, sulphur and phosphorus.
The most readily accessible reservoir of carbon is carbon dioxide, which currently makes
up 0.038% of the atmosphere. One-third of this is removed each year, so life can only
continue if there is a sequence of processes that replaces it: the carbon cycle.…read more

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Photosynthesis captures light energy using pigments such as chlorophyll. It
converts low-energy substances such as carbon dioxide and water into high-energy
carbohydrates that can be stored for later use. Oxygen is a by-product.
Respiration releases the energy from high-energy substances such as
carbohydrates that can be used to drive metabolic processes. Aerobic respiration
can break down organic compounds more fully to carbon dioxide and release much
more energy than anaerobic respiration.…read more

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Anaerobic gut bacteria in the intestines of livestock release methane into the atmosphere,
which may be subsequently oxidised to carbon dioxide.
Anaerobic bacteria in rice paddy fields release methane.
Soil disturbances by ploughing increase the rate of decomposition, which release more
carbon dioxide into the atmosphere.
Global climate change increases the rate of decomposition, which release more carbon
dioxide into the atmosphere. It also increases the rate of photosynthesis, which absorbs
more carbon dioxide from the atmosphere.…read more

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Ionising phenomena are the events that provide the energy for atmospheric nitrogen and
oxygen to react and produce oxides of nitrogen, such as lightning and forest fires. The
oxides of nitrogen can dissolve in rainwater and produce dilute nitric acid. When this is
washed into the soil it will react with minerals in the soil to form nitrates.
The chemical reduction of nitrogen to ammonia is carried out by some micro-organisms
during nitrogen fixation.…read more

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The Phosphorus cycle
Phosphorus is an important component of bones, proteins, ATP, DNA and RNA.
The phosphorus cycle is very different from the carbon and nitrogen cycles in that the
atmosphere is not involved. Phosphorus compounds, usually as phosphates, are not very
and are often the limiting factor on plant growth, especially in the deep oceans where
no rocks to release phosphates during weathering.
Dissolved phosphates are absorbed by plants and are passed along food chains.…read more

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