Nitrogen cycle

Nitrogen cycle

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Nitrogen cycle
7.4 The role of decomposers, and recycling nitrogen within an ecosystem
Decomposing organic material
Materials and energy are lost when an organism dies or excretes, and this dead and waste organic material can be broken
down by decomposers, including fungi and bacteria (and a small amount of animals). These microorganisms involved in
decomposition feed in a way which is different from animals, which is described as saprotrophically, and the organisms
themselves are described as saprophytic.
These organisms secrete enzymes onto dead and waste matter, which digest the material into small molecules which can
be absorbed into the decomposers' bodies. Once absorbed, as with other organisms, the molecules are either used for
respiration (and other life processes) or stored.
If bacteria and fungi did not break down dead organisms, energy and valuable nutrients would remain trapped within
them. By digesting such materials, microbes get a big enough supply of energy to sustain life and grow, and the trapped
nutrients are not wasted, but recycled. Microbes have a particularly important role to play in the global carbon cycle, and
also the global nitrogen cycle.
The nitrogen cycle
Living things need nitrogen to make proteins and nucleic acids. Nitrogen is cycled between the biotic and abiotic
components of an ecosystem in a cycle known as the nitrogen cycle. The four major processes which occur as part of the
cycle are nitrogen fixation, ammonification, nitrification and denitrification.
nitrogen fixation by N2 denitrification by bacteria
Rhizobium bacteria nitrogen gas
in the air
nitrogen fixation nitrogen
by the Haber fixation by
process lightning
ammonium NH4
organic organic organic and nitrate NO3
- NO
nitrogen in nitrogen in nitrogen in nitrogen oxide
ions in fertiliser
legumes plants animals nitrogen
fixation by
bacteria in
the soil Air
nitrogen in UREA NH4+ NO2- NO3-
soil ammonium nitrites nitrates
oxidation by oxidation by leaching
bacteria bacteria
nitrogen flows into
absorption by rivers and streams
root plants
nitrogen fixation
ammonification (decomposition)

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Nitrogen fixation
Nitrogen, whilst making up almost 80% of the atmosphere, is very unreactive and plants are unable to use it directly,
instead, they must have a supply of materials where nitrogen is fixed, such as in ammonium ions (NH4 ) or nitrate ions
(NO3 ). Nitrogen fixation occurs when lightning strikes, or through the Haber process, but these only account for 10% of
total nitrogen fixation ­ the majority comes from nitrogen-fixing bacteria.…read more


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