Transcript of Nitrogen Cycle The nitrogen cycle is the movement of nitrogen through different environmental...
- Slide 1
- Nitrogen Cycle The nitrogen cycle is the movement of nitrogen
through different environmental segments.
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- Nitrogen Nitrogen is essential for life. It is found in amino
acids, proteins, and genetic material.
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- Nitrogen in our environment Nitrogen is the most abundant
element in the atmosphere (~78%). However, gaseous nitrogen must be
'fixed' into another form so that it can be used by living
organisms.
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- Forms of Nitrogen useable for plants Urea CO(NH 2 ) 2 Ammonia
NH 3 (gaseous) Ammonium NH 4 Nitrate NO 3 Nitrite NO 2
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- Nitrogen Fixation Nitrogen fixation means making atmospheric
nitrogen (N 2 ) available for plant consumption through ammonia and
nitrates. Nitrogen fixation occurs naturally in the following ways.
Lightening Biological fixation I. Ammonificaiton II.
Nitrification
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- Fixation by Lightning The energy from lightning causes nitrogen
(N 2 ) and water (H 2 O) to combine to form ammonia (NH 3 ) and
nitrates (NO 3 ). Precipitation carries the ammonia and nitrates to
the ground, where they can be assimilated by plants.
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- Biological Fixation About 90% of nitrogen fixation is done by
bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium.
N 2 + 3H 2 2NH 3 Ammonia can be used by plants directly. Ammonia
and ammonium may be further reacted in the nitrification
process.
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- Ammonification When plants and animals die, bacteria convert
nitrogen nutrients back into ammonium salts and ammonia. This
conversion process is called ammonification. Amino acids + 1 1 / 2
O 2 CO 2 + H 2 O + NH 3 + 736kJ This process liberates a lot of
energy which can be used by the Saprotrophic microbes
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- Nitrification Nitrification occurs by the following reactions:
2 NH 3 + 3 O 2 2 NO 2 + 2 H + + 2 H 2 O 2 NO 2 - + O 2 2 NO 3 -
Aerobic bacteria use oxygen to convert ammonia and ammonium.
Nitrosomonas bacteria convert nitrogen into nitrite (NO 2 - ) and
then nitrobacter convert nitrite to nitrate (NO 3 - ).
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- The nitrogen fixers Cyanobacteria are nitrogen fixers that also
fix carbon (these are photosynthetic) Rhizobium bacteria are
mutualistic with certain plant species e.g. Legumes They grow in
root nodules Azotobacter are bacteria associated with the rooting
zone (the rhizosphere) of plants in grasslands
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- Nitrogen fixation by bacteria Atmospheric nitrogen is
chemically fixed by bacteria to ammonium (NH 4 +1 ), which is used
by plants as a fertilizer. Some bacteria exist in a symbiotic
relationship with plants (legumes and some root-nodule species).
Plants utilize the nitrate as a nutrient. Animals obtain nitrogen
by eating plants or plant-eating animals.
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- Only prokaryotes show nitrogen fixation These organisms possess
the nif gene complex which make the proteins, such as nitrogenase
enzyme, used in nitrogen fixation Nitrogenase is a metalloprotein,
protein subunits being combined with an iron, sulphur and
molybdenum complex The reaction involves splitting nitrogen gas
molecules and adding hydrogen to make ammonia N 2 2N- 669 kJ 2N +
8H + NH 3 + H 2 + 54 kJ This is extremely energy expensive
requiring 16 ATP molecules for each nitrogen molecule fixed The
microbes that can fix nitrogen need a good supply of energy
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- Nitrification This involves two oxidation processes The ammonia
produced by ammonification is an energy rich substrate for
Nitrosomas bacteria They oxidise it to nitrite: NH 3 + 1 1 / 2 O 2
NO 2 - + H 2 O + 276kJ This in turn provides a substrate for
Nitrobacter bacteria oxidise the nitrite to nitrate: NO 3 - + 1 / 2
O 2 NO 3 - + 73 kJ This energy is the only source of energy for
these prokaryotes so they are called chemoautotrophs.
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- Return to the atmosphere: Denitrification Nitrates and nitrites
can be used a source of oxygen for Pseudomonas bacteria Favourable
conditions: Cold waterlogged (anaerobic) soils 2NO 3 - 3O 2 + N 2
2NO 2 - 2O 2 + N 2 providing up to 2385kJ The liberated oxygen is
used as an electron acceptor in the processes that oxidise organic
molecules, such as glucose These microbes are, therefore,
heterotrophs
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- Industrial N-Fixation The Haber-Bosch Process N 2 + 3H 2 2NH 3
- 92kJ The Haber process uses an iron catalyst High temperatures
(500C) High pressures (250 atmospheres) The energy required comes
from burning fossil fuels (coal, gas or oil) Hydrogen is produced
from natural gas (methane) or other hydrocarbon
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- Nitrogen Cycle
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- Atmospheric Pollution The exhaust emissions of cars contribute
a lot to atmospheric pollution in the form of NO x These compounds
form photochemical smogs They are green house gases They dissolve
in rain to contribute to acid rain in the form of nitric acid The
rain falling on soil and running into rivers They contribute to the
Eutrophication of water bodies
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- How to make things better? The need for synthetic fertilisers
can be reduced by cultural practices Avoiding the use of soluble
fertilisers in sandy (free draining soil) prevents leaching
Rotating crops permits the soil to recover from nitrogen hungry
crops (e.g. wheat) Adding a nitrogen fixing crop into the rotation
cycle Ploughing aerates the soil and reduces denitrification
Draining water logged soil also helps reduce denitrification