Nitrogen cycle
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Transcript of Nitrogen cycle
THE NITROGEN CYCLE
Nitrates are essential for plant growth
Root uptake
Nitrate NO3
-
Plant protein
© 2008 Paul Billiet ODWS
Nitrates are recycled via microbes
Nitrification
Nitrification
Ammonium NH4+
Ammonification
Nitrite NO2-
Soil organic nitrogen
Animal protein
Root uptake
Nitrate NO3
-
Plant protein
© 2008 Paul Billiet ODWS
Ammonification
Nitrogen enters the soil through the decomposition of protein in dead organic matter
Amino acids + 11/2O2 CO2 + H2O + NH3 + 736kJ This process liberates a lot of energy which
can be used by the saprotrophic microbes
© 2008 Paul Billiet ODWS
Nitrification
This involves two oxidation processes The ammonia produced by ammonification is an
energy rich substrate for Nitrosomas bacteriaThey oxidise it to nitrite:
NH3 + 11/2O2 NO2- + H2O + 276kJ
This in turn provides a substrate for Nitrobacter bacteria oxidise the nitrite to nitrate:
NO3- + 1/2O2 NO3
- + 73 kJ
This energy is the only source of energy for these prokaryotes
They are chemoautotrophs
© 2008 Paul Billiet ODWS
Root uptake
Nitrate NO3-
Plant protein
Soil organic nitrogen
Nitrogen from the atmosphere
Biological fixation
Atmospheric fixationOut
gassing
Atmospheric Nitrogen4 000 000 000 Gt
© 2008 Paul Billiet ODWS
Atmospheric nitrogen fixation Electrical storms Lightning provides sufficient energy to split
the nitrogen atoms of nitrogen gas, Forming oxides of nitrogen NOx and NO2
© 2008 Paul Billiet ODWS
Atmospheric Pollution
This also happens inside the internal combustion engines of cars
The exhaust emissions of cars contribute a lot to atmospheric pollution in the form of NOx
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
© 2008 Paul Billiet ODWS
Biological nitrogen fixation
Treatments Yield / g
Oats Peas
No nitrate & sterile soil 0.6 0.8
Nitrate added & sterile soil 12.0 12.9
No nitrate & non-sterile soil 0.7 16.4
Nitrate added & non-sterile soil 11.6 15.3
© 2008 Paul Billiet ODWS
Conclusion
Adding nitrate fertiliser clearly helps the growth of both plants
The presence of microbes permits the peas to grow much better than the oats
The peas grow better in the presence of the microbes than they do with nitrate fertiliser added
The difference is due to the present of mutualistic nitrogen fixing bacteria which live in the pea roots.
© 2008 Paul Billiet ODWS
University of Sydney
Alafalfa (Medicago sativa)
USDA - ARS
Root nodules
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
N2 2N - 669 kJ2N + 8H+ NH3 + H2 + 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
© 2008 Paul Billiet ODWS
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
© 2008 Paul Billiet ODWS
Nitrate NO3-
Atmospheric fixation
Out gassin
g
Plant protein
Atmospheric Nitrogen
Ammonium NH4
+
Soil organic nitrogen
The human impact
Biological fixation
Industrial fixation
© 2008 Paul Billiet ODWS
Industrial N-Fixation The Haber-Bosch Process
N2 + 3H2 2NH3 - 92kJ The Haber process uses an iron catalyst High temperatures (500°C) High pressures (250 atmospheres) The energy require comes from burning fossil
fuels (coal, gas or oil) Hydrogen is produced from natural gas
(methane) or other hydrocarbon
© 2008 Paul Billiet ODWS
The different sources of fixed nitrogen
Sources of fixed nitrogen Production / M tonnes a-1
Biological 175
Industrial 50
Internal Combustion 20
Atmospheric 10
© 2008 Paul Billiet ODWS
Eutrophication Nutrient enrichment of water bodies Nitrates and ammonia are very soluble in
water They are easily washed (leached) from free
draining soils These soils tend to be deficient in nitrogen When fertiliser is added to these soils it too
will be washed out into water bodies There algae benefit from the extra nitrogen This leads to a serious form of water pollution
© 2008 Paul Billiet ODWS
Fertilisers washed into river or lake
New limiting factor imposes itselfSewage or other organic
waste
Eutrophication
© 2008 Paul Billiet ODWS
Increased Biochemical Oxygen Demand (BOD)
Hot water from industry
(Thermal pollution)
Pollution from oil or detergents
Reduction in dissolved O2
Making things worse!
© 2008 Paul Billiet ODWS
The death of a lake
Death/emigration of freshwater
fauna
Methaemoglobinaemia in infantsStomach cancer link
(WHO limit for nitrates 10mg dm-3)
Increased nitrite levels
NO3- NO2
-
Reduction in dissolved O2
© 2008 Paul Billiet ODWS
The future of industrial nitrogen fixation Food production relies heavily upon synthetic
fertilisers made by consuming a lot of fossil energy
Food will become more expensive to produce Nitrogen fixing microbes, using an enzyme
system, do the same process at standard temperatures and pressures essentially using solar energy
Answer: Genetically engineered biological nitrogen fixation?
© 2008 Paul Billiet ODWS
Making 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
© 2008 Paul Billiet ODWS
Return to the atmosphere: Denitrification Nitrates and nitrites can be used a source of
oxygen for Pseudomonas bacteria Favourable conditions: Cold waterlogged
(anaerobic) soils
2NO3- 3O2 + N2providing up to 2385kJ
2NO2- 2O2 + N2
The liberated oxygen is used as an electron acceptor in the processes that oxidise organic molecules, such as glucose
These microbes are, therefore, heterotrophs
© 2008 Paul Billiet ODWS
Sediments 10 Gt
Nitrification
Root uptake
Biological fixation
Nitrification
Ammonium NH4+
Ammonification
Nitrite NO2-
Dissolved in water6000 Gt
Denitrification
LeachingNitrate
NO3-
Soil organic nitrogen 9500 Gt
Atmospheric fixation
Out gassin
g Industrial fixation
Plant protein3500 Gt
Animal protein
Atmospheric Nitrogen4 000 000 000 Gt
© 2008 Paul Billiet ODWS