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THE NITROGEN CYCLE

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Nitrates are essential for plant growth

Root

uptake

Nitrate

NO3-

Plant

protein

2008 Paul Billiet ODWS
http://www.saburchill.com/IBbiology/bio_hp.htmlhttp://www.saburchill.com/IBbiology/bio_hp.html

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Nitrates are recycled via microbes

Nitrification

Nitrification

Ammonium NH4+

Ammonification

Nitrite NO2-

Soil organic nitrogen

Animal

protein

Root

uptake

Nitrate

NO3-

Plant

protein


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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


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Nitrification

This involves two oxidation processes

The ammonia produced by ammonification is anenergy rich substrate for Nitrosomasbacteria

They oxidise it to nitrite:

NH3+ 11/2O2 NO2-+ H2O + 276kJ

This in turn provides a substrate for Nitrobacterbacteria oxidise the nitrite to nitrate:

NO3-+ 1/2O2 NO3

- + 73 kJ

This energy is the only source of energy forthese prokaryotes

They are chemoautotrophs


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Root uptake

Nitrate NO3-

Plant

proteinSoil organic

nitrogen

Nitrogen from the atmosphere

Biological

fixation

Atmospheric

fixationOut

gassin

g

Atmospheric Nitrogen

4 000 000 000 Gt


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Atmospheric nitrogen fixation Electrical storms

Lightning provides sufficient energy to split

the nitrogen atoms of nitrogen gas,

Forming oxides of nitrogen NOxand NO2


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Atmospheric Pollution

This also happens inside the internal combustionengines of cars

The exhaust emissions of cars contribute a lot toatmospheric pollution in the form of NOx

These compounds form photochemical smogs They are green house gases

They dissolve in rain to contribute to acid rainin theform of nitric acid

The rain falling on soil and running into rivers They contribute to the eutrophicationof water

bodies


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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


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Conclusion

Adding nitrate fertiliser clearly helps the growth ofboth plants

The presence of microbes permits the peas to growmuch better than the oats

The peas grow better in the presence of themicrobes than they do with nitrate fertiliser added

The difference is due to the present of mutualisticnitrogen fixing bacteria which live in the pea roots.


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University of Sydney

Alafalfa (Medicago sativa)

USDA - ARS

Root nodules
http://www.bio.usyd.edu.au/DavidDay/root_nodules.jpghttp://www.ars.usda.gov/main/site_main.htm?modecode=36401000http://www.ars.usda.gov/main/site_main.htm?modecode=36401000http://www.ars.usda.gov/main/site_main.htm?modecode=36401000http://www.ars.usda.gov/main/site_main.htm?modecode=36401000http://www.ars.usda.gov/main/site_main.htm?modecode=36401000http://www.bio.usyd.edu.au/DavidDay/root_nodules.jpg

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Only prokaryotes show nitrogen

fixation These organisms possess the nifgene complex which make the

proteins, such as nitrogenase enzyme, used in nitrogen fixation

Nitrogenaseis a metalloprotein, protein subunits beingcombined with an iron, sulphur and molybdenum complex

The reaction involves splitting nitrogen gas molecules and adding

hydrogen to make ammonia

N2 2N - 669 kJ

2N + 8H+ NH3+ H2 + 54 kJ

This is extremely energy expensive requiring 16 ATP moleculesfor each nitrogen molecule fixed

The microbes that can fix nitrogen need a good supply of energy


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The nitrogen fixers

Cyanobacteriaare nitrogen fixers that also

fix carbon (these are photosynthetic)

Rhizob iumbacteria are mutualisticwithcertain plant species e.g. Legumes

They grow in root nodules

Azotobacterare bacteria associated with the

rooting zone (the rhizosphere) of plants ingrasslands


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Nitrate NO3-

Atmospheric

fixation

Out

gassin

g

Plant

protein


Ammonium

NH4+

Soil organic

nitrogen

The human impact

Biological

fixation

Industrial

fixation


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Industrial N-Fixation

The Haber-Bosch ProcessN2+ 3H2 2NH3 - 92kJ

The Haber process uses an iron catalyst

High temperatures (500C) 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


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The different sources of fixed nitrogenSources of fixed nitrogen Production / M tonnes a-1

Biological 175

Industrial 50

Internal Combustion 20

Atmospheric 10


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Eutrophication Nutrient enrichment of water bodies Nitrates and ammonia are very soluble in

water

They are easily washed (leached) from freedraining 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


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Fertilisers washed into river or lake

New limiting factor imposes itselfSewage or

other organic

waste

Eutrophication


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Increased Biochemical

Oxygen Demand (BOD)

Hot water

from industry

(Thermal

pollution)

Pollution

from oil ordetergents

Reduction in dissolved O2

Making things worse!


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The death of a lake

Death/emigration

of freshwater

fauna

Methaemoglobinaemia in infants

Stomach cancer link

(WHO limit for nitrates 10mg dm-3)

Increased nitrite

levels

NO3- NO2

-

Reduction in dissolved O2


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The future of industrial nitrogen fixation Food production relies heavily upon synthetic

fertilisers made by consuming a lot of fossilenergy

Food will become more expensive to produce

Nitrogen fixing microbes, using an enzymesystem, do the same process at standardtemperatures and pressures essentially usingsolar energy

Answer: Genetically engineered biologicalnitrogen fixation?


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Making things better

The need for synthetic fertilisers can be reduced bycultural practices

Avoiding the use of soluble fertilisers in sandy (free

draining soil) prevents leaching

Rotating crops permits the soil to recover fromnitrogen 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


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Return to the atmosphere:Denitrification Nitrates and nitrites can be used a source of

oxygen for Pseudomonasbacteria

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 oxidiseorganic molecules, such as glucose

These microbes are, therefore, heterotrophs


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Sediments 10 Gt

Nitrification

Root

uptake

Biological

fixation

Nitrification

Ammonium NH4+

Ammonification

Nitrite NO2-

Dissolved in water

6000 Gt

Denitrification

LeachingNitrate

NO3-

Soil organic nitrogen9500 Gt

Atmospheric

fixation

Outgassin

g Industrialfixation

Plant protein

3500 Gt

Animal

protein


4 000 000 000 Gt

O S

03 N-CYCLE (1)

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