03 n 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


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


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


Root uptake

Nitrate NO3-

Plant protein


Nitrogen from the atmosphere

Biological fixation

Atmospheric fixationOut

gassing

Atmospheric Nitrogen4 000 000 000 Gt


Atmospheric nitrogen fixation Electrical storms Lightning provides sufficient energy to split

the nitrogen atoms of nitrogen gas, Forming oxides of nitrogen NOx and NO2


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


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


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.


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


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


Nitrate NO3-

Atmospheric fixation

Out gassin

g

Plant protein

Atmospheric Nitrogen

Ammonium NH4

+


The human impact

Biological fixation

Industrial fixation


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


The different sources of fixed nitrogen

Sources of fixed nitrogen Production / M tonnes a-1

Biological 175

Industrial 50

Internal Combustion 20

Atmospheric 10


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


Fertilisers washed into river or lake

New limiting factor imposes itselfSewage or other organic

waste

Eutrophication


Increased Biochemical Oxygen Demand (BOD)

Hot water from industry

(Thermal pollution)

Pollution from oil or detergents

Reduction in dissolved O2

Making things worse!


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


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?


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


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


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


Technology

03 n cycle