Biological nitrogen fixation

An experiment carried out on the growth of legumes (peas) and non-legumes (oats) in the 19^th century, showed the following results:

• 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.

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

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.

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. The plant provides shelter and sugars (through photosynthesis), the bacterium provides fixed nitrogen.

Azotobacter are bacteria associated with the rooting zone (the rhizosphere) of plants in grasslands. Once again the plants provide organic compounds via material which exudes from the roots.

Industrial nitrogen fixation

The Haber-Bosch Process was discovered by Fritz Haber in 1909.

After the World War I it was used more and more extensively as a way of fabricating synthetic fertilisers:

The Haber process uses an iron catalyst. Despite using a catalyst the process still requires high temperatures (500°C) and high pressures (250 atmospheres). This makes it very expensive. The energy required comes from burning fossil fuels (coal, gas or oil). In addition the hydrogen is produced from natural gas (methane) or other hydrocarbons.

Industrial nitrogen fixation has no doubt increased global food production. Nitrates and ammonia are very soluble in water so they are easily washed (leached) from free draining soils. These soils tend to be deficient in nitrogen. Nitrogen is an essential element for plant growth. However, 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 eutrophication of water bodies. A serious form of water pollution.

Atmospheric nitrogen fixation

This occurs during electrical storms. Lightning provides sufficient energy to split the nitrogen atoms of nitrogen gas, forming oxides of nitrogen NO_x and NO₂.

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 NO_x. These compounds form photochemical smogs and 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 also contribute to the eutrophication of water bodies.

The estimated contributions of the different sources of fixed nitrogen

Ammonification

Nitrogen enters the soil through the decomposition of protein in dead organic matter.

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 bacteria. They oxidise it to nitrite:
 

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

This energy is the only source of energy for these prokaryotes. They are chemoautotrophs

Denitrification

Finally nitrates and nitrites can be used a source of oxygen for Pseudomonas bacteria living in cold waterlogged (anaerobic) soils.

The liberated oxygen is used as an electron acceptor in the processes that oxidise organic molecules, such as glucose. These microbes are, therefore, heterotrophs. Their source of reduced carbon comes from other organisms.