Nitrogen fixation

Life on Earth depends heavily on microorganisms that are able to convert elemental nitrogen into compounds that are soluble in water and can be absorbed and used by plants. Once in the food chain, animals can use these nitrogen compounds for their needs.

The process where by elemental nitrogen is converted to soluble nitrogen compounds is called nitrogen fixation.


Certain microorganisms, such as the Rhizobium bacteria, use enzymes to reduce elemental nitrogen to ammonia(NH3) or ammonium ions(NH4+). Rhizobium bacteria invade the roots of legumes such as (peas and beans) forming nodules such as those shown on the right. (web source unknown). These forms of nitrogen are highly soluble in water and can be readily absorbed by plants.

Another source of nitrogen fixation, albeit very small, is lightning. Lightning provides the energy to oxidise elemental nitrogen into nitric oxide and nitrogen dioxide according to the reaction below.

N2(g) + O2(g) --> 2NO(g)
2NO(g) + O2(g) -->2NO2(g)

Nitrogen dioxide reacts with water in the atmosphere to produce nitric acid according to the reaction below.

2NO2(g) + H2O(l) --> 2HNO3(aq)

When dead matter is decomposed by microorganisms, known as denitrifying bacteria , nitrogen compounds are converted back into elemental nitrogen. These bacteria are anaerobic, hence only work in the absence of oxygen. Such bacteria are found in water logged soil where oxygen is scarce.