Nitrogen Cycle

Nitrogen Cycle
The nitrogen is an essential part of protoplasm, proteins, nucleic acids, chlorophyll etc and is therefore, necessary for living organisms. The exchange of nitrogen between soil and atmosphere and its circulation through bodies of living organisms takes place through inter-related processes known as nitrogen cycle.
The atmosphere contains about 78 percent nitrogen gas. It is an inert gas and the organisms cannot use it directly. Therefore, nitrogen is first changed to soluble nitrogen compounds such as nitrates (NO₃) which the plants can absorb from the soil. Conversion of atmospheric nitrogen to soluble nitrogen compounds is called nitrogen fixation.
Source of Nitrates
There are three sources of nitrates for living organisms
i. Nitrogen Fixation by Thunderstorm
ii. Nitrogen Fixation by Living Organisms
iii. Decomposition of Dead Organic Material
i. Nitrogen Fixation By Thunderstorm
The first source of nitrates is thunderstorm and lightening. Nitrogen can combine with atmospheric oxygen at very high energy level. The lightening during rains provides this energy which is used to combine atmospheric nitrogen and oxygen to form nitrogen oxides. These oxides dissolve in water to form nitric acid which falls down into the soil and react with mineral ions there to forms nitrates.
ii. Nitrogen Fixation by Living Organisms
The second source of nitrates are bacteria and cynobacteria (blue, green, algae) which can covert free atmospheric nitrogen into soluble nitrates and nitrates. There are two types
a. Symbiotic Nitrogen Fixation
Symbiotic bacteria, such as Rhizobium, live in the nodulated roots of legumes such as bean, pea etc. These bacteria have the ability to absorb free atmospheric nitrogen and convert it to organic compounds such as nitrates which enter the soil.

b. Non-Symbiotic Nitrogen Fixation
It takes place by the activity of free living soil bacteria like clostridium and Azobaeter and blue green algae like Nostoc etc. They absorb free nitrogen from air and convert it to nitrates of the soil.
Nitrates from Decomposition
The third source of nitrogen for organisms is soil nitrates from decomposition of dead organisms and excretory wastes. This process occurs in two steps
i. Ammonification
ii. Nitrification
i. Ammonification
The dead animals, plants and excretory waste is decomposed by the saprophytic bacteria and fungi to form simple compounds like amino acids, water and carbondioxide. The amino acids are then changed to ammonia and ammonium ions. This process is called ammonification.
ii. Nitrification
Ammonia and ammonium, formed as a result of ammonification is converted to nitrites and nitrates by a process known as nitrification. It takes place by the activity of two groups of bacteria together called the nitrifying bacteria. Nitrite bacteria, such as Nitrosomonas, first convert ammonia to nitrites. The second group the nitrate bacteria such as Nitrobacter, then convert nitrites into nitrates.
The nitrates from all the above sources are taken up from the soil by the plants and are used for formation of plant proteins. These proteins are then taken up by animals as food and converted to animals protein. The dead plants, animals, the excretory waste is decomposed and the nitrates are recycled as described a above.