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The Nitrogen Cycle

Nitrogen is the most common element in the atmosphere at about 78%. Nitrogen is also vital for all life on Earth

because amino acids (constituent of proteins) and nucleic acids (constituent of DNA) would not exist without nitrogen.

In the atmosphere nitrogen exists as a very stable molecule (N2) which is unusable by plants and animals. The

process of fixing nitrogen so that it can be used by plants and animals is carried out by bacteria. Nitrogen fixing

bacteria are specialized in that they can use atmospheric nitrogen (N2) and as a byproduct release ammonia (NH4).

Next, nitrite-forming bacteria combine the ammonia with oxygen, forming nitrites (NO2-). Another group of bacteria then

converts the nitrites to nitrates (NO3-). Nitrites can be absorbed and used by green plants. In plants the nitrates are

reduced to ammonium (NH4+

) which is used to build amino acids. Animals receive their needed nitrogen from plants.Nitrogen reenters the atmosphere primarily by the actions of decomposers which break down; dead organisms, leaves

that fell off in the winter, skin, hair, urine etc. Decomposers use the nitrates (ammonia and ammonium) and produce a

byproduct of nitrogen gas, either N2 or N2O.

Nitrogen Cycle, the series of natural processes by which certain nitrogen-containing substances from air and

soil are made useful to living things, are used by them, and are returned to the air and soil. All living things

must have nitrogen to build proteins. Because of the chemical nature of nitrogen gas, however, they cannot

obtain that element directly from the air. Instead, food-making organisms such as plants obtain it from the

soil by absorbing nitrates (various nitrogen compounds containing oxygen) and ammonium compounds

(various nitrogen compounds containing hydrogen). The nitrogen cycle is essential to plants in unfertilized

soils because in such soils the nitrogen compounds are not available to the plants in any other way.

Animals, and other living things that do not make their food, depend on the nitrogen cycle indirectly. Most

animals, for example, eat plants or eat plant-eating animals.

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The nitrogen cycle consists of four natural processes: nitrogen fixation, nitrification, denitrification, and

decay.

Nitrogen Fixationis the process in which nitrogen gas from the air is continuously made into nitrogen compounds. These

compounds (primarily nitrates and ammonium compounds) are made by nitrogen-fixing microorganisms in

the soil and by lightning.

Nitrification

is the process in which ammonia in the soil is converted to nitrates. Nitrification is performed by nitrifying

bacteria. Plants absorb the nitrates and use them to make proteins.

Denitrification

is the reverse of the combined processes of nitrogen fixation and nitrification. It is the process by whichnitrogen compounds, through the action of certain bacteria, give up nitrogen gas that then becomes part of

the atmosphere. The amount of gas released by this process is relatively small.

Decay Processes

are those by which the organic nitrogen compounds of dead organisms and waste material are returned to the

soil. These compounds are chiefly proteins and urea. The many bacteria and fungi causing decay convert

them to ammonia and ammonium compounds in the soil.

Thus, through the nitrogen cycle, food-making organisms obtain the necessary nitrogen through nitrogen

fixation and (to a greater extent) through nitrification. At the same time, nitrogen compounds are returned tothe soil through decay and nitrogen is returned to the air through denitrification.

In soils in which many plants are raised and few are left to decay (as in farm soils), the nitrogen cycle does

not supply enough nitrogen to support plant growth. In these soils natural or artificial fertilizers, containing

nitrates or ammonium compounds, are needed.