The Nitrogen Cycle (With Diagram) | Soil Microbiology

In this article we will discuss about the nitrogen cycle with the help of diagram.

Nitrogen forms the main bulk of the atmosphere (78%) as well as the biological systems. Various nitrogenous compounds, e.g., proteins, enzymes, chlorophylls, nucleic acids, etc. play vital roles in the life processes of organisms.

The atmospheric nitrogen is chemically inert and is not directly taken by most of the living organisms. The latter, therefore, depend on a source of combined nitrogen or organic nitrogen compounds for their growth.

They obtain nitrogenous compounds from soil and convert them into essential biomolecules needed for their healthy development. In addition, a part of the great reservoir of the atmospheric nitrogen is converted into an organic form by certain free living microorganisms and by plant-microorganism associations which make it available to the plants.

Animals obtain it from plants. The percentage of nitrogen in the atmosphere remains constant by the operation of a nitrogen-cycle in nature.

Nitrogen is continually entering in the air by the action of denitrifying bacteria and continually returning to the cycle through the action of nitrogen-fixing microorganisms, lightning, and industrial production of artificial fertilizer.

These sequence of changes from free atmospheric nitrogen to fixed inorganic nitrogen, to simple organic compounds, to complex organic compounds in the tissues of microorganisms, plants and animals, and the eventual release of this nitrogen back to atmospheric nitrogen is dealt under the ‘nitrogen cycle’ (Fig. 30.2).

Microorganisms which play a significant role in the operation of nitrogen cycle may be divided into following four major groups:

Group 1:

Those microorganisms which are capable of fixing atmospheric nitrogen (biological nitrogen fixation), i.e., of combining free nitrogen with other elements or compounds.

Group 2:

Those microorganisms which bring about the production of ammonia, i.e., ammonification.

Group 3:

Those microorganisms which oxidise ammonia to nitrite and nitrite to nitrate, i.e., nitrification.

Group 4:

Those microorganisms which are capable of transforming nitrates to nitrogen or nitrous oxide, i.e., denitrification.

1. Fixation of Free Nitrogen (Biological N2-fixation):

A large number of microorganisms are known to have the ability to reduce atmospheric nitrogen into nitrogenous compounds. This conversion of molecular (atmospheric) nitrogen into nitrogenous compounds by microorganisms is called ‘biological nitrogen fixation’.

2. Ammonification:

Proteins and nucleic acids of the dead remains of plants and animals, and the excretory products of animals are degraded by microorganisms with the liberation of ammonia. This process is called ‘ammonification’.

Two following different steps are involved in ammonification:

(i) Proteolysis, and

(ii) Amino acid degradation.

(i) Proteolysis:

Breakdown of protein into its simpler forms is called proteolysis. A number of bacterial species, e.g., Clostridium spp. Pseudomonas, Proteus, Bacillus, and soil actinomycetes, and many fungi are extremely proteolytic.

They secrete extracellular enzyme, namely, ‘proteases’ that convert the protein to smaller units (peptides) which are then attacked by other proteolytic enzyme, namely, ‘peptidases’ resulting ultimately in the release of amino acids.

The overall process can be summarized in the form of reactions as follows:

(ii) Amino Acid Degradation:

The amino acids now undergo degradation by microbial attack. They are deaminated (i.e., removal of the amino group) to yield ammonia. Microorganisms exhibit several variations of deamination reactions in soil, one of the end products is always ammonia.

3. Nitrification:

Ammonia produced by the degradation of manures and organic matter may not be directly available even to those plants which can use it, for it is readily leached from soil and is usually converted to nitrate with the help of certain microorganisms. This conversion (oxidation) of ammonia to nitrate is called ‘nitrification’. Nitrification is carried out in two stages by specific bacteria.

(i) Oxidation of Ammonia to Nitrite:

Nitrosomonas are the most important agents of oxidation of ammonia to nitrite in soil. In addition, certain other bacteria, e.g., Nitrosococcus, Nitrosolobus, Nitrosospira, Nocardia and Streptomyces have been known to oxidize NH₂to nitrite.

2NH₃ + 3O₂ → 2HNO₂ + 2H₂O

(ii) Oxidation of Nitrite to Nitrate:

This oxidation of nitrite to nitrate in soil is dependent on the activities of bacteria belonging mainly to the genera Nitrobacter. In addition, certain fungi, e.g., Cephalosporium, Aspergillus and Penicillium have been reported able to carry out nitrification. Nitrification was discovered to be a biological process by Schloesing and Muntz (1877); Winogradsky isolated the bacteria responsible for biological nitrification in 1890.

HNO₂ + ½ O₂ → HNO₃

4. Denitrification:

The transformation (reduction) of nitrates to nitrogen gas or nitrous oxide by certain microorganisms is called ‘denitrification’. This process depletes the soil of an essential nutrient, the nitrogen, for plant growth and, therefore, is not desirable.

Some important microorganisms responsible for this process are Thiobacillus denitrificans, Micrococcus denitrificans and some species of Serratia, Pseudomonas, Bacillus, Achromobacter, and Paracoccus. The process of denitrification is completed by means of various steps in presence of ‘reductase’ enzymes.

The overall reaction is as follows:

Denitrification does not occur to any significant degree in well-aerated soils with moderate amounts of nitrates and organic matter. It occurs seriously in water logged anaerobic soils with a high organic matter content.