The following points highlight the six main steps involved in nitrogen cycle. The steps are: 1. Nitrogen Fixation 2. Nitrogen Assimilation 3. Ammonification 4. Nitrification 5. Denitriflcation 6. Sedimentation.
Nitrogen Cycle: Step # 1. Nitrogen Fixation:
Conversion of free nitrogen of atmosphere into the biologically acceptable form or nitrogenous compounds is referred to as nitrogen flxation.
This process is of two types:
(i) Physicochemical or non-biological nitrogen fixation, and
(ii) Biological nitrogen fixation.
In physico-chemical process, the atmospheric nitrogen combines with oxygen (as ozone) during lightning or electrical discharges in the clouds and produces different nitrogen oxides.
The equations are as follows:
These nitrogen oxides get dissolved in rain water, and on reaching earth surface they react with mineral compounds to form nitrates and other nitrogenous compounds:
During combustion of various types, some nitrogenous compounds are formed, which are washed down along with rain water.
Biological nitrogen fixation is carried out by certain prokaryotes. The cyanobacteria (blue- green algae) fix significant amounts of nitrogen in the oceans, lakes and soils.
Symbiotic bacteria (Rhizobium) inhabiting the root rodules of legumes and symbiotic cyanobacteria, such as Nostoc, Anabaena, etc., found in free state, or in thalli of Anthoceros (bryophyte), Azolla (water fern), coralloid roots of Cycas (gymnosperm) fix atmospheric nitrogen.
Certain free living nitrogen fixing bacteria, such as Azotobacter, Clostridium, Beijerinckia, etc., also fix free nitrogen of atmosphere in the soil. Frankia, an actinomycetous fungus found in the roots of higher plants, such as Alnus and Casuarina, also fix nitrogen.
Nitrogen fixing organisms combine the gaseous nitrogen of atmosphere with hydrogen obtained from respiratory pathway to form ammonia, which then reacts with organic acids to form amino acids.
Biological nitrogen fixation is the major source of fixed nitrogen up-to 140 – 700 mg/m2 year as against 35 mg/m2/year by electrical discharge and photochemical fixation.
Nitrogen Cycle: Step # 2. Nitrogen Assimilation:
Inorganic nitrogen in the form of nitrates, nitrites and ammonia is absorbed by the green plants and converted into nitrogenous organic compounds. Nitrates are first converted into ammonia which combines to organic acids to form amino acids. Aminoacids are used in the synthesis of proteins, enzymes, chlorophylls, nucleic acids, etc.
Animals derive their nitrogen requirement from the plant proteins. Plant proteins are not directly utilised by the animals. They are first broken down into amino-acids during digestion and then the amino-acids are absorbed and manipulated into animal proteins, nucleic acids, etc.
Nitrogen Cycle: Step # 3. Ammonification:
The dead organic remains of plants and animals and excreta of animals are acted upon by a number of microorganisms, especially actinomycetes and bacilli, such as Bacillus ramosus, B. vulgaris, etc. These organisms utilise organic compounds in their metabolism and release ammonia. This process is called ammonification. After meeting their own metabolic requirement, these microbes release the excess ammonia in the soil.
Nitrogen Cycle: Step # 4. Nitrification:
In next step of ammonia formation, ammonia is converted into nitrate by a group of chemo- autotrophic bacteria through a two-step process called nitrification.
Certain bacteria such as Nitrosomonas, Nitrococcus and Nitrospira in oceans and soils convert ammonia into nitrites and then nitrites into nitrates. These bacteria primarily use the energy of dead organic matter in their metabolism.
The equation is as follows:
Conversion of nitrites to nitrates is brought about by several microbes, such as Penicillium (a fungus), Nitrobacter, etc.
Some nitrates are also made available through weathering of nitrate containing rocks.
Nitrogen Cycle: Step # 5. Denitriflcation:
Ammonia and nitrates are converted into free nitrogen by certain microbes. This process is referred to as denitriflcation. Pseudomonas, the most common denitrifying bacterium, thrives best under poorly aerated and detritus-rich conditions. Denitrifying bacteria transform nitrate nitrogen to nitrous and nitric oxides, and ultimately to gaseous nitrogen, which goes to atmosphere. ‘
Denitrification by denitrifying bacteria.
Nitrogen Cycle: Step # 6. Sedimentation:
Nitrates of the soil are washed away to the sea or leached deep into the earth along with percolating water. Nitrates thus lost from the soil surface are locked up in the rocks. This process is called sedimentation of nitrogen. Nitrogen of rock is released only when the rocks are exposed and weathered.
Thus a large part of nitrogen is fixed up and stored up in plants, animals and microbes. Most higher plants absorb nitrate from the soil; the absorbed nitrate is ultimately converted to organic nitrogen. A fraction of nitrogen incorporated in plant tissues is used by consumers, and ultimately all dead remains convert into detritus and used by decomposers. Thus, complex nitrogen cycle completes.