NITROGEN CYCLE
Nitrogen is very important for plants and animals being an essential constituent of chlorophyll and all proteins. It is present in abundance (79%) in the atmosphere but it is never taken directly by any organism unless it is converted to a reduced (NH4+) or oxidized water soluble form.
Only a few bacteria and blue green algae are able to convert the gaseous nitrogen into nitrates and make it available for other organisms. The conversion of gaseous nitrogen into nitrates is termed nitrogen fixation. The atmospheric nitrogen is fixed symbiotically (bacteria living symbiotically with leguminous plant roots e.g., Rhizobium )as well as asymbiotically (independently living bacteria e.g.,
Azotobacter, beijerinckia, Clostridium, Aerobacter, blue green algae ). The lighting process also fix some proportion of atmospheric nitrogen.
These nitrates are absorbed by the plants for the manufacture of complex nitrogenous compounds.
Plants in turn are eaten by animals. When the plants or animals die, certain bacteria, fungi and other microbes start acting on them and decompose the protein of dead organisms into ammonia. Such bacteria are called ammonifying bacteria. Ammonia is converted into soluble ammonium compounds
which are released in the soil or water. Ammonia or the ammonium compounds are converted into nitric acid and then into nitrate bynitrifying bacteria.
Reactins eqections In reverse reaction, some bacteria break of nitrates, nitrates and ammonium compounds to molecular
nitrogen which is returned to atmosphere. Such bacteria are called denitrifying bacteria (e,g,. Pseudomonas denitrificans, Thiobacillus denitrificans, Micrococcus and Bacillus licheniformis ). This inorganic nitrogen is again recycled into the organic system upon absorption by the plants.
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