The Nitrogen Cycle
Free N2 cannot be made use of by the living organisms. The living organisms must obtain their nitrogen in some combined form, such as nitrates, ammonia, or more complex compounds like amino acids. However such combined forms of nitrogen are very scarce in surface water and soil. Nitrogen is freely available in its gaseous state in the atmosphere. In fact nitrogen constitutes 78% of the atmosphere.
Let us now see in a step-wise manner what happens to the atmospheric N2.
Free nitrogen of air is converted to nitrates and ammonia by the nitrogen-fixing bacteria in soil and by the blue-green algae in water. This process is called N2-fixation.
Green plants get their nitrogen from soil, in the form of nitrates. They assimilate this form of nitrogen and convert it to amino acids and proteins which gets incorporated into the living protoplasm.
A = Nitrogen fixation by microbes C = Nitrification by nitrifying bacteria
B = Ammonification by decaying microbes D = Denitrification by denitrifying bacteria.
Through trophic levels, the heterotrophs get their nitrogen from plant foods.
Animals give out ammonia, urea and uric acids as major excretory products. All of these are nitrogenous compounds. They are again converted to ammonia by bacterial activity. Also the death and decay of plant bodies releases ammonia. This process is called ammonification.
Ammonia is converted to nitrites and nitrates by soil bacteria. These are the nitrifying bacteria and the process is called nitrification. These salts of nitrogen may again be cycled into green plants, as they absorb it from the soil.
Part of the nitrites and nitrates of the soil are acted upon by another class of bacteria called the denitrifying bacteria which release the free nitrogen back into the air. This process is called denitrification.
So without the processes of nitrogen fixation and denitrification, the abundant nitrogen of the atmosphere is of no use to the living world. The conversion processes are carried out by various microbes, without whose activity life would be impossible. The below table gives you some names of these extremely important microbes.
All this time we were talking about biological fixation of nitrogen, which is done by bacteria. You have also seen that these bacteria may either be symbiotic as in leguminous roots, or free living. It is known that certain non-leguminous plants also have some other kinds of nitrogen-fixing microbes in their roots, as in some orchids. Plenty of nitrogen is also fixed industrially by manufacturing fertilizers (NPK), urea and other compounds. This is called industrial fixation. In fact industrial fixation nearly equals biological fixation. However an excessive use of fertilizers is harmful to aquatic life. The excess of nitrogen compounds get washed out to rivers and lakes. There they cause excessive growth of algae and phytoplankton. These in turn hinder the life of other aquatic animals.
The other materials too circulate (P, S, Ca, etc.) in the biosphere between organisms and their abiotic environments. Material cycle are excellent examples to understand the balancing mechanisms and self-supporting qualities of the biosphere.