1. Nitrogen in its gaseous form constitutes 79 percent of the atmosphere. However, it cannot be used directly by most forms of life.
2. It must be first “ fixed ” before it can be utilised by plants and animals. By fixation, nitrogen is converted into it’s chemical compounds, largely nitrates (NO3) and ammonia (NH3).
3. The fixation of nitrogen takes place through both physicochemical and biological means although the latter is by far the much bigger contributor.
4. The biological fixation is limited to a few , but abundant organisms like the free living bacteria AZETOBACTER and CLOSTRIDIUM, nodule bacteria on leguminious plants like Rhizobium and some Blue-Green Algae.
5. These are the keys to the movement of nitrogen from the atmospheric reservoir into the cycle as shown in figure.
6. The nitrates are assimilated to form amino acids, urea and other organic residues in the Producer, Consumer & Decomposer cycles.
7. The amino acids and urea are then converted to ammonia through a process called “Ammonification”.
8. To complete the cycle, Denitrifying bacteria convert the ammonia into nitrites, then into nitrates, and then back into gaseous nitrogen.
9. In this way, under normal circumstances, the total amount of Nitrogen Fixed equals the total amount returned to the atmosphere as gas.
10. Man has interfered with this natural cycle by industrially fixing nitrogen.
11. This includes production of Nitrogen Fertilisers and oxidation of nitrogen during Fossil Fuel combustion.
12. Most of the excess nitrogen is carried off into rivers and lakes and ultimately reaches the Ocean.
13. This increased runoff has greatly increased the productivity in many aquatic environments and has contributed to the process of EUTROPHICATION.
Reference Books :
Environmental Pollution Control Engineering, C.S. Rao .