With the aid of a labelled diagram, describe briefly the nitrogen cycle.

The nitrogen cycle is a natural process that involves the transformation of nitrogen from one form to another. Nitrogen is a crucial element required for the growth and development of plants and other living organisms. However, nitrogen in its gaseous form (N₂) cannot be used by plants directly. Therefore, it has to be converted into other forms such as ammonia, nitrate, and nitrite, which plants can absorb and use.

The nitrogen cycle can be divided into several steps:

1. Nitrogen Fixation: The first step in the nitrogen cycle is nitrogen fixation. Nitrogen gas (N₂) is converted into ammonia (NH₃) by nitrogen-fixing bacteria such as Rhizobium found in the root nodules of legumes like beans and peas. Some free-living bacteria like Azotobacter and Clostridium also fix atmospheric nitrogen.
2. Nitrification: The next step is nitrification, where ammonia (NH₃) is converted into nitrite (NO₂^-) and then into nitrate (NO₃^-) by nitrifying bacteria. Nitrate is the form of nitrogen that is most commonly taken up by plants.
3. Assimilation: Assimilation is the process by which plants and animals absorb nitrogen in the form of ammonia, nitrate, and other nitrogen compounds to synthesize amino acids, proteins, and other nitrogen-containing organic molecules.
4. Ammonification: Ammonification is the process by which decomposers like bacteria and fungi break down nitrogen-containing organic matter such as dead plants and animals and release ammonia into the soil.
5. Denitrification: Denitrification is the final step in the nitrogen cycle, where nitrate (NO₃^-) is converted back into gaseous nitrogen (N₂) by denitrifying bacteria under anaerobic conditions in soil and water.

Here is a labelled diagram of the nitrogen cycle:

            Nitrogen Fixation
                N2 + 3H2 -> 2NH3

                        NH3
                         |
                     Nitrification
                    NH3 -> NO2- -> NO3-

                    Assimilation
                         |
                    NH3, NO3-
                         |
                    Amino acids, proteins, etc.

                 Ammonification
                      Organic nitrogen
                          |
                          NH3

                 Denitrification
                        NO3-
                         |
                NO2-, NO, N2O, N2
                         |
                         N2

Overall, the nitrogen cycle is an essential process that enables the conversion of atmospheric nitrogen into forms that