THE NITROGEN CYCLE
Lightning reacts with nitrogen gas in the atmosphere to form nitrogen dioxide. This nitrogen dioxide (NO2) is then partly dissolved in water during rainfall to form nitric acid. When the rainfall hits the ground, nitrates are formed which are then absorbed by the roots of plants.
Nitrogen fixing bacteria (present on the root nodules of leguminous plants) converts nitrogen in the atmosphere to ammonium salts through a process call ammonification. Ammonification is also done by decomposers (saprophytes) which convert dead plants and animals as well as faecal matter into ammonium salts. The ammonium salts are then converted into nitrates by nitrifying bacteria. This process is known as nitrification.
Some of the nitrates present in the soil are absorbed by plants. These nitrates in the plants are eaten by animals. The animals in turn release faecal matter and eventually die and decompose, returning nitrates to the soil. The addition of fertilizers to the soil also produces soil nitrates.
The nitrates in the soil can also be converted into nitrogen gas by denitrifying bacteria present in the soil. This process is called denitrification. The nitrogen is returned to the atmosphere and the cycle continues.