Nitrogen can be fixed by
(i) Lightning is a natural process that converts nitrogen into a usable form.
(ii) Rhizobium bacteria is involved in the fixation of nitrogen in leguminous plants. These bacteria live in the root nodules of leguminous plants such as beans and peas, with which it has a symbiotic relationship.
(iii) Some of the blue-green algae have capacity to fix nitrogen, e.g. Anabaena, Nostoc, etc.
The atmospheric nitrogen although continuously used up by plants and animals, remains constant. This happens through a cycle of processes grouped together called nitrogen cycle.