Answer:
Nitrogen fixation
Step-by-step explanation:
Certain soil bacteria, e.g., Azobacter spp can combine free nitrogen of the atmosphere with oxygen to form nitrates. This is called nitrogen fixation. Other nitrogen-fixing bacteria such as Rhizobium form symbiotic unions with the roots of leguminous plants called root nodules. They fix nitrogen to form nitrates which are used up by the host plant. Nitrifying soil bacteria, e.g., Nitrobacter convert nitrites to nitrates in a process called nitrification.