(a) Nitrogen-fixing bacteria (Biological
Nitrogen-fixation) They live either freely (e.g. Azotobacter) or as symbionts (e.g. Rhizobium in root nodules of legumes). The bacteria pick up nitrogen gas from atmosphere and reduce it to nitrates, which is then used by plants.
(b) Lightning (Physical Nitrogen-fixation) By its energy, it helps to combine a small quantity of nitrogen with oxygen of the atmosphere forming nitrogen oxides. The latter dissolves in rainwater and enter the soil producing nitrite and nitrate.