(a) a biological process of nitrogen-fixing bacteria present in the soil
Nitrogen gas makes up 78% of our atmosphere and nitrogen is also a part of many molecules essential to life like proteins, nucleic acids (DNA and RNA) and some vitamins. Nitrogen is thus an essential nutrient for all life-forms. The nitrogen molecules present in air can be converted into nitrates and nitrites by a biological process of nitrogen-fixing bacteria present in the soil. The process of biological fixation of nitrogen is called as nitrogen-fixation.