Which one of the following is a freeliving bacterium that helps in nitrogen fixation in soil ?

Nitrogen fixation is the process where soil organisms convert atmospheric nitrogen into chemical forms usable by plants [5]. Nitrogen-fixing bacteria are categorized into symbiotic and free-living (non-symbiotic) types [1]. Azotobacter is a primary example of a free-living, aerobic bacterium that inhabits the soil and fixes nitrogen without requiring a direct physical interaction with plant roots [4]. In contrast, Anabaena and Nostoc are cyanobacteria that often form symbiotic associations; for instance, Anabaena azollae lives symbiotically within the aquatic fern Azolla. While some strains of Anabaena and Nostoc can exist in a free-living state, Azotobacter is the classic representative of a free-living soil bacterium in biological contexts [2]. Azolla itself is a fern, not a bacterium. Therefore, Azotobacter is the most accurate answer for a free-living bacterium fixing nitrogen in soil.

Sources

1. [2] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45
2. [5] https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419/
3. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20
4. [4] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/nitrogen-fixing-bacterium