Which of the following is the major source of nitrogen fixation?

The correct answer is: Biological nitrogen fixing organisms

• Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria, such as Azotobacter, and archaea.
• Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes. 
• Plants of the pea family, known as legumes, are some of the most important hosts for nitrogen-fixing bacteria, but a number of other plants can also harbor these helpful bacteria.