Which organisms are primarily responsible for nitrogen fixation in soil?

Nitrogen fixation is a crucial process in the nitrogen cycle, where atmospheric nitrogen (N2) is converted into ammonia (NH3), a form that can be utilized by plants. The organisms primarily responsible for this process are nitrogen-fixing bacteria and archaea. These microorganisms possess the necessary enzymatic machinery, particularly the enzyme nitrogenase, which allows them to break the stable triple bond of nitrogen gas, enabling the conversion to bioavailable forms of nitrogen.

Nitrogen-fixing bacteria can be free-living in the soil, such as species from the genera Azotobacter and Clostridium, or they can form symbiotic relationships with the roots of certain plants, particularly legumes, such as rhizobia. Archaea also contribute to nitrogen fixation in various environments, including extreme conditions where bacteria might not thrive.

In contrast, fungi and algae, while important in their own right for various ecological processes, do not typically engage in nitrogen fixation. Insects and earthworms play a significant role in soil aeration and organic matter decomposition but are not directly involved in the nitrogen fixation process. Lastly, not all plants have the capability to fix nitrogen; only specific families, particularly legumes, have the associated bacteria for this function. Therefore, nitrogen-fixing bacteria