What Role Do Legumes Play in the Nitrogen Cycle?

In the nitrogen cycle, legumes form a symbiotic relationship with certain types of bacteria that are involved in a process known as “nitrogen fixation.” The legumes provide the microorganisms with the required energy to initiate the process, while the bacteria fixes captured nitrogen from the air into a usable form, which is absorbed by the legumes for sustenance.

Nitrogen is an important component in the production of amino acids and proteins that drive essential life processes. The nitrogen cycle refers to the movement of nitrogen from the atmosphere into living organisms, where the nitrogen eventually cycles back into the environment. Higher organisms, such as plants and animals, cannot directly use the form of nitrogen that is abundant in air. The nitrogen must be fixed by bacteria and algae before it can be utilized in the body.

The nitrogen cycle is divided into three steps: nitrogen fixation, nitrification and denitrification. The key participants in nitrogen fixation are legumes, such as alfalfa, clover and soybeans, and nitrogen-fixing bacteria, such as rhizobium, bradyrhizobium and azorhizobium. The bacteria infect the legumes to form root nodules. Once they are firmly embedded in the nodules, the bacteria become immobile. The nodules are the sources of the carbon skeletons, or photosynthate, the bacteria needs to drive the process of nitrogen-fixation. After providing the energy , these microorganisms capture the nitrogen from the air and convert it into ammonia. Thelegumes take up the ammonia to be used as nutrients.