However, legumes (peas and beans) can obtain nitrogen from their associations with rhizobia, many kinds of bacteria that live within the roots of the plants. The bacteria are brought to the roots when the plants release chemicals into the soil. These chemicals also encourage the activation of genes within the bacteria that form a nodule, a structure found on the roots of nitrogen-fixing plants that contain symbiotic bacteria that fix nitrogen. The bacteria enter the roots by way of an infection thread, which is a tube the bacteria use to move into the plant root. The bacteria enter into the cytoplasm of the cells of the nodule, where they can fix nitrogen from the atmosphere.
The main function of rhizobia is to pull nitrogen from the air and convert it into a usable form for the plant. Plants cannot use atmospheric nitrogen because the triple bond holding the nitrogen molecules together is very stable and strong. It requires a lot of energy to break it apart. The rhizobia engage the conversion of atmospheric nitrogen into ammonia, a form that is more usable by plants through the process of nitrogen fixation. This happens because of the actions of an enzyme called nitrogenase. It adds three pairs of hydrogen ions (H+) to the nitrogen gas to form two ammonia molecules. This effort requires a lot of energy from ATP, and a reducing agent (a molecule that donates an electron) must be present.Only legumes form these symbiotic associations, but the legumes store a great deal of fixed nitrogen in their root nodules, which break down. The nitrogen in their tissues is released back into the environment to be taken in again by new plants. This process, along with the mycorrhizal associations, provides sufficient nitrogen for all plants.