What organism carries out nitrogen fixation?

Nitrobacteria

BACTERIA !!

Certain types of bacteria carry out nitrogen fixation in plants.

Do a web search for legumes and soil improvement.  Clover, alfalfa, peas, and beans are examples of legumes.  These plants host bacteria in their roots that extract nitrogen from the air and supply it to their host plants so that the plants can build their own protein.  Farmers often inoculate their legume seeds with the bacteria before planting.  That way they harvest protein rich plants or fertilize their ground by plowing these plants back into the ground so that it will be available for whatever cash crop they plant next.

Certain types of bacteria that form symbiotic relationships with plants called legumes.

bacteria

It's NOT all bacteria. It is only a certain group of bacteria that can use N2.

Rhizobium in leguminous plants during symbiotic nitrogen fixation.