Nitrogen fixation is performed naturally by certain types of anaerobic bacteria. Legumess such as clover contain symbiotic bacteria of this type within nodules in their root systems, producing nitrogen compounds that help to fertilize the soil. See George Washington Carver.
Nitrogen can also be artificially fixed for use as fertilizer or in other industrial processes. The most popular method is by the Haber process. Artificial fertilizer production has achieved such scale that it is now the largest source of fixed nitrogen in the Earth's ecosystem.