固氮作用
Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria such as "Azotobacter" and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. It occurs naturally in the air by means of NOx production by lightning.