Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial and biological processes is called as nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed '', in a usable form to be taken up by plants. Between 5x10 and 10x10 g per year are fixed by lightning strikes, but most fixation is done by free - living or symbiotic bacteria known as diazotrophs. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds. Most biological nitrogen fixation occurs by the activity of Mo - nitrogenase, found in a wide variety of bacteria and some Archaea. Mo - nitrogenase is a complex two - component enzyme that has multiple metal - containing prosthetic groups. An example of free - living bacteria is Azotobacter. Symbiotic nitrogen - fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen - poor soils. A few non-legumes can also form such symbioses. Today, about 30 % of the total fixed nitrogen is produced industrially using the Haber - Bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.
what is the first step in the nitrogen cycle when gaseous nitrogen is converted into ammonia
nitrogen fixation nitrogen fixation