II.

Biological Fixation

The most widely used and most productive of the soil microorganisms capable of nitrogen fixation are symbiotic bacteria of the genus Rhizobium, which colonize and form nodules on the roots of leguminous plants such as clover, alfalfa, and peas (see Legume). These bacteria obtain food from the legume, which in turn is supplied with abundant nitrogen compounds. Soils are sometimes inoculated with a particular species of Rhizobium to increase a legume crop, which is often planted to replenish the nitrogen depleted by other crops.

Much smaller amounts of nitrogen are fixed in the soil by nonsymbiotic (free-living) bacteria such as the aerobes, which function in the presence of oxygen, and bacteria of the genera Klebsiella and Bacillus, which function without oxygen. Some forms of cyanobacteria (formerly known as blue-green algae) also fix nitrogen, such as the alga Anabaena, which, in symbiosis with the water fern Azolla pinnata, is said to markedly increase rice yields, as was the case in paddies in the Thái Bình region of northern Vietnam. The need for fixed nitrogen in agriculture today is far greater than can be supplied by natural biological processes, and the production of nitrogen compounds from atmospheric nitrogen is a major chemical industry.

Scientists recently discovered archaebacteria that can fix nitrogen. Archaebacteria or archaea are distinct from true bacteria and include organisms that can survive in extreme conditions such as deep-sea hydrothermal vents. The enzyme that archaebacteria use to fix nitrogen appears to be very ancient and may come from a life form older than archaebacteria or bacteria.