Soil bacteria are essential in getting nitrogen from air to plant cells

The essential mineral nutrient most commonly in short supply, in both natural and agricultural situations, is nitrogen. This is surprising because elemental nitrogen (N2) makes up almost four-fifths of Earth’s atmosphere. However, plants cannot use N2 directly as a nutrient. The triple bond linking the two nitrogen atoms is extremely stable, and a great deal of energy is required to break it; thus N2 is a highly unreactive substance.

Some prokaryotes have an enzyme that enables them to convert N2 into a more reactive and biologically useful form by a process called nitrogen fixation:

N2 + 6 H+ + 6 e– → 2 NH3

This equation is simplified: actually, H2 gas is a byproduct. Nitrogen fixers, including those present in root nodules, fix approximately 170 million metric tons of nitrogen per year. Humans use industrial methods to fix about 80 million metric tons per year. In addition, about 20 million metric tons per year are fixed in the atmosphere by nonbiological means such as lightning, volcanic eruptions, and forest fires. Rain brings these atmospherically formed products to the ground.

Two types of organisms can fix nitrogen:

  1. Free-living organisms living in soil and water (e.g., Azotobacter bacteria and Nostoc cyanobacteria [sometimes called blue-green algae])

  2. Symbiotic organisms living in other organisms (e.g., rhizobia in roots of legumes, and Anabaena cyanobacteria in aquatic ferns)