Only certain bacteria, lightning, and volcanic eruptions can convert gaseous nitrogen into a form in which it can enter food webs. Nitrogen is often one of the ecosystem factors that most limits plant growth. Humans, however, do not need to wait for bacteria to turn atmospheric nitrogen into something useful. Another method is the production of cyanamide by passing nitrogen over heated calcium carbide in the presence of a catalyst. Humans do these things principally to create fertilizer, explosives, and cyanide poison.
Humans are now purposefully and accidentally pouring vast quantities of nitrogen into the biosphere.
This has only been going on for about 50 years, but the quantities are astonishing. It has been recently estimated that 5 times the pre-industrial amount of nitrogen is deposited in the North Atlantic basin every year. Biologically speaking, nitrogen moves from the atmosphere to plants and then to other organisms via the actions of soil bacteria. First, nitrogen fixation describes the process whereby gaseous nitrogen is converted to ammonia. In assimilation and biosynthesis, other organisms use ammonia and ammonia derived ammonium ion to build their molecules and tissues.