The nitrogen cycle refers to the geophysical, chemical, and biological processes involved in converting atmospheric nitrogen into a form that plants can use, recycling those nitrogen compounds within ecosystems, and returning them to the atmosphere. Of all the fundamental biogeo-chemical cycles, the nitrogen cycle is the most complex as it depends on the balanced activity of several different kinds of organisms, ecosystems, and atmospheric processes. It is also the cycle most affected by human activity. Indeed, the manipulation and transportation of nitrogen compounds by humans is now a major component of this fundamental Earth system, transforming what was once a predominantly local cycle into more of a regional and even global flow of material.

Flows of nitrogen through the biosphere are important because the availability of ...