Nitrogen is critical to life on Earth: Every amino acid of every protein molecule in every living thing contains at least one N atom. The N2 molecules that make up most of the atmosphere, however, are nearly inert, and the element must be converted into some other chemical form before organisms can use it. That conversion can be accomplished in a few ways: lightning; N fixation by various species of bacteria and algae; and, more recently, industrial synthesis of ammonia for fertilizer. Fossil-fuel combustion, too, adds to the supply of chemically available N by releasing organic material from long-dead organisms previously trapped underground.
Human activities are known to have a sizeable and deleterious effect on the global N cycle. (See the article by Ann Kinzig and Robert Socolow, Physics Today, November 1994, page 24.) In many cases N availability is the limiting factor that determines how fast...
