Almost 80 percent of the atmosphere is nitrogen gas. It is essential for plant growth, but before plants can absorb nitrogen, it must become "fixed" – bonded with carbon, hydrogen, or oxygen. Certain kinds of algae, soil bacteria and lightning accomplishes this naturally. Before human beings began to alter the nitrogen cycle, these mechanisms provided 90-150 million metric tons of fixed nitrogen (nitrates and nitrogen oxides) a year. Now human activity adds 130-150 million more tons. Half the industrial nitrogen fertilizer used in human history has been applied since 1984. As a result, coastal waters and estuaries bloom with toxic algae while oxygen concentrations dwindle, killing fish; as a result, nitrous oxide traps solar heat. And once the gas is in the air, it stays there for a century or more.
The major human intrusion into the nitrogen cycle involves inputs of nitrogen oxide into the atmosphere and nitrates into aquatic ecosystems (through improper use of nitrogen fertilizer, animal wastes and sewage).