Nitrogen passes between living organisms and the nonliving world in a biogeochemical cycle known as the global nitrogen cycle. In organisms, this element is one of the most abundant and is required in DNA, RNA, and proteins. In our largest nitrogen reservoir—the atmosphere—nitrogen gas (N2) is the most abundant gas, making up 78% of the air we breath. Although it is ubiquitous in the environment, nitrogen gas is not accessible to most living organisms. Only certain types of bacteria—the nitrogen fixers—can break the N2 triple bond and thereby tap this atmospheric reservoir. In addition to nitrogen-fixing bacteria, humans have also been converting nitrogen gas into other chemical forms to make fertilizers. Both natural and industrial nitrogen fixation convert nitrogen into usable forms that the rest of life on Earth can use.
In the global nitrogen cycle, most of the nitrogen fluxes take place among organisms or between organisms and the atmosphere. Nitrogen fixers convert N2 into NH4+. Other bacteria convert NH4+ to NO3-. Plants take up these nitrogenous compounds and use them to build DNA, proteins, and other organic molecules. These organic molecules then provide nitrogen to the rest of life on Earth.
As in the carbon cycle, humans have interfered significantly in the global nitrogen cycle. Before the production and mass use of fertilizers, usable nitrogen-containing compounds were in short supply in the environment. Nitrogen-fixing bacteria produced most of the available nitrogen. However, humans are currently supplying as much fixed nitrogen in the form of fertilizers as is fixed by natural processes. Usable nitrogen is now overly abundant in the environment and this is having negative consequences. The excess nitrogen from fertilizers contaminates groundwater, lakes, rivers, and oceans. In many aquatic ecosystems, the excess nitrogen upsets the balance of life, favoring the growth of algae at the expense of other organisms.
Textbook Reference: Concept 45.3 Certain Biogeochemical Cycles Are Especially Critical for Ecosystems