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.
Nitrogen gas (N2) is the most abundant gas in the atmosphere, making up 78% of the air we breathe, yet it is commonly in short supply in biological communities. Most organisms cannot use nitrogen gas to make their essential, nitrogen-containing molecules, such as proteins and DNA. The two nitrogen atoms in N2 are connected by an extremely stable triple bond, which does not easily break.
Just a few species of bacteria and archaea, called nitrogen fixers, can break the N2 triple bond. In a process called nitrogen fixation, these microbes convert N2 into ammonium (NH4+). Other microbes convert NH4+ into nitrite (NO2-) and nitrate (NO3-), and then plants use NO3- or NH4+ to produce nitrogen-containing organic molecules. By consuming plants, animals also obtain usable forms of nitrogen.
Much of the nitrogen taken up by plants is recycled locally through decomposition of dead organic matter in soils, which again releases ammonium and nitrate. Some nitrogen, however, is leached from soils and carried away by the movement of water, and some of this accessible nitrogen ultimately enters lakes and oceans.
In aquatic systems, primary productivity occurs in surface waters, and some nitrogen is recycled locally in these waters. Much of the nitrogen, however, is recycled at greater depths in the water column, as organisms in deep waters intercept and consume sinking detritus. Some of the nitrogen sinks to the bottom and accumulates in sediments, with a small amount later returned to the surface by upwelling.
Human activities are affecting nitrogen fluxes and pools. Fossil fuels such as coal and petroleum contain nitrogen, so burning them releases oxides of nitrogen, as does the burning of biomass. The oxides of nitrogen contribute to atmospheric smog and acid rain.
Rice cultivation and livestock production also release nitrogen oxides (NOx), nitrous oxide (N2O), and ammonia (NH3) into the atmosphere. This atmospheric nitrogen can be deposited locally or far from the source, in some areas adding as much nitrogen as farmers place on their crops.
Humans also fix nitrogen by an energy-consuming industrial process in order to manufacture fertilizer and explosives. The total of industrial nitrogen fixation and fixation by legume crops such as soybeans now rivals the rate of natural terrestrial fixation.
When organisms die, on land and in the oceans, they liberate their organic stores of nitrogen. Some of this nitrogen is converted to inorganic nitrogen compounds, such as NH4+, NO2-, and NO3-. However, these molecules are for the most part taken up again by organisms and used to create organic compounds. In a process called denitrification, a number of species of bacteria convert some of the inorganic nitrogen compounds back into N2, which enters the atmosphere.
Nitrogen lost from fertilized croplands and deforested areas by water runoff or wind is increasingly getting deposited into aquatic systems. Too much nitrogen can be a bad thing.
Phytoplankton in aquatic systems reproduce rapidly in response to the nutrient inputs—a process called eutrophication. They reproduce so rapidly that consumers cannot keep up by eating them. The phytoplankton population consumes oxygen during respiration. Also, decomposers eventually process the dead bodies of phytoplankton and consume oxygen in the process. As a result, the oxygen levels in the water plummet. Many aquatic organisms, including fish and crustaceans, cannot survive the low oxygen, resulting in a dead zone devoid of aquatic life. A dead zone occurs in the Gulf of Mexico, where the Mississippi River discharges nutrient-rich agricultural runoff.
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.