Nitrogen is an element fundamental to all life on Earth, forming the structural backbone of amino acids (proteins) and nucleic acids (DNA and RNA). Despite its omnipresence, most organisms cannot directly utilize the abundant nitrogen gas (N₂) found in the atmosphere. The nitrogen cycle is the complex biogeochemical process that converts this inert atmospheric form into reactive, biologically available compounds, allowing it to move through terrestrial and aquatic ecosystems. This movement dictates the productivity of ecosystems, as the availability of usable nitrogen frequently limits the growth of plants and other primary producers. Understanding this global cycle is important because it connects the atmosphere, the soil, and the oceans in a series of transformations driven largely by microbial activity.
Where Nitrogen Is Stored Globally
The largest reservoir of nitrogen on the planet is the atmosphere, composed of approximately 78% nitrogen gas (N₂). This pool represents an inert form largely inaccessible to most life forms.
The terrestrial biosphere stores nitrogen primarily as organic compounds within soil, living biomass, and dead organic matter. Soil nitrogen also exists as inorganic ions, such as ammonium and nitrate, which are available for plant uptake. The marine reservoir contains nitrogen in dissolved inorganic forms like nitrate, nitrite, and ammonium within the water column. A significant amount is also locked away in deep-ocean sediments and sedimentary rock, sequestered from the active biological cycle for long periods.
The Essential Biochemical Steps
The critical transformations within the nitrogen cycle are predominantly mediated by specialized microorganisms. The entry point is nitrogen fixation, which converts inert N₂ gas into ammonia (NH₃). This conversion is carried out by bacteria, such as Rhizobium in legume root nodules or free-living species like Azotobacter.
Once nitrogen is incorporated into biomass, ammonification (or mineralization) occurs. Bacteria and fungi decompose dead organic matter and animal waste, releasing the nitrogen back into the soil or water as ammonium (NH₄⁺).
The next step is nitrification, a two-stage oxidation process performed by chemoautotrophic bacteria. First, ammonia-oxidizing bacteria, like Nitrosomonas, convert ammonium into nitrite (NO₂⁻). Then, nitrite-oxidizing bacteria, such as Nitrobacter, rapidly convert the nitrite into nitrate (NO₃⁻). Nitrate is the most readily absorbed form of nitrogen for many plants and is taken up through assimilation.
The final major transformation is denitrification, which completes the cycle by returning nitrogen to the atmosphere. Denitrifying bacteria thrive in anaerobic (oxygen-poor) conditions. These microbes use nitrate as an electron acceptor during respiration, converting it back into gaseous forms, including nitrous oxide (N₂O) and eventually N₂ gas. This step balances the input of fixed nitrogen.
Nitrogen Dynamics in Land Ecosystems
The soil matrix is the center of nitrogen activity in terrestrial ecosystems, where microbial communities drive nutrient availability. Plants primarily absorb dissolved inorganic nitrogen from the soil, favoring the highly mobile nitrate (NO₃⁻) or ammonium (NH₄⁺). Rapid nitrogen recycling is maintained by the mineralization of organic matter, where soil organisms convert dead plant material into ammonium that can be reused.
Biological nitrogen fixation often involves a symbiotic relationship. For example, Rhizobium bacteria form nodules on the roots of leguminous plants, such as peas and clover, to directly supply them with fixed nitrogen. Excess nitrate that is not assimilated or denitrified can be transported out of the soil profile through leaching and surface runoff.
Nitrogen Dynamics in Ocean Ecosystems
In the open ocean, nitrogen often limits the productivity of the marine food web. Phytoplankton, the microscopic primary producers, consume dissolved inorganic nitrogen compounds to fuel their growth. The primary source of new nitrogen to the surface waters is upwelling, where deep, cold, nutrient-rich water is brought to the sunlit euphotic zone.
In oxygen-poor zones, specialized microbial processes remove fixed nitrogen. While traditional denitrification occurs, Anaerobic Ammonium Oxidation (Anammox) is a significant contributor to nitrogen loss. Anammox bacteria convert ammonium and nitrite directly into N₂ gas, removing a substantial amount of fixed nitrogen, particularly in oxygen-minimum zones.
Impact of Human Activities on the Cycle
Human activities have dramatically accelerated the rate at which inert atmospheric nitrogen is converted into reactive forms, overwhelming the natural cycle. The largest source of this reactive nitrogen is the Haber-Bosch process, an industrial method synthesizing ammonia for agricultural fertilizers. This synthetic fixation, combined with nitrogen inputs from fossil fuel combustion, has more than doubled the amount of nitrogen entering the global cycle compared to natural terrestrial fixation.
The resulting excess nitrogen has environmental consequences, particularly in aquatic systems. Nitrogen runoff from fertilized fields leads to eutrophication, where nutrient enrichment causes massive algal blooms in coastal waters and lakes. Decomposition of these blooms depletes oxygen, creating widespread “dead zones” that cannot support marine life. The increased nitrogen load also contributes to the release of nitrous oxide (N₂O) during denitrification, a powerful greenhouse gas.