Nitrogen is a fundamental element for all life on Earth. It is constantly in motion, cycling through Earth’s various systems. This element resides in several major reservoirs, shifting between different chemical forms. Understanding where nitrogen is stored helps us comprehend its availability and impact on global ecosystems.
The Atmosphere’s Vast Reservoir
The largest store of nitrogen is found in Earth’s atmosphere. Approximately 78.08% of the air we breathe consists of dinitrogen gas (N2). This form of nitrogen is incredibly stable due to a strong triple bond, making it largely unreactive and unusable by most plants and animals.
Vast quantities, estimated at 3.866 x 10^21 grams, are stored as this inert gas. Its unreactive nature necessitates other processes to convert it into biologically accessible compounds. This atmospheric reservoir acts as a massive buffer, regulating the global nitrogen supply.
Nitrogen in Earth’s Soils
Terrestrial soils represent a significant reservoir for nitrogen, storing it in both organic and inorganic forms. Organic nitrogen is primarily found within decaying plant and animal matter, known as humus, and within soil microorganisms. This organic pool, with over 98% of all nitrogen in soil existing in an organic form, is highly dynamic.
Inorganic nitrogen exists as ammonium (NH4+) and nitrates (NO3-). These inorganic ions are dissolved in soil water or adsorbed onto soil particles. Nitrates are readily taken up by plant roots, making them a crucial nutrient. Soil bacteria play a significant role in converting atmospheric nitrogen and organic nitrogen into these plant-available forms.
Oceans and Aquatic Nitrogen
Nitrogen also accumulates in marine and freshwater environments. It is present as dissolved inorganic nitrogen (DIN), including nitrates, nitrites, and ammonium, which are essential nutrients for aquatic primary producers. Dissolved organic nitrogen (DON) also contributes to this aquatic pool, originating from the decomposition of organic matter.
Within aquatic ecosystems, nitrogen is incorporated into the living tissues of phytoplankton, algae, and marine animals. Significant amounts of nitrogen are also sequestered in bottom sediments over long periods. The deep ocean holds a substantial reservoir of dissolved inorganic nitrogen.
Nitrogen Within Living Organisms
All living organisms contain nitrogen as a fundamental component of their biological structures. Nitrogen is a building block of proteins, which perform cellular functions and form structural components. It is also indispensable for nucleic acids (DNA and RNA), which store and transmit genetic information.
Other biomolecules, such as adenosine triphosphate (ATP), the energy currency of cells, and chlorophyll, the pigment essential for photosynthesis, also contain nitrogen. The nitrogen stored within living biomass is a relatively smaller and more transient reservoir compared to the atmosphere or oceans. Nitrogen is continuously acquired by organisms and then released back into the environment upon death and decomposition.