Nitrogen, a fundamental element, plays a foundational role in sustaining life on Earth. It is a core component of amino acids, the building blocks of proteins, and of nucleic acids like DNA, which carry genetic information. In this scientific context, a “reservoir” refers to a significant natural store of a substance. This article explains Earth’s largest natural nitrogen reservoir and other important compartments.
The Atmosphere: Earth’s Primary Nitrogen Store
The Earth’s atmosphere is the largest nitrogen reservoir. Nitrogen gas (N₂) constitutes approximately 78% of the atmosphere by volume, making it the most abundant gas in the air we breathe. Its vast volume contributes to its capacity as a massive repository for nitrogen.
Nitrogen gas is stable due to the strong triple bond between its two atoms, making it largely unreactive under normal atmospheric conditions. This chemical inertness prevents it from readily reacting or being removed from the atmosphere, allowing it to accumulate over geological timescales. While molecular nitrogen is abundant, most organisms cannot directly utilize this gaseous form, requiring specialized processes like nitrogen fixation by certain bacteria to convert it into usable compounds. This stability ensures the atmosphere functions as a long-lasting and dominant nitrogen reservoir.
Other Key Nitrogen Compartments
Beyond the atmosphere, other significant, but smaller, nitrogen reservoirs exist across Earth’s systems. Oceans contain nitrogen both dissolved in water and incorporated into marine organisms. While a large portion of oceanic nitrogen is in the form of inert N₂ gas, similar to the atmosphere, fixed nitrogen compounds are also present, playing a role in marine productivity. Nitrogen dissolved in seawater, particularly in the form of nitrate, is essential for the growth of plankton and algae, which form the base of ocean food webs.
Terrestrial systems, including living biomass and soil, also hold substantial amounts of nitrogen. Nitrogen is found in living organisms such as plants, animals, and microorganisms, and in organic matter within the soil. For instance, healthy plants can contain 3 to 4 percent nitrogen in their above-ground tissues. In soils, nitrogen exists in organic compounds, ammonium, and nitrate ions, with organic forms accounting for a large percentage of potentially available nitrogen. This terrestrial nitrogen is more dynamic, constantly cycling through biological processes like decomposition and uptake by plants.
The lithosphere, or Earth’s crust, represents another long-term nitrogen reservoir, with nitrogen trapped within rocks and minerals. While primary igneous rocks contain little nitrogen, it is stored in sedimentary and crystalline rocks, often as ammonium. The lithosphere holds a considerable amount of nitrogen, though much of it is not readily available to ecosystems.