Earth’s atmosphere is actually about 80 percent nitrogen. Most of the rest is oxygen, with tiny amounts of other stuff thrown in. Nitrogen is essential to all living systems, which makes the nitrogen cycle one of Earth’s most important nutrient cycles.
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth’s gravity. The common name given to the atmospheric gases used in breathing and photosynthesis is air. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. Although air content and atmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants and terrestrial animals currently is only known to be found in Earth’s troposphere and artificial atmospheres.
To understand the abundance of Nitrogen in the atmosphere, it is useful to compare it to Oxygen (the next most abundant element in the atmosphere). Compared to Oxygen, Nitrogen is 4 times as abundant in the atmosphere. However, we must also consider the relative abundances of Oxygen and Nitrogen over the entire Earth (oxygen is about 10,000 times more abundant). These earthly abundances overall reflect the composition of the material from which the Earth originally formed and the process of Earth’s accretion. Oxygen is a major component of the solid earth. Nitrogen is not stable as a part of a crystal lattice, so it is not incorporated into the solid Earth. This is one reason why nitrogen is so enriched in the atmosphere relative to oxygen. The other primary reason is that, unlike oxygen, nitrogen is very stable in the atmosphere and is not involved to a great extent in chemical reactions that occur there. Thus, over geological time, it has built up in the atmosphere to a much greater extent than oxygen. It is important to know that both nitrogen and oxygen are intimately involved with the cycle of life on the planet.
Atmospheric nitrogen becomes part of living organisms in two ways. The first is through bacteria in the soil that form nitrates out of nitrogen in the air. The second is through lightning. During electrical storms, large amounts of nitrogen are oxidized and united with water to produce an acid that falls to Earth in rainfall and deposits nitrates in the soil.
Plants take up the nitrates and convert them to proteins that then travel up the food chain through herbivores and carnivores. When organisms excrete waste, the nitrogen is released back into the environment. When they die and decompose, the nitrogen is broken down and converted to ammonia. Plants absorb some of this ammonia; the remainder stays in the soil, where bacteria convert it back to nitrates. The nitrates may be stored in humus or leached from the soil and carried into lakes and streams. Nitrates may also be converted to gaseous nitrogen through a process called denitrification and returned to the atmosphere, continuing the cycle.
Nitrogen Cycle Facts
Human activities cause increased nitrogen deposition in a variety of ways, including
- burning of both fossil fuels and forests, which releases nitrogen into the atmosphere
- fertilizing crops with nitrogen-based fertilizers, which then enter the soil and water
- ranching, during which livestock waste releases ammonia into the soil and water
- allowing sewage and septic tanks to leach into streams, rivers, and groundwater
Harmful Effets of Nitrogen
The consequences of human-caused nitrogen deposition are profound and influence many aspects of the Earth system, including
ecosystems: Nitrogen additions to the soil can lead to changes that favor weeds over native plants, which in turn reduces species diversity and changes ecosystems. Research shows that nitrogen levels are linked with changes in grassland species, from mosses and lichens to grasses and flowers.
precipitation: Nitrogen oxides react with water to form nitric acid, which along with sulfur dioxide is a major component of acid rain. Acid rain can damage and kill aquatic life and vegetation, as well as corrode buildings, bridges, and other structures.
air quality: High concentrations of nitrogen oxides in the lower atmosphere are a precursor to tropospheric ozone which is known to damage living tissues, including human lungs, and decrease plant production.
water quality: Adding large amounts of nitrogen to rivers, lakes, and coastal systems results in eutrophication, a condition that occurs in aquatic ecosystems when excessive nutrient concentrations stimulate blooms of algae that deplete oxygen, killing fish and other organisms and ruining water quality. Parts of the Gulf of Mexico, for example, are so inundated with excess fertilizer that the water is clogged with algae, suffocating fish and other marine life.
carbon cycle: The impacts of nitrogen deposition on the global carbon cycle are uncertain, but it is likely that some ecosystems have been fertilized by additional nitrogen, which may boost their capture and storage of carbon. Sustained carbon sinks are unlikely, however, because soil acidification, ozone pollution, and other negative effects eventually compromise nitrogen-enhanced carbon uptake.