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The nitrogen cycle
Human activities and the nitrogen cycle
Harmful effects of nitrogen deposition

This illustration shows how nitrogen travels through the living and non-living parts of the Earth system. (©UCAR. This image is freely available for nonprofit and media use.)

The nitrogen cycle

Nitrogen is essential to all living systems, which makes the nitrogen cycle one of Earth's most important nutrient cycles.

Eighty percent of Earth's atmosphere is made up of nitrogen in its gas phase.

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.

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Human activities and the nitrogen cycle

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

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Harmful effects of nitrogen deposition

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.

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Nitrogen Links

UCAR Center for Science Education: The Changing Nitrogen Cycle

Examples of how humans are changing the nitrogen cycle and how the changing nitrogen cycle affects humans and ecosystems.

PhysicalGeography.net: The Nitrogen Cycle

Information designed for high school and college students about the nitrogen cycle, plus additional reading and related links, may be found here.

Windows to the Universe: The Nitrogen Cycle

A vast resource students, teachers, and the public for learning about the Earth and Space sciences.

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Scientific adviser: Elisabeth Holland
Writer: Nicole Gordon
Last updated : March 2015

Backgrounders provide supplementary information and should not be considered comprehensive sources.

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