Topics

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

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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NCAR experts

Elisabeth Holland
NCAR Scientist
303-497-1433
Home Page

Beth Holland is a biogeochemist who studies the link between the chemistry of the atmosphere and ecosystems on Earth. As leader of NCAR's Biogeosciences Program, she brings a biological perspective to geophysics and atmospheric research. Holland's research focuses on the regional and global nitrogen cycles and their interactions with the carbon and water cycles, including how land ecosystems are affected by air pollution and climate change. She also works on the Intergovernmental Panel on Climate Change (IPCC) assessments and will serve as a lead author on the next report, due in 2007.

Jean-Francois Lamarque
NCAR Scientist
303-497-1495

Jean-Francois Lamarque uses global models to study chemistry-climate interactions.   Some of his work on nitrogen involves simulating the deposition of nitrogen for conditions today and in 2100. The results, which show nitrogen deposition increasing threefold by the end of the century, provide an upper limit on the possible impact of increased emissions. Lamarque's research includes studying how this nitrogen deposition can lead to increased carbon uptake. 

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Nitrogen Links

Ecological Society of America: Human Alteration of the Global Nitrogen Cycle

This report in the ESA's Issues in Ecology series gives an overview of how human activities are changing the global nitrogen cycle and outlines the consequences.

Ecological Society of America: Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen

Summaries of eutrophication, health effects, sources of nonpoint pollution, as well as discussion of what can be done to remediate such pollution are discussed in this Issues in Ecology report.

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.

Visionlearning: The Nitrogen Cycle

This learning module from a site supported by the National Science Foundation for high school and college students provides an overview of the nitrogen cycle and the chemical changes that govern it.

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Scientific adviser: Elisabeth Holland
Writer: Nicole Gordon
Last updated : March 2005

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