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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.
Human activities cause increased nitrogen deposition in a variety of ways, including
The consequences of human-caused nitrogen deposition are profound and influence many aspects of the Earth system, including
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 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.
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.
Scientific adviser: Elisabeth Holland
Writer: Nicole Gordon
Last updated : March 2005
Backgrounders provide supplementary information and should not be considered comprehensive sources.
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.