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