nitrogen cycle

Science Dictionary
nitrogen cycle  
The continuous process by which nitrogen is exchanged between organisms and the environment. Nitrogen is an essential nutrient, needed to make amino acids and other important organic compounds, but most organisms cannot use free nitrogen, which is abundant as a gas in the atmosphere. Gaseous nitrogen is broken apart and fixed (converted to stable, biologically assimilable inorganic compounds) in the process of nitrogen fixation. Some atmospheric nitrogen is fixed naturally during lightning strikes and some by industrial processes. Cyanobacteria and certain other species of bacteria, especially those living as symbionts in the roots of legumes, fix atmospheric nitrogen biologically in ammonium ions. Ammonia and ammonium ions are also produced by the ongoing decay of organic materials. Ammonia can be absorbed directly by plant cells, and certain bacteria living in soil and water convert ammonia and ammonium ions into nitrites and nitrates in the process known as nitrification. The nitrates are easily absorbed by plant roots. In this way, nitrogen is passed into the food chain and ultimately returned to the soil, water, and atmosphere by the metabolism and decay of plants and animals.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
Cite This Source
Previous Definition: nitrogen base
Next Definition: nitrogen fixation
Words Near: nitrogen cycle
More from
Synonyms and Antonyms for nitrogen cycle
More from
Search for articles containing nitrogen cycle
More from Translator Word FAQs presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.

Copyright © 2015, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature