GEF

Unit 4   |  The Global Ecosystem 94 The nitrogen cycle is fundamental to the functioning of life on Earth, and like other cycles has also been substantially affected by human activity. Nitrogen is found in the atmosphere as nitrogen gas (N 2 ). Before organisms can use it, atmospheric nitrogen must first be “fixed” by free-living or symbiotic bacteria. These bacteria are found inside the roots of plants, such as clover and beans. The bacteria convert nitrogen gas into ammonium (NH 4 +) or nitrate (NO 3 -), which is useful to organisms. When organisms die, other bacteria continue the cycle and nitrogen returns to the atmosphere in the process of decomposition. SYMBIOTIC A vital and close relation- ship between two species of organisms.