The atmosphere is made up of about 78% nitrogen gas (N2). Nitrogen is important to ecosystems because it limits the growth of organisms. Lightning is one of the many ways nitrogen can enter the cycle.
Nitrogen is found in proteins. The largest concentration of nitrogen is found in the atmosphere. But plants and animals cannot use nitrogen directly from the atmosphere. The nitrogen must first undergo nitrogen fixation before plants can use it.
Nitrogen is returned to the soil in several ways. When an animal urinates, nitrogen returns to the water or soil and can be reused by plants. When an organism dies, decomposers transform the nitrogen in proteins and other compounds back into ammonium. Organisms in the soil convert ammonium into nitrogen-oxygen compounds that can be used by plants called nitrification.
Finally, a process called denitrification is done by other soil bacteria and converts nitrogen-oxygen compounds back into nitrogen gas, which returns to the atmosphere.
