The Nitrogen Cycle includes important process like fixation, mineralization, nitrification and denitrification. In the Earth's atmosphere, it is approximately 78% of Nitrogen making it the largest pool of nitrogen. But, the atmospheric nitrogen has limited availability for biological useage, leading to a lack of usable nitrogen in many types of ecosystems.
Helpful video: http://www.ehow.com/video_4801947_nitrogen-cycle_.html