The interaction of nitrogen and oxygen in the atmosphere can only occur with the temperatures and pressures associated with lightning, however, and is not the primary means of converting nitrogen to a usable form (Elmhurst 2009). Instead, nitrogen fixing (as the process is known) is carried out by certain bacteria in the soil, which convert nitrogen gas and other forms of nitrogen waste in the soil to usable form of nitrogen, especially ammonia (NH4). Plants then absorb these usable forms of nitrogen and integrate the element into their own structure through growth. When animals -- like human beings -- eat these plants (or eat other animals that have eaten these plants), the nitrogen moves from the plant body to the animal body. When these higher organisms die, the nitrogen they have stored is broken down by other bacteria and fungi into nitrites (NO2) and nitrates (NO3) through a process known as nitrification (Pidwirny 2006). The...

In the natural nitrogen cycle, however, these substances are wither further broken down and returned to the atmosphere as nitrogen gas, or are reabsorbed by plants and bacteria, entering the cycle again immediately.
References

Elmhurst. (2010). "nitrogen cycle." Accessed 5 January 2010. http://www.elmhurst.edu/~chm/onlcourse/chm110/outlines/nitrogencycle.html

Killpack, S. & Bulchholz, D. (2010). "Nitrogen cycle." Accessed 5 January 2010. http://extension.missouri.edu/publications/DisplayPub.aspx?P=WQ252

Pidwirny, M. (2006). "The nitrogen cycle." Accessed 5 January 2010. http://www.physicalgeography.net/fundamentals/9s.html