Nitrogen (chemical symbol: N7) is the most abundent atmospheric element in the Earth's atmosphere, making up 78% of the total atmospheric gas. Mars, however, has very little nitrogen in it's atmosphere. At only 3% of the total atmospheric gas, this lack of nitrogen will cause problems for colonists hoping to extract nitrogen in any great quanitity from the atmosphere.
The "nitrogen cycle" is an essential terrestrial process that produces organic compounds intrinsic to life on Earth. "Fixing" by lightning strikes or bacterial processes combine atmospheric nitrogen with other elements (such as hydrogen, producing ammonia) producing organic compounds useful to plants (sustaining growth and used in photosynthisis), thereby supporting ecosystems. Nitrogen can be found in amino acids, proteins and DNA, making it an essential component of life as we know it.