Biology currently dominates nitrogen cycling on the Earth. However, the non-biological chemistry of nitrogen is important to understanding the Early Earth and other terrestrial planets, such as Mars. Nitrogen is necessary for compounds proteins, DNA, RNA, and for life as we know it. To understand the origin of life we need to understand the prebiotic sources of nitrogen. Similarly, life, in turn, affects nitrogen cycling. For example, nitrous oxide has been proposed as biosignature on extrasolar planets. Nitrogen cycling will also impact general planetary habitability. For example, lack of significant nitrogen in the atmosphere (as on Mars) can contribute to an atmospheric pressure too low to allow liquid water to exist on the surface.