Nitrogen is one of the essential major elements for all life forms, and is often one of the limiting nutrients for biological growth in various environment. In the same time, nitrogen cycle is one of the few natural cycles that is dominated by biological processes. As a result, nitrogen cycle provides us information on the interaction between biology and the Earth environment.

This is a graduate level seminar class on nitrogen cycles occurring in natural environment. We will review the classical studies in understanding nitrogen cycle to learn the basic knowledge as well as the classical tools. We will also select recent papers of new discoveries and new techniques.