Figure 57.17 The Global Nitrogen Cycle The largest pool of nitrogen is in the atmosphere in the form of nitrogen gas, N2. Nitrogen cycles through the biosphere primarily via the processes of nitrogen fixation, which converts inorganic nitrogen to an organic form usable by plants, and denitrification, which returns N2 to the atmosphere. The estimated pools in major compartments (white boxes) and the annual fluxes between compartments (arrows) are expressed in units of 1012 grams.