Nitrogen Fixation

Nitrogen makes up about 78% of the atmosphere in its molecular form, N2, but the two nitrogen atoms in this molecule are held together by an unusually strong triple covalent bond, which makes N2 chemically inert and unusable by plants and animals directly. The process of breaking this bond and converting atmospheric N2 into ammonia - the chemical form living organisms can actually use - is called nitrogen fixation. It happens by two broadly different routes, shown in Figure 12.5: biological nitrogen fixation, carried out by living organisms (further divided into symbiotic an …