Nitrogen is an essential element of biological molecules, such as amino acids, nucleotides, proteins, and DNA (Figure 20.1). All organisms can convert ammonia (NH3) to organic nitrogen compounds (substances containing C-N bonds). The reduction of N2 to NH3, on the other hand, which is called biological nitrogen fixation, can only be carried out by certain microorganisms, sometimes in symbiotic relationship with plants. The reduction of NO3^- to NH3, however, is widespread among both plants and microorganisms.

Within the biosphere a balance is maintained between total inorganic and total organic forms of nitrogen. The conversion of inorganic nitrogen to organic nitrogen, which starts with nitrogen fixation and nitrate reduction, is counterbalanced by catabolism, denitrification, and decay (Figure 20.2). Nitrogen availability is the factor limiting the fertility of most soils.

Nitrosomonas oxidizes ammonia to nitrite and Nitrobacter oxidizes nitrite to nitrate. Other bacteria, the denitrifying bacteria, convert ammonia to N2-just the opposite of the nitrogen-fixing bacteria.