Nitrogen is an essential component of amino acids and nucleic acids, which are the building blocks of all living things. Nitrogen makes up most of the atmosphere, but the N2 in the atmosphere cannot be used by most organisms. Nitrogen fixation is the process by which atmospheric nitrogen is transformed into forms that are more usable in biological processes. Although life needs nitrogen, only a few organisms are actually able to fix nitrogen out of the atmosphere. The biochemical reactions of the few types of bacteria that contribute to the recycling of nitrogen may be discussed. All other life is dependent on these organisms for the continuation of the essential nitrogen cycle. Human activity which has an impact on the cycling of nitrogen may also be discussed.
Although the atmosphere consists of 80% nitrogen gas (N2), plants can utilize nitrogen only in the form of ammonium (NH4+) or nitrate (NO3-).
Nitrogen fixation, the conversion of N2 to NH4+, occurs by nitrogen-fixing bacteria that live in the soil, or in the root nodules of certain plants such as legumes. In turn, nitrification occurs by certain nitrifying bacteria that convert NH4+ to NO2- (nitrite) and by other nitrifying bacteria that convert NO2- to NO3-. In addition, some N2 is converted to NO3- by the action of lightning. On the other hand, denitrification occurs when denitrifying bacteria convert NO3- back to N2.
From the NH4+ or NO3- absorbed plants make amino acids and nucleic acids. When animals eat the plants or other animals, they, in turn, obtain a form of nitrogen that they can metabolize. When animals break down proteins, they produce ammonia (NH3). Since NH3 is toxic, many animals such as aquatic animals excrete it directly. Other animals convert NH3 to less toxic forms, such as urea (mammals) or...