The Nitrogen Cycle

"Nitrogen is a crucial component of crucial organic molecules such as DNA and proteins" (SIRS Researcher, 2003, The Nitrogen Cycle). The nitrogen cycle is "a matter cycle in which, through the processes of nitrogen fixation, synthesis, decomposition, and denitrification, nitrogen atoms move from nitrogen gas in the atmosphere, to inorganic form in the soil, to organic form in living things, and then back to inorganic form in the soil and nitrogen gas in the atmosphere; organic compounds that contain nitrogen include amino acids (proteins) and DNA" (Ritter, 2001, pg.712).

Nitrogen fixation is the process of atmospheric nitrogen converting to nitrates as nitrates are the only forms of nitrogen that are useful to organisms. This can be done in two ways: by lightning or bacteria.

Lightning converts a small amount of nitrogen into nitrates. "The energy from the lightning causes nitrogen gas to react with oxygen in the air" (Ritter, 2001, pg.66). Plant cells obtain nitrates as the nitrates dissolve in rain or surface water, enter the soil, and move into plants through their roots. Plants cells use nitrates to make DNA and also convert nitrates into amino acids, which intermingle to produce proteins.

A selected variety of bacteria are capable of fixing nitrogen by providing plants with built-in supplies of useful nitrogen, while plants supply the nitrogen-fixing bacteria with the sugar needed to make nitrates.

"Decomposers break down the nitrogen-containing chemicals in the waste or body into simpler chemicals such as ammonia. Other bacteria convert ammonia into nitrites, and still others convert the nitrites into nitrates. These bacteria all require oxygen to function. The nitrates then continue the cycle when they are absorbed by plant roots and converted into cell proteins and DNA" (Ritter, 2001, pg.67).

Denitrification is the process of denitrifying bacteria breaking down nitrates into nitrites, and then nitrites into nitrogen gas. "Denitrification ensures the balance among soil nitrates, nitrites, and atmospheric nitrogen, and completes the nitrogen cycle" (Ritter, 2001, pg.67). This concept can only be carried out by denitrifying bacteria that do not need oxygen. The overall "process speeds up when the soil is very acidic or water-logged (oxygen content is low)" (Ritter, 2001, pg.67).