The Nitrogen Cycle

Renewable resources can be recycled for reuse through the interactions of natural processes of metabolism. Microorganisms are essential in the webs of metabolic activities that renew the earth's natural resources. Among the most important biogeochemical cycles is the nitrogen cycle.

Nitrogen is a key cellular element of amino acids, purines, pyrimidines, and certain coenzymes. The element accounts for about 9 to 15 percent of the dry weight of a cell. Proteins and other organic compounds of life could not be formed without nitrogen.

Ammonification. In the nitrogen cycle, many organisms obtain their nitrogen from organic sources such as amino acids or purines, while others obtain it from inorganic compounds such as nitrogen gas (N 2), ammonia (NH 3), or nitrate (NO 3 ‐1). Before nitrate or nitrogen gas can be used, however, the nitrogen in the compounds must be changed into ammonia, a process called ammonification. The ammonia is then brought into the living system by an enzyme‐catalyzed pathway in which glutamic acid and glutamine form. These amino acids are then used to synthesize other nitrogen compounds in the cell (Figure ).

Figure 1

The complex interactions of the nitrogen cycle as it occurs in the soil.

Nitrogen fixation. The principal reservoir of nitrogen on earth is the atmosphere, which contains about 80 percent nitrogen. In the process of nitrogen fixation, nitrogen gas from the atmosphere is used to form ammonia by the chemical process of reduction. Nitrogen fixation is performed by free-living bacteria as well as by bacteria growing in symbiosis with leguminous plants (plants that bear their seeds in pods, such as peas, beans, alfalfa, clover, and soybeans).

Nitrogen fixation is accomplished by species of Rhizobium inhabiting the roots of leguminous plants in a mutually beneficial (symbiotic) relationship. These Gram-negative bacteria penetrate the root hairs and form an infection thread that becomes a root nodule. Here the bacteria fix atmospheric nitrogen, while deriving nutrients from the plant.

There are many genera of free-living bacteria that exist apart from legumes and fix nitrogen in the soil. Among the important ones are species of Azotobacter, Azospirillum, Bacillus, Beijerinckia, and numerous species of cyanobacteria.

Once nitrogen has been incorporated into ammonia, the ammonia is used for various organic substances. Later, when plants, animals, and microorganisms die, the nitrogen is recycled by forming ammonia once again in the process of ammonification. For example, proteins and nucleic acids are broken down first to amino acids and purines and then to acids, gases, and ammonia. Ammonification also occurs from animal excretory products such as urea, the major component of the urine. The urea is broken down by urea-digesting bacteria, and ammonia is released.

Nitrification. The conversion of ammonia to nitrate (NO3-1) is the process ofnitrification. Nitrifying bacteria, such as species of Nitrosomonas andNitrosococcus, are involved. Nitrosomonas species convert ammonia to nitrite(NO2-1); then Nitrosococcus species convert the nitrite to nitrate (NO3-1). Nitrification occurs in soils, fresh water, and marine environments. The nitrate that results serves as an important nitrogen source for plants.

Denitrification. Denitrification is the process in which the nitrogen of nitrate is released as gaseous nitrogen. This process makes nitrogen available to bacteria that use it for nitrogen fixation. Denitrification is accomplished by numerous bacteria that reduce nitrite (NO2-1) to nitrous oxide (N2O) and then to atmospheric nitrogen (N2).