Nitrogen Cycle
Nitrogen is a crucial nutrient for the survival of all living organisms. It is the building block of proteins, DNA and chlorophyll. Most nitrogen atoms exist as tightly bound pairs, forming dinitrogen gas (N2), which represents 78% of the atmosphere. Despite its abundance, only a few microorganisms can break the powerful triple bond of N2, utilize it, and convert it into a more accessible form (reactive nitrogen) such as ammonia (NH3), nitrate (NO3-), or organic nitrogen.
The movement of nitrogen through the atmosphere, biosphere, and geosphere in different forms is called the nitrogen cycle.
Nitrogen fixation
The process of converting N2 into biologically available and reactive nitrogen is called nitrogen fixation. The triple bonds between the nitrogen atoms in N2 make this compound very stable. Only certain organisms, known as nitrogen fixing organisms, are able to expend the energy required to break the triple bond of N2. Examples of nitrogen fixing organisms are bacteria from the genus Rhizobium, Azotobacter, Clostridium and cyanobacterias. Rhizobium forms a symbiotic relationship with plants, primarily from the legume family (beans, clover and peas). The bacteria inhabit the plant’s root nodules and provide organic nitrogen in exchange for carbohydrates.
Nitrogen fixation can also happen abiotically (without organisms) through lightning, cosmic radiation and forest fires. In these processes, N2 combines with oxygen to form the nitrogen oxides NO and NO2-. These nitrogen oxides mix with rain and fall to the earth’s surface as nitric acid (HNO3).
However, humans have contributed more to the introduction of reactive nitrogens into the environment than natural processes have. Humans have done this by burning fossil fuels and over-using synthetic nitrogen fertilizers. Most man-derived nitrogen, such as nitrogen fertilizers, is the result of the Haber-Bosch process.
Nitrification
Nitrification is the conversion of ammonia into nitrite (NO2-) and then into nitrate (NO3-). Most nitrification occurs aerobically and is carried out exclusively by prokaryotes. Species of ammonia-oxidizing and nitrite-oxidizing bacteria are ubiquitous in aerobic environments. They include Nitrosomonas, Nitrosospira, and Nitrosococcus, which oxidize ammonia to nitrite; and Nitrobacter, Nitropinam, Nitrospira, and Nitrococcus, which oxidize nitrite to nitrate.
The first step of nitrification involves two different reactions. First, an enzyme called ammonia monooxygenase converts ammonia to hydroxylamine. Second, an enzyme called hydroxylamine oxidoreductase converts hydroxylamine to nitrite.
Finally, the resulting nitrite is further oxidized by a nitrite oxidizer into nitrate.
Ammonia oxidation can also occurs anaerobically. This process is called anammox (anaerobic ammonia oxidation), and is carried out by the bacteria such as Brocadia anammoxidans.
Ammonification
Nitrogenous waste in dead plants and animals is converted into inorganic ammonia (NH3) by bacteria and fungi in a process called ammonification. The resulting ammonia can be used by plants for growth.
Denitrification
Denitrification is the process that converts nitrate to nitrogen gas, thus removing bio-available nitrogen and returning it to the atmosphere.
Haber - Bosch process
German scientists Fritz Haber and Carl Bosch developed a method to convert nonreactive atmospheric nitrogen into ammonia. In this method nitrogen from the air is directly combined with hydrogen under high pressures and temperatures. Currently, the Haber-Bosch process is used to produce about 100 teragrams (Tg) reactive nitrogen (35% of total reactive nitrogen) per year worldwide, most of which is used to produce nitrogen fertilizers.
Global Nitrogen Cycle
The inert nature of N2 limits the primary productivity, such as in plants, of many ecosystems. Increases in the human population can alter the global nitrogen cycle, which greatly affects biodiversity, global warming, water quality and human health.