Denitrification
Denitrification
The biology of denitrification
Denitrification is a microbial process by which fixed nitrogen is lost from soil or aquatic systems to the atmosphere. This loss occurs when bacteria convert nitrogen-containing molecules, in particular, nitrate (NO3−) and nitrite (NO2−), to gaseous nitrous oxide (N2 O) and dinitrogen (N2 ).
The biology of denitrification
Respiration is a chemical process in which energy is released when electrons are passed from a donor molecule to an acceptor molecule. In addition to energy being released, the respiratory process results in the donor molecule being converted to an oxidized molecule, meaning it has lost electrons, and the acceptor molecule being converted to a reduced molecule, meaning it has gained electrons. Typically, the acceptor molecule is oxygen, but in anaerobic environments, which lack oxygen, bacteria may reduce molecules other than oxygen that have high reduction potentials or the ability to accept electrons in a process known as anaerobic respiration.
Denitrification occurs when bacteria reduce nitrate or nitrite by this process. In a sequence of four reductions, nitrate is converted to dinitrogen gas, the molecular form in which nitrogen escapes from soils and aquatic systems. The four-step sequence is: 1.) Nitrate is reduced to nitrite. 2.) Nitrite is reduced to nitric oxide. 3.) Nitric oxide (NO) is reduced to nitrous oxide. 4.) Nitrous oxide is reduced to dinitrogen. Depending on its physiological capabilities, a single organism may carry out all of these reductions, or it may carry out only a few.
In addition to dinitrogen, small amounts of nitrous oxide leave aquatic and soil systems. This happens because not all bacteria that produce nitrous oxide can subsequently reduce it to dinitrogen. Therefore, some nitrous oxide can leak out of cells into the atmosphere, if it is not first reduced to dinitrogen by other organisms. Nitric oxide is also a gas, but organisms that have the ability to reduce nitrite to nitric oxide always have the ability to reduce nitric oxide to nitrousoxide. For this reason, nitric oxide is not an important product of denitrification.
In aquatic and soil systems fixed nitrogen primarily exists as a component of three inorganic molecules; nitrate, nitrite, and ammonium (NH4+), and in the proteins and other types of organic molecules that comprise living and dead organisms. Although only nitrogen from the molecules nitrate and nitrite is converted to a gaseous form and removed from these systems, nitrogen from proteins and ammonium can also be removed if it is first oxidized to nitrate or nitrite. This conversion begins in a process termed ammonification, when nitrogen is released from the biomass of dead organisms, which produces ammonium. Ammonium can then be converted to nitrate in an aerobic respiratory reaction called nitrification, in which the ammonium serves as an electron donor, and oxygen an electron acceptor.
Importance
Along with dinitrogen fixation, ammonification, and nitrification, denitrification is a major component of the nitrogen cycle. Estimates of nitrogen fluxes from terrestrial and marine ecosystems to the atmosphere as a result of microbial denitrification range from 90 × 1012 to 243 × 1012 grams per year for terrestrial systems and 25 × 1012 to 179 × 1012 grams
KEY TERMS
Aerobic respiration— Respiration in which oxygen serves as the electron acceptor.
Anaerobic respiration— Respiration in which a molecule other than oxygen serves as the electron acceptor.
Dinitrogen fixation— Process in which dinitrogen reacts to form a new nitrogen compound such as ammonium or ammonia. Most nitrogen is fixed as a result of microbial dinitrogen fixation, chemical synthesis by humans and lightning.
Fixed nitrogen— Nitrogen that occurs in molecular forms other than dinitrogen such as that found in ammonium, nitrite, nitrate, organic molecules, and nitrous oxide.
per year for marine systems. Scientists generally agree that less than 10% of these fluxes occur with nitrogen as a component of nitrous oxide. The range in these estimates reflects the difficulty researchers face in measuring denitrification and extrapolating the measurements to a global scale.
Humans are primarily interested in denitrification because this process is responsible for fixed nitrogen being removed from sewage and lost from cropland. Environmentally harmful nitrate concentrations in sewage discharge can be reduced by storing wastes under denitrifying conditions before releasing them into the environment. Although denitrification is a beneficial process in sewage treatment, it is considered a problem in agriculture. Farmers increase their crop yields by applying nitrogen-containing fertilizers to their land. As a result of denitrification, crop yields may be reduced because much of the added nitrogen is lost to the atmosphere. This loss of fixed nitrogen may have global consequences. Increased denitrification from cropland is responsible for increased amounts of nitrous oxide in the atmosphere. Although nitrous oxide is not the major end product of denitrification, it is highly reactive and may contribute to the depletion of ozone in the stratosphere.
Resources
BOOKS
Kupchella, Charles, and Margaret Hyland. Environmental Science: Living Within the System of Nature. 2nd ed. Boston: Allyn and Bacon, 1989.
Prescott, L., J. Harley, and D. Klein. Microbiology 5th ed. New York: McGraw-Hill, 2002.
OTHER
Rensselaer Polytechnic Institute “Denitrification” <http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Environmental/DENITE/denite.htm> (accessed November 16, 2006).
Steven MacKenzie
Denitrification
Denitrification
Denitrification is a microbial process by which fixed nitrogen is lost from soil or aquatic systems to the atmosphere. This loss occurs when bacteria convert nitrogen-containing molecules, in particular, nitrate (NO3-) and nitrite (NO2-), to gaseous nitrous oxide (N2O) and dinitrogen (N2).
The biology of denitrification
Respiration is a chemical process in which energy is released when electrons are passed from a donor molecule to an acceptor molecule. In addition to energy being released, the respiratory process results in the donor molecule being converted to an oxidized molecule, meaning it has lost electrons, and the acceptor molecule being converted to a reduced molecule, meaning it has gained electrons. Typically, the acceptor molecule is oxygen , but in anaerobic environments, which lack oxygen, bacteria may reduce molecules other than oxygen that have high reduction potentials or ability to accept electrons in a process known as anaerobic respiration. Denitrification occurs when bacteria reduce nitrate or nitrite by this process. In a sequence of four reductions, nitrate is converted to dinitrogen gas, the molecular form in which nitrogen escapes from soils and aquatic systems. The four-step sequence is: 1.) Nitrate is reduced to nitrite. 2.) Nitrite is reduced to nitric oxide. 3.) Nitric oxide (NO) is reduced to nitrous oxide. 4.) Nitrous oxide is reduced to dinitrogen. Depending on its physiological capabilities, a single organism may carry out all of these reductions, or it may carry out only a few.
In addition to dinitrogen, small amounts of nitrous oxide leave aquatic and soil systems. This happens because not all bacteria that produce nitrous oxide can subsequently reduce it to dinitrogen. Therefore, some nitrous oxide can leak out of cells into the atmosphere, if it is not first reduced to dinitrogen by other organisms. Nitric oxide is also a gas, but organisms that have the ability to reduce nitrite to nitric oxide always have the ability to reduce nitric oxide to nitrousoxide. For this reason, nitric oxide is not an important product of denitrification.
In aquatic and soil systems fixed nitrogen primarily exists as a component of three inorganic molecules; nitrate, nitrite, and ammonium (NH4+), and in the proteins and other types of organic molecules that comprise living and dead organisms. Although only nitrogen from the molecules nitrate and nitrite is converted to a gaseous form and removed from these systems, nitrogen from proteins and ammonium can also be removed if it is first oxidized to nitrate or nitrite. This conversion begins in a process termed ammonification , when nitrogen is released from the biomass of dead organisms, which produces ammonium. Ammonium can then be converted to nitrate in an aerobic respiratory reaction called nitrification , in which the ammonium serves as an electron donor, and oxygen an electron acceptor.
Importance
Along with dinitrogen fixation, ammonification, and nitrification, denitrification is a major component of the nitrogen cycle . Estimates of nitrogen fluxes from terrestrial and marine ecosystems to the atmosphere as a result of microbial denitrification range from 90 × 1012 to 243 × 1012 grams per year for terrestrial systems and 25 × 1012 to 179 × 1012 grams per year for marine systems. Scientist generally agree that less than 10% of these fluxes occur with nitrogen as a component of nitrous oxide. The range in these estimates reflects the difficulty researchers face in measuring denitrification and extrapolating the measurements to a global scale.
Humans are primarily interested in denitrification because this process is responsible for fixed nitrogen being removed from sewage and lost from cropland. Environmentally harmful nitrate concentrations in sewage discharge can be reduced by storing wastes under denitrifying conditions before releasing them into the environment. Although denitrification is a beneficial process in sewage treatment , it is considered a problem in agriculture. Farmers increase their crop yields by applying nitrogen containing fertilizers to their land. As a result of denitrification, crop yields may be reduced because much of the added nitrogen is lost to the atmosphere. This loss of fixed nitrogen may have global consequences. Increased denitrification from cropland is responsible for increased amounts of nitrous oxide in the atmosphere. Although nitrous oxide is not the major end product of denitrification, it is highly reactive and may contribute to the depletion of ozone in the stratosphere.
Resources
books
Kupchella, Charles, and Margaret Hyland. Environmental Science: Living Within the System of Nature. 2nd ed. Boston: Allyn and Bacon, 1989.
Prescott, L., J. Harley, and D. Klein. Microbiology. 5th ed. New York: McGraw-Hill, 2002.
Steven MacKenzie
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Aerobic respiration
—Respiration in which oxygen serves as the electron acceptor.
- Anaerobic respiration
—Respiration in which a molecule other than oxygen serves as the electron acceptor.
- Dinitrogen fixation
—Process in which dinitrogen reacts to form a new nitrogen compound such as ammonium or ammonia. Most nitrogen is fixed as a result of microbial dinitrogen fixation, chemical synthesis by humans and lightning.
- Fixed nitrogen
—Nitrogen that occurs in molecular forms other than dinitrogen such as that found in ammonium, nitrite, nitrate, organic molecules, and nitrous oxide.
Denitrification
Denitrification
A stage in the nitrogen cycle in which nitrates in the soil or in dead organic matter are converted into nitrite, nitrous oxide , ammonia, or (primarily) elemental nitrogen . The process is made possible by certain types of bacteria, known as denitrifying bacteria. Denitrification is a reduction reaction and occurs, therefore, in the absence of oxygen. For example, flooded soil is likely to experience significant denitrification since it is cut off from atmospheric oxygen. Although denitrification is an important process for the decay of dead organisms, it can also be responsible for the loss of natural and synthetic fertilizers from the soil.