Nitrogen cycle

Nitrogen is a part of vital organic compounds in microrganisms, such as amino acids, proteins and DNA. The gaseous form of nitrogen (N₂), makes up 78% of the troposphere. One might think this means we always have plenty of nitrogen available, but unfortunately it does not work that way. Nitrogen in the gaseous form cannot be absorbed and used as a nutrient by plants and animals; it must first be converted by nitrifying bacteria, so that it can enter food chains as a part of the nitrogen cycle.

During the conversion of nitrogen cyano bacteria will first convert nitrogen into ammonia and ammonium, during the nitrogen fixation process. Plants can use ammonia as a nitrogen source.

Nitrogen fixation is carried out according to the following reaction:
N₂ + 3 H₂ -> 2 NH₃

After ammonium fixation, the ammonia and ammonium that is formed will be transferred further, during the nitrification process. Aerobic bacteria use oxygen to convert these compounds. Nitrosomonas bacteria first convert nitrogen gas to nitrite (NO₂^-) and subsequently nitrobacter convert nitrite to nitrate (NO₃^-), a plant nutrient.

Nitrification is carried out according to the following reactions:
2 NH₃ + 3O₂ - > 2 NO₂ + 2 H⁺ + 2 H₂O
2 NO₂^- + O₂ -> 2 NO₃^-

Plants absorb ammonium and nitrate during the assimilation process, after which they are converted into nitrogen-containing organic molecules, such as amino acids and DNA.
Animals cannot absorb nitrates directly. They receive their nutrient supplies by consuming plants or plant-consuming animals.
When nitrogen nutrients have served their purpose in plants and animals, specialized decomposing bacteria will start a process called ammonification, to convert them back into ammonia and water-soluble ammonium salts. After the nutrients are converted back into ammonia, anaerobic bacteria will convert them back into nitrogen gas, during a process called denitrification.

Denitrification is carried out according to the following reaction:
NO₃^- + CH₂O + H⁺ -> ½ N₂O + CO₂ + 1½ H₂O

Finally, nitrogen is released into the atmosphere again. The whole process starts over after release.

A schematic representation of the nitrogen cycle is shown here:

Nitrogen as a limiting factor

Although the nitrogen conversion processes often occurs and large quantities of plant nutrients are produced, nitrogen is often a limiting factor for plant growth. Water flowing across the soil causes this error. Nitrogen nutrients are water-soluble and as a result they are easily drained away, so that they are no longer available for plants.

The annamox reaction

In 1999 researchers at the Gist-Brocades in Delft, The Netherlands, discovered a new reaction to be added to the nitrogen cycle; the so-called annamox reaction. This is now found to occur in the Black Sea, as well. The reaction implies conversion of nitrite and ammonium to pure nitrogen gas (N₂), which than escapes to the atmosphere. The reaction mechanism is triggered by a newly discovered bacterium, called Brocadia anammoxidans. This appears to be a compartmentalized bacterium; within the cell membrane two compartments can be found which are also surrounded by a membrane, a very rare phenomenon. Intermediate products of the reaction included hydroxylamine, and toxic hydrazine compounds. The bacterial membranes were found to consists of badly permeable membranes, which are thought to function as a barrier for hydrazines produced within the cell. This discovery has major consequences, as it alters the entire contribution of oceans to the nitrogen balance.

Source: NRC Handelsblad, 12-04-2003

For more information on nitrogen, move to the periodic chart

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