Nitrogen cycle

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All organisms require nitrogen, which is an essential component of DNA, RNA, and proteins.

Nitrogen (N2) is the dominating component of the atmosphere (78%), however due to the strong triple bond between the N atoms in N2 molecules, which makes it relatively inert, or unreactive, it is unavailable to most organisms.

For plants and animals to be able to utilize nitrogen, the N2 gas must first be converted into a more chemically available form such as ammonium (NH4+), nitrate (NO3-), or organic nitrogen (e.g., urea (NH2) 2CO).

The inert nature of N2 means that biologically available nitrogen is in short supply in nature, limiting plant growth.

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms.

Five main processes of the nitrogen cycle, cycle nitrogen through the biosphere, atmosphere, and geosphere: nitrogen fixation, nitrogen uptake through organismal growth, nitrogen mineralization through decay, nitrification, and denitrification.

Microorganisms, particularly bacteria, play major roles in the nitrogen cycle and in all of the principal nitrogen transformations. Because these processes are microbially mediated, or controlled by microorganisms, these nitrogen transformations tend to occur faster than geological processes like plate motion, a very slow, purely physical process that is a part of the carbon cycle. Instead, rates are affected by environmental factors that influence microbial activity, such as temperature, moisture, and resource availability.

Nitrogen cycle processes.

Nitrogen fixation – Fixation is primarily done by free-living or symbiotic bacteria known as diazotrophs.

N2 →NH4+.
N2 is converted to ammonium (NH4+).

This is the only way that organisms can attain nitrogen directly from the atmosphere.

These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds.

Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates.

Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form such symbioses.

Lightneing, fires and volcanic activity also may cause fixation of nitrogen

About 30% of the total fixed nitrogen is produced industrially using the Haber-Bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.

The amount of fixed nitrogen in the biosphere has over the past century significantly increased from sources such as burning fossil fuels, utilization of nitrogen fertilizers and cultivating legumes.

Nitrogen assimilation – Plants take nitrogen from the soil by absorption through their roots as nitrate ions, nitrite ions, or ammonium ions.

Nitrogen ammonification or mineralization – From dead plants or animals or animal waste, the initial form of nitrogen is organic.

Organic N →NH4+.

Bacteria or fungi convert the organic nitrogen within the remains back (decay) into ammonium (NH4+).

Nitrification – Conversion of ammonia to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria.

NH4+ →NO3

In the primary stage ammonia (NH4+) is converted into nitrites (NO2−) by bacteria such as the Nitrosomonas species.

Other bacterial species such as Nitrobacter are responsible for the oxidation of the nitrites into nitrates (NO3−).

Ammonia needs to be converted into nitrates or nitrites since ammonia gas is toxic to plants.

Denitrification – Reduction of nitrates back into nitrogen gas (N2), completing the nitrogen cycle.

NO3– →N2+ N2O

This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions (e.g. in water or waterlogged soils) using nitrate as an electron acceptor in the place of oxygen during respiration.

They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.

Once converted to dinitrogen, nitrogen is rapidly lost to the atmosphere and unlikely to be reconverted to a biologically available form.

Denitrification removes nitrogen from ecosystems, roughly balancing the amount of nitrogen fixed during nitrogen fixation.

Anaerobic ammonia oxidation – Nitrite and ammonia are converted directly into molecular nitrogen (N2) gas.

This process makes up a major proportion of nitrogen conversion in the seas.

NH4+ + NO2-→N2 + 2H2O