Nitrogen cycle

The nitrogen cycle is a complex system necessary for living beings.

The nitrogen cycle, more specifically called the biogeochemical nitrogen cycle, operates as a multi-stage process involving the transformation and movement of atmospheric nitrogen molecules (N₂) and other nitrogen-containing compounds (e.g., nitrate, NO₃^-; nitric oxide, NO; nitrogen dioxide, NO₂; nitrous oxide, N₂O; ammonium, NH₄⁺) into the Earth's soil and water compartments through rainfall, and back again into the atmosphere after soil- and water-bound nitrogen atoms have passed through many different organic and inorganic compounds by many different chemical processes. The biogeochemical cycling of nitrogen atoms serves to sustain living systems, nitrogen contributing one component of many of life's essential molecules, such as amino acids, proteins, and nucleic acids like DNA and RNA, and a necessary element for photosynthetic organisms, upon which nearly all Earth's living systems depend.

While nitrogen is abundant in air as dinitrogen molecules (N₂) and prevents oxygen from combustion, the air-born nitrogen is generally not useful to living beings; it must be transformed to other compounds to enable its use by living systems. Since di-nitrogen has a triple chemical covalent bond, it is relatively inert and requires considerable energy to render it biologically available. This occurs naturally as well as by human activity. Nitrogen is an important element in the production of food, as all organisms require it to maintain their living state.

Thus, on Earth, atoms of the chemical element, nitrogen (atomic symbol, N; number of protons, 7; standard atomic weight, 14.0067) — an essential element for Earth's living systems — undergo cyclical movement through the atmosphere, the crust (lithosphere), water compartments (hydrosphere), living systems, and non-living organic matter, a multi-stage processes effected by natural reactions, but influenced significantly as well by man-made activity (see below).

When animals defecate or when plants decompose, they produce manure, sewage waste, compost, rotting leaves and sticks and other matter, and the result is waste that contains organic nitrogen. The result is organic soil material called humus. In addition, inorganic nitrogen comes from different sources as well; for example, certain minerals have nitrogen, as well as nitrogen coming from precipitation such as rain or snow, and fertilizers have nitrogen as well. This added nitrogen in the soil makes it easier for living plants to grow healthy. While plants can't use some types of organic nitrogen, it's necessary for microbes living in the soil to convert the organic nitrogen into inorganic nitrogen which enables the plants to use it for their benefit.

Plants use a variety of inorganic or "fixed" nitrogen, which means that they have their nitrogen incorporated into compounds such as nitrate ions, such as:

• Soil-based inorganic nitrogen such as ammonium NH4+
• Urea (NH2)2CO
• Nitrate NO3- (or nitrate ions)
• Nitrite or NO3-

Some nitrates and nitrites can leach out into the soil into the groundwater and change into nitrogen gases such as N2, NO, N20, and ammonia gas (NH3). When they escape out of the soil, it is called volatizing. It goes into the atmosphere of the Earth.

Different parts of the nitrogen cycle

These include:

• Ammonification is when nitrogen in organic form is converted by microorganisms into ammonium or NH4+. It has a positive charge. This lets it become adsorbed and fixated to the negatively charged soil particles, or be taken in by plants. When a plant or animal dies or makes waste, the nitrogen is organic and must be changed by either bacteria or fungi into ammonium (NH4+), a process called ammonification or mineralization. Some microorganisms are not properly called bacteria but are called archaea, which have a genetic makeup similar to humans, and are classified as eukaryotes, and they sometimes have the ability to live in "extreme" environments such as inside the mouths of active volcanoes, or in extreme cold such as Antarctica, or in very acidic or very saline environments (eg the Dead Sea.)

• Decay is a process of transforming excrement which contains organic nitrogen by microorganisms; the molecules are broken down, so that excretions and dead organisms turn into ammonia.

• Denitrification is when microbes transform nitrogen in both nitrate and nitrite form into gases called nitrous oxide and nitrogen gas. For this to happen, it requires soils to be extremely wet, or poorly drained. This is why wetlands are valuable since they contribute an important role in the nitrogen cycle. The resulting gases volatize into the atmosphere, but then they're no longer available for use by plants. Denitrification involves the reduction of nitrates back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacteria like Pseudomonas and Clostridium in anaerobic conditions.

• Erosion is when the soil is worn away by running water, wind, ice, gravity, human activity such as bulldozers, or by activities such as earthquakes or tsunamis or tornadoes or other geological processes. When the soil is moved by these forces, nitrogen in both organic and inorganic form in the soil are carried along.

The intense energy of a lightning strike can fix nitrogen, but the overall amount of nitrogen fixation by lightning strikes is small compared with the action of bacteria.

• Fixation, sometimes better known as Nitrogen fixation, is when atmospheric nitrogen is changed or fixed to make it useful by plants. It can happen by:

1. Lightning strikes, which have enormous energy to break down inert nitrogen molecules, allowing them to combine with oxygen making nitrogen oxides; these can dissolve in rain, forming "nitrates", that are carried by gravity to the earth.
2. Symbiotic bacteria which have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their organic compounds needed by the bacterium for internal processes. Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes and work with the plant as a form of symbiosis and produce ammonia in exchange for carbohydrates. If a soil is lacking valuable nutrients, it's possible to plant legumes to add the nitrogen quotient. It's possible for nitrogen fixing bacteria to take nitrogen from the air and convert it into ammonia or NH3. It goes into a variety of amino acids and helps make proteins.
3. Industrial fixation is a third way. Under great pressure at 600°C, and using a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3) which can be used directly as fertilizer, but for economic reasons, mostly it's processed further to yield urea and ammonium nitrate (NH4NO3).

• Haber Process is a synthetic process in which nitrogen and hydrogen react under great pressure and temperator in the presence of a catalyst to make ammonia. It was named after Fritz Haber, a Nobel laureate who helped transform food production worldwide.

• Immobilization is when inorganic forms of nitrogen are converted into organic forms as microbes, and when plants die. It's the opposite of mineralization. When nitrogen enters the soil in mineral form -- that is when it's not from a plant or animal source -- it's inorganic.

• Inorganic nitrogen is added to the soil by rain or snowfall or as fertilizers. This is why it's better to have actual rain water be the water source for indoor plants, since they get a generous helping of nitrogen as a result; using tap water usually doesn't bring this added benefit. In the soil, microorganisms convert organic nitrogen into inorganic forms, and this process is known as biological nitrogen fixation. Afterwards, the inorganic nitrogen is ready to be used by plants.

• Leaching is when nitrogen, when it's in the form of nitrate, is negatively charged. As a result, it's not attracted to the negatively charged clay and humus in the soil, that is, it's repelled, and the nitrogen won't be absobed by the clay. Rather, it moves down through the soil into the groundwater where streams and drinking water can become contaminated. Ammonia is highly toxic to fish and, as a result, it's important to monitor the amount of ammonia in sewage treatment. One way to prevent this is to conduct nitrification before discharging materials downstream.

• Mineralization can happen to nitrogen, like it can happen to any element, when it's converted from an organic to an inorganic form by microbes, and can be used by plants for growing.

• Nitrification happens under certain conditions. Microbes in the soil use ammonium nitrate in the soil for energy and therefore oxidize ammonium nitrogen first into Nitrite Nitrogen (NO2-) and then into Nitrate Nitrogen (NO3-). It's primarily done by bacteria in the soil such as Nitrosomonas. It's very important for nitrites to be converted to nitrates because the former are toxic to plants; if nitrites keep building up, it will eventually harm plant life. Because nitrates are highly soluble, it's easy for them to enter the groundwater, but it can affect drinking water; it's more dangerous for young human babies because it can interfere with their blood-oxygen levels and cause a condition known as methemoglobinemia. Also, when there is too much nitrates in streams, it can lead to excessive growth of algae which can lead to the death of aquatic life such as fishes because of the algae's excessive demand for oxygen.

• Organic nitrogen can't be used by plants. But it's formed when living matter dies, such as plant roots, leaves, sticks, animals including humans and insects. It's also formed when animals excrete and make manure, compost, and sewage sludge. The process of decomposition is when organic nitrogen is broken down into inorganic nitrogen by microbes.

Organic nitrogen is useless to plants, but must be converted into inorganic form to once again be useful.

• Runoff happens when soil becomes so wet that it can no longer hold water; the water runs off the surface of the soil. If the soil has a protective layer of grasses or trees with strong roots, then the runoff will be mostly water; but if there isn't a protective covering, the runoff can erode substantial amounts of soil. This process can take both forms of nitrogen along with it -- both inorganic and organic. Sometimes this causes pollution and ends up in streams, lakes, rivers, reservoirs, and bays, and it can cause harm to aquatic life such as fishes.

• Uptake is the opposite of runoff. It's sometimes called assimilation. It happens when plant roots absorb inorganic nitrogen and other ingredients in the soil, and this helps the plants grow. Some plants get nitrogen from the soil, and by absorption of their roots in the form of either nitrate ions or ammonium ions. When animals eat plants, they get nitrogen via this pathway. Some plants can absorb nitrate or ammonium ions from the soil via their root hairs.

• Volatilization is when nitrogen is moved out of the soil and into the Earth's atmosphere. It becomes nitrogen in the form of a gas such as NH3, NO, N2O, or N2.

Nitrogen cycle processes (N=nitrogen)

This process...	turns these things...	into these things
Ammonification	Organic N	Ammonium N
Denitrification	Nitrate N or nitrite N	Nitrous oxides and nitrogen gas
Erosion	Organic or inorganic N (on soil particles)	Surface water or other parts of the landscape
Immobilization	Inorganic N (ammonium N, nitrate N)	Organic N
Leaching	Nitrate N	Groundwater
Mineralization	Organic N	Inorganic N (ammonium N, nitrate N)
Nitrification	Ammonium N	Nitrite N and then to nitrate N
Runoff	Nitrate N or ammonium N	Surface water
Uptake	Nitrate N or ammonium N	Plant roots
Volatilization	Ammonium N	Ammonia N (gas form)

Source: The Nitrogen Cycle: Nitrogen Transformations in Soil, Water, and Air by Soil Science Education Home Page (via NASA)

Impact of humans

There is concern that a number of activities by humans have influenced the nitrogen cycle, and it is not clear what ramifications these changes have had or will continue to have. Of interest is the growing of certain types of plants such as soy, alfalfa and clover, the use of chemical fertilizers in soil to enhance crop yields, and pollution emitted by cars and trucks and planes and industrial activity. There is speculation that N2O, or nitrous oxide can have negative affects in the atmosphere; this is believed to be exacerbated by agricultural use of fertilizers, the burning of biomass and cattle and feedlots, and industrial output. There is concern that nitrous oxide can have a negative affect on the atmospheric layer known as ozone, since the ozone layer is believed to shield the earth from dangerous rays from the sun. Further, changes in the cycle may have resulted in more ammonia in the air which can decrease air quality and cling to water droplets, possibly resulting in acid rain. Last, there is concern that changes in the cycle have led to contamination by certain areas of nitrates, with speculation about the safety of drinking water, that is, they pollute drinking water.

There is a movement among biogeochemists to see the nitrogen cycle as it relates to the cycles of other compounds in the earth, such as the water cycle, or the cycling of other chemical compounds. A reporter explained:

A biogeochemical cycle is a pathway by which a chemical element, such as carbon, or compound, like water, moves through Earth's biosphere, atmosphere, hydrosphere and lithosphere. In effect, the element is "recycled," although in some cycles the element is accumulated or held for long periods of time. Chemical compounds are passed from one organism to another, and from one part of the biosphere to another, through biogeochemical cycles. Water, for example, can go through three phases (liquid, solid, gas) as it cycles through the Earth system. It evaporates from plants as well as land and ocean surfaces into the atmosphere and, after condensing in clouds, returns to Earth as rain and snow. Researchers are discovering that biogeochemical cycles--whether the water cycle, the nitrogen cycle, the carbon cycle, or others--happen in concert with one another. Biogeochemical cycles are "coupled" to each other and to Earth's physical features.

See also

View images of nitrogen cycle by right-clicking these links link1 link2 & selecting 'open in new tab'.

• Water cycle
• Carbon cycle