Nitrogen cycle

2007 Schools Wikipedia Selection. Related subjects: General Biology; General Chemistry

The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature.

The basics

Earth's atmosphere is about 78% nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; it is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA. In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth(Smil, 2000).

Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make its own organic compounds. Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses.

Other plants get nitrogen from the soil by absorption at their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain.

Ammonia

The source of ammonia is the decomposition of dead organic matter by bacteria called decomposers, which produce ammonium ions (NH₄⁺). In well-oxygenated soil, these ions are then oxygenated first by nitrifying bacteria into nitrite (NO₂^-) and then into nitrate (NO₃^-). This two-step conversion of ammonium into nitrate is called nitrification (Smil, 2000).

Ammonium ions readily bind to soils, especially to humic substances and clays. Nitrate and nitrite ions, due to their negative electric charge, bind less readily since there are less positively charged ion-exchange sites (mostly humic substances) in soil than negative. After rain or irrigation, leaching (the removal of soluble ions, such as nitrate and nitrite) into groundwater can occur. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome (Vitousek et al, 1997). Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process leading to high algal and blue-green bacterial populations and the death of aquatic life due to excessive demand for oxygen. While not directly toxic to fish life like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.

Ammonia is highly toxic to fish life and the water discharge level of ammonia from wastewater treatment plants must often be closely monitored. To prevent loss of fish, nitrification prior to discharge is often desirable. Land application can be an attractive alternative to the mechanical aeration needed for nitrification.

During anaerobic (low oxygen) conditions, denitrification by bacteria occurs. This results in nitrates being converted to nitrogen gas and returned to the atmosphere. Nitrate can also be reduced to nitrite and subsequently combine with ammonium in the anammox process, which also results in the production of dinitrogen gas.

Processes of the Nitrogen Cycle

Nitrogen Fixation

Conversion of N₂

There are four ways to convert N₂ (atmospheric nitrogen gas) into more chemically reactive forms (Smil, 2000):

The conversion of dinitrogen (N₂) from the atmosphere into a form available to plants and hence to animals and humans. This is an important step in the terrestrial nitrogen cycle:

1. Biological fixation : some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen and assimilate it as organic nitrogen. An example of mutualistic nitrogen fixing bacteria are the Rhizobium bacteria, which live in plant root nodes. These species are diazotrophs.
2. Industrial N-fixation ; in the Haber-Bosch process, N₂ is converted together with hydrogen gas (H₂) into ammonia (NH₃) fertilizer.
3. Combustion of fossil fuels : automobile engines and thermal power plants, which release NOx.
4. Other processes : Additionally, the formation of NO from N₂ and O₂ due to photons and lightning, are important for atmospheric chemistry, but not for terrestrial or aquatic nitrogen turnover.

As a result of extensive cultivation of legumes (particularly soy, alfalfa, and clover), use of the Haber-Bosch process in the creation of chemical fertilizers and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of nitrogen into a biologically available form (Vitousek et al, 1997). This has occurred to the detriment of aquatic and wetland habitats through eutrophication.

Assimilation

In plants which have a mutualistic relationship with Rhizobium, some nitrogen is assimilated in the form of ammonium ions from the nodules. All plants however, can absorb nitrate from the soil via their root hairs. These are then reduced to nitrite ions and then ammonium ions for incorporation into amino acids, and hence protein, which forms part of the plants or animals that they eat (Smil, 2000).

Ammonification

Nitrates are the form of nitrogen most commonly assimilated by plant species, which, in turn are consumed by heterotrophs for use in compounds such as amino and nucleic acids. The remains of heterotrophs will then be decomposed into nutrient-rich organic material. Bacteria or in some cases, fungi, will convert the nitrates within the remains back into ammonia.

Nitrification

The conversion of ammonia to nitrates is performed primarily by soil-living bacteria. The primary stage of nitrification, the oxidation of ammonia (NH₃) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO₂^-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO₃^-) (Smil, 2000)

Anaerobic Ammonium Oxidation

In this biological process, nitrite and ammonium are converted directly into dinitrogen gas. This process makes up a major proportion of dinitrogen conversion in the oceans.

Denitrification

Denitrification is the reduction of nitrates back into the largely inert nitrogen gas (N₂), completing the nitrogen cycle. This process is performed by bacterial species such as the Pseudomonas (Smil, 2000) .

Nitrogen Cycle in Aquariums

Schematic representation of the flow of Nitrogen through a common aquarium.

One of the primary goals of the aquarist is to reproduce parts the nitrogen cycle on a small scale. While similar to the nitrogen cycle in natural environments, the aquarist must supplement some of the components necessary for the cycle to complete. When an aquarium is initially setup, there is insufficient beneficial bacteria to break down fish waste and uneaten food, which allows for unhealthy levels of ammonia and nitrite to build up. Hobbyists refer to this situation as "New Tank Syndrome"; it is a leading cause of fish deaths with newcomers to the hobby. Over time the addition of fish waste, carbon dioxide, light, and plant fertilizers will begin to build large colonies of beneficial bacteria that will ensure the aquarium remains healthy and active (The New Tank Syndrome 2006).

The primary source of ammonia (NH₃) is created when fish consume food and oxygen (O₂) and create waste and carbon dioxide (CO₂). The fish waste then decays into Ammonia. Other sources include excess food that is not eaten as well as decaying plants and dead fish. The rise in ammonia triggers the growth of Nitrosomonas which produce nitrites (NO₂). The nitrites trigger the growth of Nitrobacter to produce nitrates (NO₃). The nitrates and carbon dioxide are consumed by plant life which produce oxygen through the process of photosynthesis. Excess nitrates are removed by water changes.

Human Influences on the Nitrogen Cycle

Humans have contributed significantly to the nitrogen cycle by artificial nitrogen fertilization (primarily through the Haber Process, using energy from fossil fuels to convert N₂ to ammonia gas (NH₃) and planting of nitrogen fixing crops (Vitousek et al., 1997). In addition, humans have significantly contributed to the transfer of nitrogen trace gases from Earth to the atmosphere. N₂O has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources (Chapin et al. 2002). N₂O has deleterious effects in the stratosphere, where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. NH₃ in the atmosphere has tripled as the result of human activities. It is a reactant in the atmosphere, where it acts as an aerosol, decreasing air quality and clinging on to water droplets, eventually resulting in acid rain. Fossil fuel combustion has contributed to a 6 or 7 fold increase in NOx flux to the atmosphere. NO actively alters atmospheric chemistry, and is a precursor of tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems (Smil, 2000). Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can kill plants(Vitousek et al., 1997). Decreases in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degredation of nitrogen-poor, species diverse heathlands (Aerts and Berendse 1988).