Nitrogen Cycle

The nitrogen cycle represents one of the most important nutrient cycles found in terrestrial and aquatic ecosystems. Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acids, proteins, and nucleic acids. Terrestrial, aquatic, and atmospheric ecosystems receive nitrogen inputs through natural processes and human activities. The Earth's atmosphere contains about 78% nitrogen. Nitrogen in the atmosphere is abundant but not in the right chemical form unless it is transformed into more chemically available forms. Nitrogen is introduced into terrestrial and aquatic ecosystems by biological and chemical nitrogen fixation and removed again by denitrification.

Fixation

Transfer of nitrogen between atmosphere and terrestrial and aquatic ecosystems begins with chemical or biological fixation of molecular nitrogen. This process is carried out by lightning, by photochemical fixation in the atmosphere, by the action of microorganisms, and industrially by the Haber-Bosch process used in the manufacture of commercial fertilizers. Nitrogen fixation is the conversion of molecular, unreactive, dinitrogen gas (N2) to nitrogen combined with other elements, such as oxygen and hydrogen, into reactive forms that readily undergo chemical reactions.

Ammonia (NH3) is the first product of nitrogen fixation. Biological nitrogen fixation is performed exclusively by bacteria and related microorganisms using an enzyme complex termed nitrogenase. Biological nitrogen fixation can be represented by the following equation, in which two moles of ammonia are produced from one mole of nitrogen gas, accompanied by the conversion of 16 molecules of adenosine triphosphate (ATP) to 16 molecules of adenosine diphosphate (ADP) and the release of one molecule of hydrogen (H2) and 16 molecules of inorganic phosphate (Pi) as by-products.

This reaction is performed exclusively by bacteria and related organisms using an enzyme complex termed nitrogenase. The most common nitrogenase consists of two proteins, one large containing molybdenum, iron, and inorganic sulfur (dinitrogenase), the other smaller containing iron and inorganic sulfur (dinitrogenase reductase).

Chemical fixation, through the Haber-Bosch process, is the reaction of nitrogen and hydrogen to produce ammonia. Nitrogen (N2 from the atmosphere) and hydrogen (H2 from water) gases are reacted over an iron catalyst (Fe3+) and aluminum oxide (Al2O3) and potassium oxide (K2O) are used as promoters. The reaction is carried out under conditions of 250 atmospheres (atm) and 450-500 °C. Delta H (AH) is the heat of reaction or enthalpy (—92.4kJmol—1 at 25 °C).

The Haber-Bosch process produces about 100 tera-grams of nitrogen fertilizer per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. That fertilizer is responsible for sustaining 40% of the Earth's population, as well as causing various deleterious environmental consequences.

Mineralization

Once nitrogen is fixed, it is subject to several chemical reactions which can convert it to different organic and inorganic forms. Plants and microorganisms incorporate fixed nitrogen into their cellular tissue. Animals receive their supply of nitrogen through the food they eat. These living organisms then use the nitrogen to manufacture amino acids and convert them into proteins. Nitrogen in living and dead organic matter and organic nitrogen fertilizers, such as urea, occurs predominantly in the amino form (NH2). Mineralization occurs in soil and sediment as microorganisms convert organic nitrogen into inorganic forms. Mineralization is a three-step process that begins with aminization, followed by ammonification, and ends with nitrification.

Aminization

Aminization is the first step of mineralization in which microorganisms break down complex proteins into simpler amino acids, amides, and amines.

(*R designates a carbon chain of indefinite length)

Ammonification

Ammonification is the second step of mineralization. Ammonification refers to any chemical reaction in which NH2 groups are converted into ammonia or its ionic form, ammonium (NH^), as an end product. Bacteria and related microorganisms derive metabolically useful energy from the oxidation of organic nitrogen to ammonium. Ammonium is then available to be assimilated and incorporated into amino acids or used for other metabolic purposes. If microorganisms produce ammonium in excess of their own metabolic requirements, the surplus is excreted into the ambient environment, such as soil or water, and is available for use as a nutrient by plants, or as a substrate for other microbial processes (e.g., nitrification). Nitrogen transformation of organic nitrogen compounds is not limited to microorganisms. Animals excrete urea or uric acid in their nitrogen-containing urine, along with diverse organic nitrogen compounds in their feces. The urea, uric acid, and organic nitrogen of feces are all substrates for ammonification. The generalized reaction for ammonification of soil organic compounds is

The generalized reaction for ammonification of urea is urease

CO(NH2 )2 + 2HOH = (NH4 )2CO3 (NH4 )2CO3 + HOH = 2NH+ + 2OH- + CO2

Nitrification

Nitrification is the final step of mineralization. During the process of nitrification, ammonia or ammonium ions are oxidized to nitrite (NO—) and then to nitrate (NO—).

Ammonia and Ammonium in the Environment

Circulation of nitrogen in soil, water, and the atmosphere results in numerous consequences as nitrogen undergoes various transformations within the nitrogen cycle. As with all components of the nitrogen cycle, the proper functioning of ammonification is essential for healthy, balanced, ecosystems.

Soil

In crop and livestock production systems nitrogen is converted into cellular tissue. In crop production systems, crop production (grain, fruit, forage, biomass) is sustained through photosynthesis and uptake of water, nitrogen, and other essential plant nutrients. In livestock production systems, animals assimilate nitrogen through the consumption of amino acids in grains and forages.

Nitrogen is reintroduced into the soil system in the form of crop residues and animal manure. In the absence of ammonification, these organic forms of nitrogen would accumulate in large quantities.

Humans have a major influence on the nitrogen cycle, especially through the use of manure and industrially manufactured fertilizers in agricultural systems. In intensively managed agricultural systems, nitrogen is often the most limiting plant nutrient. Under nitrogen-limited conditions, crop producers increase the availability of soil nitrogen by applying nitrogen fertilizer. Conventional cereal crop producers generally supply just over half the crop nitrogen needs as industrially manufactured fertilizer and/or manure, with the other half supplied by recycled nitrogen from crop residue, soil organic matter, atmospheric deposition, and biological nitrogen fixation.

The most common nitrogen sources of industrially manufactured fertilizers contain nitrogen in the nitrate and/or ammonium form or as urea. In some agricultural systems, compost or other organic materials (e.g., animal manure) may be added to soils as a nitrogen fertilizer source. Whether nitrogen is supplied by industrially manufactured fertilizer or from animal manure, ammonia and the organic forms of nitrogen must be converted to available ammonium through ammonification before they are available for plant and/or microbial assimilation.

In soil environments ammonia is rapidly converted to ammonium and subsequently to nitrate. The majority of nitrogen assimilated by plants is usually in the nitrate form; however, under flooded conditions, such as in rice production, the soil is devoid of oxygen, biological nitrification is limited, and most nitrogen stays in the ammonium form. Ammonium nitrogen, due to its positive charge, may be strongly adsorbed by ion-exchange reactions to negatively charged surfaces of clay minerals or organic matter. Consequently, ammonium is not leached very effectively by water as it percolates downward through the soil. In contrast, nitrate is highly soluble in soil water and is readily leached which can lead to surface and groundwater degradation. In situations where rates of nitrogen fertilization are greater than crop demand, the ability of the ecosystem to assimilate the nitrogen input becomes saturated. High-nitrate ground-water poses risks for human health, while surface waters may experience an increased productivity through eutrophication.

Nitrogen loss in the gaseous ammonia form into the atmosphere is called ammonia volatilization. Ammonia emissions occur from livestock housing and manure storage systems, manure applied to soil, and ammonia containing industrially manufactured fertilizers. Ammonia losses from soil increase as soil moisture and pH (acidity) increase. Ammonia emissions can also occur when urea fertilizer granules are applied on the surface of high-pH soils or when applied to soil with large amounts of crop residue on the soil surface, such as in no-till crop production. Ammonia, urea, and manure banded below the soil surface or incorporated by tillage operations within 3 days can minimize ammonia volatilization and promote conversion of available ammonia to ammonium through ammonification.

In saturated soil, under anaerobic conditions, nitrogen can undergo respiratory denitrification, a microbial process wherein nitrate is transformed to dinitrogen gas. Denitrification results in the production of molecular nitrogen or nitrogen oxides (i.e., nitrous oxide). These nitrogen gases are reintroduced into the atmosphere. Formation of molecular nitrogen is favored where there is an available energy supply (i.e., carbon). The rate of denitrification is influenced by pH, being much slower in acid than in neutral or alkaline systems. Nitrate may also be removed from saturated soil and sediment by dissim-ilatory nitrate reduction. Dissimilatory nitrate reduction results in nitrate ammonification (nitrate reduction to ammonium). The ammonification pathway results in microbial excretion of ammonium into the environment where it is available for use as a plant nutrient, or as a substrate for nitrification. Compared to denitrification, nitrate ammonification is a less significant process for nitrate reduction.

Water

Nitrogen cycling in aquatic ecosystems requires consideration ofnitrogen inputs and the fate ofthose inputs. The major controls on nitrogen supply to a water body include land-use practices, landscape vegetation, atmospheric loading, soil processes, and hydrology, including artificial drainage. The nitrogen status of a water body will largely depend on nitrogen storage, uptake, release, and exchange by abiotic and biotic processes, within and between sediments and the water column, and subsequent transport longitudinally downstream, in the case of flowing water. Biologically available forms of nitrogen (ammonia-, nitrate-, and organic-nitrogen) are subject to an extensive combination of physical and biogeochemical processes in aquatic systems. Once in an aquatic system, nitrogen is highly chemically and biologically active, undergoing numerous transformations and partitioning between the dissolved and particulate phases, between sediment and water column, and between the biotic and abiotic environments.

The ammonification of organic nitrogen to ammonia or ammonium in water is similar to the process that occurs in soil. In aquatic environments, cyanobacteria (e.g., Anabaena spp.; Nostoc spp.) are important atmospheric nitrogen-fixing bacteria. Organic nitrogen in aquatic systems may be dissolved in the water column or it may be associated with organic material deposited in sediments.

In either case, ammonification converts organic nitrogen to ammonia and ammonium which, under aerobic conditions, are rapidly converted to nitrate by nitrification. Ammonia is known to be acutely toxic to some freshwater vertebrates and invertebrates. Equilibrium exists in water between toxic ammonia and nontoxic ammonium. The dynamic equilibrium between ammonia and ammonium is affected by water temperature and pH. At a pH of 6.0 the ratio of ammonia to ammonium is about 1 to 3000 but decreases to 1 to 30 when the pH rises to 8.0 (becomes less acidic). Warm water will contain more toxic ammonia than cool water. Biological assimilation of ammonium by bacteria, biofilms, and aquatic plants is preferred to nitrate assimilation.

Nitrate disappearance in aquatic ecosystems is generally due to either respiratory denitrification or dissimilatory nitrate reduction to ammonium. Both these processes occur exclusively in sediments. Regardless of the amount of nitrogen entering aquatic systems from terrestrial sources, any substantial reduction in nitrogen will decrease the impacts of nitrogen loading.

Atmosphere

The atmosphere receives and circulates nitrogen as air emissions of nitrogen oxide (NO*), ammonia and ammonium (NHX), and nitrous oxide (N2O) from aquatic and terrestrial ecosystems. These gases have relatively short residence times in the atmosphere and are reintroduced into soil and water ecosystems usually within hours to days. Regional accumulation of ammonia and ammonium can occur in the lower atmosphere. Atmospheric ammonia reacts to form ammonium aerosol which is associated with decreased atmospheric visibility, acid rain, soil acidification, eutrophication, and human health impacts.

See also: Acidification; Atmospheric Deposition; Biodegradation; Decomposition and Mineralization; Denitrification; Nitrification; Nitrogen Cycle.

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