• The nitrogen cycle is the set biogeochemical processes by which nitrogen undergoes chemical reactions, changes form, and moves through difference reservoirs on earth, including living organisms.• Nitrogen is required for all organisms too live and grow because it is the essential component of DNA, RNA, and protein. However, most organisms cannot use atmospheric nitrogen, the largest reservoir.• The five processes in the nitrogen cycle -- fixation, uptake, mineralization, nitrification and denitrification -- are all driven by microorganisms.• Humans influence the global nitrogen cycle primarily through the use of nitrogen-based fertilizers.
• Nitrogen (N) is an essential component of DNA, RNA, and proteins, the building blocks of life. All organisms require nitrogen to live and grow. Although the majority of the air we breathe is N2, most of the nitrogen in the atmosphere is unavailable for use by organisms.• This is because the strong triple bond between the N atoms in N2 molecules makes it relatively inert.
• In fact, in order for plants and animals to be able to use nitrogen, N2 gas must first be converted to more a 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 often in short supply in natural ecosystems, limiting plant growth and biomass accumulation.
• Nitrogen is an incredibly versatile element existing in both inorganic and organic forms as well as many different oxidation states. The movement of nitrogen between the atmosphere, biosphere and geosphere in different forms is described by the nitrogen cycle (Figure 1), one of the major biogeochemicalcycles. Similar to the carbon cycle, the nitrogen cycle consists of various storage pools of nitrogen and processes by which the pools exchange nitrogen (arrows) (see our The Carbon Cycle module for more information).
The nitrogen cycle. Yellow arrows indicate human sources of nitrogen to theenvironment. Red arrows indicate microbial transformations of nitrogen. Blue arrowsindicate physical forces acting on nitrogen. And green arrows indicate natural, non-microbial processes affecting the form and fate of nitrogen. Ecology & Ecosystem
• Five main processes cycle nitrogen through the biosphere, atmosphere, and geosphere: nitrogen fixation, nitrogen uptake (organismal growth), nitrogen mineralization (decay), nitrification, and denitrification. Microorganisms, particularly bacteria, play major roles in all of the principal nitrogen transformations
• . As microbial mediated processes, 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 fixation• N2 NH4+ Nitrogen fixation is the process wherein N2 is converted to ammonium, essential because it is the only way that organisms can attain nitrogen directly from the atmosphere. Certain bacteria, for example those among the genus Rhizobium, are the only organisms that fix nitrogen through metabolic processes. Nitrogen fixing bacteria often form symbiotic relationships with host plants. This symbiosis is well-known to occur in the legume family of plants (e.g. beans, peas, and clover).
• In this relationship, nitrogen fixing bacteria inhabit legume root nodules and receive carbohydrates and a favorable environment from their host plant in exchange for some of the nitrogen they fix. There are also nitrogen fixing bacteria that exist without plant hosts, known as free-living nitrogen fixers. In aquatic environments, blue-green algae (really a bacteria called cyanobacteria) is an important free-living nitrogen fixer.
• In addition to nitrogen fixing bacteria, high- energy natural events such as lightning, forest fires, and even hot lava flows can cause the fixation of smaller, but significant amounts of nitrogen (Figure 3). The high energy of these natural phenomena can break the triple bonds of N2 molecules, thereby making individual N atoms available for chemical transformation.•
• Within the last century, humans have become as important a source of fixed nitrogen as all natural sources combined. Burning fossil fuels, using synthetic nitrogen fertilizers, and cultivation of legumes all fix nitrogen. Through these activities, humans have more than doubled the amount of fixed nitrogen that is pumped into the biosphere every year , the consequences of which are discussed below.
• Nitrogen uptake• NH4+ Organic N The ammonia produced by nitrogen fixing bacteria is usually quickly incorporated into protein and other organic nitrogen compounds, either by a host plant, the bacteria itself, or another soil organism. When organisms nearer the top of the food chain (like us!) eat, we are using nitrogen that has been fixed initially by nitrogen fixing bacteria.
• Nitrogen mineralization• Organic N NH4+ After nitrogen is incorporated into organic matter, it is often converted back into inorganic nitrogen by a process called nitrogen mineralization, otherwise known as decay. When organisms die, decomposers (such as bacteria and fungi) consume the organic matter and lead to the process of decomposition.
• During this process, a significant amount of the nitrogen contained within the dead organism is converted to ammonium. Once in the form of ammonium, nitrogen is available for use by plants or for further transformation into nitrate (NO3-) through the process called nitrification.
• Nitrification• NH4+ NO3- Some of the ammonium produced by decomposition is converted to nitrate via a process called nitrification. The bacteria that carry out this reaction gain energy from it. Nitrification requires the presence of oxygen, so nitrification can happen only in oxygen-rich environments like circulating or flowing waters and the very surface layers of soils and sediments. The process of nitrification has some important consequences.
• Ammonium ions are positively charged and therefore stick (are sorbed) to negatively charged clay particles and soil organic matter. The positive charge prevents ammonium nitrogen from being washed out of the soil (or leached) by rainfall. In contrast, the negatively charged nitrate ion is not held by soil particles and so can be washed down the soil profile, leading to decreased soil fertility and nitrate enrichment of downstream surface and groundwaters.
• Denitrification• NO3- N2+ N2O Through denitrification, oxidized forms of nitrogen such as nitrate and nitrite (NO2- ) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas. Denitrification is an anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the following sequence: NO3- NO2- NO N2O N2.
• Nitric oxide and nitrous oxide are both environmentally important gases. Nitric oxide (NO) contributes to smog, and nitrous oxide (N2O) is an important greenhouse gas, thereby contributing to global climate change.• Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically available form because it is a gas and is rapidly lost to the atmosphere. Denitrification is the only nitrogen transformation that removes nitrogen from ecosystems (essentially irreversibly), and it roughly balances the amount of nitrogen fixed by the nitrogen fixers described above.
Human alteration of the N cycle and its environmental consequences• Early in the 20th century, a German scientist named Fritz Haber figured out how to short circuit the nitrogen cycle by fixing nitrogen chemically at high temperatures and pressures, creating fertilizers that could be added directly to soil. This technology has spread rapidly over the past century, and, along with the advent of new crop varieties, the use of synthetic nitrogen fertilizers has led to an enormous boom in agricultural productivity. This agricultural productivity has helped us to feed a rapidly growing world population, but the increase in nitrogen fixation has had some negative consequences as well. While the consequences are perhaps not as obvious as an increase in global temperatures or a hole in the ozone layer, they are just as serious and potentially harmful for humans and other organisms.
• Not all of the nitrogen fertilizer applied to agricultural fields stays to nourish crops. Some is washed off of agricultural fields by rain or irrigation water, where it leaches into surface or ground water and can accumulate. In groundwater that is used as a drinking water source, excess nitrogen can lead to cancer in humans and respiratory distress in infants. The U.S. Environmental Protection Agency has established a standard for nitrogen in drinking water of 10 mg per liter nitrate-N. Unfortunately, many systems (particularly in agricultural areas) already exceed this level. By comparison, nitrate levels in waters that have not been altered by human activity are rarely greater than 1 mg/L. In surface waters, added nitrogen can lead to nutrient over-enrichment, particularly in coastal waters receiving the inflow from polluted rivers. This nutrient over-enrichment, also called eutrophication, has been blamed for in
• creased frequencies of coastal fish-kill events, increased frequencies of harmful algal blooms, and species shifts within coastal ecosystems.• Reactive nitrogen (like NO3- and NH4+) present in surface waters and soils, can also enter the atmosphere as the smog-component nitric oxide (NO) and the greenhouse gas nitrous oxide (N2O). Eventually, this atmospheric nitrogen can be blown into nitrogen-sensitive terrestrial environments, causing long-term changes
For example, nitrogen oxides comprise a significant portion of the acidity in acid rain which has been blamed for forest death and decline in parts of Europe and the Northeast United States.Increases in atmospheric nitrogen deposition have also been blamed for more subtle shifts in dominant species and ecosystem function in some forest and grassland ecosystems
• Currently, much research is devoted to understanding the effects of nitrogen enrichment in the air, groundwater, and surface water. Scientists are also exploring alternative agricultural practices that will sustain high productivity while decreasing the negative impacts caused by fertilizer use. These studies not only help us quantify how humans have altered the natural world, but increase our understanding of the processes involved in the nitrogen cycle as a whole.
• In 1958, atmospheric carbon dioxide at Mauna Loa was about 320 parts per million (ppm), and in 2010 it is about 385ppm.• Future CO2 emission can be calculated by the kaya identity
• The environmental sulphur cycle involves many physical, chemical and biological agents.• As such, the figure indicates the relationships between sulphur, S, hydrogen sulphide, H2S, sulphur dioxide, SO2, and the sulphate ion, SO4--. In mineral form sulphur may be present as sulphides (e.g. pyrite, FeS2, chalcopyrite, FeS.CuS, pyrrhotite, FeS) and/or sulphates (e.g. gypsum, CaSO4.2H2O, barite, BaSO4). Sulphur in minerals may move through the cycle as a result of the oxidation of sulphides to sulphate and/or the dissolution of sulphates. For example, oxidation of pyrite to sulphuric acid may be immediately followed, in situ, by acid neutralization by calcium carbonate (calcite) to form calcium sulphate (gypsum). The reaction of hydrogen sulphide with dissolved metal ions may precipitate metallic sulphides which are chemically indistinguishable from naturally occurring sulphide minerals.
• At some mines, sulphur is added to the cycle as sulphur dioxide in processes such as the Inco/SO2 process for cyanide destruction in the treatment of tailings. This added sulphur is oxidized to sulphate ion (Ingles & Scott, 1987), most of which remains free, but some of which combines with lime, CaO, in the tailings to form gypsum.• For information on the sulphur cycle with respect to water quality monitoring see Canadian Council of Environment Ministers (1987).
• The Role of Micro-organisms in the Sulphur Cycle• Micro-organisms (most frequently bacteria) are often integrally involved in the chemical alteration of minerals. Minerals, or intermediate products of their decomposition, may be directly or indirectly necessary to their metabolism. The dissolution of sulphide minerals under acidic conditions (ARD), the precipitation of minerals under anaerobic conditions, the adsorption of metals by bacteria or algae, and the formation and destruction of organometallic complexes are all examples of indirect micro-organism participation. Where minerals are available as soluble trace elements, serve as specific oxidizing substrates, or are electron donors/acceptors in oxidation-reduction reactions, they may be directly involved in cell metabolic activity.
• There are three categories of oxidation-reduction reactions for minerals with micro-organisms:• Oxidation by autotrophic (cell carbon from carbon dioxide) or mixotrophic (cell carbon from carbon dioxide or organic matter) organisms. Energy derived from the oxidation reaction is utilized in cell synthesis.• Electron acceptance by minerals (reduction) for heterotrophic (cell carbon from organic matter) and mixotrophic bacteria. Chemical energy is used to create new cell material from an organic substrate.• Electron donation by minerals (oxidation) for bacterial or algal photosynthesis (reaction is fuelled by photon energy).• Natural Oxidation in the Sulphur Cycle• Oxidation of sulphur or sulphides for energy production is restricted to the bacterial genus Thiobacillus, the genus Thiomicrospira, and the genus Sulfolobus. These bacteria all produce sulphuric acid (i.e. hydrogen ions, H+, and sulphate ions, SO4-- ) as a metabolic product. Extensive reviews of these bacteria and their behaviour have been written by Brierley (1978) and Trudinger (1971).• It is these bacteria that are known to accelerate the generation of Acid Rock Drainage (ARD) from pyritic and pyrrhotitic rocks under suitable conditions. Evangelou & Zhang (1995) report that sulphide oxidation catalysed by bacteria may have reaction rates six orders of magnitude (i.e. 1,000,000 times) greater than the same reactions in the absence of bacteria. Photomicrographs 1, 2 and 3, from LeRoux, North & Wilson (1973), illustrate the shape and appearance of T. ferrooxidans: The bacteria develop flagella only if they are required for mobility in accessing energy sources.• http://technology.infomine.com/enviromine/ard/microorganisms/roleof.htm
Almost all living things need oxygen. They use this oxygen during the process of creating energy in living cells.
The flow of sulphur compounds in our environment.Scheme: Elmar Uherek, adapted and modified from an water cycleillustration of the Center for Space Research, Univ. of Austin, Texas Please click the picture for a larger view! (150 K)
• We find many sulphur compounds on Earth.• These include sulphur dioxide, elemental sulphur, sulphuric acid, salts of sulphate or organic sulphur compounds such as dimethylsulphide and even amino acids in our body.• All these chemical compounds do not last forever. They are transported by physical processes like wind or erosion by water, by geological events like volcano eruptions or by biological activity.• They are also transformed by chemical reactions. But nothing is lost. Changes often take place in cycles. Such cycles can be chemical cycles in which a sulphur compound A reacts to form B, B to C, C to D and D to A again.• At the same time there are spatial / geographical cycles. One example is when sulphur compounds move from the ocean to the atmosphere, are transported to the land, come down with the rain and are transported by rivers to the ocean again.•
• Oxidation and reduction• In chemical cycles, sulphur is usually oxidised in the air from organic sulphur or elemental sulphur to sulphur oxides like SO2 and SO3 ending up as sulphate in sulphate salts M(II)SO4, M(I)2SO4 or sulphuric acid H2SO4. The sulphate compounds dissolve very well in water and come down again with the rain, either as salts or as acid rain.• In chemical cycles oxidized compounds must also be reduced again. This process does not take place in the atmosphere but on the ground and in the oceans and is carried out in complicated chemical reactions by bacteria. The most important products are elemental sulphur, hydrogen sulphide (H2S), which smells awful and is very unhealthy, and organic sulphur compounds.
• Sulphur compounds play a big role for our environment and the climate system. On the one hand they contribute to acid rain.• But they are also important for the formation of clouds. Finally, a lot of sulphur is brought into the air by volcanic eruptions.• If it was a strong eruption, the emitted particles can go up to the stratosphere (9 - 12 km of altitude) and cool down half our planet by 1-2°C. http://www.atmosphere.mpg.de/enid/Nr_6_Feb __2__6_acid_rain/C__The_sulphur_cycle_5i9.ht ml