1. The nitrogen cycle is the set biogeochemical processes by whichnitrogen undergoes chemical reactions, changes form, and movesthrough difference reservoirs on earth, including living organisms. Nitrogen is required for all organisms too live and grow because it isthe essential component of DNA, RNA, and protein. However, mostorganisms cannot use atmospheric nitrogen, the largest reservoir. The five processes in the nitrogen cycle -- fixation, uptake,mineralization, nitrification and denitrification -- are all driven bymicroorganisms. Humans influence the global nitrogen cycle primarily through theuse of nitrogen-based fertilizers.

2. Nitrogen (N) is an essential component of DNA,RNA, and proteins, the building blocks of life. Allorganisms require nitrogen to live and grow.Although the majority of the air we breathe is N2,most of the nitrogen in the atmosphere isunavailable for use by organisms. This is because the strong triple bond betweenthe N atoms in N2 molecules makes it relativelyinert. 3. In fact, in order for plants and animals to beable to use nitrogen, N2 gas must first beconverted to more a chemically available formsuch as ammonium (NH4+), nitrate (NO3-), ororganic nitrogen (e.g. urea - (NH2)2CO). The inert nature of N2 means that biologicallyavailable nitrogen is often in short supply innatural ecosystems, limiting plant growth andbiomass accumulation. 4. Nitrogen is an incredibly versatile elementexisting in both inorganic and organic forms aswell as many different oxidation states. Themovement of nitrogen between the atmosphere,biosphere and geosphere in different forms isdescribed by the nitrogen cycle (Figure 1), one ofthe major biogeochemicalcycles. Similar to thecarbon cycle, the nitrogen cycle consists ofvarious storage pools of nitrogen and processesby which the pools exchange nitrogen (arrows)(see our The Carbon Cycle module for moreinformation). 5. 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 6. Nitrogen cycle 7. Five main processes cycle nitrogen throughthe biosphere, atmosphere, and geosphere:nitrogen fixation, nitrogen uptake (organismalgrowth), nitrogen mineralization (decay),nitrification, and denitrification.Microorganisms, particularly bacteria, playmajor roles in all of the principal nitrogentransformations 8. . As microbial mediated processes, thesenitrogen transformations tend to occur fasterthan geological processes like plate motion, avery slow, purely physical process that is a partof the carbon cycle. Instead, rates are affectedby environmental factors that influencemicrobial activity, such as temperature,moisture, and resource availability 9. Nitrogen fixation N2 NH4+ Nitrogen fixation is the process whereinN2 is converted to ammonium, essential becauseit is the only way that organisms can attainnitrogen directly from the atmosphere. Certainbacteria, for example those among the genusRhizobium, are the only organisms that fixnitrogen through metabolic processes. Nitrogenfixing bacteria often form symbiotic relationshipswith host plants. This symbiosis is well-known tooccur in the legume family of plants (e.g. beans,peas, and clover). 10. In this relationship, nitrogen fixing bacteriainhabit legume root nodules and receivecarbohydrates and a favorable environmentfrom their host plant in exchange for some ofthe nitrogen they fix. There are also nitrogenfixing bacteria that exist without plant hosts,known as free-living nitrogen fixers. In aquaticenvironments, blue-green algae (really abacteria called cyanobacteria) is an importantfree-living nitrogen fixer. 11. In addition to nitrogen fixing bacteria, high-energy natural events such as lightning, forestfires, and even hot lava flows can cause thefixation of smaller, but significant amounts ofnitrogen (Figure 3). The high energy of thesenatural phenomena can break the triple bondsof N2 molecules, thereby making individual Natoms available for chemical transformation. 12. Within the last century, humans have becomeas important a source of fixed nitrogen as allnatural sources combined. Burning fossil fuels,using synthetic nitrogen fertilizers, andcultivation of legumes all fix nitrogen. Throughthese activities, humans have more thandoubled the amount of fixed nitrogen that ispumped into the biosphere every year , theconsequences of which are discussed below. 13. Nitrogen fixation 14. Nitrogen uptake NH4+ Organic N The ammonia produced bynitrogen fixing bacteria is usually quicklyincorporated into protein and other organicnitrogen compounds, either by a host plant, thebacteria itself, or another soil organism. Whenorganisms nearer the top of the food chain (likeus!) eat, we are using nitrogen that has beenfixed initially by nitrogen fixing bacteria. 15. Nitrogen mineralization Organic N NH4+ After nitrogen is incorporatedinto organic matter, it is often converted backinto inorganic nitrogen by a process callednitrogen mineralization, otherwise known asdecay. When organisms die, decomposers(such as bacteria and fungi) consume theorganic matter and lead to the process ofdecomposition. 16. During this process, a significant amount ofthe nitrogen contained within the deadorganism is converted to ammonium. Once inthe form of ammonium, nitrogen is availablefor use by plants or for further transformationinto nitrate (NO3-) through the process callednitrification. 17. Nitrification NH4+ NO3- Some of the ammonium produced bydecomposition is converted to nitrate via aprocess called nitrification. The bacteria thatcarry out this reaction gain energy from it.Nitrification requires the presence of oxygen, sonitrification can happen only in oxygen-richenvironments like circulating or flowing watersand the very surface layers of soils andsediments. The process of nitrification has someimportant consequences. 18. Ammonium ions are positively charged andtherefore stick (are sorbed) to negatively chargedclay particles and soil organic matter. The positivecharge prevents ammonium nitrogen from beingwashed out of the soil (or leached) by rainfall. Incontrast, the negatively charged nitrate ion is notheld by soil particles and so can be washed downthe soil profile, leading to decreased soil fertilityand nitrate enrichment of downstream surfaceand groundwaters. 19. Denitrification NO3- N2+ N2O Through denitrification, oxidizedforms of nitrogen such as nitrate and nitrite (NO2-) are converted to dinitrogen (N2) and, to a lesserextent, nitrous oxide gas. Denitrification is ananaerobic process that is carried out bydenitrifying bacteria, which convert nitrate todinitrogen in the following sequence:NO3- NO2- NO N2O N2. 20. Nitric oxide and nitrous oxide are both environmentallyimportant gases. Nitric oxide (NO) contributes to smog,and nitrous oxide (N2O) is an important greenhousegas, thereby contributing to global climate change. Once converted to dinitrogen, nitrogen is unlikely to bereconverted to a biologically available form because itis a gas and is rapidly lost to the atmosphere.Denitrification is the only nitrogen transformation thatremoves nitrogen from ecosystems (essentiallyirreversibly), and it roughly balances the amount ofnitrogen fixed by the nitrogen fixers described above. 21. Human alteration of the N cycle and its environmentalconsequences Early in the 20th century, a German scientist named Fritz Haberfigured out how to short circuit the nitrogen cycle by fixing nitrogenchemically at high temperatures and pressures, creating fertilizersthat could be added directly to soil. This technology has spreadrapidly over the past century, and, along with the advent of newcrop varieties, the use of synthetic nitrogen fertilizers has led to anenormous boom in agricultural productivity. This agriculturalproductivity has helped us to feed a rapidly growing worldpopulation, but the increase in nitrogen fixation has had somenegative consequences as well. While the consequences areperhaps not as obvious as an increase in global temperatures or ahole in the ozone layer, they are just as serious and potentiallyharmful for humans and other organisms. 22. Not all of the nitrogen fertilizer applied to agricultural fields stays tonourish crops. Some is washed off of agricultural fields by rain orirrigation water, where it leaches into surface or ground water andcan accumulate. In groundwater that is used as a drinking watersource, excess nitrogen can lead to cancer in humans andrespiratory distress in infants. The U.S. Environmental ProtectionAgency has established a standard for nitrogen in drinking water of10 mg per liter nitrate-N. Unfortunately, many systems (particularlyin agricultural areas) already exceed this level. By comparison,nitrate levels in waters that have not been altered by humanactivity are rarely greater than 1 mg/L. In surface waters, addednitrogen can lead to nutrient over-enrichment, particularly incoastal waters receiving the inflow from polluted rivers. Thisnutrient over-enrichment, also called eutrophication, has beenblamed for in 23. 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 insurface waters and soils, can also enter theatmosphere as the smog-component nitric oxide(NO) and the greenhouse gas nitrous oxide (N2O).Eventually, this atmospheric nitrogen can beblown into nitrogen-sensitive terrestrialenvironments, causing long-term changes 24. For example, nitrogen oxides comprise asignificant portion of the acidity in acid rainwhich has been blamed for forest death anddecline in parts of Europe and the NortheastUnited States.Increases in atmospheric nitrogen depositionhave also been blamed for more subtle shiftsin dominant species and ecosystem functionin some forest and grassland ecosystems 25. Currently, much research is devoted tounderstanding the effects of nitrogen enrichmentin the air, groundwater, and surface water.Scientists are also exploring alternativeagricultural practices that will sustain highproductivity while decreasing the negativeimpacts caused by fertilizer use. These studiesnot only help us quantify how humans havealtered the natural world, but increase ourunderstanding of the processes involved in thenitrogen cycle as a whole. 26. http://www.visionlearning.com/library/module_viewer.php?mid=98&mcid=&l= 13/10/10 27. In 1958, atmospheric carbon dioxide atMauna Loa was about 320 parts per million(ppm), and in 2010 it is about 385ppm.[3] Future CO2 emission can be calculated by thekaya identity 28. The environmental sulphur cycle involves many physical, chemicaland 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 maymove through the cycle as a result of the oxidation of sulphides tosulphate 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 formcalcium sulphate (gypsum). The reaction of hydrogen sulphide withdissolved metal ions may precipitate metallic sulphides which arechemically indistinguishable from naturally occurring sulphideminerals. 29. At some mines, sulphur is added to the cycle assulphur dioxide in processes such as the Inco/SO2process for cyanide destruction in the treatmentof tailings. This added sulphur is oxidized tosulphate ion (Ingles & Scott, 1987), most of whichremains free, but some of which combines withlime, CaO, in the tailings to form gypsum. For information on the sulphur cycle with respectto water quality monitoring see Canadian Councilof Environment Ministers (1987). 30. The Role of Micro-organisms in the Sulphur Cycle Micro-organisms (most frequently bacteria) are oftenintegrally 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 anaerobicconditions, the adsorption of metals by bacteria or algae,and the formation and destruction of organometalliccomplexes are all examples of indirect micro-organismparticipation. Where minerals are available as soluble traceelements, serve as specific oxidizing substrates, or areelectron donors/acceptors in oxidation-reduction reactions,they may be directly involved in cell metabolic activity. 31. 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 carbondioxide or organic matter) organisms. Energy derived from the oxidation reaction is utilized in cellsynthesis. 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 organicsubstrate. Electron donation by minerals (oxidation) for bacterial or algal photosynthesis (reaction is fuelledby photon energy). Natural Oxidation in the Sulphur Cycle Oxidation of sulphur or sulphides for energy production is restricted to the bacterial genusThiobacillus, the genus Thiomicrospira, and the genus Sulfolobus. These bacteria all producesulphuric acid (i.e. hydrogen ions, H+, and sulphate ions, SO4-- ) as a metabolic product. Extensivereviews 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) frompyritic and pyrrhotitic rocks under suitable conditions. Evangelou & Zhang (1995) report thatsulphide 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, 2and 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 32. Oxygen cycle 33. Almost all living things need oxygen. They use this oxygen during the process of creating energy in living cells. 34. 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) 35. We find many sulphur compounds on Earth. These include sulphur dioxide, elemental sulphur, sulphuric acid, salts ofsulphate or organic sulphur compounds such as dimethylsulphide andeven amino acids in our body. All these chemical compounds do not last forever. They are transported byphysical processes like wind or erosion by water, by geological events likevolcano 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 inwhich a sulphur compound A reacts to form B, B to C, C to D and D to Aagain. At the same time there are spatial / geographical cycles. One example iswhen sulphur compounds move from the ocean to the atmosphere, aretransported to the land, come down with the rain and are transported byrivers to the ocean again. 36. Oxidation and reduction In chemical cycles, sulphur is usually oxidised in the airfrom organic sulphur or elemental sulphur to sulphuroxides like SO2 and SO3 ending up as sulphate insulphate salts M(II)SO4, M(I)2SO4 or sulphuric acidH2SO4. The sulphate compounds dissolve very well inwater and come down again with the rain, either assalts or as acid rain. In chemical cycles oxidized compounds must also bereduced again. This process does not take place in theatmosphere but on the ground and in the oceans and iscarried out in complicated chemical reactions bybacteria. The most important products are elementalsulphur, hydrogen sulphide (H2S), which smells awfuland is very unhealthy, and organic sulphur compounds. 37. Sulphur compounds play a big role for ourenvironment and the climate system. On the onehand they contribute to acid rain. But they are also important for the formation ofclouds. Finally, a lot of sulphur is brought into theair by volcanic eruptions. If it was a strong eruption, the emitted particlescan go up to the stratosphere (9 - 12 km ofaltitude) and cool down half our planet by 1-2C.http://www.atmosphere.mpg.de/enid/Nr_6_Feb__2__6_acid_rain/C__The_sulphur_cycle_5i9.html