NITROGENCYCLEAug23,2015

Nitrogenisoneoftheprimarynutrientscriticalforthesurvivalofalllivingorganisms.Itisanecessarycomponentofmanybiomolecules,including proteins, DNA, and chlorophyll. Although nitrogen is veryabundant in the atmosphere as dinitrogen gas (N2), it is largelyinaccessible in this form tomostorganisms, making nitrogen ascarce resource and often limiting primary productivity in manyecosystems. Only when nitrogen is converted from nitrogen gasinto ammonia (NH3) does it become available to primaryproducers,suchasplants.

Inaddition toN2andNH3,nitrogenexists inmanydifferent forms,including both inorganic (e.g., ammonia, nitrate) and organic (e.g.,amino and nucleic acids) forms. Thus, nitrogen undergoes manydifferent transformations in theecosystem,changing fromone formto another as organisms use it for growth and, in some cases,energy.Themajortransformationsofnitrogenarenitrogenfixation,nitrification, denitrification, anammox, and ammonification Thetransformation of nitrogen into its many oxidation states is key toproductivity in the biosphere and is highly dependent on theactivities of a diverse assemblage of microorganisms, such asbacteria,archaea,andfungi.

Themovement of nitrogen between the atmosphere, biosphere,andgeosphereindifferentformsiscalledthenitrogencycle,oneofthemajorbiogeochemicalcycles.Similartothecarboncycle,thenitrogen cycle consists of various reservoirs of nitrogen andprocessesbywhichthosereservoirsexchangenitrogen

Source:http://tygae.weebly.com/

ProcessesinthenitrogencycleFive main processes cycle nitrogen through the biosphere,atmosphere, and geosphere nitrogen fixation, nitrogen uptakethrough organismal growth, nitrogen mineralization throughdecay, nitrification, and denitrification. Microorganisms,particularlybacteria,playmajorrolesinalloftheprincipalnitrogentransformations.Because theseprocessesaremicrobiallymediated,or controlled by microorganisms, these nitrogen transformationstend to occur faster than geological processes like plate motion, averyslow,purelyphysicalprocessthatisapartofthecarboncycle.Instead, rates are affected by environmental factors that influencemicrobial activity, such as temperature, moisture, and resourceavailability.

MajorTransformationinNitrogenCycle

Source:Nature.com

1. NitrogenFixation

The nitrogenmolecule (N2) is quite inert. To break it apart so thatits atoms can combine with other atoms requires the input ofsubstantialamountsofenergy.Threeprocessesare responsible formostof thenitrogen fixation inthebiosphere:

atmosphericfixationbylightning

biological fixation by certain microbes alone or in asymbioticrelationshipwithsomeplantsandanimals

industrialfixation

AmosphericFixation

The enormous energy of lightning breaks nitrogen molecules andenables their atoms to combine with oxygen in the air formingnitrogen oxides. These dissolve in rain, forming nitrates, that arecarriedtotheearth.Atmospheric nitrogen fixation probably contributessome58%ofthetotalnitrogenfixed.

IndustrialFixation

Under great pressure, at a temperature of 600C, andwith the useof a catalyst, atmospheric nitrogen and hydrogen (usually derivedfrom natural gas or petroleum) can be combined to form ammonia(NH3).Ammoniacanbeuseddirectlyasfertilizer,butmostof its isfurtherprocessedtoureaandammoniumnitrate(NH4NO3).

BiologicalFixation

The ability to fix nitrogen is found only in certain bacteria andarchaea. Some nitrogenfixing organisms are freeliving whileothers are symbiotic nitrogenfixers, which require a closeassociation with a host to carry out the process. Most of thesymbiotic associations are very specific and have complexmechanisms that help tomaintain the symbiosis. For example, rootexudatesfromlegumeplants(e.g.,peas,clover,soybeans)serveasa signal to certain species ofRhizobium, which are nitrogenfixingbacteria. This signal attracts the bacteria to the roots, and a verycomplex series of events then occurs to initiate uptake of thebacteria into the rootand trigger theprocessofnitrogen fixation innodulesthatformontheroots

2. Nitrification

Nitrification is the process that converts ammonia to nitrite andthentonitrateand isanother importantstep in theglobalnitrogencycle. Most nitrification occurs aerobically and is carried outexclusively by prokaryotes. There are two distinct steps ofnitrificationthatarecarriedoutbydistincttypesofmicroorganisms.Thefirststepistheoxidationofammoniatonitrite,whichiscarriedout by microbes known as ammoniaoxidizers. Aerobic ammoniaoxidizers convert ammonia to nitrite via the intermediatehydroxylamine, a process that requires two different enzymes,ammoniamonooxygenaseandhydroxylamineoxidoreductase(Figure4).Theprocessgeneratesaverysmallamountofenergyrelativetomany other types of metabolism as a result, nitrosofiers arenotoriously very slow growers. Additionally, aerobic ammonia

oxidizers are also autotrophs, fixing carbon dioxide to produceorganic carbon, much like photosynthetic organisms, but usingammoniaastheenergysourceinsteadoflight.

3. Denitrification

The three processes above remove nitrogen from the atmosphereandpassitthroughecosystems.

Denitrificationreducesnitratesandnitritestonitrogengas,thusreplenishing the atmosphere. In the process several intermediatesareformed:

nitricoxide(NO)

nitrous oxide (N2O)(a greenhouse gas 300 times as potent asCO2)

nitrousacid(HONO)

In this agents are Bacteria. They live deep in soil and inaquatic sediments where conditions are anaerobic. They usenitrates as an alternative to oxygen for the final electronacceptorintheirrespiration.

Ecological Implications of Human Alterations to the NitrogenCycle

Many human activities have a significant impact on the nitrogencycle. Burning fossil fuels, application of nitrogenbased fertilizers,and other activities can dramatically increase the amount ofbiologically available nitrogen in an ecosystem. And becausenitrogen availability often limits the primary productivity of manyecosystems, largechanges intheavailabilityofnitrogencanleadtosevere alterations of the nitrogen cycle in both aquatic andterrestrial ecosystems. Industrial nitrogen fixation has increasedexponentially since the 1940s, and human activity has doubled theamountofglobalnitrogenfixation

In terrestrial ecosystems, the addition of nitrogen can lead tonutrientimbalanceintrees,changesinforesthealth,anddeclinesinbiodiversity. With increased nitrogen availability there is often achange in carbon storage, thus impactingmore processes than justthe nitrogen cycle. In agricultural systems, fertilizers are usedextensively to increase plant production, but unused nitrogen,usually in the form of nitrate, can leach out of the soil, enterstreams and rivers, and ultimately make its way into our drinkingwater. The process of making synthetic fertilizers for use inagriculture by causing N2 to react with H2, known as the HaberBosch process, has increased significantly over the past several

decades. In fact, today,nearly80%of thenitrogen found inhumantissuesoriginatedfromtheHaberBoschprocess

Much of the nitrogen applied to agricultural and urban areasultimately enters rivers and near shore coastal systems. In nearshoremarinesystems,increasesinnitrogencanoftenleadtoanoxia(no oxygen) or hypoxia (low oxygen), altered biodiversity, changesinfoodwebstructure,andgeneralhabitatdegradation.Onecommonconsequence of increased nitrogen is an increase in harmful algalblooms Toxic blooms of certain types of dinoflagellates have beenassociatedwithhighfishandshellfishmortalityinsomeareas.Evenwithout such economically catastrophic effects, the addition ofnitrogencanleadtochangesinbiodiversityandspeciescompositionthatmay lead tochanges inoverallecosystemfunction.Somehaveevensuggestedthatalterationstothenitrogencyclemayleadtoanincreased risk of parasitic and infectious diseases among humansandwildlife(Johnsonetal.2010).Additionally,increasesinnitrogenin aquatic systems can lead to increased acidification in freshwaterecosystems.