- 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