The document summarizes the nitrogen cycle, which is the biogeochemical process by which nitrogen is converted between different chemical forms and moves through ecosystems. Key points include: (1) nitrogen is essential for life but most organisms cannot use atmospheric nitrogen, (2) microorganisms drive five processes - fixation, uptake, mineralization, nitrification, and denitrification - that convert nitrogen into biologically available forms, and (3) human activity such as fertilizer use has increased nitrogen fixation and altered the global nitrogen cycle, with environmental consequences like water pollution and coastal eutrophication.

1. • 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.

2. • 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.

3. • 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.

4. • 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).

5. The nitrogen cycle. Yellow arrows indicate human sources of nitrogen to the
environment. Red arrows indicate microbial transformations of nitrogen. Blue arrows
indicate physical forces acting on nitrogen. And green arrows indicate natural, non-
microbial processes affecting the form and fate of nitrogen. Ecology & Ecosystem

7. Nitrogen cycle

8. • 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

9. • . 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

10. 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).

11. • 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.

13. • 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.
•

14. • 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.

15. Nitrogen fixation

16. • 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.

17. • 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.

18. • 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.

19. • 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.

20. • 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.

21. • 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.

22. • 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.

23. 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.

24. • 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

25. • 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

26. 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

27. • 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.

28. • http://www.visionlearning.com/library/modul
e_viewer.php?mid=98&mcid=&l= 13/10/10

32. • In 1958, atmospheric carbon dioxide at
Mauna Loa was about 320 parts per million
(ppm), and in 2010 it is about 385ppm.[3]
• Future CO2 emission can be calculated by the
kaya identity

36. • 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.

37. • 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).

38. • 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.

39. • 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

40. Oxygen cycle

41. Almost all living things need oxygen. They use this oxygen during
the process of creating energy in living cells.

44. The flow of sulphur compounds in our environment.
Scheme: Elmar Uherek, adapted and modified from an water cycle
illustration of the Center for Space Research, Univ. of Austin, Texas
Please click the picture for a larger view! (150 K)

45. • 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.
•

46. • 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.

48. • 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
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