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Environmental Sciences Course
Biogeochemical Cycles
Dr.-Eng. Hasan Hamouda
Eng. Osama Othman
Outline
Definitions
Carbon Cycle
Nitrogen Cycle
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Definitions
Biogeochemical Cycles
The Earth is a closed system for matter, except for small
amounts of cosmic debris that enter the Earth's atmosphere.
This means that all the elements needed for the structure and
chemical processes of life come from the elements that were
present in the Earth's crust when it was formed billions of
years ago. This matter, the building blocks of life,
continually cycle through Earth's systems, the atmosphere,
hydrosphere, biosphere, and lithosphere, on time scales that
range from a few days to millions of years.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Biogeochemical Cycles
These cycles are called biogeochemical cycles, because they
include a variety of biological, geological, and chemical
processes. Many elements cycle through ecosystems,
organisms, air, water, and soil. Many of these are trace
elements. Other elements, including carbon, nitrogen, oxygen,
hydrogen, sulfur, and phosphorous are critical components of
all biological life. Together, oxygen and carbon account for
80 percent of the weight of human beings. Because these
elements are key components of life, they must be available
for biological processes.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Biogeochemical Cycles
Carbon, however, is relatively rare in the Earth's crust, and
nitrogen, though abundant in the atmosphere, is in a form
that is not useable by living organisms. The biogeochemical
cycles transport and store these important elements so that
they can be used by living organisms. Each cycle takes
many different pathways and has various reservoirs, or
storage places, where elements may reside for short or long
periods of time. Each of the chemical, biological, and
geological processes varies in their rates of cycling. Some
molecules may cycle very quickly depending on the
pathway.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Biogeochemical Cycles
Carbon atoms in deep ocean sediments may take hundreds to
millions of years to cycle completely through the system. An
average water molecule resides in the atmosphere for about
ten days, although it may be transported many miles before
it falls back to the Earth as rain. How fast substances cycle
depends on its chemical reactivity and whether or not it can
be found in a gaseous state. A gaseous phase allows
molecules to be transported quickly. Phosphorous has no
gaseous phase and is relatively unreactive, so it moves very
slowly through its cycle.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Biogeochemical Cycles
Phosphorous is stored in large amounts in sediment in the oceans
or in the Earth's crust and is recycled back to the surface only
over very long periods of time through upwelling of ocean waters
or weathering of rocks. Biogeochemical cycles are subject to
disturbance by human activities. Humans accelerate natural
biogeochemical cycles when elements are extracted from their
reservoirs, or sources, and deposited back into the environment
(sinks). For example, humans have significantly altered the
carbon cycle by extracting and combusting billions of tons of
hydrocarbons in fossil that were buried deep in the Earth's crust,
in addition to clearing vegetation that stores carbon.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Biogeochemical Cycles
Global release of carbon through human activities has
increased from 1 billion tons per year in 1940 to 6.5 billion
tons per year in 2000. About half of this extra carbon is
taken up by plants and the oceans, while the other half
remains in the atmosphere.
In addition to carbon cycle, humans have altered the
nitrogen and phosphorous cycles by adding these elements
to croplands as fertilizers, which has contributed to overfertilization of aquatic ecosystems when excess amounts are
carried by runoff into local waterways.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Carbon Cycle
Carbon is one of the fundamental building blocks of life;
we are carbon-based life forms. Carbon cycles through
the oceans, atmosphere, the lithosphere, and the
biosphere over both short and long term time scales. The
geological Carbon cycle takes place over hundreds of
millions of years and involves the cycling of carbon
through the layers of the Earth. The biological/physical
carbon cycle occurs over days, weeks, months and years
and involves the absorption, conversion, and respiration
of carbon by living organisms.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Carbon Cycle
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Nitrogen Cycle
Nitrogen is both the most abundant element in the
atmosphere and, as a building block of proteins and
nucleic acids such as DNA, a crucially important
component of all biological life. The nitrogen cycle is
a complex biogeochemical cycle in which nitrogen is
converted from its inert atmospheric molecular form
(N2) into a form that is useful in biological processes.
The nitrogen cycle contains several different stages.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Nitrogen Cycle
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Nitrogen Cycle
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3316 Dr. Hasan Hamouda Eng. Osama Othman
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Nitrogen fixation
Atmospheric nitrogen occurs primarily in an inert form (N2)
that few organisms can use. (It takes a great deal of energy
to split the N2 molecule.) Therefore, it must be converted to
an organic form, or fixed, in a process called nitrogen
fixation. A small amount of nitrogen is fixed through high
energy fixation, primarily lighting strikes that convert
atmospheric nitrogen into ammonia (NH4+) and nitrates
(NO3-). But most atmospheric nitrogen is fixed through
biological processes. First, the nitrogen is deposited from
the atmosphere into soils and surface waters.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Nitrogen fixation
This is done by certain microorganisms, which fall under
three broad categories: bacteria living in symbiotic
relationships with certain plants, free anaerobic bacteria, and
algae. Crops such as alfalfa and beans are often planted in
order to remedy nitrogen-depletion in soils. Nitrogen fixing
bacteria employ an enzyme, known as nitrogenase, to do the
energy-intensive work of splitting the atmospheric nitrogen
molecule into individual atoms for combination into other
compounds. Nitrogen is also fixed by man-made processes,
primarily industrial process that create ammonia and
nitrogen-rich fertilizers
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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Nitrification & Assimilation
Nitrification: While ammonia can be used by some plants,
most nitrogen taken up by plants is converted by bacteria
from ammonia, which is highly toxic to many organism,
first into nitrite (NO2-), and then into nitrate (NO3-). This
process is called nitrification, and these bacteria are known
as nitrifying bacteria.
Assimilation: compounds such as nitrate, nitrite,
ammonia, and ammonium are taken up from soils by plants.
The nitrogen in these compounds is used in the formation
of plant and animal proteins.
University Of Palestine UNI
3316 Dr. Hasan Hamouda Eng. Osama Othman
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