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The Biogeochemical Cycles
The biogeochemical cycle is the pathway
an element takes through the Earth system.
This cycle includes the atmosphere, ocean
soil, living organisms, etc…
The cycle includes bio, because chemicals
cycle through living things.
The cycle includes geo because the Earth
itself is part of the cycle. (air water, land)
The 4 key components of the cycle are:
Solid Earth (rocks, soils)
Air (atmosphere)
Water (both fresh and salt)
Living systems ( organisms )
Chemicals cycle within an ecosystem
and are exchanged with the biosphere.
Organisms exchange chemicals with the
nonliving environment.
Chemicals are transferred among different
pools, and remain in those pools for varying
lengths of time, depending on the ART.
Biogeochemical Cycles and Life
Of the 109 known elements, only 24 are
necessary for life.
These 24 are divided into macronutrients,
which are needed in large amounts, and
micronutrients, which are needed in
small amounts.
Macronutrients
The macronutrients mostly consist of 6
elements; Carbon, Hydrogen, Nitrogen
Oxygen, Phosphorous, and Sulfur.
Other elements are also macronutrients,
but play a lesser role. They are; Calcium
Sodium, and Potassium
If a particular element is not present in the
proper amounts, it can become a limiting
factor.
A limiting factor is the one key element,
which if supplied in the proper amount
would allow the organism or population
to grow.
There will always be a limiting factor on
a population.
Micronutrients
Many of the micronutrients are necessary
in low concentrations.
Micronutrients may become toxic if the
concentration is too high.
Ex: Copper is an essential micronutrient
but is toxic at higher levels.
High concentrations of Cu are sometimes
used as a pesticide because of its toxicity
Geologic Cycles
Cycles responsible for formation of Earth
are referred to as geologic cycles.
1)The Tectonic Cycle: creation,
destruction, and re-creation of outer crust
of the Earth.
The outer crust of the Earth is called the
Lithosphere.
Lithospheric plates move, on average,
2 to 15 cm/year.
Environmental Effects:
Physical: determines the physical makeup
of continents, locations of oceans, change
ocean and atmospheric currents,
creation of islands (evolution) .
Ex: Movement of S. America away from
Africa
Chemical: At plate boundaries, new
materials are created from within the
Earth, materials are buried and become oil
and coal deposits.
Ex: deep sea hydrothermal vents that
support varied life, spreading sea floor
created from magma from within the
mantle of Earth.
2) The Hydrologic Cycle: movement of
water from oceans to atmosphere,
atmosphere to land, land to ocean,
The total volume of water on Earth is
3
about 1.3 billion km .
Major storage pools include the oceans,
glaciers and ice caps, ground and surface
fresh water, and the atmosphere.
Important Environmental Impact:
Most of the water is stored in the oceans.
As water evaporates from the oceans, most
is returned directly via precipitation.
458/505 km3/year, or 90.6%
47km3/year is moved onto land, and falls
as precipitation.
119km3/year falls as precipitation on
land surfaces.
Of that total, 60%, or 72km3/year, is lost
to evaporation back to the atmosphere.
Only 40%, or 47 km3/year, is sent into
surface and sub-surface storage and
runoff.
What are the implications for future
population growth, based on the
hydrologic cycle?
Remember the concept of a limiting factor.
How will the availability of water,
(especially clean water) effect the
growth of the human population?
Are there any answers?
3) The Rock Cycle: depends on both the
Tectonic cycle ( for energy) and the
Hydrologic cycle (for water).
The rock cycle produces both rock and soil,
and recycles minerals.
Three types of rock are produced: igneous
sedimentary, and metamorphic.
Igneous rocks are formed via molten
material from Earth’s mantle.
Ex: Granite, obsidian, lava
Weathering (erosion) of these rocks
produces sediments, including sand, silt,
clay, gravel, and pebbles.
Weathering also produces dissolved
minerals and elements.
Weathered materials accumulate, and
become sedimentary rock when the
conditions are correct.
Heat, pressure, and chemistry all lead to
sediments becoming rock.
Ex: sandstone, shale, chalk
Some sedimentary rock is biologically
produced, by coral reefs.
Ex: Limestone
Metamorphic rock is formed from
both igneous and sedimentary rock.
Immense pressure and heat form
metamorphic rock.
The pressure creates rock that is unlike
the parent rock, hence the name.
Ex: chalk (S) into marble (M), Shale (S)
into slate (M), Granite (I) into gneiss (M)
Different Types of Cycles
Materials and elements cycle into and
out of an Ecosystem at varying rates.
One of the largest factors in determining
the rate of cycling is whether or not
the element has a phase that is taken
into the atmosphere.
Metals as a rule are not moved into the
atmosphere, therefore they cycle slowly
For example, Calcium does not form a gas,
so does not have a large component in the
atmosphere.
Sulfur ( a non metal) does form a gas, so
does have a large pool in the atmosphere
(hydrogen sulfide, sulfur dioxide)
Elements that do not pool in the atmosphere
tend to be limiting factors, especially in
land ecosystems.
Chemical Cycling and Natural Balance
For an ecosystem to sustain life, energy
must be continuously added, and storage of
essential elements must not decline.
Elements are not always in a steady state,
but must be replaced as they are used up
or lost.
Rate of gain must at least equal rate of
loss. G > L, or G = L
Unfortunately, mankind has disrupted the
natural flow of many elements, or raised
the rate of loss until it is much greater
than the rate of gain. (G < L)
-Removal of tropical rainforest trees,
which are the main pool of elements in
the tropical forest. (minerals, N,P,K)
-Increased erosion of all sorts leads to
loss of essential nutrients from soil, much
faster than they are replaced.
Some Major Chemical Cycles
1)Carbon Cycle: an important element,
the basic building block of all life on Earth.
Although basic for life, it is one of the
least abundant elements by weight.
It constitutes 0.032% of the weight of the
Earth’s crust
Carbon has a gas phase (CO2, CH4), so
is present in the atmosphere.
Carbon enters the atmosphere via the
respiration of organisms, the burning of
organic material, and via diffusion from
the oceans.
It is removed from the atmosphere via
photosynthesis of green plants and
photosynthetic bacteria.
The cycling of carbon is fairly rapid, with
15% of the total carbon in the atmosphere
being taken up and released annually.
Inorganic carbon occurs as several forms:
-as carbonate and bicarbonate (limestone,
shells of marine animals)
Carbon is taken into the ocean via diffusion
of CO2 from the atmosphere. It is then
taken up by organisms and converted into
carbonate and bicarbonate.
Aquatic and marine plants also use CO2
directly in photosynthesis.
Carbon is transferred from land to ocean by
erosion and movement in rivers.
Carbon compounds moved this way are
inorganic (CO2) and many types of organic
materials (living matter, fine particles of
organic matter)
Winds also blow organic materials into the
ocean.
Human Effect on Carbon Cycle
Increased burning of fossil fuels has added
about 3 billion metric tons/year to the
atmosphere. (Mostly CO2)
Deforestation leads to conversion of organic
carbon into inorganic CO2. (burning and
decomposition).
Carbon-Silicate Cycle
Although carbon does cycle fairly rapidly
from atmosphere to ocean and organic life,
it does get tied up for geologically long
periods in the carbon-silicate cycle.
1)Carbon dioxide in the atmosphere
dissolves in water, forming carbonic acid.
H2CO3. This is why all rain is slightly
acidic.
2) Chemical weathering of silicate rocks
in the Earth’s crust releases bicarbonate
ions (HCO3-) as well as calcium ions
(Ca++)
3) Eventually these ions are transported to
the ocean, where marine organisms use
the bicarbonate and calcium to construct
new shells.
4) When these organisms die, they fall
to the bottom, and are incorporated into
the sediment.
5) These sediments eventually become
rock, and are transported via plate
tectonics, and the cycle starts over.
The carbon thus trapped is held for a very
long time as rock.
The Nitrogen Cycle
Nitrogen is a biologically important element
but cannot be used directly by organisms.
It must first be converted to nitrate (NO3-)
or the ammonium ion (NH4+).
This is accomplished either by lightning,
which oxidizes atmospheric N into nitrate,
or by bacteria, which fix N into ammonium.
Nitrogen Facts
About 79% of the atmosphere consists
of nitrogen gas (N2)
Nitrogen is virtually impossible to
metabolize by itself, because the gas
is held by a triple bond.
Because of this, nitrogen is often a
limiting factor in nature.
The 5 Steps of the Nitrogen Cycle
1)Nitrogen Fixation
2) Nitrification
3)Assimilation
4)Ammonification
5)Denitrification
1)Nitrogen Fixation
-converting molecular nitrogen (N2) to
Ammonia (NH3)
Most is converted via biological
activity.
-fixation by beneficial bacteria on
plant roots, and in soil.
Rhizobium (bacteria) is associated with
roots of many plants, most notably
soy beans.
-symbiotic relationship
Farmers often alternate soy beans with
other crops that have high nitrogen needs,
because the soybeans (and their associated
bacteria) add nitrogen to the soil.
2) Nitrification: when water is added
to ammonia (NH3), it is converted to
nitrate ions (NO3-).
3) Assimilation: Plants absorb NH4
NH3 and NO3 to form amino acids
and proteins.
+,
4) Ammonification: Plants convert
NO3- into ammonia (NH3).
Ammonia is the required form of N
for plants to use.
Ammonium Nitrate fertilizer
(NH4NO3) is often used because of its
high Ammonia content.
5) Denitrification: some bacteria use
Nitrogen compounds (NH3, NO3)
as a part of their metabolism, and
give off N2 as waste.
These bacteria (Pseudomonas,
Enterobacter) are particularly useful
in sewage treatment to lower Nitrogen
output in wastewater.