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Cycles in Nature
• Hydrologic Cycle
• Carbon Cycle
• Nitrogen Cycle
• Phosphorus Cycle
Let’s Focus on the Hydrologic Cycle
First!
Life on Earth would not exist
without the hydrologic cycle.
As you can see, from space our planet
looks like a big blue marble, and is
therefore nicknamed the water planet
•Earth is covered by one of our
most precious resources.
•Nearly 97% of the Earth’s water is
tied up in its oceans, however!
How do we obtain fresh water
resources then? Where does drinkable
water come from?
To understand, we need to turn to
the hydrologic cycle.
This is the process, powered by the
sun’s energy, which moves water
between the oceans, the sky, and
the land.
Let’s first look at the oceans...
The sun causes evaporation of
water on the surface of the
oceans.
Approximately
80% of all
evaporation on
Earth occurs
over oceans
Evaporation is the process whereby
liquid water changes to a gas
Once the gaseous water (water vapor)
is in our atmosphere, it condenses into
tiny droplets which cling to dust
particles.
The tiny dust
and dirt
particles known
as “aerosols” are
called
“condensation
nuclei”
Condensation is the process whereby
water is changed from a gaseous state
into a liquid state.
These tiny water droplets form clouds.
Once heavy enough, these droplets fall
to Earth as precipitation.
• rain
• sleet
• snow
• hail
• freezing rain
Precipitation is the primary mechanism for
transporting water from the atmosphere to
the surface of the Earth
Some of the Earth’s precipitation is
absorbed in the process of infiltration.
Water that has
penetrated
Earth’s surface
becomes
groundwater
When the ground is particularly
impermeable, the water may runoff
causing flooding
Some of the Earth’s water is used and
recycled by plants in a process known as
transpiration.
Transpiration is the
evaporation of
water into the
atmosphere from
the leaves and
stems of plants
through openings
called “stomata”.
Evapotranspiration accounts for
approximately 10% of all evaporating
water.
• More of Earth’s water flows into
streams, and rivers, and eventually back
to the oceans, where the entire process
starts over again.
• The amount of precipitation and
evaporation is balanced throughout
the world.
• Sometimes, one part of the world is
experiencing a drought, while another
is experiencing flooding rains.
• On a global scale, over the years,
however, everything balances out.
It can take as little as a few days, to
thousands of years for a single water
molecule to complete the cycle.
It can be trapped in
glacial ice sheets for
a very long time.
To recap, the five main processes
included in the hydrologic cycle
are:
• Condensation
• Precipitation
• infiltration
• runoff
• Evapotranspiration
Combines the
processes of
transpiration and
evaporation
Considering so little of the water on Earth is
drinkable to people, it is amazing the supply
has survived as long as it has.
The newest problem faced by the
planet is not simply the consumption of
freshwater, but the pollution of
freshwater.
Estimates show that 100 million billion
gallons a year are cycled through the
hydrologic cycle.
Perhaps we should treat it as the
precious commodity that it is, and stop
taking it for granted.
Carbon and Nitrogen Cycles
•The nitrogen cycle represents one of
the most important nutrient cycles
found in both terrestrial, and aquatic
ecosystems
• nitrogen is critically important in forming
amino acids which in turn form the proteins
of your body. Proteins make up skin and
muscle, among other important structural
portions of your body, and all enzymes are
proteins. Since enzymes carry out almost all
of the chemical reactions in your body, it's
easy to see how important nitrogen is.
Just like the hydrologic cycle had processes
at each stage, so does the nitrogen cycle.
This cycle is carried out through abiotic and
biotic means.
• Fixation
• Nitrification
• Assimilation
• Ammonification
• Denitrification
The chief reservoir of nitrogen is the atmosphere,
which is about 78% nitrogen
= Atmospehric nitrogen
N2
•Nitrogen gas can be taken from the atmosphere
(fixed…and made useable to organisms) in two
basic ways:
1. lightning provides enough energy to
"burn" the nitrogen and fix it in the form
of nitrate, which is a nitrogen with three
oxygens attached NO3
2. And nitrogen fixing bacteria called
Rhizobium, found in soil and often
associated with the root systems of legumes.
• All nitrogen fixing bacteria fix nitrogen
either in the form of nitrate (NO3) Like the lightning did
•or in the form of ammonia (nitrogen
with 3 hydrogens attached) (NH3)
• Ammonia is rather toxic, but, fortunately there
are nitrite bacteria in the soil and in the water
which take up ammonia and convert it to nitrite,
which is nitrogen bound to two oxygens. (NO2)
• Nitrite is also somewhat toxic, but another type
of bacteria, nitrate bacteria, take nitrite and
convert it to nitrate, which can be taken up by
plants to continue the cycle. (NO3)
This is the process whereby plants absorb
ammonium (NH3) , ammonia ions (NH4), and
nitrate ions (NO3-) through their roots.
• Most autotrophs can take up
nitrate and convert it to amino
acids, which are, of course, the
building blocks of proteins..
•Heterotrophs acquire all of their
amino acids when they eat plants (or
other animals that consume plants)
•When plants or animals die (or release waste) the
nitrogen is returned to the soil, usually in the form
of ammonia.
 This ammonia (NH3) or ammonium (NH4) is a product
of decomposing bacteria, and in this state, may be
reused by plants.
•If this is indeed a cycle…how is N2 returned to
the atmosphere?
 It turns out that there are denitrifying bacteria
(usually anaerobic) which take the nitrites and nitrates
and combine the nitrogen back into nitrogen gas, and
nitrous oxide. (N2), (N2O)
The carbon
cycle models
the movement
and storage of
carbon in the
biosphere,
lithosphere,
hydrosphere
and
atmosphere.
•Carbon is stored in the biosphere as living
organisms.
•in the atmosphere as carbon dioxide gas
•in the lithosphere as soil organic matter
as fossil fuel deposits,
and as sedimentary rock deposits
•and in the oceans as dissolved carbon
dioxide gas and as calcium carbonate shells
in marine organisms
Some of the processes that move carbon in
the cycle include:
> photosynthesis
> respiration
> biomass combustion
> fossil fuel burning
> fossil fuel creation
> sedimentary rock formation
Humans have altered the carbon cycle
through fossil fuel burning, deforestation,
and land-use change.
The net result of these processes is an
increasing concentration of carbon dioxide
in the atmosphere.
Sink
Amount in Billions of Metric Tons
Atmosphere
Soil Organic Matter
Ocean
Marine Sediments and
578 (as of 1700) - 766 (as of 1999)
1500 to 1600
38,000 to 40,000
66,000,000 to 100,000,000
Sedimentary Rocks
Terrestrial Plants
Fossil Fuel Deposits
540 to 610
4000
Judging from the data above, where on Earth is the largest
Carbon Sink found? Oceans!
• Ecosystems gain most of their carbon
through atmospheric CO2 (through
what processes?) Photosynthesis and Respiration
This is the simplest of all the nutrient cycles, as
Phosphorous doesn’t exist in the atmosphere for the most
part…but found in soil, rock, and sediment. Also, of all the
cycles, this one is more “local” than the others.
 It is released from the rocks and sediment through
chemical weathering.
It is usually released from sediments in the form of
phosphate (PO4-3), which is very soluble and can be absorbed
from the soil by plants. Phosphorus is a limiting factor in
the growth of plants, so plants that are exposed to minimal
amounts of phosphorus are usually stunted.
Phosphorus is an important
nutrient because it is an
essential building block of
nucleic acids.
This can result in fish-kills.
Humans have affected the natural cycling
of this nutrient by mining phosphorous rich
rocks for fertilizers. When we add those
fertilizers to soil it is leached out, and
washed to rivers and eventually into oceans
where it can cause artificial eutrophication.
The phosphorus is
simply returned to the
sediment when
organisms that have
incorporated it into
their systems
decompose.
• Through
geological
processes, these
sediments then
either become
rocks again…or are
washed away and
redeposited
elsewhere.