BIOE 103
Nitrogen cycle
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Please answer daily question.
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A personal story
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Today’s goal: To understand how fertilizer works,
how it is made, and how it affects environment.
Fertilizer is good for plants, so should be good
for the environment, right?
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Lecture Outline:
1. How fertilizer works
2. How to make fertilizer
3. Fertilizer pollution
4. Not just nitrogen
5. Political connections
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Fertilizer often increases plant growth
Result from a fertilizer study in 1942
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Recall: Everything in the world is
made of atoms
Examples:
Nitrogen (N)
Phosphorus P)
Potassium (K)
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Recall: Plants use carbon and oxygen
molecules to make glucose
C
C
O
O
C
O
O
C
O
C
O
O
C
O
O
C
O
O
C
C
C
Carbon dioxide
O
C
C
Glucose
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Recall that plants get MOST of their raw
material from CO2 in the air
But plants need more than carbon and oxygen to
make all the molecules they need to function. E.g.
nitrogen is needed to make protein molecules
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Background information: Protein
• Protein is an important
component of all
organisms (not just
animals)
• Proteins can be part of
structure of organisms
(e.g. muscle) or can be
enzymes* that facilitate
chemical reactions
*An enzyme is a protein speeds up chemical reactions in organisms. You don’t need to worry about
this for this course, but biology majors will study enzymes in great detail
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Plant cells have protein in them
Plants can’t grow if they can’t make protein
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Protein molecules are ‘glob-like’ molecules made of a
string of amino acids ‘balled up’
Each circle is an amino acid
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Amino acids are simple molecules that all have
at least one Nitrogen atom in them
Plants and animals can’t make amino acids if they don’t have Nitrogen.
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Plant growth in many ecosystems is limited
by nitrogen availability
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Fertilizer has high levels of
Nitrogen (N), Phosphorus (P) , Potassium (K)
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N–P–K
Nitrogen
Phosphorus
Potassium
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Why fertilizer makes plants grow better
• Growing plants need nitrogen
• Plants absorb nitrogen from the
soil.
• In most ecosystems, including
farms, there is a shortage of
nitrogen.
• This limits how much plants can
grow.
• Adding nitrogen fertilizer lets
plant grow faster.
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Venus fly traps live in soils with very little
nitrogen
They capture insects to get nitrogen.
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Tropical rain forests often have nitrogen-poor soil
Most nitrogen in ecosystem is in plants
If trees are cut down, nitrogen is removed. Forest can’t grow back
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Which of these ingredients for chocolate
chip cookies is like nitrogen?
Recipe requires
Pantry contains
————————————————————————
2 cups flour
51 cups
½ cup sugar
14 cups
½ cup brown sugar
7 cups
1 cup butter
3 cups
2 eggs
12 eggs
1 teaspoon baking powder
34 teaspoons
1 cup chocolate chips
1/2 cup
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2. How to make fertilizer
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It is ironic nitrogen availability limits plant
growth: 80% of the atmosphere is nitrogen
N2
O2
N2
N2
O2
N2
N2
N2
O2
O2
N2
N2
CO2
N2
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Nitrogen atoms are present in different
molecules
Atmosphere has
this molecule:
This form of nitrogen is
completely useless to
most organisms
Soil has
this molecule (ammonia)
Plants (and animals) can
use this form
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This process is called “fixing” nitrogen.
Requires lots of energy.
Note: This reaction isn’t balanced.
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Fertilizer production requires energy
and complex industrial facilties
Which is why many poor countries don’t have
sufficient fertilizer.
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Increasing availability of synthetic fertilizer availability
in Africa could result in increased food supply
But synthetic fertilizers are not the only fertilizers
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Some types of plants (e.g. legumes) can “fix”
nitrogen (convert it to a useable form)
Legumes: alfalfa, clover, peas, beans, soybeans, lentils,
peanuts
Nitrogen fixing
bacteria live in
root nodules a
legume
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Nitrogen fixing bacteria in soybean
root nodule
Bacteria
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Nitrogen fixing plants can be used as a
“green manure”
Clovers and other legumes
can add nitrogen to soil
naturally
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And of course, there are organic fertilizers
Manure fertilizes growth because it has high levels of nitrogen
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3. What can happen when an
ecosystem is fertilized
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88,000,000 acres of corn are planted each year
in the United States
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Corn requires up to 200 pounds of
fertilizer per acre
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15% of that fertilizer ends up in
streams and rivers
Mississippi River (showing nearby farmland)
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Nitrogen application in Mississippi River Basin
have increased steadily
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Fertilizer in Mississippi River
ends up in Gulf of Mexico
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Ocean is filled with phytoplankton
Small plant-like organisms floating in the water
column
Phytoplankton thrive in presence of nitrogen.
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Fertilizer runoff can cause blooms
of algae or plankton
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Plankton die, sink to bottom and
decompose
Decomposition uses up oxygen
C6H12O6 + 6O2  6CO2 + 6H2O
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Aquatic organisms need oxygen
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Oxygen concentrations in the Gulf of Mexico
(Red indicates low levels)
8000 squares miles
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This process is called
“Eutrophication”
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Three billion dollar fishery in Gulf of
Mexico threatened
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Gulf of Mexico “Dead Zone”
- Where Mississippi River
enters Gulf
- 8000 square miles
- Occurs every summer
- Kills all bottom dwellers
(shellfish, crabs, snails)
- Cause: Nitrogen fertilizer in
Mississippi watershed
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Fertilizers may have bigger impact on the Gulf
of Mexico than Deepwater Horizon oil spill
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Much of this research done by university
scientists, e.g., Dr. Nancy Rabalais
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The Potash & Phosphate Institute
& International Plant Nutrition Institute specifically
challenged these claims about the Gulf
Some of their
counter
arguments
sound
convincing
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These are trade groups funded by
fertilizer industry
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How should I
know who to
believe?
IPNI or
university
scientists?
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You might use the baloney
detection kit?
1. Peer reviewed journal?
2. Is the data shown? Results
clear?
3. Work replicated?
4. Consensus in community?
5. Does claim make sense?
6. Alternatives considered?
7. Who provided funding?
8. Is there an agenda?
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What does this teach us?
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Dead zones are growing in size
exponentially around world
~400 Dead zones
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Riparian buffer zones can help keep
fertilizer out of streams
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Lawn
fertilizer has
same effect
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Untreated sewage also causes
eutrophication
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4. Other limiting nutrients
Nitrogen does not always limit plant growth
Other nutrients that may limit growth include:
phosphorus, potassium, iron, and many other
relatively rare substances.
E.g., Iron limits growth in some parts of ocean.
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5. Political connections
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US Environmental Protection Agency
works to protect Gulf
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Many in the ‘Tea party’ movement
have proposed abolishing EPA
“…the EPA is again goosestepping even more into
our every day lives with
arbitrary edicts.”
—Cleveland Tea Party (in response
to EPA announcement to regulate
mercury in dental fillings)
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Study guide
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•
•
•
•
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Read Chapter 5.
Review what is an atom and molecule.
Be able to identify what limits plant growth and explain why.
Be able to explain the difference between nitrogen in the
atmosphere and nitrogen that is useful for plants.
Be able to explain two ways that nitrogen in the atmosphere
is converted to ammonia.
Be able to list 3 ways to fertilize a field.
Be able to describe eutrophication.
Be able to extract a lesson from the history of the “dead
zone” in the Gulf of Mississippi (see textbook for additional
details).
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Sample exam questions
Explain in one or two sentences why adding fertilizers to crops
increases plant growth.
What is a “dead zone”? What causes them?
A septic tank is an underground tank used to break down solid
wastes from homes that are not connected to a municipal sewer
system. Because a septic tank is underground, it can be difficult
to locate if you do not know where it was installed. Folk
knowledge says that the grass over septic will be greener. Is this
likely to be true? Why?
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The End
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