Transcript Nitrogen Cycle

The Nitrogen Cycle
Nitrogen Cycle
• Nitrogen in
Atmosphere = 79%
• Problem is getting N2
into a form that plants
can use.
• Most N in soil used by
plants for growth and
for Agriculture.
Nitrogen FixationConversion of N2 into ammonium (NH4+) or
nitrate (NO3-)
A . Non-Biological Fixation
-Air Pollution -
Nitrate is primarily the
result of gases from motor vehicle exhaust and
stationary fuel combustion sources like electric
utilities and industrial boilers
- rainfall additions from
electrical discharge (lightning)
2-5 lbs....../acre/year
N2 -----> NO3-
B . Biological Fixation
1. Non-Symbiotic Bacteria - An
independent organism that lives in the
soil - They fix about 5-50 lbs/acre
/year of nitrogen.
2. Symbiotic Bacteria - mutually
beneficial for host organism and
bacteria. It is a complex plant bacteria interaction
Symbiotic N- Fixation
Bacteria = Rhizobia
Forms the relationship
with a Legume.
Eg. peas, clover, alfalfa,
peanuts, beans, soybeans
Alfalfa - 200
lbs....../acre/year
Soybeans - 100
lbs......./acre/year
Beans - 40
lbs...../acre/year
Symbiotic N Fixation
• Bacteria invades the root of a host
plant.
• Response of host plant root is to
grow a nodule for the bacteria to
live in.
• Bacteria takes N2 from the air and
converts it some of it into NH4+
which the plant is able to use.
Symbiotic N Fixation
• Fate of N Fixed by Rhizobium:
1) used by host plant,
2) leaks out of root to become
available to surrounding plants,
3) as roots and nodules are
sloughed-off, heterotrophic
organisms immobilize (take in) the
N and it eventually becomes part
of the Soil Organic Matter.
The Nitrogen Cycle
• After nitrogen from the atmosphere has been
fixed into the soil, plants can now use it to make
proteins.
• This is how nitrogen enters the food chain.
Consumers must get their nitrogen from
consuming plants or other organisms. They are
unable to utilize any of the nitrogen from the
atmosphere.
• The route N takes into the food chain forms a sub
cycle within the larger N cyle.
Nitrification
• Like other nutrients, nitrogen is returned to
the ecosystem through animal wastes and
dead organisms.
• Decomposers produce ammonia which is
used by some plants.
• Ammonia can be converted back to nitrates
by nitrifying bacteria, in a process called
nitrification.
• Nitrates are the source of most of the
nitrogen for plants.
Denitrification
• To complete the nitrogen cycle, the
nitrates are then converted back to
nitrogen gas by denitrifying bacteria.
• This process is called denitrification.
The Nitrogen Cycle
Human Impacts on the N Cycle
• Most wild plants are adapted to thrive in low
levels of N that are found in the soil and
water.
• The development of industrial fertilizer and
the burning of fossil fuels has disrupted the
natural N balance in many ecosystems.
• Commercial fertilizers are now commonly
used to increase the growth and yield of
agricultural crops.
Human Impacts on the N Cycle
• Modern farming and the
combustion of fossil fuels now
move about 140 million tonnes of
extra nitrogen through the
environment each year.
• More nitrogen is being added to the
environment than what plants can
absorb and use.
Effects on the Soil
• With a surplus of nitrogen, plant growth soon becomes
limited by the scarcity of other resources such as
phosphorous, calcium and water.
• The plants are not able to use any more N so it washes from
the soil into streams and groundwater without being
absorbed by organisms.
• This extra N can harm forests by stunting growth, turning
trees yellow or by killing mycorrhizal fungi which is
essential for some tree roots to absorb nutrients.
• This occurs because the acidity of the soil changes with
extra nitrogen.
Effects on the Atmosphere
• Industry and automobiles are huge contributors of
nitrogen containing gases in the atmosphere.
• When these compounds dissolve into water
droplets in the atmosphere, it forms acid
precipitation.
• This precipitation has affected lakes and forests in
Eastern Canada by killing many species of fish,
amphibians and trees.
• The only cure right now is to dump powdered
limestone into the acidified lakes to rebalance the
pH.
Effects on Freshwater Ecosystems
•
Much of the excess N from the land ends up in
freshwater ecosystems due to runoff.
• These nutrients build up in these ecosystems
and create a process called eutrophication.
What is Eutrophication?
1. Runoff carries nitrates from farms and cities
into water systems.
2. An increased growth of plants on the surface of
the water blocks sunlight from penetrating to
deeper waters.
What is Eutrophication?
3. Plants below the surface are unable to carry out
photosynthesis. They begin to die and
therefore stop producing oxygen.
4. As the plants die, the population of
decomposers explodes, feeding on the extra
decaying matter. The decomposers cause a
further decline in oxygen levels through their
cellular respiration.
5. The altered conditions kill fish and other
animals that require high oxygen levels
Nitrogen Pollution and Eutrophication
• Nitrates can also be a problem with human
drinking water sources, especially in rural areas.
• The excess nitrogen leaches into ground aquifers
and wells where many rural communities and
residences get their drinking water.
• When excess nitrates are ingested, they are
converted to nitrites which reduce the blood’s
ability to carry oxygen and produce a form of
anemia.
• This is especially a concern with infants and
young children.
Effects on Marine Ecosystems
• Excess nitrates in a marine ecosystem that has
warm surface water can create a population
explosion of algae called an algal bloom.
• As the algae die they sink into cooler waters and
are broken down by bacteria.
• Colder water holds less oxygen, and the use of
that oxygen by the bacteria can cause death of
many organisms that need higher levels of
oxygen such as fish and shellfish.
Baltic Sea between
Russia and Sweden
Effects on Biodiversity
• Biodiversity is a measure of the variety of species
on Earth.
• Biodiversity is not the same everywhere on Earth.
• As a rule, biodiversity is reduced in extreme
conditions.
• This means that only a few species are able to
thrive in an environment that have very high levels
of nutrients.
• Only these few organisms will survive and become
dominant, the others will die off because they
cannot compete for the other scarce nutrients.
Assignment
• Read pages 52 – 69
• Answer questions 1-6 pg. 65 & 1-5 pg. 68
• Use the chapter at a glance (p. 69) to review
the material from the chapter.