The Nitrogen Cycle

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The Nitrogen Cycle
Objective (1)The Learner will identify the
components of the Nitrogen Cycle. (2) TLW explain
how nitrogen is fixed by bacteria. (3) TLW
model symbiosis and explain the relationship of
N2 fixing bacteria and their mutual relationship
with certain plants.

• By Mr. John DeWeese

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Nitrogen Just the Facts

• All life requires nitrogen-compounds, e.g.,
  proteins and nucleic acids.
• Air, which is 79 nitrogen gas (N2), is the major
  reservoir of nitrogen.
• But most organisms cannot use nitrogen in this
  form.
• Plants must secure their nitrogen in "fixed"
  form, i.e., incorporated in compounds such as
• nitrate ions (NO3-)
• ammonia (NH3)
• urea (NH2)2CO
• Animals secure their nitrogen (and all other)
  compounds from plants (or animals that have fed
  on plants)

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Four Nitrogen Processes

• Four processes participate in the cycling of
  nitrogen through the biosphere
• nitrogen fixation
• decay
• nitrification
• denitrification
• Microorganisms play major roles in all four
  processes.

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Nitrogen Fixation

• The nitrogen molecule (N2) is quite inert
  (stable). To break it apart so that its atoms can
  combine with other atoms requires the input of
  substantial amounts of energy.
• Three processes are responsible for most of the
  nitrogen fixation in the biosphere
• atmospheric fixation by lightning
• biological fixation by certain microbes - alone
  or in a symbiotic relationship with plants
• Industrial Fixation
• Under great pressure, at a temperature of 600C,
  and with the use of a catalyst, atmospheric
  nitrogen and hydrogen (usually derived from
  natural gas or petroleum) can be combined to form
  ammonia (NH3). Ammonia can be used directly as
  fertilizer, but most of its is further processed
  to urea and ammonium nitrate (NH4NO3).

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Atmospheric Fixation

• The enormous energy of lightning breaks nitrogen
  molecules and enables their atoms to combine with
  oxygen in the air forming nitrogen oxides. These
  dissolve in rain, forming nitrates, that are
  carried to the earth.
• Atmospheric nitrogen fixation probably
  contributes some 5-8 of the total nitrogen
  fixed.

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Decay

• The proteins made by plants enter and pass
  through food webs just as carbohydrates do.
• At each trophic level, their metabolism produces
  organic nitrogen compounds that return to the
  environment, chiefly in excretions.
• The final beneficiaries of these materials are
  microorganisms of decay.
• They break down the molecules in excretions and
  dead organisms into ammonia.

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Nitrification

• Ammonia can be taken up directly by plants -
  usually through their roots. However, most of the
  ammonia produced by decay is converted into
  nitrates. This is accomplished in two steps
• Bacteria of the genus Nitrosomonas convert NH3 to
  nitrites (NO2-).
• Bacteria of the genus Nitrobacter oxidize the
  nitrites to nitrates (NO3-).
• These two groups or autotrophic bacteria are
  called nitrifying bacteria. Through their
  activities (which supply them with all their
  energy needs), nitrogen is made available to the
  roots of plants.

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The Chemical Formula

• N2 2H -gt NH3 H2

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Organisms that Can Fix N2

• A microbial mat at a hot spring in eastern
  Oregon. Some of the organisms in the mat are
  cyanobacteria that can fix nitrogen.
  Cyanobacteria are found in a variety of
  terrestrial and aquatic habitats. Many, but not
  all, are capable of nitrogen fixation.

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Azolla

• The aquatic fern Azolla is the only fern that can
  fix nitrogen. It does so by virtue of a symbiotic
  association with a cyanobacterium (Anabaena
  azollae).
• Azolla is found worldwide and is sometimes used
  as a valuable source of nitrogen for agriculture.
  The plants shown here are about 2 cm across.
• The pale yellow plant has been deprived of cobalt
  (essential for the cyanobacterial symbiont) and
  thus is showing typical signs of N deficiency.

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Lichens

• Lobaria pulmonaria, a common N-fixing lichen in
  Pacific Northwest forests. The nitrogen-fixing
  symbiont is the cyanobacterium Nostoc which is to
  be found in pockets within the lichen referred to
  as cephalodia.
• Lichens such as this are a major source of N in
  old growth forests.

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Alder

• Young plants of red alder (Alnus rubra). Alder is
  a N-fixing plant that forms a symbiotic
  association with bacteria (more specifically an
  actinomycete) of the Genus Frankia.

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Nodules on the root mass

• Note the nodules on the root mass.
• Bacteria live inside the nodules.

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Legumes

• Many crops are grown to produce food, but also
  return nitrogen to the soil for future crops to
  use.
• Include peanuts, peas, alfalfa, soybeans and
  clovers.

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Denitrification

• The three processes (nitrification, decay
  atmospheric fixation) remove nitrogen from the
  atmosphere and pass it through ecosystems.
• Denitrification reduces nitrates to nitrogen gas,
  thus replenishing the atmosphere.
• Once again, bacteria are the agents. They live
  deep in soil and in aquatic sediments where
  conditions are anaerobic. They use nitrates as an
  alternative to oxygen for the final electron
  acceptor in their respiration.
• Thus they close the nitrogen cycle.

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What you LEARNED!

• All life requires nitrogen-compounds
• To make proteins
• and nucleic acids
• Air, which is 79 nitrogen gas (N2), is the major
  reservoir of nitrogen.
• Most organisms cannot use nitrogen from the
  atmosphere.

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You Learned

• Plants must secure their nitrogen in "fixed"
  form, i.e., incorporated in compounds such as
• nitrate ions (NO3-)
• ammonia (NH3)
• urea (NH2)2CO

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And

• Lightning breaks nitrogen molecules and enables
  their atoms to combine with oxygen in the air
  forming nitrogen oxides.
• Nitrogen oxides in rain dissolve to form nitrates
  that are carried to the earth.
• Atmospheric nitrogen fixation contributes 5-8 of
  the total nitrogen fixed.

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And

• Animals secure their nitrogen (and all other)
  compounds from plants (or animals that have fed
  on plants)

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The End