The Nitrogen Cycle

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The Nitrogen Cycle

• Ch 7 Part 3

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Nitrogen Cycle

• Nitrogen cycle the routes that nitrogen atoms
  take through the environment
• Nitrogen gas (N2) is the most abundant gas in the
  atmosphere (78)
• Nitrogen gas is inert cannot be used directly
  by most organisms.
• Nitrogen is cycled through animals, soil,
  gases.
• Organisms use nitrogen (in various forms) to make
  vital organic compounds.
• Nitrogen is an important component of biological
  molecules DNA, RNA, proteins.
• Nitrogen is a plant nutrient acts as a limiting
  factor for plant growth.
• Too much nitrogen in aquatic systems can cause
  eutrophication.

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• Several steps
• Nitrogen fixation
• nitrogen to ammonia
• Nitrification
• ammonia to nitrates
• Assimilation
• nitrates to proteins, DNA, RNA, etc.
• Ammonification
• decomposition
• Denitrification
• ammonia nitrates to nitrogen
• Abbreviations to know
• nitrogen gas (N2)
• ammonia (NH3)
• ammonium ions (NH4)
• nitrite ions (NO2-)
• nitrate ions (NO3-)

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Nitrogen Fixation

• Even though N2 is the most abundant gas in the
  atmosphere (78) it must be fixed or converted
  into a usable form before it can be absorbed and
  used by multicellular organisms.
• Ways of converting N2
• 1. lightning
• causes reaction with O2 to form NO3-
• 2. Nitrogen fixation N2 is combined (fixed) with
  H2 to become ammonia, NH3
• whose water-soluble ions of ammonium, NH4, can
  be taken up by plants
• Done by nitrogen-fixing bacteria

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Nitrogen Fixation

• Specialized bacteria convert N2 gas to ammonia
  (NH3)
• N2 3H2 ? 2NH3
• Once converted into ammonia, water-soluble ions
  in the form of ammonium (NH4) can be taken up by
  plants
• Done mostly by
• Cyanobacteria in soil water
• Rhizobium bacteria in nodules on roots of wide
  variety of plants
• (ex. legumes, soybeans,
• alfalfa)
• Requires large amounts of energy

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Nitrification

• Nitrification bacteria convert ammonium ions,
  NH4 (the water soluble form of ammonia, NH3,
  which is usually produced by decay) through a
  2-step process into NO2- (nitrite) NO3-
  (nitrate)
• Done by 2 groups of specialized aerobic
  nitrifying bacteria (for use by plants) to
• 1. Nitrite ions (NO2-)
• toxic to plants
• Nitrosomonas
• Nitrosococcus
• 2. Nitrate ions (NO3-)
• easily taken up by plants
  as a
  nutrient through roots
• Nitrobacter

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Assimilation

• Plant roots absorb inorganic ammonia (NH3),
  ammonium ions (NH4), nitrate ions (NO3-).
• Formed by nitrification (NH3 ? NH4 ? NO2- ?
  NO3-) nitrogen fixation (N2 3H2 ? 2NH3).
• Ions are used to make
• N-containing organic
• molecules such as
• DNA RNA
• Amino Acids
• Proteins
• Animals get their N by eating plants or
  plant-eating animals

Assimilation
NH3
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Ammonification

• Bacteria in soil water decompose N-rich organic
  compounds, wastes, cast-off particles dead
  organisms into
• 1. simpler N-containing inorganic compounds such
  as ammonia (NH3)
• 2. water-soluble salts containing ammonium ions
  (NH4)

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Denitrification

• Dentrification other specialized bacteria
  convert ammonia back into gaseous nitrogen
  through a multistep process
• Ammonia (NH3) ammonium ions (NH4) ? nitrite
  ions (NO2-) nitrate ions (NO3-) ? nitrogen gas
  (N2) nitrous oxide gas (N2O)
• Done by denitrifying bacteria deep in soil
  aquatic sediments
• Anaerobic conditions So use nitrates as an
  alternative to oxygen for the final electron
  acceptor in their respiration.
• Replenish atmosphere close the nitrogen cycle

11
Humans Affect the Nitrogen Cycle

• Haber-Bosch process synthetic production of
  fertilizers by combining nitrogen hydrogen to
  synthesize ammonia
• Dramatically changed the nitrogen cycle since
  devised in 1950s
• huge increases in crop yields (due to
  fertilization)cheaply
• Humans are fixing more nitrogen than nature does
• Reduced biodiversity of plants adapted to
  low-nitrogen soils
• Calcium potassium in soil washed out by
  fertilizers (depletion)
• Acidified water soils
• Disposing of N-rich municipal sewage
• Raising cattle in feedlots adjacent to waterways
  (wastes in runoff)
• Changed estuaries coastal ecosystems
  fisheries
• N in run-off causes bloom in algae aquatic
  plants (eutrophication)
• When photosythesizers die in the water their
  decomposition by bacteria removes O2 from the
  water (hypoxia) causes death of other
  organisms.
• Burning fossil fuels grasslands deforestation
  removes N from the soil produces NOx (a
  precursor for acid rain ozone depletion)

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Effects of Human Activities on the Nitrogen Cycle

• Human activities such as production of
  fertilizers now fix more nitrogen than all
  natural sources combined.

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Human Inputs of N Into the Environment
Fully half of nitrogen entering the environment
is of human origin
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The Nitrogen Cycle
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Nitrogen Cycle Animation
N Cycle Tutorial
N Cycle Animation