Nitrogen and the Nitrogen Cycle

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Nitrogen and the Nitrogen Cycle

• Brett Cunningham

Raven, Peter H., Ray F. Evert, and Susan E.
Eichhorn. Biology of Plants. 7th ed. New York,
New York W.H. Freeman and Company, 2005. 653-660.
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Nitrogen

• N2 makes up 78 of the atmosphere
• Most living things rely on ammonium and nitrate
  in soil for nitrogen
• Shortage of nitrogen in the soil is usually the
  limiting factor in plant growth
• Nitrogen cycle process by which the limited
  amount of nitrogen is circulated and recirculated
  throughout the living world

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Organic material decays and ammonium is released

• Saprophytic bacteria and fungi decompose
  nitrogenous compounds from dead organisms into
  amino acids, proteins and NH4 or NH3
• Plants take up NH4 from soil and use for
  synthesis of plant protein

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Nitrogen converting bacteria

• Chemosynthetic autotrophic bacteria oxidize NH3
  or NH4 - nitrification
• Nitrification yields energy used to reduce CO2
• Nitrosomonas bacteria oxidizes ammonium to
  nitrite 2NH4 3O2 2NO2- 4H 2H2O
• Nitrobacter oxidizes nitrite to nitrate ions and
  release energy 2NO2- O2 2NO3-
• Most crops absorb nitrogen as nitrate
• Common fertilizer has NH4 or urea which breaks
  down into NH4
• Nitrification turns NH4 into NO3-

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Carnivorous plants

• A few species can use animal proteins directly as
  a source of nitrogen (common bladderwort, sundew,
  Venus flytrap, and butterwort)
• Most are found in bogs
• These plants also absorb other organic compounds
  and minerals (potassium and phosphate)
• After organism is trapped enzymes secreted by
  plant and bacteria digest the animal

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common bladderwort
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sundew
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Soil-Plant system

• Major loss of nitrogen is by denitrification
  nitrate reduced to N2 and N2O anaerobically by
  microorganisms
• High nitrate concentrations occur because of the
  lack of an energy source
• Nitrogen is also lost by removal of plants, soil
  erosion, plant burning, and leaching
• Nitrates and nitrites can be washed from the root
  zone by water

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Nitrogen replacement

• Nitrogen fixation certain bacteria reduce N2
  into NH4
• Nitrogenase enzyme that catalyzes nitrogen
  fixation
• ATP is the energy source for nitrogenase

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Nitrogen-fixing bacteria

• 2 kinds free-living and symbiotic
• Symbiotic bacteria fix more and most common are
  Rhizobium and Bradyrhizobium which invade roots
  of legumes
• Bacteria provide nitrogen and plant provides
  energy source and carbon containing molecules to
  make nitrogenous compounds
• Crop rotation rotate leguminous crop with non
  leguminous ones to increase nitrogen content in
  soil

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Roots produce nodules

• Rhizobium and Bradyrhizobium bacteria rhizobia
• Rhizobia attach to root hairs of leguminous
  plants because of chemical attractants
• Infection threads are formed by invasion of
  rhizobia into root cells and cortical cells
• Cortex undergoes cell division because bacteria
  and Rhizobia are released into envelopes from the
  host-cell plasma membrane

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Roots produce nodules continued

• Rhizobia develop into bacteroids
• Nodules are formed from bacteroid and cortical
  cell proliferation
• Nodules eventually produce other compounds such
  as asparagine (amino acid)
• Leghemoglobin O2 binding heme protein
  regulates O2 and is found in cytosol of infected
  cells
• Leghemoglobin buffers O2 concentration within
  nodule and acts as an O2 carrier to bacteroids

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Rhizobia attached to root hair
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Root hair with infection threads
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Infection thread containing rhizobia
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Bacteroids each surrounded by membrane from
infected root nodule cell
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Specific relationship

• Bacteria that induce nodules in clover roots will
  not induce nodules on soybean roots
• Two groups of genes in bacteria are necessary for
  nodule formation
• nod genes are involved in host-specific response
  and nodule formation
• nif genes are involved in nitrogen fixation
• Other nod products activate Nod genes
• Nod genes encode proteins called nodulins which
  aid in cell division and nodule growth and
  function

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Nodules on roots of a soybean
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Section through mature soybean root nodule
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Non leguminous Nitrogen fixation

• Alder trees, sweet gale, sweet fern, and mountain
  lilacs form nodules with actinomycetes
• Azolla (floating water fern) and Arabaena
  (nitrogen-fixing cyanobacterium) form another
  symbiotic relationship found on rice paddies

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Azolla, which grows with Anabaena
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Filaments of Anabaena with megagametophyte of
Azolla
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Free living bacteria

• Nonsymbionts Azotobacter, Azotococcus,
  Beijerinckia, and Clostridium genera fix nitrogen
• These saprophytes rely on the oxidation of
  organic matter in the soil for energy

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Haber process

• N2 is reacted with H2 at high temperature and
  pressure with metal catalysts to form ammonia
• H2 comes from natural gas, petroleum, or coal

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Assimilation

• Nitrate enters a cell, reduced to ammonia,
  incorporated into organic compounds through
  glutamine synthetase-glutamate synthase pathway
• Process takes place in cytosol and leaf
  chloroplasts
• Nitrate reduced in plastids of roots when nitrate
  is low
• Organic nitrogen transported in xylem as amino
  acids

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Assimilation continued

• Organic nitrogen is important for plants in
  nitrogen limited ecosystems such as the Arctic
• In heather ectomycorrhizae and mycorrhizae break
  down proteins in organic soil matter and absorb
  and transfer amino acids to the plant directly

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glutamine synthetase-glutamate synthase pathway