Where does N come from

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(No Transcript)
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Where does N come from?
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• N inputs
• In pristine systems, biological N fixation is the
  main source

• Exceptions water or animal transport into system

• Humans have increased global N fixation by almost
  3 X

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Changing Global Nitrogen Cycle
About half of humans additions come from
fertilizer production
About a quarter from increasing amount of
biological N fixation
The other quarter inadvertently from fossil fuel
combustion
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Global N fixation
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Global patterns of human-caused N deposition
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• N inputs in biological systems
• Biological N fixation

• Carried out by microbes using enzyme nitrogenase
• N2 8H 8e- 16 ATP 2NH3 H2 16ADP 16
  Pi
• High C cost high P cost in ATP

• Symbioticassociation between plant or fungi and
  heterotrophic microbes
• Plant or fungi feeds microbe C, gets fixed N

• Asymbioticfree-living heterotrophic or
  photoautotrophic bacteria
• C from litter, root exudates, or CO2

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Parkinsonia-Rhizobium
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Alnus-Frankia
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Sacchrum-Acetobacter
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Cycad-Nostoc
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Azolla-Nostoc
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Peltigera (fungus-Nostoc)
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• N inputs
• Highest rates of fixation found in symbioses with
  higher plants

• Up to 25 of Net Ps goes to feed microbes
• 2 to 4 x higher than cost of N uptake from soil
• 5-100 g N m-2 year-1

• Lower rates in free-livers

• Photoautotrophs gt heterotrophs
• Nostoc (living free as a phototroph) 2.5 g N
  m-2 year-1
• Heterotrophs 0.1 0.5 g N m-2 year-1

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If N limits growth in many ecosystems, why
doesnt N fixation occur everywhere?
Why doesnt N fixation alleviate N limitation?
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1. Light (energy) availability constrains N
fixation

• Costs more C than taking up ions

• More fixers early in succession (high light)

• More fixers in open-canopy systems

• More fixers in shallow water

• More fixers on high C quality litter

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2. Other elements limit fixation

• High P requirement of ATP for fixation

• High requirement of enzyme co-factors Mo, Fe,
  and S

• These may ultimately be the master elements
  that limit primary productivity, even through N
  is proximally limiting

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3. Animals prefer N-fixers
4. Biogeography N-fixers didnt get there