Chapter 9 Terrestrial Nutrient Cycling

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Chapter 9Terrestrial Nutrient Cycling

• Principles of
• Terrestrial Ecosytem Ecology
• Chapin, Matson and Mooney

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Nutrient inputs to ecosystems

• Weathering
• Biological fixation of atmospheric N
• Deposition (rain, dust, gases)

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Nitrogen deposition is small in unpolluted
ecosystems Substantial in agricultural and
industrial areas Human activities are THE major
source of N deposition
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Nitrogen fixation

• Conversion of atmospheric N2 to NH4
• Requires abundant energy and P
• Inhibited by oxygen
• Carried out by bacteria
• Symbiotic N fixation (e.g., legumes, alder)
• Heterotrophic N fixation (rhizosphere and other
  carbon-rich environments)
• Phototrophs (bluegreen algae)

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Paradox of nitrogen limitation

• Nitrogen is the element that most frequently
  limits terrestrial NPP
• N2 is the most abundant component of the
  atmosphere
• Why doesnt nitrogen fixation occur almost
  everywhere???
• Why dont N fixers have competitive advantage
  until N becomes non-limiting?

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Environmental limitations to N fixation

• Energy availability in closed-canopy ecosystems
• N fixers seldom light-limited in well-mixed
  aquatic ecosystems (e.g., lakes)
• Nutrient limitation (e.g., P, Mo, Fe, S)
• These elements may be the ultimate controls over
  N supply and NPP
• Grazing
• N fixers often preferred forage

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