Soil Carbon in Global Perspective

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Soil C Residence Times

• Calculate residence times of C pools in previous
  figure. Measure of response time to
  perturbations.
• Table below shows approximate average range vs
  climate

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Soil Carbon in Global Perspective

• Two main anthropogenic C inputs are form fossil
  fuel and soil/plants
• Main C sinks are atmosphere, oceans and (???)
  ecosystems

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Effect of Cultivation on Soil C Storage

• Inputs
• can increase or decrease depending on cultural
  practices
• Losses/loss rate
• almost always increase due to
• disruption of aggregates which releases
  physically protected C to microbes
• increases in soil temperature (increasing
  surface albedo)
• increase physical erosion which may more rapidly
  oxide soil C
• Soil C storage vs. population

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Paired plot (left) and time series (below)
analyses of soil C vs. cultivation. Apparent
gains in soils of low original C may be due to
enhanced inputs by irrigation, etc.
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Soil C Trends with Time (natural and after
cultivation models
At start of cultivation
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Can Soil C be Restored Under Improved Ag
Practices major UN/Kyoto issue

• Approaches
• Reduce/eliminate tillage which allows C to be
  stored in soil structure and away from microbes
• Major possibility for USA in Great Plains
• Minimum tillage reduces ka in previous diagram
  back toward original value (and soil C increases)
• Return marginal lands to native vegetation
• Important also in Great Plains
• Again, k changes and soil C rebuilds (rate is
  dependent on many things, including climate, N
  availability, etc).
• Better manage grazing lands, establish
  agro-forestry in tropics etc.

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Limitations to Agricultural Storage of CO2

• Rate is likely to be originally high, but should
  rapidly decline on decadal time scales.
• Total potential storage in all cultivated soils
  is about 70 Gt, which about 12 years of fossil
  fuel burning
• Restored C can be easily and rapidly lost through
  changes in land management (back to plowing,
  etc.).

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