Richard T' Conant

1
Carbon saturation in soil fractions How can it
occur?

• Richard T. Conant
• Johan Six

• Keith Paustian
• Eldor A. Paul

Natural Resource Ecology Laboratory Colorado
State University
2
Expected response to changes in C inputs
Semiarid Sites
Mesic Sites

• Linear relationships
• R2 range from 0.81 to 0.98
• Similar for mesic and semiarid sites

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• Common characteristics of SOM models
• 1st order decay dynamics
• Decay controlled by litter quality, climate
• Kinetically defined soil C pools

metabolic C
metabolic C
Litter quality
Structural C
CO2
Structural C
Active C
Active C
Litter quality
CO2
CO2
CO2
CO2
Slow C
Lignin
Carbon content
Slow C
CO2
Texture
Passive C
CO2
Carbon input
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Soil C response to C inputs Melfort, Sask.
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Soil C response to different C input rates
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Questions

• Why do soils become saturated with respect to C?
• How common is soil C saturation?
• What factors determine whether soils are
  saturated?
• Can soil C fractions become saturated?
• How does saturation influence soil C
  sequestration rates and amounts?

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Silt and clay protected C
g silt clay C kg-1 soil
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Microaggregate protected C

• Aggregate stability increases to a maximum level
  with
• increasing clay content and free Al- and Fe-
  oxide content.

2) Macroaggregate structure (gt 250 µm) exerts
minimal amount of physical protection (Pulleman
and Marinissen, 2001)
Microaggregates protect SOM more (Skjemstad et
al. 1996 Six et al. 2000)
Biochemically protected C
C pool stabilized by its inherent or acquired
biochemical resistance to decomposition capacity
seems limited since the mean age is much younger
than the pedogenic age of the soil.
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Unprotected (light fraction) C response to C
inputs Melfort, Sask.
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Four identified pools with saturation properties
Carbon content
Saturation level
Non-protected
Protection level
Biochemically protected
Protective capacity
Microaggregate protected
Silt clay protected
Carbon input
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Model with measurable pools that have
saturation-type behavior
CO2
Unprotected Soil C
Unprotected soil C
Litter quality
Physically protected Soil C
Aggregate turnover
Adsorption/desorption
CO2
Microaggregate associated soil C
Silt- and clay-associated soil C
Condensation/complexation
CO2
Chemically protected Soil C
Non-hydrolyzable soil C
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Conclusions I
It seems that inherent soil physical
characteristics define C stabilization
capacity that limits sequestration of C with
increases in C inputs gt at saturation C input
C respiration
Carbon content

input respiration
Saturation level
input ? respiration
Carbon input
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Conclusions II

• The four C pools all follow saturation dynamics
• Physically separable C pools whose responses of
  the various pools to different types of
  management may be evaluated.
• Fractions are protected from decay by varying
  degrees and are not well-described by 1st order
  decay dynamics.
• Soil physical properties define stabilization
  cap.
• Silt- and clay-associated C is linked directly to
  silt and clay content.
• Aggregation is related to soil texture and
  mediates C stabilization within aggregates
• Few soils are saturated
• Maximum C sequestration potential is equal to
  stabilization capacity minus current C content
  (not native C content!).
• Silt- plus clay-C are likely to be saturated
  under most conditions
• Some soil fractions may be saturated while others
  are not differences related to variable
  stabilization affinity.
• C sequestration rate is related to the saturation
  deficit (difference between capacity and content).