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Carbon Isotope Systematics in Soil-or-Plant
Poo and Microbe Farts
Justin Yeakel, UCSC
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Soil Pathway Summary

• Organic matter finds its way to the forest floor
  and decomposes
• SOM (Soil Organic Matter) is further decomposed
  by microbes which emit CO2 as a biproduct
• Under certain conditions, water and this carbon
  dioxide can form carbonates

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• Each step in this sequence is met with a
  fractionation of the original carbon.
• Diffusion of carbon dioxide and mixing with the
  atmosphere is responsible for the drop in d13C
  values of CO2 and CaCO3 as you descend through
  the soil layers.
• Cause of rise in d13C values with depth in SOM is
  debated.

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• Since C3 and C4 plants are distributed based on
  specific environmental parameters
• And the carbon that is cycled through soils is
  from these plants
• ? values in the carbon found in soils will be
  representative of these respective photosynthetic
  pathways

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What else could possibly contribute a significant
amount of carbon to this process?
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What else could possibly contribute a significant
amount of carbon to this process?
Parent material
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• Most carbon in soil carbonates is from soil gas
  (vegetation, respiration, atmosphere) and not
  from the parent material that soil is found on.

Altitude (meters)
? 13C Soil Carbonates
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Phase 1 Enrichment of SOM d13C values
-Trees respire -Roots respire -Litter on the
forest floor gets eaten by bacteria This CO2 is
representative of the particular photosynthetic
pathway (C3 or C4), and is therefore about -27
or -13 respectively.
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Initially, ?13C values of the SOM look exactly
like the plant. But with time and/or depth, SOM
?13C values rise.
CO2
CO2
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Initially, ?13C values of the SOM look exactly
like the plant. But with time and/or depth, SOM
?13C values rise.
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Depth (cm)
-100
-30
0
-20
?13C
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Torn et al. 2001
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Why the SOM ?13C values rise?

1. Suess effect - recent drop in d13C values of
   plants due to fossil fuel burning, etc.
2. Differential loss of components (lipids, lignin,
   protein, etc.) with different d13C values.
3. Progressive 13C-enrichment of residual SOM due to
   respiratory loss of 12C-enriched CO2.
4. Carboxylation in soils in the presence of soil
   gas that is 13C-enriched relative to typical
   plant matter!!

Depth
?13C
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• Although there is a rise in the ?13C values of
  SOM material at greater depths, most carbon is
  digested by microbes and released into the soil
  as CO2, leaving little remaining carbon in deeper
  SOM.
• This 13C-enrichment in deeper SOM does not have a
  great effect on the ?13C values of soil CO2
  because is occurs at such low concentration.

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Phase 2 Soil CO2
Via decay of SOM and root respiration, soils are
a huge source of CO2, which diffuses into the
atmosphere.
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Phase 2 Soil CO2
Via decay of SOM and root respiration, soils are
a huge source of CO2, which diffuses into the
atmosphere.
The diffusion of CO2 has a direct impact on the
?13C values, and is a affected by properties of
the soil. The pattern produced by diffusion can
be modeled to estimate the ?13C values of the CO2
at differing depths.
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• There is an initial 4.4 enrichment due to
  diffusion of CO2 out of the soils
• Based on soil properties, atmospheric CO2 plays a
  role at differing depths in a soil
• ?13C values at depth reflect the respiration,
  whereas values nearer the surface are more
  affected by the atmosphere

Cerling et al. 1991
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Per mil value of CO2 in the soil as a function
of depth
Per mil value for atmospheric CO2
Ratio of 13C in diffused CO2

• Soil Properties
• Production rate of respired CO2
• Diffusion coefficient for CO2 in soil
• Depth
• Soil Tortuosity

Concentration of CO2 in the atmosphere
Per mil value for soil-respired CO2
Constructed to put number into delta format
13C/12C ratio of isotopic standard PDB
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This equation is set by parameters that limit the
function. -The uppermost boundary condition is
that soil CO2 concentration equals that of the
atmosphere (z0) -CO2 production is modeled with
depth and an impermeable lower boundary at some
finite z.
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In more productive soils (red), the effects of
the atmosphere are only seen in the top few cm.
Soil gas has a ?13C value respired CO2 offset
due to diffusion. In less productive soils,
atmospheric affects gas isotope values deep in
the soil (blue).
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Phase 3 Carbonate Formation
In the presence of water, the soil CO2 goes
through a number of chemical reactions that can
result in the formation of soil carbonates.
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CO2 water ? H2CO3 ? H HCO3- ? Ca2 2HCO3-
? CaCO3 CO2 H2O
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CO2 water ? H2CO3 ? H HCO3- ? Ca2 2HCO3-
? CaCO3 CO2 H2O
Acid formed by hydration of soil CO2 dissolves
minerals near the soil surface and is slowly
buffered by this process. At deeper levels in
the soil, either because of high Ca2
concentration (due to mineral dissolution or
seasonal evaporation) or simply due to the
degassing of CO2 from soils, the reaction above
is driven to the right, leading to soil carbonate
precipitation.
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Fractionations that occur along these reactions
are temperature-sensitive. Between the 4.4
impact of diffusion and these equilibrium
reactions, soil carbonate is 13C-enriched by 13.5
to 16.5 relative to soil organic matter.
CO2 water ? H2CO3 ? H HCO3- ? Ca2 2HCO3-
? CaCO3 CO2 H2O
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CO2 water ? H2CO3 ? H HCO3- ? Ca2 2HCO3-
? CaCO3 CO2 H2O
In addition, the ?13C is strongly correlated to
that of the SOM and overlaying flora
Cerling Quade 93
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The fractionations that occur during carbonate
formation enrich the ? values from 13.5 and 16.5
per mil between 0ºC and 25ºC respectively.
CO2 water ? H2CO3 ? H HCO3- ? Ca2 2HCO3-
? CaCO3 CO2 H2O
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How deep a carbonate is when it forms also has an
impact on its carbon isotope value. This is
directly correlated to the ?13C value of soil
CO2, which differs according to depth.
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Cerling Quade 93
The model described previously is used in the
graph above, and shows a desertine C4 environment.
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14 enrichment in 13C
Progression of ?13C values in soil CO2 to soil
carbonate
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Overall Process
Soil Carbonates
13C-enrichment with depth Cause unclear, but no
large effect overall on d13C of soil CO2
SOM
As soil CO2 reacts to form soil carbonate, C
isotopes are fractionated differently according
to temperature. Overall fractionations amount to
13.5 to 16.5 enrichment relative to plants or
soil CO2.
Microbial digestion of SOM and root respiration
releases CO2 into soil. Diffusion leads to 13C
enrichment of 4.4 relative to respired CO2 .
Soil CO2
With increasing depth, atmosphere affects ?13C of
soil CO2 less and less, and soil CO2 is more
representative of vegetation
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All Summed up
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• T H E E N D

T H E E N D