Prairie Ridge Soil Profile
What do we expect to see and why?
What we do see, and how come?
What can we conclude?
In the soil profile segment of carbon cycling system, we are primarily dealing with decay.
In the upper portion, several cm to 2 m, there is living root mass.
There is no photosynthesis in the soil, thus the primary processes are respiration and
decay.
Litter (undecomposed)
Organic layer, fermented
Organic layer, humified
Mineral layer with organic
carbon and leached
minerals
Mineral layer with precipitation
of oxides/hydroxides and/or
carbon
Unaltered parent substrate
Source: Gleixner, G. 2005. Stable isotope composition
of soil organic matter. In Stable isotopes and
biosphere-atmosphere interactions. ed. Flanagan,
L.B., E.J. Ehleringer and D.E. Patake.
Figure 1
Cellulose
Monosaccharide
(e.g. glucose)
Lipid
Lignin
Lignin
monomer
Cellulose
Lipid
Monosaccharide
(e.g. glucose)
Lignin
Lignin
monomers
Intermediates
Intermediates
(e.g. acetic acid)
CO2
Source: Gleixner, G. 2005. Stable isotope
composition of soil organic matter. In Stable
isotopes and biosphere-atmosphere interactions.
ed. Flanagan, L.B., E.J. Ehleringer and D.E.
Patake.
Protein
Amino acid
Protein
Amino acid
Ammonium
(e.g. acetic acid)
CO2
Humic
Substances
Source: Gleixner, G. 2005. Stable isotope composition of
soil organic matter. In Stable isotopes and biosphereatmosphere interactions. ed. Flanagan, L.B., E.J. Ehleringer
and D.E. Patake.
Humic
Substances
Nitrites/Nitrates
N2, N2O
s
Ammonium
Nitrites/Nitrates
N2, N2O
Fig 2
There is an order to the decay of organic matter. The first compounds to decay tend to be
the carbon fractions depleted in 13C, the tannins, non-polar compounds, and water-soluble
compounds, though lignin, also depleted in 13C, is conserved at this point. Cellulose,
enriched in 13C, is the next to decay. The most recalcitrant of the carbon fraction
compound is lignin, though lignin’s rate of decay can be increased by the addition of
simple sugars (Melillo 1989). There is some evidence that nitrogen may decrease
lignin’s decay rate (Fenn etal 1981, Keyser etal 1978).
Given this order of decay, it is fairly well established that the soil profile should have
certain biogeochemical characteristics, i.e., we expect to see 13C increase with depth
(Fig 3), % C decrease with depth (Fig 4), C/N decrease with depth (Fig 5), and carboxylic
and aromatic groups present in organic layers, increase with depth. % N may either
increase/decrease with depth. (Wynn and Ehleringer??)
Fig 3 Carbon isotopic composition profiles.
Undisturbed site
Disturbed (agricultural) site
(Fig 2 middle, J.G. Wynn, et al., 2006)
Fig 4 Carbon concentration profiles.
Undisturbed site
Disturbed (agricultural) site
“Kink” in the Cz curve reflects root depth or productivity zone
(Fig 2. Top, J.G. Wynn, et al., 2006)
Fig 5 Source: C/N of soil organic matter from different depth intervals
(Gleixner, 2005)
There are several hypotheses to explain these trends. Both Ehleringer and Wynn have
laid them out quite nicely. The first is the Suess effect (Fig 6), by which older, deeper
soil organic matter (SOM) originated when atmospheric 13C was more positive (CO2
was heavier). From 1744 to 1993, difference in 13C is approximately -1.3 ‰. The
typical soil profile 13C differences are approximately 3 ‰, so the Suess effect is not the
entire explanation.
Fig 6. Mixing of SOC derived
from the modern atmosphere
versus that derived from a preIndustrial Revolution
atmosphere. (Fig. 1A, J.G.
Wynn, et al., 2006)
The second hypothesis explaining the organic matter trends in the soil profile is soil
carbon mixing. The mixing has several possible mechanisms. Surface litter, depleted in
13
C may mix with root derived (enriched) SOM (Fig 7). Soil organic carbon (SOC)
derived from C3 communities will vary from that derived from C4 communities. If these
communities are in proximity, or vary over time and space, the SOM will mix and vary as
well (Fig 8). Some of the carbon incorporated into SOM by the decomposing fauna has
an atmospheric or soil gas, not SOM, source. Atmospheric CO2 in the soil is 4.4 ‰
heavier than CO2 metabolized by decomposition (Wedin, 1995)
Fig 7 Mixing of leaf litter-derived SOC
and root-derived SOC. (Fig. 1B, J.G.
Wynn, et al., 2006)
Fig 8 Mixing of SOC formed under two
different vegetation communities, e.g.
C3 vs C4. Slope could vary from
positive to negative depending on
direction of shift. (Fig. 1C, J.G. Wynn,
et al., 2006)
Hypothesis three is that there is microbial decomposers preferentially select 13C enriched
compounds. Sugars, amino acids, hemi-cellulose, pectin are all 13C enriched, while
lipids, lignin, cellulose are 13C depleted with respect to the whole plant. lipids and lignin
are preferentially accumulated in early decomposition, which actually works against soil
depth enrichment of 13C. More C than N is lost from soil as SOM decomposes due to
internal recycling of N.
The final hypothesis is kinetic fractionation, in which microbes preferentially respire 12C
CO2, preserving 13C enriched decomposition products. While Ehleringer, 2000, claims
that there is no direct evidence of this, Wynn, 2006, makes use of a Rayleigh distillation
analysis in his argument for this hypothesis (Fig 9 and 10). In his analysis, Wynn
assumed that he had an open system in which all components decompose and contribute
to soil-respired CO2 at same rate with depth, that is, FSOC  fSOC .
Fig 9. 13C distillation during decomposing SOM. The gray lines show the model with
varying fractionation factors from 0.997 to 0.999. (Fig. 1D, J.G. Wynn, et al., 2006)

  13Cf



1
 1000

1

F 



 13Ci

 1    1  e t  1   1 
 1000
   e  1t  1  t  
Fig 10. Rayleigh distillation equation.
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•
•
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fraction of remaining soil organic matter (SOC) – approximated by the
calculated value of fSOC
13Cf isotopic composition of SOC when sampled
13Ci isotopic composition of input from biomass
α
fractionation factor between SOC and respired CO2
e
efficiency of microbial assimilation
t
fraction of assimilated carbon retained by a stabilized pool of SOM
F
Wynn studied two areas, one an undisturbed forested area, and one a highly disturbed
agricultural area. While the results for the forested area conform fairly well to expected
curves, those for the agricultural area do not. Wynn proposed various reasons that
disturbed land might not conform to the nice regression curve in fig 9 (Wynn fig 9 ), such
as the introduction of C4 plants (e.g. corn), enriched in 13C, into an area formerly
vegetated exclusively by C3 plants (Fig 11B); cropping, which removes new, low 13C
material, leading to surface enrichment (Fig 11C); erosion, which removes the upper soil
layer, moving the whole curve up (Fig 11D); and a reintroduction of soil organic carbon
through better management practices. reversing the trends in B, C, and D (Fig 11E).
1. A – natural
2. B – introduce C4 plants, enriched in 13C
3. C – Cropping – removes new, low 13C material, leading to surface
enrichment
4. D – Erosion – removes upper layer, moving the whole curve up
5. E – Reintroduce soil organic carbon (better management practices) –
reverses the trends in C, D, and E
Results
• 13C – increase 3 ‰ to 8 cm (PRS 18 = anomaly)
• C/N – increases to 8 cm, then decreases
• % C – decrease with depth (PRS 15 = anomaly)
• % N – decrease with depth (PRS 15 = anomaly)
• PRS 7 and PRS 15, both surface soils, have similar absorbencies
• All soils have peak at wavelength 1032
• All 5 spectra have similar peaks, though not necessarily similar absorbencies
• In our bulk and heavy samples, are the mineral spectra masking the organics, as
in Poirier’s M-SOM?
Discussion and conclusions
– Random protocol on soil sampling at the site (i.e. depth interval, mass of
soil)
– Inconsistent sample preparation procedure (i.e. different mass, subjective
sorting)
– Poor implementation of IRMS protocols (i.e. sample size, standard
calibration)
– Insufficient samples for statistical accuracy
– 13C – increase 3 ‰ to 8 cm as expected
– C/N – decreases in lower portion of profile as expected
– % C – decrease with depth as expected
– % N – decrease with depth
– Don’t have a real soil profile or enough samples or rigorous sampling method
– Many studies examine only the organic layer – 5 cm only
– Minerals swamp the organics in our FTIR results
– Look at methods to extract organics
References
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Ehleringer, James R., Buchmann, N., Flanagan, L.B., 2000 Carbon Isotope Ratios
in Belowground Carbon Cycle Processes, Ecological Applications, vol. 10, no. 2,
p. 412-422
Gleixner, G. 2005. Stable isotope composition of soil organic matter. In Stable
isotopes and biosphere-atmosphere interactions. ed. Flanagan, L.B., E.J.
Ehleringer and D.E. Patake.
Melillo, Jerry M., Aber, J.D., Linkins, A.E., Ricca, A., Fry, B., Nadelhoffer, K.J.,
1989, Carbon and nitrogen dynamics along the decay continuum: Plant litter to
soil organic matter, Plant and Soil, vol. 115, p. 189-198
Poirier, N., Sohi, S.P., Gaunt, J.L., Mahieu, N., Randall, E.W., Powlson, D.S.,
Evershed, R.P., 2005, The chemical composition of measurable soil organic
matter pools, Organic Geochemistry, vol. 36, p. 1174-1189
Wedin, David A., Tieszen, L.L., Dewey, B., Pastor, J., 1995, Carbon Isotope
Dynamics During Grass Decomposition and Soil Organic Matter Formation,
Ecology, vol. 76, no. 5, p. 1383-1392
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Wynn, J.G., Harden, J.W., Fries, T.L., 2006, Stable carbon isotope depth profiles
and soil organic carbon dynamics in the lower Mississippi Basin, Geoderma, vol.
131, p. 89-109