Transcript δ13C Variation from Plants to Soil

13
δ C
Variation from
Plants to Soil
Jonathan Harris
MEA 760 NCSU
13
δ C
within Plants and Soil

C3 Plants
depleted ~
21 – 30 per mil

C4 Plants
depleted ~
10 – 15 per mil
Plants at Prarie Ridge, NC
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Current Plot currently dominated by bermuda
grass, a C4 grass.
Historically maintained as a cow pasture
dominated by fescue, with some orchard grass,
both C3 grasses.
Area underwent manipulation starting 2 years
ago.
The Plant to Soil Transition

How δ13C changes from plant to soil

Disequilibrium factor
Difference in atmospheric CO2 between time of soil
carbon production and current plant utilized CO2
 Modern CO2 averages -8 per mil while 30-100 years ago
CO2 averaged -6.5 per mil.
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Decomposition is likely not a factor
How δ
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13C
changes with depth
Many studies have
indicated that δ 13C
becomes enriched with
depth.
(Nisselbaum et. al 1974, Deines 1980, Ehleringer et.
al 2000)

Also that soil organic
matter decreases in size
and amount with depth.
What causes this?

4 hypotheses from Ehleringer et. al 2000
Atmospheric CO2 change over time
 Microbial fractionation during decomposition
 Preferential microbial decomposition of litter
 Mixing of C in the soil
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Plant δ
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
13C
Relative to Surface
The mean δ 13C of all the plant samples
collected was -22.21 per mil
The mean δ 13C of all the surface soil samples
(PRP-5,7) was -19.33
Clearly an enrichment has occurred from just
the transition of living plant material to soil
organic matter.
δ

13C
change in the Soil Profile
Ignoring the isotopic values or large size plant
fragments removed from soil the δ 13C changed
as follows
O horizon -19.05 per mil
 A horizon -18.83 per mil
 Ap horizon -15.76 per mil
 B horizon -22.94 per mil

Do our results match the model?
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All of the data collected matched the idea of
soil organic matter becomes enriched in δ 13C
with an increase in depth.

The one exception was the B horizon which in
both samples showed a more depleted δ 13C
than any other soil.
Conclusions
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δ 13C decreases with soil depth at Prairie Ridge,
NC
Likely a combination of processes
Soil mixing
 Disequilibrium effect
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A great deal of information can be learned from
this site if coupled with a definite time of
transition from C3 to C4 dominated.
References
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Balesdent et. al, 1993. Site-related δ13C of Tree Leaves and Soil Organic
Matter in a Temperate Forest. Ecology 74: 1713-1721.
Ehleringer et. al, 2000. Carbon Isotope Ratios in Belowground Carbon Cycle
Processes. Ecological Application 10: 412–422.
Keyworth, Amy J. An Investigation into the Fate of Carbon, Nitrogen, and
their Isotopes in a Former Cow Pasture in the Piedmont Region of North
Carolina
Yamakura,T. and Sahunalu, P. 1990. Soil Carbon/nitrogen Ratio as a Site
Quality Index for Some South-east Aasian Forests. Journal of Tropical Ecology. 6:
371-378.