Folks,

I am characterizing particulate plant derived organic matter input to 
the soil.  The approach I have taken is planting C3 plants into a 
soil with a long C4 history.  The particulate organic matter in the 
soil at the start of the experiment had a del vPDB signature of 
-14.000 and the new C3 input has a del vPDB of -28.000.  My problem 
is that the protocol for isolating the organic matter requires that I 
co-collect the organic matter with the sand fraction of the soil, 
which contains about 18% carbonate with a del vPDB of 0. I'm 
wondering if anybody can help me with a three source mixing model 
that can be run using only one isotope?  Using a separate analysis of 
the soil I can calculate the amount of carbonate in each sample but I 
need to determine the proportion of each plant source (C3 or C4) in 
the remaining organic carbon.
Ideally I would have removed the carbonate prior to isotopic analysis 
but at the time of sample prep I didn't realize how much of an issue 
the carbonate would be.

Thanks
jude

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Jude Maul
Department of Horticulture
Cornell University
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(607)255-0883 (office)
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