Hi, forget about a 3-component mixing model. Re-analyse your samples after proper removal of the carbonate. Good luck, Michael > 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 > > -- > #################### > Jude Maul > Department of Horticulture > Cornell University > [log in to unmask] > (607)255-0883 (office) > #################### >