TESTING FOR KINETIC FRACTIONATION OF SOIL ORGANIC CARBON WITHIN HOLOCENE SOILS

Carbon isotopes are currently used as a proxy to determine the relative contribution of C3 and C4 plant functional types to the soil organic matter (SOM) pool on a landscape and observe how they have changed over time. But what role, if any, do degradation processes have on the fidelity of ¹²C/¹³C ratios in buried SOM? At the Claussen site, just west of Topeka, Kansas, an exposure of Holocene alluvium contains paleosols ranging in age from 9,200 to 810 yrs B.P. Analyzing 11 samples taken at various depths from the buried soils and surface soil, I have conducted bulk soil carbon and C-isotope analyses. The SOM’s physically protected carbon is in soil microaggregates, samples of which, when extracted with NaCl solutions plus sonification and pH adjustment as dissolved and suspended fractions, can be subjected to C-isotope analyses and compositional analysis using thermal and Fourier infrared methods. Comparing the results of the thermal and infrared analyses to those of the stable C-isotope analyses sheds light on whether the isotopic signature of the SOM might be affected by kinetic fractionation during decomposition.