Distribution of radiocarbon ages of soil organic matter by thermal fractionation

TitleDistribution of radiocarbon ages of soil organic matter by thermal fractionation
Publication TypeConference Paper
Year of Publication2011
AuthorsPlante, A. F., Beaupre, SR, Fernandez, JM, Roberts, M, Baisden, WT
Corporate AuthorsPlante, A. F.
Date Published2011
PublisherAmerican Geophysical Union, Washington, DC, United States (USA)
Conference LocationUnited States (USA)

Radiocarbon analysis has become an important tool in quantifying the dynamics of soil organic matter within the terrestrial carbon cycle (Trumbore 2009). Measuring radiocarbon concentrations in bulk soil samples provides only the mean age or residence time of the associated organic matter, but it is well recognized that representing soil organic matter as a single, homogeneous pool is inadequate. Significant effort has gone into separating soil organic matter into pools with different intrinsic turnover rates (i.e., radiocarbon concentrations), but the success of these separation methods has been mixed. Using thermal analysis techniques to characterize soil organic matter is rooted in the hypothesized link between the thermal and biogeochemical stability of the organic matter (Plante et al. 2011). Two grassland topsoil samples under contrasting land uses (native vegetation and long-term cultivation) were sampled in 2005 and submitted for radiocarbon analysis. Results of bulk (super 14) C showed a significant shift in radiocarbon age (94 to 79 pMC in the soil from Indian Head, Saskatchewan; 104 to 89 pMC in the soil from Akron, Colorado), attributable to the depletion of labile organic matter during long-term cultivation. Thermogravimetry and evolved gas analysis of these soil samples also showed distinct patterns in mass loss and CO (sub 2) release during thermal analysis, suggesting significant changes in organic matter composition. Four to five "fractions" associated with different CO (sub 2) -evolution regions have been identified and will be analyzed for radiocarbon via NOSAMS's established "dirt burner" method (Rosenheim et al. 2008), consisting of the recently developed discrete CO (sub 2) fraction collector interface between NOSAMS's dirt burner and microwave gas ion source (GIS) continuous flow AMS system (CFAMS). The research question is: Do the differing patterns in CO (sub 2) release during thermal analysis reflect organic matter of different ages Our working hypothesis is that thermally labile soil organic matter (combusting at low temperatures) will consist of younger carbon than thermally resistant organic matter. References: Plante, A. F., J. M. Fernandez, M. L. Haddix, and R. T. Conant. 2011. Biological, chemical and thermal indices of soil organic matter stability in four grassland soils. Soil Biol. Biochem. 43: 1051-1058. Rosenheim, B. E., M. B. Day, E. Domack, H. Schrum, A. Benthien, and J. M. Hayes. 2008. Antarctic sediment chronology by programmed-temperature pyrolysis: Methodology and data treatment. Geochemistry Geophysics Geosystems 9: 1-16. Trumbore, S. 2009. Radiocarbon and soil carbon dynamics. Annu Rev Earth Pl Sc 37: 47-66.