Tuesday, November 14, 2006 - 9:00 AM

Controls Over Molecular C Dynamics and Turnover Rates of Soil Fractions in Geochemically Distinct Soils.

A.S. Grandy and J.C. Neff. University of Colorado at Boulder, Department of Geological Sciences, Boulder, CO 80309-0399

The heterogeneous nature of soil organic matter and the complex interactions between microbial and physical processes make it difficult to predict soil organic matter dynamics.  Physical soil fractionation methods that separate soil into density fractions coupled with enzyme analyses and chemical characterization of these size fractions may provide considerable insight into C transformations and stabilization.  We used pyrolysis gc/ms to determine the molecular chemistry of light and heavy soil factions from 24 samples at two elevations (alpine and forest) in two geochemically distinct basins (volcanic and sedimentary).  We also measured the activity of 10 decomposition enzymes in these soils to determine their relationship to molecular C structure across soil fractions.  Principal components analysis revealed clear distinctions between light and heavy fraction samples; light fraction organic matter across sites and elevations was characterized by high lignin content whereas heavy fraction samples were characterized by an abundance of polysaccharides.  Distinct lignin biomarkers such as guaicol, vinylguaicol, vanillin, and isoeugenol were all greater in light fraction samples.  Stabilized polysaccharides including furfural, furfural 5-methyl, and furfural 3-methyl were highly concentrated in the heavy fraction.  Nitrogen rich compounds such as pyrrole and pyridine were also primarily found in heavy fraction samples.  Our results demonstrate that there is a clear, knowable progression of chemical changes as C transitions from plant litter into stabilized SOM and that enzyme activities may be linked to the transition of specific compounds between light and heavy fraction soil organic matter.  They also suggest that the molecular structure of heavy fraction C is similar but not homogenous across soils and is dominated by polysaccharides, raising questions about the relative contribution compounds derived from lignin, protein and carbohydrate make to persistent soil C pools.