Thermal analysis of soil organic matter in density fractions

Association of soil organic matter with the mineral matrix is considered a major control on organic matter quality and stability. Fractionation of soils by density separation separates organic matter more or less associated with mineral matter, where the lighter fractions have generally been found to be more C-enriched with higher C/N ratios while heavier fractions have less C and narrower C/N ratios. The objective of this study was to apply thermal analysis techniques to characterize the organic matter in density fractions from several contrasting soils. We recovered six density fractions ranging from < 1.6 g cm^-3 to > 2.6 g cm^-3, which were also characterized for mineralogy. Density fraction mass and organic C distributions differed significantly between soils. Thermal analysis of the density fractions showed that the organic matter associated with different density fractions differed within soils, as well as between soils. Our results demonstrate the importance of mineralogy and organic matter quality in soil organic matter stabilization through the formation of organo-mineral complexes.