Methodological considerations for using thermal analysis in the characterization of soil organic matter

Thermal analysis is primarily used in the field of materials science, but has a long history in the geosciences. Soil organic matter (SOM) has received a great deal of recent scientific interest because of its role in the global carbon cycle. Conventional methods of characterizing SOM quality are unsatisfactory because they do not adequately capture the complete quality continuum that SOM comprises or the various mechanisms that act to stabilize it in the soil matrix. Thermal analysis techniques have the potential to capture this quality continuum, but are dependent on numerous experimental conditions that limit the comparability of results among different studies. Published methodology on thermal analysis of soils and sediments has largely focused on the characterization of the mineral component, while the organic component has received little attention. We tested several experimental conditions for their effects on the exothermic region of curves generated by thermal analysis of easily dispersed soil clay fractions and non-protected light-density particulate organic matter fractions isolated from the surface horizon of a forest soil. Results were found to be highly repeatable but strongly sensitive to crucible material, heating rate, and sample amount, and relatively insensitive to the use of a reference material. Thermal analysis is an important addition to the set of analytical tools used to characterize SOM quality because it provides direct, quantitative information of the energy potentially available for microbial metabolism. However, users will need to balance the needs of specific scientific objectives with the need for standardized methods and comparability between studies.