Investigation of thermal reactions and energy content of building blocks of soil organic matter using simultaneous thermal analyses

Soils are multi-component, open systems and play a decisive role in natural energy and matter fluxes, e.g. in the storage and bioenergetic control of carbon. A key component in this system is soil organic matter (SOM), as it determines the functionality of the soil. SOM is formed by building blocks from biomass, plant and animal detritus. Therefore, SOM itself is a complex, supramolecular mixture of different components. This property complicates the thermodynamic characterization of SOM and consequently the determination of the energy content. The latter is an important piece of the puzzle for a thermodynamic description of energy fluxes in soil systems, which is necessary for a holistic understanding of SOM turnover and stabilization.

In soil science, simultaneous thermal analysis (STA) is used to determine the energy content of SOM. The soil sample is heated in crucibles in a defined temperature program (e.g. 30-1000 °C, 10 °C/min) under an oxidative atmosphere. During heating, the mass loss (thermogravimetry, TG) and heat flux caused by SOM combustion (dynamic scanning calorimetry, DSC) are measured simultaneously. The STA data can be used to determine the energy content of the SOM during combustion and to identify SOM fractions of different thermal stability.

To develop a deeper understanding of the reactions taking place during STA of SOM, we investigated the combustion of building blocks of SOM and examined the influence of different crucible setups (Al₂O₃ with and without lid, Pt-Rh-Al₂O₃ with lid) on the measured energy content. The selection of building blocks included well-defined compounds like glucose, cellulose, chitin, etc. and complex compounds like maize straw, peptidoglycan, humic acid and lignin (organosolv). The STA was coupled with an evolved gas analyzer (mass spectrometer, MS) to draw also conclusions about the combustion reactions of the building blocks by monitoring H₂O and CO₂.

Our results show two main points: First, the crucible setup has a huge impact on the measured energy content. A better thermal conductivity (Pt-Rh > Al₂O₃) and the use of a lid lead to an increase in the measured energy content. Secondly, we observe two distinguishable thermal reactions for most of the building blocks, which were mainly revealed by the release of H₂O and CO₂. The first reaction is a decomposition at low temperatures (< 400 °C) with the formation of char, which was then further oxidized at higher temperatures (> 400 °C). The change in the ratio between the H₂O- and CO₂-MS signal allows a clear allocation of different thermal reactions.

In summary, the investigation of building blocks of SOM by STA coupled with MS provides a better understanding of the combustion of SOM in soil samples and thus allows a more reliable interpretation of the measured energy contents. Further STA studies should focus on the interaction between SOM building blocks and soil minerals to identify other possible thermal reactions that could affect the measured energy content.