Both soil minerals and organic material contribute to the energy content of soil – Insights from an artificial soil experiment and calorimetric analyses

The thermodynamic perspective on soil systems gets more and more in the research focus and has the potential to take us a substantial step toward a holistic understanding of soil organic matter (SOM) turnover and stabilization. An integral part of new bioenergetic concepts and models is the energy content of SOM, but its determination particularly in mineral soils is challenging. One of the most promising techniques in this respect is thermogravimetry combined with differential scanning calorimetry (TG-DSC), where the heat of combustion is related to the mass losses of soil material during a temperature ramp from 50 to 1000°C under an oxidative atmosphere. Heat and mass changes in the range from 180-600°C are usually interpreted as the result from the exothermic reaction of SOM and thus used to obtain the energy content (combustion enthalpy, ∆_CH) of SOM. Overlapping exo- and endothermic reactions by other non-oxidizing processes (e.g. dehydroxylation/-carboxylation and desorption of soil minerals etc.) in that temperature range are often neglected because their distinction and quantification from the rather strong exothermic oxidation reactions from SOM is challenging.

To investigate this, we determined the ∆_CH of an organic substrate (cellulose) and soil minerals (quartz sand, quartz silt, goethite, illite, montmorillonite) 1) individually, 2) intensively mixed in the dry state, and 3) intensively mixed after several wetting-drying cycles. Furthermore, the minerals were mixed to create a silt loam texture and combined with cellulose to mimic an artificial soil. Calorimetric analyses were conducted using a TG-DSC coupled with a mass spectrometer (MS) to analyze the evolved gases during combustion.

First results show that the ∆_CH value obtained by TG-DSC is lower for the organic substrate compared to reference values obtained by combustion calorimetry as the standard method. Furthermore, ∆_CHdiffers when mineral compounds are mixed with cellulose indicating that thermal reactions by mineral soil compounds affect the determination of the energy content by the TG-DSC standard procedure described above. This is supported by the analyses of the pure mineral compounds, which revealed that all investigated minerals show exothermic and/or endothermic side reactions in the range from 180-600°C affecting the TG-DSC signal. In dependence on the mineral composition of the soil, the energy content of SOM by the classical TG-DSC approach can be substantially over- or underestimated.

From this first data set, we identified options to improve both the measurement and the data evaluation procedures. Building on this, we aim to develop a procedure for the accurate measurement of the energy content of SOM in (mineral) soils by TG-DSC(-MS), taking into account the contribution of mineral oxidation and the effect of organo-mineral associations on the energetic signatures derived from the thermograms. This is crucial if energy flows and sinks in soil systems are to be quantified to better understand OM turnover and stabilization in soil.