Contribution of minerals and organic material to the energetic signatures of soil – Insights from an artificial soil experiment and calorimetric analyses

International audience

Inglés. The thermodynamic perspective on soil systems is receiving increasing attention and has thepotential to contribute to a holistic understanding of soil organic matter (SOM) dynamics. Anintegral part of new bioenergetic concepts is the energy content of SOM, but its determinationparticularly in mineral soils is challenging. A promising technique in this respect is thermogravimetry combined with differential scanning calorimetry (TG-DSC). Here, the heat flow and mass losses are measured simultaneously in a controlled combustion of the soil material during a temperature increase from 50 to 1000 °C under an oxidative atmosphere. Heat and mass changes in the range of 180-600°C are usually interpreted as the result from the exothermic combustion reaction of SOM and thus used to derive the energy content (combustion enthalpy, ΔCH) of SOM. Overlapping exo- and endothermic reactions by other non-oxidizing processes (e.g. dehydroxylation/-carboxylation and desorption of soil minerals) in that temperature range are often neglected because their quantification and differentiation from the rather strong exothermic oxidation reactions of SOM is challenging. To disentangle these reactions, we determined the ΔCH of cellulose as model substrate for SOM and additional soil minerals (quartz sand, quartz silt, goethite, illite, montmorillonite) 1) individually, 2) intensively mixed in the dry state, and 3) intensively mixed after several wettingdrying cycles. The minerals were mixed to create a silt loam texture and combined with cellulose to mimic a soil. Calorimetric analyses were conducted using a TG-DSC coupled with a mass spectrometer to analyze the gases (H2O, CO2) evolved during combustion. First results show that the ΔCH value obtained by TG-DSC is lower for cellulose to reference values obtained by combustion calorimetry as the standard method. Furthermore, ΔCH differs when mineral compounds are mixed with cellulose, indicating that thermal reactions by mineral compounds affect the determination of the energy content by TG-DSC. This is supported by the analyses of the pure minerals, which revealed exothermic and/or endothermic side reactions for all minerals in the range from 180-600°C affecting the TG-DSC signal. Depending on the mineral composition of the soil, the energy content of SOM can be substantially over- or underestimated by TG-DSC. This is particularly important for soils rich in oxides and clay minerals.