IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 8 KL Thermodynamic and Calorimetric Quantification of Soil Organic Matter Turnover Thomas Maskow1, Du Yuan1, Shiyue Yang1, Christian Fricke2,3, Marcel Lorenz4,5, Eliana di Lodovico1,2, Anja Miltner5, Mathias Kästner5 1 Helmholz Centre for Environmental Research-UFZ, Department Microbial Biotechnology, Permoserstr. 15, 04318 Leipzig, Germany, e-mail: thomas.maskow@ufz.de 2 RPTU Kaiserslautern-Landau, iES Landau, Institute for Environmental Sciences, Group of Environmental and Soil Chemistry, R, Fortstraße 7, 76829 Landau, Germany 3 Duale Hochschule Sachsen - Staatliche Studienakademie Riesa, University of Cooperative Education, Rittergutstraße 6, 01591, Riesa, Germany 4 Trier University, Soil Science, FB VI, Behringstraße 21, 54296 Trier, Germany 5 Helmholz Centre for Environmental Research-UFZ, Department Molecular Environmental Biotechnology, Permoserstr. 15, 04318 Leipzig, Germany Soil organic matter (SOM) turnover is commonly described in terms of carbon pools, microbial activity, and mass balances. However, organic matter entering soil is not only a source of carbon and nutrients, but also a carrier of chemical energy. During microbial decomposition, this energy is partially lost trough respiratory processes and dissipation as heat, while the remaining fraction is conserved in microbial biomass, necromass, and ultimately stabilized SOM. Quantifying these energy transformations may therefore provide an additional perspective on soil carbon cycling and on the mechanisms controlling SOM persistence [1]. In this contribution, I will present how calorimetric and thermodynamic approaches can be used to complement classical carbon-based descriptions of SOM turnover. Isothermal calorimetry and calorespirometry allow microbial heat production to be linked with CO2 or O2 fluxes, thereby coupling stoichiometric balances with enthalpy balances and providing access to reaction enthalpies, and energy dissipation during microbial turnover in combination with thermodynamc modelling. These measurements can be further used to estimate thermodynamic state functions such as Gibbs energies and entropies, and use efficiencies of carbon (CUE) and energy (EUE). By combining carbon and energy balances, these approaches make it possible to distinguish whether SOM turnover primarily results in respiratory losses, heat dissipation, or the formation of microbial biomass and necromass. In addition, combustion calorimetry and thermal analysis provide information on the chemical energy stored in SOM itself, which is required to close energy balances and to develop thermodynamically consistent models of soil carbon turnover.
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