Thermodynamic control of microbial turnover of organic substrates in soils

Microbial turnover of organic substrates is a key process in soil organic matter formation and turnover. As microorganisms require both carbon and energy for growth and maintenance, carbon and energy fluxes in soils are tightly coupled. On the level of cellular metabolism, the substrates have to be allocated to catabolism and anabolism according to the requirements of the cells. In the soil system, additional processes have to be considered such as multiple substrate use, recycling of biomass components, interaction between different organisms and abiotic processes. As most of the energy flux in catabolism is created by the reduction of the terminal electron acceptors, the availability of the electron acceptors strongly affects carbon use efficiency and energy use efficiency. Here, we present a thermodynamic concept that combines experimental approaches of calorimetry and turnover mass balances paving the way for a better understanding of microbially mediated organic matter turnover and stabilization in soil. Mass balances in soil systems need to be set up for exemplary substrates using isotope labelled compounds. They should be combined with information on energy fluxes, which can be obtained using calorimetric methods for thermodynamic calculations. Recently, calorimetric methods have been introduced into soil studies, e.g. differential scanning calorimetry or isothermal reaction calorimetry. Alternatively, enthalpies of combustion or formation must be known or estimated, e.g. based on the nominal oxidation state of the substrates and reaction products. All of these methods have their strengths and weaknesses, which need to be considered when assessing and interpreting the results. From a thermodynamic perspective, it is crucial to define the system boundaries and to use thermodynamic state variables such as reaction enthalpy, entropy and Gibbs free energy. If applied properly, the predictive power of thermodynamics can be fully utilized for process evaluation. In particular, this approach will enable us to identify whether a particular process is thermodynamically feasible or not under the given conditions. In summary, linking mass balances and thermodynamics will allow us to better understand and predict soil organic matter turnover and sequestration.