Soil organic matter (SOM) and is crucial for maintaining soil functionality. Carbon (C) and energy (E) fluxes during turnover of SOM are closely related, and they depend on environmental conditions and resource availability. However, we still need a solid theoretical framework for thermodynamic assessment, combined with accurate mass balances, of soil processes. The Gibbs energy release of a mineralization reaction is dominated by the reduction half reaction and thus by the terminal electron acceptor (TEA), but often only aerobic conditions (or combustion enthalpy) are considered. The availability of microbial biomass building blocks (BB) may alter the C and E balance of microbial turnover because their use will save E for biosynthesis. Finally, the abundance of active microorganisms (relative to substrate) alters substrate degradation kinetics, but little is known about its effect on C and E balances. TherMic-II aims to (1) analyse C and E fluxes under different redox regimes (oxic, anoxic, and alternating) and the effect of TEA availability on the link between C and E flux, (2) elucidate how the availability of biomass BB changes C and E fluxes, (3) study the effect of biocatalyst abundance relative to substrate availability on the C and E balances of substrate degradation, and (4) develop a concept to proceed from enthalpy to Gibbs energy.TherMic-II comprises experiments studying the effect of resource availability (TEA, biomass BB, biocatalysts/substrate) on E and C balances of substrate degradation and refinement of thermodynamic concepts for soils. The effect of TEA availability will be studied by soil incubations tracing the fate of cellobiose at oxic, anoxic and alternating redox conditions. The resulting C and E balances will be assessed with respect to different TEAs. We will study how the availability of biomass BB (e.g tryptophan) relative to cellobiose changes the C and E balances of cellobiose degradation. Incubations of soil amended with different concentrations of cellobiose will reveal the effect of the biocatalyst-to-substrate ratio on C and E balances of degradation. All experimental data will be used for thermodynamic calculations and modelling. We will also explore how we can proceed from enthalpy measured by calorimetry to Gibbs energy, the driving force of processes. TherMic-II will additionally lead an interlaboratory comparison for testing the variability of calorimetric data and thus pave the way towards synthesis of calorimetric data from different research groups. TherMic-II will also significantly contribute to the joint experiment E-ComPLEX by analysing remaining substrate and biomass/necromass formation during degradation of four selected substrates with different combustion enthalpies and metabolic functions, and to SOM Battery with PLFA analyses. The results from TherMic-II will contribute to improved prediction of organic compound turnover in soil at different environmental conditions.
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