Thermodynamics of microbial turnover of organic compounds in soil - matter and energy flux under varying environmental conditions (water, nutrient availability, temperature) – TherMic

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Soil microbial communities are both drivers of soil organic matter (SOM) dynamics and dominant contributors to SOM. They modulate soil carbon dynamics and fluxes through microbial biomass along different flux channels finally resulting in stabilised necromass in SOM. The basic hypothesis of SPP 2223 thus is that most of the C flux towards SOM pass through microbial biomass, which needs energy for growth and maintenance. Energy consideration of soil processes based on thermodynamic principles is needed, since huge amounts of solar energy pass through soil and are partly conserved in SOM. During microbial metabolism, parts of the organic substrates are used for energy delivery (catabolism) and energy conservation by biomass formation (anabolism). In addition, small amounts of nutrients and biomolecules in soils may result in higher biomass yields from a growth substrate, because less energy is needed for biomass compound synthesis. Microbial energy use efficiency (EUE) and carbon use efficiency (CUE), i.e. the balance of anabolic versus catabolic processes, are thus the overarching determinants of microbial turnover of organic substrates in soils and retention of C and energy in SOM. Boundary conditions, including temperature, soil moisture, bulk density (gas diffusion), and nutrient availability, strongly effect the relation between matter and energy flux and thus CUE and EUE. Therefore, the present core project TherMic aims at directly linking thermodynamics to microbial mass turnover and growth and alterations of microbial communities as well as at studying the effect of boundary conditions on matter and energy turnover in soil systems. TherMic addresses this objective by laboratory incubation studies with simultaneous analysis of carbon and energy fluxes from selected substrates (cellulose, peptidoglycan). We use isotope-labelled substrates to follow the flux of C through the system combined with calorimetry to assess energy balances. Incubations at different boundary conditions allow to study the effect of temperature, soil moisture and nutrient availability on mass and energy fluxes and balances. In addition, for broader application of simultaneous analysis of matter and energy fluxes, we develop a calorimeter for application in standardised soil tests. Finally, all our results are used to develop models describing and predicting C and energy fluxes on the basis of energy and mass balance data.The results from TherMic are complemented by collaboration within the SPP, in particular with the other core projects analysing microbial communities and trophic networks in the proposed experiments. At the end, TherMic will allow a better understanding of turnover processes and estimating how much of the potentially available energy can actually be used under given soil conditions.