Substrate affects microbial driven distribution of energy and matter among organic carbon functional pools in soil - Driver Pool

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Microbial transformation of organic matter (OM) is a necessity for long-term storage of SOM. Thereby, microorganisms metabolize OM through enzymatic activity, converting it into own biomass or respiring it for energy consumption. This results in a flux of energy and matter that is modulated and retarded by continuous recycling of matter and residual energy. The carbon use efficiency (CUE) of transformation depends on the properties of the various substrates (quality of provided energy), on the microbial metabolism, the connectivity within the microbiome, and environmental factors. In order to model that complex, combined turnover of matter and energy a more generalized conception is required. This asks for a thermodynamic-based approach describing transformations and interactions on the level of OC turnover and distribution among different functional pools in soil. Additionally, process kinetics (rates) must be considered because the optimum trade-off between efficiency and rate marks the process optimum.Hence, the overall aim of the project Driver Pool is to understand the interplay between energy fluxes/thermodynamic balances and substrate turnover pathways, modulated by shifts in microbial abundance, community structure and functioning. Which are the most relevant thermodynamic driving forces directing OC transformation processes and distribution among OM functional pools in soil? We hypothesize that OM turnover is driven by an energy flux-controlled interplay between three process types varying in their rate, energy balance and tendency to dissipate, keep or accumulate Gibbs energy. Furthermore, OC from substrates may be distributed among various functional pools in soil that can be energetically distinguished in “enthalpy storage pools” representing ordered structures like biomass and “(temporary) entropy storage pools” like supramolecular OM and necromass.The project is part of a group of coordinated core-projects. Joint microcosm incubation experiments were planned that are coordinated between the three PIs. The turnover of seven substrates, mostly with well-defined molecular and thermodynamic properties, added to soil will be tested in five experiments going across four work packages that are assigned to the three PIs. We aim to determine (i) complete balances of energy and matter for microbial turnover, (ii) kinetics of microbial driven processes and (iii) quantities of the distribution of substrates among soil functional pools, in order (iv) to find out which substrate properties direct fluxes of energy and matter into which pools, resulting in substrate specific CUE, EUE, and heat flow respectively. Finally, (v) a balance-based thermokinetic model will be developed integrating the combined data of WP 1-3. This model will be based on the law of Hess representing the connection between material and energy fluxes (formulated for enthalpy, entropy, Gibbs energy).