Thermodynamics of Soil Microbial Metabolism: Applications and Functions

Abstract

The thermodynamic characterization of soils would help to study and to understand their strategies for survival, as well as defining their evolutionary state. It is still a challenging goal due to difficulties in calculating the thermodynamic state variables (enthalpy, Gibbs energy, and entropy) of the reactions taking place in, and by, soils. Advances in instrumentation and methodologies are bringing options for those calculations, boosting the interest in this subject. The thermodynamic state variables involve considering the soil microbial functions as key channels controlling the interchange of matter and energy between soil and the environment, through the concept of microbial energy use efficiency. The role of microbial diversity using the energy from the soil organic substrates, and, therefore, the who, where, with whom, and why of managing that energy is still unexplored. It could be achieved by unraveling the nature of the soil organic substrates and by monitoring the energy released by the soil microbial metabolism when decomposing and assimilating those substrates. This review shows the state of the art of these concepts and the future impact of thermodynamics on soil science and on soil ecology.

Microbial Necromass in Soils—Linking Microbes to Soil Processes and Carbon Turnover

“…For soil they have to be related to complex and diverse energyconsuming processes and including the formation of microbial biomass, necromass, and subsequently SOM (Chakrawal et al, 2020;Barros, 2021). Thermodynamic properties of low-molecular-weight organic compounds have been related to their elemental composition and the nominal oxidation state of C (NOSC) in organic compounds (including SOM) and were suggested as a proxy for the potential release of Gibbs energy during the oxidative degradation of these compounds (LaRowe and van Cappellen, 2011).…”

“…The CUE concept relates C turnover to microbial growth and provides much deeper insight into the relevant processes of C dynamics when considering biomass and the resulting necromass formation as substrate and nutrient resources in soil. However, over the last decade the role of energy fluxes has hardly been given any consideration, with the main focus having been on the turnover of C, N, and P. Combined mass turnover and energy balances enable evaluation of the amount of energy derived from C sources degradation as well as the amount of energy retained in soil by microbial biomass (Barros and Feijóo, 2003;Barros, 2021).…”

“…The CUE concept relates C turnover to microbial growth and provides much deeper insight into the relevant processes of C dynamics when considering biomass and the resulting necromass formation as substrate and nutrient resource in soil. Combined mass turnover and energy balances enable evaluation of the amount of energy derived from C sources degradation as well as the amount of energy retained in soil microbial biomass (Barros and Feijóo, 2003;Barros, 2021). Energy is needed for microbial biomass and necromass formation, but it is also stored in the soil by necromass retention.…”

The effect of extreme temperatures on soil organic matter decomposition from Atlantic oak forest ecosystems

“…γ C yields the Gibbs energy change for the SOM combustion in the DSC by the following relation ( Sandler and Orbey, 1991 ): Δ c G = γ C (−110.23 kJ Cmol –1 ) Where Δ c G is the Gibbs energy change for the combustion of organic substrates. Considering results in previous works, SOM at a higher degree of reduction would involve more negative ΔG values ( Barros et al., 2020 ; Barros, 2021 )that would be interpreted as increased potential for spontaneous SOM combustion as temperature increases. Here, what we find is that SOM more reduced than carbohydrates in H and M samples, and therefore, with more negative Gibbs energy values and higher potential to combust spontaneously, are the ones that did not resist the heatwave.…”

“…It allows determining ΔG through several models connecting the enthalpies and Gibbs energy to the degree of reduction of SOM ( Roels, 1983 ; Sandler and Orbey, 1991 ). Q SOM would yield the degree of reduction of SOM by the following equation ( Sandler and Orbey, 1991 ): Q SOM ≈Δ c H SOM = γ C ( – 109 kJ Cmol –1 ) Where Δ c H SOM is the enthalpy of SOM combustion, γ C represents the degree of reduction of Carbon, C, in SOM, and −109 kJ Cmol −1 is the oxycaloric quotient for the aerobic reaction ( Sandler and Orbey, 1991 ; Gary et al., 1995 ; Barros et al., 2020 ; Barros, 2021 ). γ C yields the Gibbs energy change for the SOM combustion in the DSC by the following relation ( Sandler and Orbey, 1991 ): Δ c G = γ C (−110.23 kJ Cmol –1 ) Where Δ c G is the Gibbs energy change for the combustion of organic substrates.…”