## 3 Results
### Thermodynamics of SOM Microbial Decomposition
#### 3.2.1 Thermodynamic State Variables and Soil Microbial Diversity
This is in agreement with the ecological hypothesis for coexisting microbial species on the same substrate by yielding different end products following thermodynamic constraints. Recent literature reveals how thermodynamics asserts coexistence of various species by the Gibbs energy change available from the metabolic conversions [44]. Results suggest that microbial coexistence, essential to keep microbial diversity, is based on supporting reactions with low Gibbs energy change when using the same substrate. Environments supporting reactions with low Gibbs energy change (more negative) would allow more metabolic diversity than environments allowing reactions at high Gibbs energy change (less negative). When microorganisms start to yield products at similar Gibbs energy as reactants (higher Gibbs energy change), the system would be evolving towards thermodynamic inhibition. Microorganisms overcome this limitation by yielding different products at low concentrations in the environment. Under that theory, mineral soils from the UK would be supporting lower microbial diversity and would be closer to the thermodynamic inhibition (Gibbs energy change closer to zero) than those from Ireland. Figure 3: Clusters of the soil samples based on their thermodynamic properties. LF samples from the UK (ROGLF, BWLF, and KLF) and Ireland (DCLF, GLF, and KLF) form a cluster separated from two clusters of mineral samples from the UK (ROGM, BWM, and NFM) and Ireland (DCM, GM, and KM). The expected natural trend for survival would be by favoring the coexistence of a high number of different metabolic conversions in the environment. The end products of these metabolic conversions are reutilized as an energy source by the existing and/or by the new microbial population, creating a niche for future microbial diversity through adaptation. At the thermodynamic equilibrium (\(\Delta\)G = 0), two species could not coexist in the same niche, as stated by the competitive exclusion principle [48]. Recent additional work connecting Gibbs energy of dissolved organic matter, DOM, with the microbial diversity [43] also links lower Gibbs energy values to higher microbial diversity. Microbial biodiversity is given by the number of OTUS of bacteria and fungi (Taxa_S) and by the Shannon index of diversity for four additional mineral soil samples. The mineral soil samples are Leonardite, a recalcitrant material without carbohydrates, a peat sample, and two Cambisols under different management (pine forest and pasture) collected in paired plots in the same location. These samples were surveyed and characterized in a previous paper [47]. Results are reported in Table 5.  
Leonardite, Pahokee peat, and Cambisol under pine have SOM with \(\Delta_{\text{c}}\)H\({}_{\text{SOM}}\) higher than carbohydrates, indicating more reduced substrates. Cambisol under pasture has a lower \(\Delta_{\text{c}}\)H\({}_{\text{SOM}}\) (absolute values) than the other samples. Leonardite and Cambisol under pine have less negative \(\Delta_{\text{r}}\)G\({}_{\text{SOM}}\) values and their CR values indicate that they are decomposing substrates more oxidized than carbohydrates, representing partial decomposition of substrates or higher component of anaerobic metabolism [31; 36]. Their \(\Delta_{\text{r}}\)G\({}_{\text{SOM}}\) values indicate microorganisms degrading substrates from SOM at similar free energy to the products. Leonardite contains no carbohydrates in its composition, Cambisol under pine, yes, but shows a CR value similar to the sample without them. A possible explanation could be that carbohydrates are physically protected in the Cambisol under pine, being less available to microorganisms for that reason. Pahokee peat and Cambisol pasture have CR values closer to their \(\Delta_{\text{c}}\)H\({}_{\text{SOM}}\) values suggesting degradation of carbohydrates and/or humic material [36]. Peats have a weak mineral matrix and therefore carbohydrates are more available to microorganisms than in Cambisol under pine.