## 3 Results
### Thermodynamics of SOM Microbial Decomposition
#### 3.2.1 Thermodynamic State Variables and Soil Microbial Diversity  
The study of SOM biodecomposition by thermodynamic state variables determined experimentally remains a distinct goal [26; 43; 44]. This subject has been discussed typically on a theoretical basis [2; 45] due to difficulties in calculating the thermodynamic state variables for SMM. Differences in soil properties and soil chemical and thermal composition yield distinct microbial structures affecting the metabolic heat rate and the kinetics of decomposition of external C sources. This is reflected in the profiles of the calorimetric plots [41; 43] that can be related to the decomposition of distinct substrates, but not to the genetic microbial structure in soils. It is demonstrated by calorimetry that different microbial structures may yield different metabolic efficiencies and that the CR may be sensitive to changes in the soil microbial composition linked to a certain soil management [39]. Some results also indicate how microbial diversity is involved in the sensitivity of the soils to temperature by calorimetry [46; 47], but there are difficulties in relating all of them to a certain microbial structure because heat rates largely depend more on the enzymatic microbial diversity than on the microbial composition. That is, diverse microorganisms contributing to microbial genetic diversity can synthesize the same enzymes and run the same metabolic paths by similar metabolic heat rates. In this sense, calorimetry would be more useful for studying soil microbial metabolic diversity because it detects soil microbial biodegradation and assimilation of different substrates constituting a method for quantitative assessment of soil microbial metabolic diversity [41; 43]. Nevertheless, none of those applications connects soil microbial diversity with thermodynamic functions. This subsection provides some initial results connecting the thermodynamic characterization of SMM with the SOM thermodynamic properties shown in Tables 2 and 3. There are some additional samples to reveal the possible connection between the thermodynamic properties and the soil microbial metabolic diversity in mineral soil samples. The enthalpy change of the microbial reaction taking place in a calorimeter can be directly determined by caloresiprometry and the CR which gives the thermodynamic state variables for Equation (4). Table 4 shows the results obtained for the soil samples handled in this review. In this case, only [PERSON]'s model is applied to obtain Gibbs energy. The reason is to avoid incongruences with the entropy change, as happened with the characterization of SOM. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & \(-\)**CR** & \(-\)**CR** & & & & \(-\Delta_{\tau}\)**GSOM** & \(-\Delta_{\tau}\)**GSOM** & \(\Delta_{\tau}\)**SSOM** & \(\Delta_{\tau}\)**SSOM** \\
**Samples** & **kJ mol\({}^{-1}\)CO\({}_{2}\)-C** & **kJ mol\({}^{-1}\)CO\({}_{2}\)-C** & **Y** & **Y** & **kJ mol\({}^{-1}\)C** & **kJ mol\({}^{-1}\)C** & **JK\({}^{-1}\)mol\({}^{-1}\)C** & **JK\({}^{-1}\)mol\({}^{-1}\)C** \\  & **LF** & **M** & & & **LF** & **M** & **LF** & **M** \\ \hline ROG & 423 \(\pm\) 30 & 280 \(\pm\) 7 & 3.88 & 2.57 & 427 & 283 & 16.0 & 10.6 \\ BW & 507 \(\pm\) 19 & 276 \(\pm\) 9 & 4.65 & 2.53 & 513 & 279 & 19.2 & 10.5 \\ NF & 312 \(\pm\) 10 & 253 \(\pm\) 12 & 2.86 & 2.32 & 316 & 256 & 11.8 & 9.6 \\ DC & 310 \(\pm\) 44 & 400 \(\pm\) 68 & 2.85 & 3.67 & 315 & 405 & 11.8 & 15.2 \\ G & 327 \(\pm\) 41 & 485 \(\pm\) 9 & 3.00 & 4.45 & 332 & 490 & 12.4 & 18.4 \\ K & 355 \(\pm\) 51 & 421 \(\pm\) 17 & 3.26 & 3.87 & 359 & 427 & 13.4 & 16.0 \\ \hline \end{tabular}
\end{table} 
Table 4: Values for the caloresiprometric ratio, CR, for the LF and M samples from the UK (ROG, BW, and NF) and Ireland (DC, G, and K); the degree of reduction of substrates being metabolized, \(\gamma_{\tau}\), is obtained By Equation (5).