## 3 Results
### Thermodynamic Characterization of SOM
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(-\mathbf{Q}_{\text{SOM}}\) & \(-\mathbf{Q}_{\text{SOM}}\) & \(-\mathbf{\Delta}_{\text{c}}\mathbf{H}_{\text{SOM}}\) & \(-\mathbf{\Delta}_{\text{c}}\mathbf{H}_{\text{SOM}}\) \\
**Samples** & **kj g\({}^{-1}\)SOM** & **kj g\({}^{-1}\)SOM** & **kj mol\({}^{-1}\)C** & **kj mol\({}^{-1}\)C** \\  & **LF** & **M** & **LF** & **M** \\ \hline ROG & 15.4 & 21.9 & 449 & 617 \\ BW & 15.1 & 20.7 & 441 & 586 \\ NF & 15.1 & 22.2 & 440 & 624 \\ DC & 15.0 & 26.4 & 437 & 734 \\ G & 16.4 & 25.0 & 475 & 697 \\ K & 15.9 & 21.3 & 461 & 602 \\ \hline \hline \end{tabular} \begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{1}{c}{\multirow{2}{*}{\begin{tabular}{c} \text{YSOM} \\ \text{LF} \\ \end{tabular} }} & \multirow{2}{*}{\begin{tabular}{c} \text{YSOM} \\ \text{LF} \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} \text{YSOM} \\ \text{LF} \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} \text{YSOM} \\ \text{M} \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} \text{YSOM} \\ \text{M} \\ \end{tabular} } \\ \hline ROG & 3.90 & 4.11 & 5.36 & 5.66 \\ BW & 3.83 & 4.05 & 5.10 & 5.39 \\ NF & 3.83 & 4.04 & 5.43 & 5.72 \\ DC & 3.80 & 4.01 & 6.38 & 6.73 \\ G & 4.13 & 4.36 & 6.06 & 6.39 \\ K & 4.01 & 4.23 & 5.23 & 5.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Thermodynamic data of samples obtained directly from DSC curves, Q\({}_{\text{SOM}}\) and \(\Delta_{\text{c}}\)H\({}_{\text{SOM}}\), and the degree of reduction of samples, \(\gamma_{\text{SOM}}\) determined by [PERSON], \(\gamma_{\text{SOM}}\)R, and [PERSON] and [PERSON], \(\gamma_{\text{SOM}}\)S&O, correlations for the LF soil layer and mineral soil samples, M. Samples ROG, BW, and NF are from the southeast of the UK. Samples DC, G, and K are from the southwest of Ireland. The reproducibility of measurements by DSC-TG is 5%. The uncertainty averaged for the degree of reduction is \(\pm\)0.2 based on the standard errors reported for both models [27; 28]. Figure 1: A summary of the procedure followed to approach the enthalpy of combustion of SOM, \(\Delta_{\text{c}}\)H\({}_{\text{SOM}}\), by simultaneous DSC–TG. The Gibbs energy change is determined by the [PERSON] and Sander and Orbey correlations, and the entropy change is from Equation (3). QSOM and \(\Delta_{\text{c}}\text{H}_{\text{SOM}}\) values in Table 2 show higher values in mineral samples than in LF samples, indicating SOM at a higher degree of reduction as soil depth increases. This involves a change in SOM nature, and it is expected that it affects the soil microbial population, too. The degree of reduction values yields the Gibbs energy change of SOM combustion, \(\Delta_{\text{c}}\text{G}_{\text{SOM}}\), and, indirectly, the entropy change, \(\Delta_{\text{c}}\text{S}_{\text{SOM}}\), for Equation (1). Results are shown in Table 3. The Gibbs energy becomes more negative in mineral samples than LF samples, independently of the models used. Although, apparently, these models yield similar values for Gibbs energy, incongrurences between them appear when calculating the entropy changes. By comparing results using the Sandler and Orbey model alone [28], it is observed that a higher degree of reduction of SOM could yield a relative increment in the entropy change, compatible with the higher structural complexity of SOM and/or higher stable material in M samples than in LF samples. Little variability of the entropy change is also obtained among the different locations.