## 3 Results and Discussion
### Bulk \(\delta^{13}\)C, \(\delta^{15}\)N IRMS, and C, N Content Analysis  
In agreement with the general CSIA trend, bulk \(\delta^{13}\)C values of maize biomass expertise a light but significant enrichment during the composing process from \(-14.64\) at the initial stage (T1) to \(-13.07\) at final stage (T4) (F = 56.835, \(p\) = 0.000) (Figure 5, Table S2). Both, average CSIA values and bulk, measurements showed \(\delta^{13}\)C values corresponding to literature reported values for maize compost [72; 73; 74]. Moreover, it could be pointed out--as a good signal in analytical terms--that the average \(\delta^{13}\)C Py-CSIA values and \(\delta^{13}\)C bulk values are highly correlated (R\({}^{2}\) = 0.8059; Figure 5). However, and accounting for the great heterogeneity of complex samples, such as compost, the variability in bulk data is greater than in compound-specific values, where a larger amount of sample is being weighted and yielded in a bulk combustion reactor. Nevertheless, that heterogeneity is considerably minimized during the sample preparation sampling and process.  
Regarding the total content of organic carbon, a significant increase can be observed till T3, at which point it starts decreasing (Table 3). Although the C content is expected to decrease due to the mineralization mechanism, this variability may be explained by the increase of the aromaticity and further condensation of the newly compounds that are being generated during the composting process, thus compensating the C content.  
Finally, the C:N ratio has been frequently used to assess compost maturity [75]. An initial C:N ratio of 30 is considered optimal for ensuring that the composting process takes place satisfactorily [9]. Then, during composting, C:N ratios decrease to 10-15, as previously reported for fully stabilized compost [76], providing additional information of the humification stage of OM of composts. C:N ratios varied from 17.4 to 21.9 at its final stage. As the starting C:N ratio is low, this ratio is increasing during the composting process, indicating a mineralization of nitrogen as supported by its total content. However, there is not a clear trend that may totally support this statement, apart from considering that values around 20 are a good proper indicator of mature composting material.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Orig 1** & **T1** & **T2** & **T3** & **T4** \\ \hline C content (\%) & 21.4 (4.7) a & 25.8 (3.8) a & 33.6 (2.5) b & 30.1 (0.5) b \\ \hline N content (\%) & 1.2 (0.2) a & 1.4 (0.1) a & 1.7 (0.01) b & 1.4 (0.01) a \\ \hline C:N ratio & 17.4 & 18.5 & 19.3 & 21.9 \\ \hline \(\delta^{13}\)C bulk (\%) & \(-\)14.6 (0.3) a & \(-\)14.3 (0.3) a & \(-\)13.4 (0.01) b & \(-\)13.07 (0.02) c \\ \hline \(\delta^{15}\)N bulk (\%) & 13.5 (0.3) b & 14.01 (0.6) b & 15.0 (0.4) c & 9.7 (0.4) a \\ \hline \hline \end{tabular} Same letters indicate no significant differences (Kruskal–Wallis test, \(p<0.05\), Dunn post-hoc); lowercase letters indicate significant differences comparing different composting times.  
\end{table}
Table 3: Description of the compost samples: total C and N contents, C to N ratios, \(\delta^{13}\)C, and \(\delta^{15}\)N bulk isotopic data. Average values are as (\(n=2\)), including standard deviations (in brackets).  
Figure 5: Correlation between \(\delta^{13}\)C mean values obtained from Py-CSIA (\(n=3\)) versus EA-IRMS values (\(n=2\)). Error bars represent mean standard deviation (horizontal: bulk measurements; vertical: Py-CSIA).