## 3 Results and Discussion  
### Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC/MS)  
A total of 97 distinct compounds were identified by analytical pyrolysis (Table S1) that can be grouped attending to their biogenic origin (Table 1). Representative pyrograms of biomass and compost material are shown in Figure 1. The pyrolizates from all samples were found dominated by lignocellulosic derived compounds; polysaccharides (PS) and lignin-derived phenols that included the main three methoxyphenol types: \(p\)-hydroxyphenyl (LH), guiaacyl (LG), and syringyl units (LS). Other compounds were also present in relative minor proportion, such as nitrogen-derived compounds (N) derived from proteins and polypeptides, aromatic compounds with unspecific origin (ARO), and some unidentified compounds (UNK). It is also remarkable the presence of mid-chain lipids (LIP), alkanoic acids (fatty acids, FA), fatty acid methyl esters (FAME), isoprenoids (ISO), series of long-chain \(n\)-alkanes (ALK), and sterols (EST). Their relative abundance, although relevant for the analysis, was low compared with that of the two major pyrolysis components (polysaccharide and lignin derived). tenedione structures (furancarboxaldehyde, furanmethanol) are known pyrolysis products of polysaccharides [45]. Other relevant compounds derived from polysaccharides and known as the most specific pyrolysis product from cellulose [46], such as furural and anhydrousars (levoglucosane), were also abundant. Differences were found in their relative abundances, with a significant decrease noticeable from T0 to T1 (Table 1), that gradually increases along with the maturation of the compost pile. The initial decrease observed indicates a fast metabolization by the action of different cellulolytic and hemicellulose enzymatic activities of decomposers fungi and bacteria, corresponding to the first stages of the composting process [47; 48]. The following increase from T1 to T4 is probably originated from new syntetizates from the microbial biomass with slower turnover rates that may indicate an ongoing humification process [49]. The lignin-derived pyrolysates observed in all samples were mainly derived from the guaiacyl-type (LG), followed by syringyl-type, compounds (LS). LG compounds are considered specific biomarkers; hence, it helps to comprehend the specific effect of decomposition processes on lignin [50]. The relative abundance of LG and LS units (Table 1; both summarized as LIG in Figure 2) significantly increases during the composting process, which reflects its resilience to degradation and the occurrence of a selective preservation of lignins during composting. On the other hand, it can be observed that there is a higher proportion of LG units in comparison to LS units, which points to increasing humification processes [51], typical from the composting process. In addition, although few hydroxyphenyl-type compounds were found, mainly 4-vinylphenol was found at high proportion. This can be originated from the pyrolysis of other plant compounds, such as hydroxycinnamic acids, i.e., \(p\)-coumaric acid [52]. The presence and high relative abundance of 4-vinylphenol found in maize biomass is in line with previous studies about lignin composition in herbaceous species [53]. To better study lignocellulosic dynamics during composting, relevant indexes were calculated from the pyrolysis results (Table 2). The total lignin/polysaccharides and total LG/polysaccharides ratios confirmed what has been found before: a conspicuous decrease of polysaccharides is observed at the beginning of the composting process from T0 to T1, and, later, a steadily increase is observed, showing a progressive slow  
Figure 2: Evolution of the relative abundance (average; %) of main biogenic compounds released by analytical pyrolysis (Py-GC/MS) during the composting process. Compounds families with (\(\lx@math@degree\)) indicate that values are 10-fold represented (\(\times 10\)). decrease in the indexes but always retaining higher values than at T0.