## 4 \({}^{13}\)C Solid-State NMR Applications to Vermicomposting
Even when the feedstock is simply constituted by CM [119; 112; 113; 114; 115], HAs composition shows some variability, as highlighted by the spread of the relative areas shown in Figure 4.  
HAs extracted from mature vermicomposts derived from CM and sugar industry waste [113; 115] showed relatively larger areas over the region including CH\({}_{3}\)O/CHN, O-alkyl, and di-O-alkyl C compared to HAs extracted from mature CM vermicomposts. The relative areas were equal to 50% in the former case and to 40% in the latter. The larger values are due to the larger amount of carbohydrates in HAs, thus reflecting the composition of the feedstock. The difference in composition of the two HAs results in a slightly lower _HB/HI_ index exhibited by the HAs originating from CM and sugar industry waste. Analogously, the progressive replacement of CM with lignocellulosic residues of flock cotton production [108] resulted in an increase in the integral intensity in the O-alkyl and di-O-alkyl C regions, where cellulose signals are observed, and in an increase in the O-aryl C region due to lignin, reflected in the lowering of the _HB/HI_ index. As expected, the dependence of the \({}^{13}\)C CP-MAS NMR spectra of extracts on feedstock is generally less pronounced for extracts obtained from mature vermicomposts. This was clearly shown by [PERSON] et al. [115], who studied HAs extracted from vermicomposts produced from different feedstocks at three different maturation stages. The integral intensities over the different spectral regions for HAs extracted from the feedstocks and the vermicomposts after 120 days of maturation (also reported in Table 5) are shown in Figure 6. The integral intensities are spread over larger intervals for HAs extracted from feedstocks compared to those extracted from vermicomposts, in particular for the alkyl, O-alkyl, and aromatic C regions. Significant changes of the integral intensity over these regions were also observed in HAs extracted from vermicompost produced with CM and plant residues [105; 121] at two different maturation stages (45 and 90 days), and in HAs extracted from vermicomposts produced with tomato plant debris and paper mill sludge [117] at the beginning and at the end of the process (180 days). The reported data suggest that vermicomposting leads to a general standardization of the Has' composition, although some specificities remain inherent to the composition of the feedstock. In an interesting study, [PERSON] et al. [114] compared the composition of HAs with that of the vermicompost fraction obtained using only water as extraction medium and of the solid residue after alkaline extraction. The authors found that aromatic structures were more abundant in HAs (20.18%) than in the aqueous extracts (11.82%) and in the solid residue (8.42%). The aqueous extracts showed the largest presence of carbonylic groups and were found to be the most hydrophobic ones according to the _HB/HI_ index values. [PERSON] et al. [122] studied the composition of HAs extracted from a vermicompost made with CM, bagasse, pressmud and trash at different maturation stages up to 60 days after the thermophilic phase. In this case, monitoring the areas of the spectral regions allowed extensive mineralization in the final stages of the process to be revealed, as indicated by an initial increase in the intensities of the aromatic and carboxyl C regions followed by a decrease after about 40 days. As an isolated case, [PERSON] et al. [116] did not detect any significant change in the composition of HAs from vermicomposts produced with CM, saw dust, and tanner waste over a 135 days vermicomposting time. The absence of significant changes seems rather extraordinary, considering that the feedstock is mainly constituted of CM, for which changes were observed by other authors [115], and that some degradation does occur, as revealed by a decrease in the total organic carbon in the vermicompost at different composite times. HA fractions characterized by different molecular dimensions were investigated by [PERSON] at al. [105].