Characterization of humic substances formed during the composting of solid wastes from wineries

Humic substances (HS) were extracted from grape marc (GM) compost (the residues of wine processing) at various stages of decomposition. Chemical analyses, carbon-13 nuclear magnetic resonance (¹³C-NMR) acquired with cross-polarization magic-angle spinning (CPMAS) and Fourier-transform infrared (FTIR) spectroscopy measurements were performed on the purified humic acids (HAs). The total level of HS slightly decreased (from 390 to 310 g kg^-1 OM) during the composting process, HAs and fulvic acid concentrations did not change (250 g kg^-1 OM and 60 g kg^-1 OM, respectively). However, the level of the non-humic fraction declined rapidly (from 100 to 40 g kg^-1 OM). The ratios between the various HS fractions were calculated and found to be useful humification indices which changed significantly during the process. These ratios were exponentially and significantly correlated with either the composting time or C/N ratio. Although the elemental composition of the GM-HAs did not change during the composting process, an increase in aliphatic C and carboxyl groups, a decrease in phenolic OH and total aromaticity as well as a decline in polysaccharide levels were recorded. The HAs extracted from GM exhibited similar features to peat-HA. Humic acids extracted from GM changed both quantitatively and qualitatively. The spectroscopic characteristics of the HAs resembled these of peat HAs and young forms of soil HAs. On the basis of this study it may be concluded that CPMAS ¹³C-NMR supported by FTIR significantly improves the characterization of HAs extracted from composts and that several maturity criteria for composts may be derived from sequential quantitative extraction and fractionation of HS.