Humic acid-like fractions in raw and vermicomposted winery and distillery wastes

Three different substrates from winery and distillery wastes including a spent grape marc (SGM), a mixture of SGM and lees cake (SGML) and a mixture of biosolid vinasse and vine shoots (BvS) were vermicomposted with Eisenia andrei for 8 months. The chemical changes occurring in the bulk substrates and in their humic acid-like (HAL) fractions were investigated. Vermicomposting produced a decrease of total organic C content and C/N ratio, and an increase of total extractable C and humic acid C contents in the three substrates. The HAL fractions in the initial substrates were characterized by a marked aliphatic character, small acidic functional group content, a marked presence of proteinaceous materials and polysaccharide-like structures, extended molecular heterogeneity and small degrees of aromatic ring polycondensation, polymerisation and humification. After vermicomposting, the chemical and structural characteristics of HAL fractions approached those typical of soil humic acid. In particular, a loss of aliphatic, polypeptidic and carbohydrate components, and an increase in oxygenated and acidic functional groups occurred. Results obtained showed that vermicomposting is a suitable treatment for transforming fresh organic matter in SGM, SGML and, especially, BvS into humified forms, thus enhancing the quality of these materials as soil organic amendments. Further, the mixture SGML yielded slightly better results in terms of HAL fraction quality and humification degree of the final product, with respect to SGM.