Effectiveness of vermicomposting for bioconversion of grape marc derived from red winemaking into a value-added product

Grape marc, the main solid by-product of the wine industry, can be used as a nutrient-rich organic amendment if treated appropriately before its application into soil. In this study, we evaluated the potential of vermicomposting to process grape marc derived from the red winemaking of Mencía grapes in order to yield a high-quality, polyphenol-free organic vermicompost that could be used as an environmentally friendly fertiliser. We observed that the grape marc from this cultivar appears to be an optimum substrate for feeding earthworms providing optimum conditions for growth and reproduction, and sufficient energy to sustain large populations. Moreover, earthworm activity favoured the stabilisation of the grape marc resulting in a final vermicompost characterised by a higher concentration of macro- and micro-nutrients and a reduced polyphenol content after 112 days of vermicomposting. Lower values of microbial activity, indicative of stabilised materials, were recorded at the end of the process. These findings highlight vermicomposting as an environmentally sound management system for processing grape marc that could easily be scaled up for industrial application.