Ontologies and folksonomies represent two alternative approaches for creating and managing knowledge on the web. Ontologies are produced by some experts in charge of formally defining terms and relationships among them in a given domain. Folksonomies emerge from a free tagging activity performed by direct participation of end users, but lack a controlled vocabulary and a formal content organisation, thus making difficult their management. To overcome the limits of folksonomies while keeping their advantages, we propose a novel approach allowing users both to relate their tags to a reference ontology and to discuss about inserted tags and relationships to reach a collective interpretation. To this aim, argumentation theory is used as the underlying model to manage persuasive dialogues among users. The proposal is substantiated by an example referred to an application for collaborative web mapping in the domain of natural heritage interpretation, called TheSilence.org.