Relic charcoal hearth soils: a neglected carbon reservoir. Case study at Marsiliana forest, Central Italy

Charcoal production in forests is one of the oldest forms of forest exploitation. The legacy of such once widespread activity is a plethora of relic charcoal hearths (RCHs1), where soil shows a thick, black, charcoal-rich top horizon. Even where very common, such as in European forests, RCHs were rarely studied to assess their relevance as C reservoir. For this purpose, as a case study, we investigated some RCHs at Marsiliana, a typical Mediterranean oak forest from Central Italy. We found that RCHs soils, in spite of representing< 0.5% of total surface, gave a substantial contribution in terms of C, i.e. 1.1% to 4.2% of total ecosystem C, including litter, the top 30 cm of soil, deadwood, aboveground and below-ground biomass. On average, soil C content in RCHs was eight times higher than the soil outside the RCHs. The environmental significance of RCHs soils appears still greater considering that, on average, 43% of their C stock was charcoal, a form of C highly recalcitrant to mineralization. These results would stress the importance of accounting for the contribution of RCHs in terms of soil C and giving an estimation of their charcoal content in future C inventories, both as macroscopic and microscopic particles in soil. This study support the necessity of safeguarding the anthropogenic soils of RCHs as a precious C reservoir as well as a memory of past land uses.