Volcanic soils, covering only ~1% of the Earth's land, store over 5% of the global soil organic C stock. The frequent burial of these soils by tephra fallout from explosive volcanic eruptions is a critical but poorly quantified C storage process in soils from volcanically active regions. Using field measurements, we demonstrate that single eruptions can bury substantial amounts of stable organic carbon in soils. We develop a modelling framework and estimate that, in Ecuador alone, at least 1.1 Pg C has been stored in volcanic soils repeatedly affected by tephra deposition during the Holocene. This stock of tephra-buried soil organic carbon exceeds the cumulative CO₂ emissions from the source eruptions. Here, we show that explosive volcanism, through the repeated burial of organic C in volcanic soils, acts as a significant regional C sink over time, ultimately averaging to net C-negative events.