Does organic carbon sequestration in volcanic soils offset volcanic CO2 emissions?

Volcanic soils contain the highest amounts of organic C among all mineral soil types. But are they important sinks in the global terrestrial C cycle? Does organic C accumulation in these soils offset volcanic CO2 emissions? To date, no global estimates of organic C sequestration rates are available for these soils, which would allow answering these questions. Here I combine data from available studies conducted on Holocene volcanic deposits in different parts of the world to assess soil organic C accumulation. I find exponentially decreasing accumulation rates with increasing soil age, dropping below 10 g C m−2 yr−1 after approximately 1000 years of soil development. The obtained trends are similar for soils of contrasting climate zones. An estimated upper limit of global organic C sequestration in volcanic soils is 20 Tg yr−1, which is at most 25% of the C degassing from terrestrial volcanoes and less than 1% of the “residual land sink” of C. Contemporary organic C sequestration in volcanic soils is less important than CO2 consumption by chemical weathering of volcanic deposits and plays a negligible role in the global terrestrial C cycle.