The charcoal carbon pool in boreal forest soils

Forest fires release significant amounts of carbon dioxide into the atmosphere¹, but also convert a fraction of the burning vegetation to charred black carbon. Black carbon is hard to break down, and formation of this reserve therefore creates a long-term soil carbon sink^{2, 3, 4, 5, 6, 7}. However, although soil black carbon pools are important for global carbon budgets, the spatial variation and dynamics of these pools are poorly understood^{6, 7, 8, 9}. Here we examine the charcoal content of 845 soil samples collected from a broad range of boreal forest landscapes and climates in Scandinavia. We show that there is considerable variation in the distribution and carbon content of soil charcoal between forest landscapes; the landscape-level amount of soil carbon stored in charcoal ranged from 0 to 222 g C m⁻², with an average of 77 g C m⁻². The carbon concentration in the soil charcoal is significantly lower than that found in recently produced fresh charcoal, suggesting that charcoal carbon content decreases with time. Indeed, the median age of a subset of ¹⁴C-dated soil charcoal particles was 652 years, implying a rapid turnover compared with the expected median age of approximately 5,000 years if charcoal is persistent. Assuming that our measurements are representative of boreal forests worldwide, we estimate that boreal forest soils store 1 Pg of carbon in the form of charcoal, equivalent to 1% of the total plant carbon stock in boreal forests.