The charcoal carbon pool in boreal forest soils (original) (raw)

Forest fires release significant amounts of carbon dioxide into the atmosphere 1 , 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 2-7. However, although soil black carbon pools are important for global carbon budgets, the spatial variation and dynamics of these pools are poorly understood 6-9. 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 14 C-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. Large areas in northern forests are struck by fire every year 10. Fire return intervals of 50-200 years are common 11 , but shorter 12 or longer 13 return intervals can also prevail locally or regionally. Each fire event reduces ecosystem amounts of organic carbon (C) by release of CO 2 to the atmosphere. Such fire-driven losses of C from northern forests are huge. A fire event typically emits 1,000-2,000 g C m −2 , and with an estimated 5-15 million hectares burning annually, the inter-annual variation of emitted C is substantial 14-17. However, during each fire event a fraction of the burning vegetation and soil organic matter is converted to black pyrogenic carbon, which exists as a continuum from partly charred plant material, via char and charcoal to soot and graphite particles 6,18. As black pyrogenic carbon is refractory and stored in soils, it represents carbon that is removed from the faster cycling pools to an extent that makes it function as a long-term carbon sink 2-7. Recent findings do, however, indicate that soil pools of black pyrogenic carbon are less refractory than previously thought 6,8,18-21. This is reasonable because the amounts of pyrogenic carbon in boreal forest soils seem rather small compared with the amounts produced at each fire event 12,19 , indicating that it is re-burnt during subsequent fires, or lost by other means of degradation. Yet information on the amounts and dynamics of black pyrogenic carbon in boreal forest soils is sparse and incomplete 5,6,8. For example, existing data cannot determine the quantity of carbon present in recently produced fresh pyrogenic