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The charcoal carbon pool in boreal forest soils

Abstract

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 sink2,3,4,5,6,7. However, although soil black carbon pools are important for global carbon budgets, the spatial variation and dynamics of these pools are poorly understood6,7,8,9. 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.

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Figure 1: Geographic distribution of study sites and appearance of charcoal in boreal forest soils.
Figure 2: Concentration of carbon in different types of charcoal.
Figure 3: Charcoal and its content of carbon in boreal forest soils.

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Acknowledgements

This research was financially supported by grants from the Research Council of Norway. We thank T. W. Swetnam for comments and J. G. Dokk, M. Haugmo, E. J. Kristoffersen, O. W. Røstad, E. Tryterud and A. N. Wist for technical assistance.

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T.Ø. and R.H. designed the approach for the spatial positioning of the soil samples and selected all sampling positions except those in sites E, L, M and O, which were selected by M.O. B.D. measured macroscopic charcoal. K.J.B. analysed the microscopic charcoal fraction. R.H. and M.O. analysed the data. M.O. wrote the paper. All authors discussed and edited the manuscript.

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Correspondence to Mikael Ohlson.

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Ohlson, M., Dahlberg, B., Økland, T. et al. The charcoal carbon pool in boreal forest soils. Nature Geosci 2, 692–695 (2009). https://doi.org/10.1038/ngeo617

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