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Environ Sci Technol. 2017 Mar 7;51(5):2621-2627. doi: 10.1021/acs.est.6b06369. Epub 2017 Feb 13.

Boreal Forests Sequester Large Amounts of Mercury over Millennial Time Scales in the Absence of Wildfire.

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Climate Impacts Research Centre, Department of Ecology and Environmental Science, Umeå University , 981 07 Abisko, Sweden.
Department of Forest Mycology and Plant Pathology, Swedish University of Agricultural Sciences , 750 07 Uppsala, Sweden.
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences , 901 83 Umeå, Sweden.
Asian School of the Environment, Nanyang Technological University , 50 Nanyang Avenue, Singapore 639798.
Department of Ecology and Environmental Science, Umeå University , 901 87 Umeå, Sweden.


Alterations in fire activity due to climate change and fire suppression may have profound effects on the balance between storage and release of carbon (C) and associated volatile elements. Stored soil mercury (Hg) is known to volatilize due to wildfires and this could substantially affect the land-air exchange of Hg; conversely the absence of fires and human disturbance may increase the time period over which Hg is sequestered. Here we show for a wildfire chronosequence spanning over more than 5000 years in boreal forest in northern Sweden that belowground inventories of total Hg are strongly related to soil humus C accumulation (R2 = 0.94, p < 0.001). Our data clearly show that northern boreal forest soils have a strong sink capacity for Hg, and indicate that the sequestered Hg is bound in soil organic matter pools accumulating over millennia. Our results also suggest that more than half of the Hg stock in the sites with the longest time since fire originates from deposition predating the onset of large-scale anthropogenic emissions. This study emphasizes the importance of boreal forest humus soils for Hg storage and reveals that this pool is likely to persist over millennial time scales in the prolonged absence of fire.

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