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

Abstract

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 (R = 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.

Potential impacts of mercury released from thawing permafrost

“…Hg has four release pathways: (1) evasion into the atmosphere after microbial decay 11 , (2) leaf stomata transpiration 14 , (3) fire 11 , and (4) leaching into groundwater followed by eventual export by rivers into the oceans 16 . Microbial decay frees Hg from organic matter, but plants and soil organic matter reabsorb 11 most of this liberated Hg. Whether leaves represent an Hg source or sink depends on the concentration gradient between the stomata and the atmosphere 14 .…”

“…Whether leaves represent an Hg source or sink depends on the concentration gradient between the stomata and the atmosphere 14 . Fire consumes soil organic matter, emitting carbon dioxide and Hg into the atmosphere 11 . Once leached into water, bound to Dissolved Organic Carbon (DOC) and Particulate Organic Carbon (POC), Hg can methylate, entering the food chain and accumulating in various species, particularly fish 8 .…”

“…Because Hg bonds to organic matter, the terrestrial carbon cycle modulates the terrestrial Hg cycle. Hg has three uptake pathways: (1) bonding to soil organic matter 8,10,11 , (2) stomatal leaf uptake 12,13 , and (3) root absorption 12,14 . Once absorbed by plants, translocation by phloem assimilates Hg into leaves and wood 12,13 .…”

“…Once absorbed by plants, translocation by phloem assimilates Hg into leaves and wood 12,13 . The deposition of dead leaves, roots, and stems transfers additional Hg to the soil 11,15 .…”

Paleolimnological Assessment of Wildfire‐Derived Atmospheric Deposition of Trace Metal(loid)s and Major Ions to Subarctic Lakes (Northwest Territories, Canada)

“…As a result of these environmental changes, it is predicted that wildfires will become increasingly frequent and severe in the near future, including in the Arctic (Hu et al, 2015; Teufel & Sushama, 2019; Wang et al, 2015; Wotton et al, 2017). These new fire regimes may alter freshwater ecosystems by rapidly liberating metal(loid) contaminants slowly accumulated in soils and biomass over centuries or millennia (Abraham et al, 2017; Giesler et al, 2017). Burned catchments result in elevated loading of sediment, organic matter, nutrients, major ions, and metal(loid)s to connected streams and lakes for decades following wildfires because catchment erosion is facilitated by fire (Dunnette et al, 2014; Ice et al, 2004; Jensen et al, 2017; Nelson et al, 2007; Rothenberg et al, 2010; Stein et al, 2012).…”