MERCURY ACCUMULATION IN OLD-GROWTH AND RECENTLY CLEAR-CUT FORESTS IN THE TEPHRA FALL ZONE OF THE 1980 MOUNT ST. HELENS ERUPTION

The 1980 eruption of Mount St. Helens provides a clear tephra marker that permits the study of mercury (Hg) and carbon (C) accumulation rates in soils under differing aboveground vegetation. Our study resides in the 600 km² area impacted by the 1980 deposition of tephra, outside the direct blast zone. Here, deposition of tephra left the forest overstory intact. Soil formed above the tephra, after the eruption, was collected, along with underlying tephra. Mercury and C were measured in old-growth forest soils and areas that had been clear-cut and replanted prior to the eruption. A significant component of the global Hg cycle is the exchange between the atmosphere, forest canopy, and soil. Here we examine the effect of old-growth forest canopy (~600 years old) on Hg accumulation and storage in underlying soils compared to forests clear-cut in the past. Mercury storage in the newly developed soil at these sites averaged 7.79 ± 3.01 g Hg ha^-1 in old-growth and 1.42 ± 1.12 g Hg ha^-1 in clear-cut sites. The enhanced Hg storage observed in the old-growth forest soils illustrates the significant role the canopy played in both scavenging Hg from the atmosphere (over the 33 years between sampling and eruption) and likewise in the enhanced Hg retention in soils via carbon addition. A stronger positive correlation between Hg and C concentrations was observed in the old-growth sites (r² = 0.63) than in those clear-cut in the past (r² = 0.49). After accounting for the initial tephra deposited by the eruption of Mount St. Helens, annual accumulation over 33 years was calculated. Annual Hg accumulations in soils at old-growth and clear-cut sites was 0.51 g Hg ha^-1 yr^-1 and 0.22 g Hg ha^-1 yr^-1 respectively, such that the old-growth canopy resulted in 0.29 g Hg ha^-1 yr^-1more Hg storage.