THE EFFECT OF FIRE ON MERCURY AND CARBON IN FOREST SOILS: RESULTS FROM NORTHERN MICHIGAN AND MINNESOTA

Soil and bedrock geochemical surveys show that mercury in forest soils in two National Parks in northern Minnesota and Michigan is largely derived from atmospheric deposition. Despite this fairly uniform source, soils in Isle Royale National Park, MI and Voyageurs National Park, MN have highly variable concentrations of Hg in ground layer (forest litter and organic soil) and A-horizon mineral soils. Hg and organic C concentrations in A-horizon soils are highly correlated (Isle Royale: n=116, r²=0.80; Voyageurs: n=42, r²=0.75). Processes that affect C concentrations apparently also affect Hg concentrations. One such process is wildland fire, which can volatilize much of the organic matter on the forest floor and in surface soil layers and liberate C, N, S, and Hg. Over time, both C and Hg levels in soils may increase, dependent on input rates, but relatively low concentrations can persist for many years. Areas of Voyageurs and Isle Royale National Parks that burned in recent time have lower C and Hg contents in soils, supporting the supposition that recent fire influences the distribution of both elements in soils. To document the impact of fire on Hg in soils, ten test plots were established in an area of the Superior National Forest, northern Minnesota that was scheduled for a prescribed fuel-reduction fire. Ground layer and mineral soil samples were collected prior to the fire and immediately afterward, before any rain had fallen. The ten sites covered a wide range of fire severity, largely controlled by soil moisture conditions. Corresponding pre-burn and post-burn samples from nine of the sites were analyzed for a number of elements, including organic C and Hg, to determine changes in the geochemistry of the soils as a result of the fire. Although the most severely burned sites lost all biomass material, underlying mineral soils did not lose measurable C or Hg immediately after the fire. Additional samples from all sites were collected in May, 2001 following the October, 2000 burn to examine geochemical changes in exposed soils over time.