Impact of fire on the geochemistry of forest soils

Laurel Woodruff (woodruff@usgs.gov), U.S. Geological Survey, Mounds View, MN (763.783.3291)

William Cannon (wcannon@usgs.gov), U.S. Geological Survey, Reston, VA

Connie Dicken (cdicken@usgs.gov), U.S. Geological Survey, Reston, VA

We are investigating on-going geochemical effects of fire on the forest floor and mineral soils in an area of the Superior National Forest, northern Minnesota, which was burned in a fuel reduction prescribed fire by the USDA Forest Service on October 11, 2000. The scheduled fire gave us the opportunity to establish 10 study sites in the proposed burn area. At each site, forest floor material (forest litter, and/or O-horizon) and mineral soil horizons (A-, E-, B- and C-horizons as available) were measured and collected.

On October 13, prior to any post-fire rainfall, we resampled the sites. Burn severity was estimated and, where possible, samples comparable to pre-burn samples were collected. Fire severity was high (100% of organic forest floor material consumed, mineral soil exposed) at 4 sites, moderate at 1 site (some forest floor material burned, mineral soil not exposed), and light at 3 sites (surface material charred with minimal loss at the forest floor). Two sites were untouched by fire. The sites were sampled again in May 2001 and May 2002.

Because the footprints of historic fires are apparent in studies of soil geochemistry in both Voyageurs and Isle Royale National Parks (Cannon et al., 2002; Woodruff and Cannon, 2001) it seems apparent that over time severe fire can lead to decreases in C and elements bound to C from the forest floor. Based on pre-burn analyses and measured thickness and density of organic material and assuming that all Hg in litter and an O-horizon is emitted during high severity burns, average Hg emissions from this prescribed fire were about 2 kg/km². This estimate only takes into account Hg bound to organic material on the forest floor and does not include burned foliage or woody fuels. Immediately after the fire, Hg and C levels of A-horizon mineral soils were unchanged, even at sites of high burn severity. However, analyses of A-horizon soils collected in May 2001 show a decrease in C but a marked increase in Hg contents. The increase in Hg may be the result of leaching of Hg from the overlying ash layer. One year later, Hg levels in A-horizon soils were lower than the previous May, while C values were little changed. In contrast, Pb in A-horizon soils was higher in samples collected immediately after the fire, compared to preburn values. Pb levels decrease in both May 2001 and May 2002, although most samples still have higher Pb values than preburn levels. The increase of Pb in soils immediately following the fire may be the result of the concentration of residual Pb remaining after the burning of forest vegetation.

References cited:

Cannon, W.F., Woodruff, L.G., Dicken, C.L., and Saari, S., 2002, Prolonged influence of wildfires on the geochemistry of forest soils, Isle Royale National Park, Michigan and Voyageurs National Park, Minnesota: Geological Society of America Abstracts with Programs, v.34, p.xxx.

Woodruff, L.G. and Cannon, W.F., 2001, The effect of fire on mercury and carbon in forest soils: results from northern Michigan and Minnesota: Geological Society of America Abstracts with Programs, v.33, p.A186.

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