IMPACT OF PRESCRIBED BURNS ON SOIL MERCURY IN THE OSSIPEE PINE BARRENS, NEW HAMPSHIRE

Prescribed burning of the Ossipee Pine Barrens to restore the natural ecosystem could also release anthropogenic Hg that has accumulated in soil organic horizons since the industrial revolution. To evaluate this hypothesis, 30 soil cores were collected from 10 sites with a range of burn histories from non-burned to burned within several weeks. Cores were segmented and extracted in the field, dried at 50°C and weighed to determine dry bulk density. Dry samples were then homogenized using an electric coffee grinder and analyzed for Hg by Thermal Decomposition/Cold Vapor Atomic Absorption using a Teledyne Leeman Labs HydraIIC. Loss on Ignition was calculated by computing the loss of mass after analysis.

The highest Hg concentrations (150 ng/g) were found in the organic horizons of the soil. Dry deposition of atmospheric Hg is concentrated by the forest canopy and is released to the forest floor via throughfall during precipitation events and litterfall at the end of the growing season. Mercury has a strong affinity for organic material so it may become sequestered in the organic soil horizons. Burning of the organic horizons could potentially volatilize Hg back into the atmosphere or it could transform it into a more soluble form that could be leached into the underlying mineral soil. Infiltration is enhanced in the Ossipee Pine Barrens as the area lies on glacial outwash deposits composed of highly permeable stratified sands and gravels.

Organic horizon Hg concentrations were significantly lower in recently (September) burned areas compared to areas burned 4 years ago (56 ng/g versus 151ng/g). There are also significantly lower amounts of total Hg (concentration times bulk density) in samples from burned areas (8.6 mg/m²) compared to unburned areas (16.6 mg/m²). Depth trends in total mercury show an increase near 15cm indicating that there is some downward leaching of Hg in these highly permeable soils. Integrating total Hg values over the length of the cores shows that the total amount of Hg in the soil from unburned areas (150 mg/m²) is about double that found in burned areas (78 mg/m²). This suggests that prescribed burns do volatilize Hg back into the atmosphere.