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