THE MINERALIZATION OF FOSSIL FUEL DERIVED LEAD IN SOILS OF NEW ENGLAND AND NORWAY

Atmospherically derived lead is a widely distributed contaminant that can inhibit a range of essential ecosystem processes in forest soils. While Clean Air Legislation essentially halted lead (Pb) emissions in the United States and much of Europe in the later half of the 20th century, the forest floor in these regions still maintains considerable Pb burdens. Bulk organic (O) horizons sampled from coniferous forests in early 2000 had Pb concentrations of 50 to 300 mg/kg, and microenvironments within the soil profile may have Pb concentrations nearly an order of magnitude higher. Here we use a combination of selective chemical extractions, Pb isotopes, and synchrotron-based X-ray studies to specifically detail the speciation, coordination environment, and mobility of anthropogenic Pb in natural O horizon samples. Combined, these studies serve to better understand the fate of lead and its impacts on ecosystem function.

Using synchrotron-based X-ray absorption spectroscopy, we give evidence that Pb is bound to iron oxides via inner-sphere adsorption throughout the soil profile. Selective chemical extractions provide further proof that Pb is adsorbed to Eh sensitive secondary oxide mineral phases: the reducing agent hydroxylamine hydrochloride in 0.02 N HCl extracts up to 50% of the total Pb in O horizons, compared to <10% extraction with straight 0.02 N HCl. The speciation of atmospherically-delivered Pb in the soil profile is dynamic and appears to be tightly coupled to organic matter decomposition and the subsequent mineralization of iron. Using selective chemical extractions and a constant-flux ²¹⁰Pb dating model (previously developed for peats) we estimate trace metal mineralization rates in the soil profile. While Pb speciation in air pollution from automobile exhaust and smelters was dominated by Pb halides and Pb sulfur species, respectively, our data indicate that anthropogenic Pb in soils becomes associated with iron oxides on a timescale of just a few years. In large part due to the strong association of Pb with iron oxides, atmospherically-delivered Pb will have a relatively long residence time in well-drained montane forest soils (>100 y), but will be of limited ecological relevance because of its stable, presumably unavailable form.