THE FATE OF GASOLINE DERIVED PB IN ORGANIC SOIL HORIZONS OF VARYING FOREST TYPE AND DEGREE OF DECOMPOSITION

A thorough understanding of the environmental variables controlling the fate of gasoline-derived anthropogenic lead in New England is needed to understand its potential effects on regional environmental quality. Lead is strongly retained in organic soil horizons but the effect of organic matter type and quality, which vary with forest management and natural environmental gradients across the northeastern United States, has not been examined. Here we determine the distribution and solid phase association of Pb in natural and experimental forest floor samples to isolate the role of different forest types and progressive decomposition on Pb fate in surface soils. We conducted a two-year decomposition experiment with Pb-spiked fresh litters of multiple forest types to examine changes in Pb association over labile carbon turnover timescales not readily observable in natural settings. Lead retention mechanisms were probed using selective extractions, synchrotron-based X-ray spectroscopy, and micro X-ray fluorescence, and the results were compared with lead speciation and inventories in forest floors. We find that that Pb speciation is extremely dynamic, with changes in Pb association occurring on the order of a few months in experimental samples. Initially, upon deposition, Pb is adsorbed to organic matter surfaces, but during progressive litter decay, an increasing fraction of this Pb becomes associated with Mn/Fe mineral phases by inner-sphere adsorption, which is observed in both artificial decomposition and natural O-horizon samples of all forests. The shift in Pb association occurs more rapidly in higher quality litter of northern hardwood forests, presumably because nutrient-rich hardwood litter is more susceptible to microbial attack. This rapid change in association of Pb from organics to colloidal mineral surfaces has important ramifications for Pb retention/transport in temperate forest soils.