Linking Mercury and Lead Retention in the Forest Floor Across the Northeastern United States.

Upland forest soils act as a reservoir of atmospherically-deposited mercury (Hg), reducing its movement to aquatic ecosystems. The retention time of Hg in the forest soils of the northeastern United States have not been well-constrained. Another atmospherically-deposited metal, lead (Pb), has been further studied and may provide insight into the retention of both metals in forest soils. Forest floor samples were collected at 16 long-term upland forest research sites across the northeastern U.S. in 1980, 1990, 2002, and 2012. Decreases in forest floor Pb concentrations were modeled with exponential regression. Lead response times (1/k) were significantly greater for frigid soils (47 ± 5 yrs) compared with mesic soils (31 ± 3 yrs). Forest floor Pb response time and mercury concentration and mass were determined to be significantly correlated with soil organic matter content, pH, forest floor depth and site longitude. When multi-regressed, 65% of the variation in response time was explained by the four variables. Mercury concentrations were also correlated with the four variables, explaining 80% of its variation. These results suggest that Pb and Hg retention in the forest floor are dependent on similar ecosystem variables. Further, Pb retention times across the study region may be indicative of Hg retention times.