Mercury in litterfall and upper soil horizons in forested ecosystems in Vermont, USA

Mercury (Hg) is an atmospheric pollutant that, in forest ecosystems, accumulates in foliage and upper soil horizons. The authors measured soil and litterfall Hg at 15 forest sites (northern hardwood to mixed hardwood/conifer) throughout Vermont, USA, to examine variation among tree species, forest type, and soils. Differences were found among the 12 tree species sampled from at least two sites, with Acer pensylvanicum having significantly greater litterfall total Hg concentration. Senescent leaves had greater Hg concentrations if they originated lower in the canopy or had higher surface:weight ratios. Annual litterfall Hg flux had a wide range, 12.6 to 28.5 µg/m(2) (mean, 17.9 µg/m(2) ), not related to forest type. Soil and Hg pools in the Oi horizon (litter layer) were not related to the measured Hg deposition flux in litterfall or to total modeled Hg deposition. Despite having lower Hg concentrations, upper mineral soil (A horizons) had greater Hg pools than organic soil horizons (forest floor) due to greater bulk density. Significant differences were found in Hg concentration and Hg/C ratio among soil horizons but not among forest types. Overall, our findings highlight the importance of site history and the benefits of collecting litterfall and soils simultaneously. Observed differences in forest floor Hg pools were strongly correlated with carbon pools, which appeared to be a function of historic land-use patterns.