Mercury Distribution across 14 U.S. Forests. Part II: Patterns of Methyl Mercury Concentrations and Areal Mass of Total and Methyl Mercury

This study characterized distribution patterns of monomethyl mercury (MeHg) and areal mass of total mercury (THg) and MeHg across U.S. upland forests. MeHg concentrations increased from surface litter (average: 0.14 μg kg^–1) to intermediate (0.47 μg kg^–1) and deeper, decomposed litter (1.43 μg kg^–1). MeHg concentrations were lower in soils (0.10 μg kg^–1 at 0–20 cm depth; 0.06 μg kg^–1 at >20 cm depth). Ratios of MeHg to THg were higher in litter compared to soils. In soils, MeHg concentrations positively correlated with THg across all sites, and MeHg concentrations also increased with C content and latitude. THg areal mass ranged from 41.6 g ha^–1 to 268.8 g ha^–1. Largest THg mass at all sites was sequestered in soils (average of 91%), followed by litter (8%) and aboveground biomass (<1%). MeHg mass (litter plus soils only) ranged from 75 to 443 mg ha^–1, of which 88% was found in soils. Both THg and MeHg mass correlated with latitude, with average mass increases of 10.6 g ha^–1 (THg) and 20 μg ha^–1 (MeHg) per degree latitude, indicating that highest THg and MeHg accumulation in upland forests are expected in northern sites.