Mercury in Soils of the Calhoun Critical Zone Observatory: Importance of Redox Features in Sequestration.

Soils of the Critical Zone perform the essential ecosystem service of sequestering both natural and anthropogenic mercury (Hg). We investigated total Hg concentrations in three soil profiles at the Calhoun Critical Zone Observatory to characterize Hg sequestration in these Southern Piedmont soils. Forest floor samples from hardwood and pine forest stands have Hg concentrations that range from 78 – 129 ng g^-1. Mineral soil Hg concentrations in three soil profiles decreased with depth from ~28 ng g^-1 in A horizons at ~10 cm down to 15 ng g^-1 in Bt and Bs horizons at ~45 cm depth. A most striking pedologic feature at Calhoun CZO are the redoximorphic features locally known as “Tyger stripes”, Liesegang-banding-like features. These were sampled in the Long-Term-Soil-Experiment plots, which have soil excavations that access the upper 2-m. In the orange Fe-enriched bands with hematite and goethite, Hg concentrations were substantially lower than in Fe-depleted kaolin-rich gray to white bands (47 vs 156 ng g^-1, respectively), demonstrating the importance of microbially driven reducing conditions on Hg sequestration in soil. Our results affirm the potential for surface horizons to sequester Hg, particularly in O horizons, but also highlight the importance of deeper soil horizons in Hg sequestration, a commonly overlooked sink. The fluctuations in redox conditions in deep soil may play an underappreciated role local and global Hg biogeochemistry.