Terrestrial Mercury (Hg) surface reservoirs: magnitude, spatial patterns, fate, and re-emission potential to the atmosphere

Terrestrial ecosystems are strong natural reservoirs that retain the bulk of atmospheric Hg deposition. As a result, a long-term legacy of past and present Hg pollution is sequestered in surface litter and soil pools. Hg shows a particular affinity to—and hence tends to accumulate in—terrestrial organic C pools. We present results from a comprehensive five-year investigation where we quantified concentrations and pool size distribution of terrestrial reservoirs of Hg in forests, and their relationships to organic C to assess the degree to which C determines net retention of Hg. A special emphasis is placed upon fate of terrestrial Hg reservoirs, including losses though wildfires, C mineralization, the potential for re-emissions back to the atmosphere, and sensitivity to climate change.

Results show that continental-scale spatial distribution of Hg in soils and litter is strongly related to that of C, and that Hg levels (concentrations and pool sizes of total Hg, as well as methylated Hg) increase with higher latitude. We calculate that at a global scale as much as 4.5 x 10⁶ Mg of Hg is sequestered in terrestrial surface reservoirs. Experimental studies and field observations to address fate of Hg show that (i) fires lead to up-to-complete Hg losses from biomass in either gaseous elemental or particulate-bound form; (ii) litter decomposition can lead to evasion losses of Hg in the range of 50% of initially-bound Hg; (iii) re-emission losses of Hg from soils to the atmosphere upon C mineralization are small (<3% of Hg bound to C) and likely driven by surface processes; (iv) in deeper soils, gaseous soil pore Hg concentrations are mostly below atmospheric levels and decoupled from CO₂ concentration profiles, indicating that the soil matrix may be an active sink for gaseous Hg rather and a strong diffusion-driven source.