Surface-air mercury fluxes and a watershed mass balance in forested and harvested catchments

Forest soils are among the world's largest repositories for long-term accumulation of atmospherically deposited mercury (Hg), and understanding the potential for remobilization through gaseous emissions, aqueous dissolution and runoff, or erosive particulate transport to down-gradient aquatic ecosystems is critically important for projecting ecosystem recovery. Forestry operations, especially clear-cut logging where most of the vegetaiton is removed, can influence Hg mobility/fluxes, foodweb dynamics, and bioaccumulation processes. This paper measured surface-air Hg fluxes from catchments in the Pacific Northwest, USA, to determine if there is a difference between forested and logged catchments. These measurements were conducted as part of a larger project on the impact of forestry operations on Hg cycling which include measurements of water fluxes as well as impacts on biota. Surface-air Hg fluxes were measured using a commonly applied dynamic flux chamber (DFC) method that incorporated diel and seasonal variability in elemental Hg (Hg-0) fluxes at multiple forested and harvested catchments. The results showed that the forested ecosystem had depositional Hg-0 fluxes throughout most of the year (annual mean: -0.26 ng/m(2)/h). In contrast, the harvested catchments showed mostly emission of Hg-0 (annual mean: 0.63 ng/m(2)/h). Differences in solar radiation reaching the soil was the primary driver resulting in a shift from net deposition to emission in harvested catchments. The surface-air Hg fluxes were larger than the fluxes to water as runoff and accounted for 97% of the differences in Hg sequestered in forested versus harvested catchments. Published by Elsevier Ltd.

Automated summary

The study found that Hg fluxes in forested ecosystems are primarily depositional, while in harvested areas they are mainly emission-driven. Differences in solar radiation were the primary driver of this shift. Surface-air Hg fluxes were larger than fluxes to water, accounting for a significant portion of the differences in Hg sequestration between forested and harvested catchments.