Impacts of Invasive Earthworms on Soil Mercury Cycling: Two Mass Balance Approaches to an Earthworm Invasion in a Northern Minnesota Forest

Invasive earthworms perturb natural forest ecosystems that initially developed without them, mainly by consuming the forest floor (an organic rich surficial soil horizon) and by mixing the upper parts of the soil. The fate of mercury (Hg) formerly contained in the forest floor is largely unknown. We used two mass balance approaches (simple mass balance and geochemical mass balance) to quantitatively assess the impact of exotic earthworms on Hg cycling in forest soils and the fate of Hg formerly contained in the forest floor. Two sets of soils, collected in 2009 and 2014 from an earthworm invasion transect in north central Minnesota, were analyzed in this study. We observed a substantial loss of Hg from the soil system following earthworm invasion. Mass balance calculations showed that 35 to 65 % of the forest floor Hg was retained in the underlying mineral soil with the remainder unaccounted. The most likely explanation for the loss of forest floor Hg from the system is microbially induced volatilization of Hg by soil microbes and possibly by earthworm gut microbes during consumption of the forest floor. Our calculations suggest that exotic earthworms can substantially perturb mercury cycling in the topsoil. Forest floor Hg may accumulate within worm biomass; however, this process can account for only a small fraction (1 to 3 %) of the lost Hg. Additionally, we calculated the mass balance of organic carbon (C) due to Hg’s high affinity for organic matter and found that a portion of the organic carbon also appears to be lost from the soil system.