MERCURY RELEASE FROM SOIL ORGANIC HORIZONS IMPACTED BY INVASIVE EARTHWORMS

Exotic Asian and European earthworm invasion in forest soils has been shown to disrupt carbon and nitrogen cycling, leading to the release of Dissolved Organic Carbon (DOC) from organic soil horizons. This increase in organic decomposition likely releases anthropogenic mercury accumulated from atmospheric deposition since the industrial revolution.

To evaluate this hypothesis, a series of microcosm experiments were conducted using O horizon soil collected from hemlock and deciduous stands in the Avery Brook Watershed, located in West Whately, Massachusetts. An equal mass of worms (the efficient composting “Red Wiggler,” Eisenia fetida) was added to five replicate microcosms of each soil type while an additional 5 replicates of each type were left without worms as controls. The microcosms were watered weekly with ultrapure water and leachate was collected and analyzed for pH, DOC, ultraviolet adsorption at 254 nanometers (UV254) and total dissolved mercury (THg). Specific Ultraviolet Adsorption (SUVA) was calculated from the UV254 and DOC results and is a measure of the aromaticity of the DOC.

Worms were added to the appropriate microcosms after the first 3 weeks of the experiment to allow organic decomposition rates to come to equilibrium with ambient lab temperature and experimental soil moisture conditions. During this time DOC and UV254 concentrations increased more quickly in leachate from the deciduous microcosms compared to the hemlock microcosms. As a result, THg concentrations were higher in deciduous leachate. The addition of worms caused a dramatic increase in DOC and UV254 levels in leachate from the hemlock microcosms and THg concentrations (63 ng/L) were double those from the non-worm hemlock controls. The response in worm bearing deciduous microcosms was not as dramatic with THg concentrations increasing from 28 ng/L to 37ng/L. Leachate THg concentrations were correlated with DOC, UV254 and SUVA but relationships were different for different soils and for the presence or absence of worms. One of the impacts of the worms is to change the relationship between DOC and UV254 such that with worms present, the values of UV254 are higher. This suggests that the worms produce leachate with higher aromaticity and this is more efficient in leaching THg from soil organic material.