Fate processes and behavior of mercury in litter and soil

Abstract

The goals of my research were two fold: first, I intended to quantify fate processes of Hg in tree litter during decomposition, particularly to assess what happens with Hg that was originally associated with tree litter during in forest floors. This study was designed as a combined laboratory and field decomposition study: in the laboratory, I measured changes in dry mass, C mass and concentration, Hg mass and concentration, N mass and concentration, Hg/C ratio, C/N ratio, Hg/N ratio, and soluble Hg in litter incubated for different time periods (0, 3, 6, 12, and 18 months); in the field, I compared Hg concentration, C/N ratio, and Hg/C ratio before and after one year of field exposure to those in the laboratory. I found significant Hg mass losses (5-23%) after 18 months of decomposition indicating gaseous Hg losses during litter decomposition. In the field, Hg concentrations were significantly enriched (8-64 %) after 12 months as compared to laboratory samples showing additional sorption of Hg in the field. The second goal of my study was to conduct the first, continuous monitoring of soil gas Hg0 concentration at different depths in the soil profile. For this study, I developed and set-up a continuous CO2 and Hg0 concentration measurements using semi-permeable Teflon tubing. Teflon tubing was placed at three different soil depths (7 cm, 20 cm, and 40 cm) in two replicate locations, and in ambient air ( total of 7 ports) and were connected to a measurement system consisting of a Tekran 2537 GEM analyzer and a LiCOR CO2 analyzer to correspondingly measure soil Hg0 and soil CO2 concentration profiles. I observed a consistently lower Hg0 concentrations in the soil pores compared to ambient air indicative of a net uptake and sorption of Hg0 in the soil. Surface horizons of the soils contained higher Hg0 concentration than deeper levels and strong seasonal variability in Hg0 concentration was observed in the top of soils. Lowest soil Hg0 levels, and hence likely highest Hg0 sorption, was associated with driest conditions in the mid-summer.