PS 25-94
Ecological controls on mercury input and storage in a Pacific Northwest temperate rainforest
Our understanding of mercury cycling in Pacific Northwest forests has been limited by insufficient sampling, though recent models suggest that terrestrial ecosystems in this region of the USA may experience elevated rates of mercury deposition. We investigated mercury storage in second growth temperate rain forest soils in the Evergreen Ecological Observation Network (EEON), in Olympia WA, where overstory species composition, canopy structure, and aboveground carbon storage had been previously determined. This approach allows us to explicitly investigate ecological controls on mercury deposition to forest ecosystems to better understand the dynamics of this toxin in terrestrial systems.
Soil cores were collected in duplicate from 10 plots within the EEON characterized by a species composition ranging from Douglas-fir-dominated to red alder-dominated. Soils were collected in increments distinguishing between the O horizon, 0-5 cm depth through the A horizon, 5-30 cm depth, and 30-55 cm depth, often reaching the C horizon. Soils were analyzed for organic carbon content, C:N ratios, and Hg storage through each profile.
Results/Conclusions
Mercury storage in O and A horizons of soils (litter and 0-5 cm depth) ranged from 17.0 g Hg/ha to 34.3 g Hg/ha within the EEON plots. Deciduous-dominated plots (up to 100% red alder) were characterized by lower mercury storage in the O horizon when compared to coniferous-dominated plots (up to 92.5% Douglas-fir). Mercury storage in the complete soil profiles (5 to 55 cm depth) was nearly 10x higher than surficial samples, ranging from 166.7 g Hg/ha to 355.5 g Hg/ha. This degree of mercury storage is relatively high for forest soils and may reflect the infrequency of fires as well as elevated Hg deposition to this region. In the deeper soils (5-55 cm depth), we did not observe differences in Hg storage among forest types.