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Abstract Title: Hg Methylation in Iron-rich Soils from Iceland with Contrasting Organic Matter Content
Presenter Name: Maureen Le Bars
Company/Institution: Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, CHN, ETH Zurich, 8092 Zurich, Switzerland
Session: Mercury in the Terrestrial Ecosystems
Co-Authors: Maureen Le Bars,Sylvain Bouchet,Laurel K. ThomasArrigo,Ruben Kretzschmar

Abstract Information :

Soils in the subarctic region accumulate Hg mainly due to the affinity of Hg for organic matter. However, previous studies have shown that the neurotoxic methylmercury (MeHg) can be produced in soils under reducing conditions and released into adjacent aquatic ecosystems. Yet, the biogeochemical parameters controlling microbial Hg methylation in soils are still not completely understood. For example, the influence of Fe and C cycles on Hg methylation in soils remains unclear.

This study focused on soils from Iceland as they are a good example of subarctic soils impacted by redox fluctuations, which are expected to be more frequent with heavier precipitation events predicted with climate change. Also, these soils are rich in Fe and organic matter, and they contain abundant short-range ordered minerals (e.g. allophanes, ferrihydrite), responsible for carbon and nutrient accumulations.

We conducted a 4-month incubation experiment with slurries made with 6 topsoils and 3 subsoils exhibiting contrasting amounts and quality of C (4-25 wt.%, C/N molar ratios from 13 to 19), Fe contents (4-11 wt.%) and C/Fe molar ratios (2 to 31). MeHg production and release in the aqueous phase were determined by monitoring Hg species (MeHg, inorganic Hg) in the solid and aqueous phases by GC-ICP-MS. Other physico-chemical parameters were measured concomitantly in the aqueous phase (e.g. Eh, pH, Fe, DOC, SO42- concentrations) to identify geochemical processes favoring Hg transformations and release.

Contrasting Hg methylation patterns were observed among the soils. Particularly, a topsoil with low C content (6%) showed the highest Hg methylation potential (10% of total Hg). Interestingly, MeHg was not measured in the subsoils and some of the topsoils despite low redox potential (-110 to 70 mV) and C availability that favor the activity of iron reducing bacteria. The results show the potential of Icelandic soils to methylate Hg with a complex combination of soil biogeochemical factors (including C availability, Fe and Mn minerals reduction) determining the fate of Hg in soils.

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