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The boreal journey of methyl mercury

from forest harvest to black alder swamps

Kronberg, Rose-Marie (2014). The boreal journey of methyl mercury. Diss. (sammanfattning/summary) Umeå : Sveriges lantbruksuniv., Acta Universitatis Agriculturae Sueciae, 1652-6880 ; 2014:11
ISBN 978-91-576-7970-3
eISBN 978-91-576-7971-0
[Doctoral thesis]

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The bioaccumulating neurotoxin methyl mercury (MeHg) is largely recognized as a problem in the boreal landscape. Both forest harvest sites and wetlands have been identified as MeHg sources. However, studies on logging effects have reported mixed results, and furthermore a black alder swamp has been recognized as a significant and consistent MeHg sink. This thesis aimed at clarifying the influence of these environments on the fate of MeHg in boreal landscapes.
The impact of forest harvest on MeHg production and export was studied in two types of landscapes, undulating and flatter terrain. Based on MeHg soil pools and fluxes of MeHg in streams, I conclude that MeHg was net produced in the soils after clear-cutting, with up to a 10-fold increase. Further, clear-cutting in undulating terrain approximately doubled the MeHg export to surface waters. The increase in load corresponded to 14% of the MeHg export from forested land in Sweden. When comparing to both forested and wetland areas, the increase in load was almost 8%. Moreover, laboratory incubation experiments using stable Hg isotopes were used to determine factors influencing methylation and demethylation rates.

Experiments with
soils from clear-cuts and reference sites suggested that the increase in methylation after clear-cutting was controlled by methanogens and sulfate reducing bacteria. In contrast, clear-cutting did not affect demethylation rates.
Furthermore, we wanted to know whether degradation or retention in the soil caused a black alder swamp to be a MeHg sink. A spatial analysis showed that MeHg was net degraded in the soil. In addition, based on snapshot budgets of nine additional swamps, net degradation of MeHg appeared as a common feature of alder swamps. Finally, incubation studies including sterile controls were performed to understand the processes affecting Hg methylation and MeHg demethylation in the black alder swamp. By targeting specific microbial guilds, we identified active microbial communities, showing that in particular methanogens were responsible for the biotic demethylation. Also, high rates of abiotic MeHg degradation were observed throughout the swamp.

Altogether, these findings are important for forestry practice recommendations and landscape planning. Forestry operations clearly increase the MeHg load while black alder swamps mitigate MeHg produced in upstream environments. Therefore, I propose that riparian buffer zones and restoration of black alder swamps situated downstream MeHg sources would decrease the MeHg load to downstream surface waters.

Authors/Creators:Kronberg, Rose-Marie
Title:The boreal journey of methyl mercury
Subtitle:from forest harvest to black alder swamps
Series Name/Journal:Acta Universitatis Agriculturae Sueciae
Year of publishing :2014
Number of Pages:46
I.Kronberg R-M, Drott A, Jiskra M, Wiederhold J G, Björn E, Skyllberg U. (2014a). Increased formation and export of methyl mercury after boreal forest harvest. Under review in Nature Geoscience.
II.Kronberg R-M, Björn E, Skyllberg U. (2014b). The role of sulfate reducing bacteria and methanogens as potential methylators in soils of boreal forest harvest sites. Manuscript.
III.Kronberg R-M*, Tjerngren I*, Drott A, Björn E, Skyllberg U. (2012). Net degradation of methyl mercury in alder swamps. Environmental Science and Technology, 46 (24), 13144-13151.
IV.Kronberg R-M, Schaefer J K, Björn E, Skyllberg U. (2014c). Methanogens and abiotic processes drive MeHg degradation in a black alder swamp. Manuscript.
UNSPECIFIED*Equal contributors
Place of Publication:Umeå
Publisher:Institutionen för skogens ekologi och skötsel, Sveriges lantbruksuniversitet
ISBN for printed version:978-91-576-7970-3
ISBN for electronic version:978-91-576-7971-0
Publication Type:Doctoral thesis
Full Text Status:Public
Agris subject categories.:K Forestry > K01 Forestry - General aspects
P Natural resources > P33 Soil chemistry and physics
T Pollution > T01 Pollution
Subjects:(A) Swedish standard research categories 2011 > 4 Agricultural Sciences > 401 Agricultural, Forestry and Fisheries > Soil Science
Agrovoc terms:mercury, boreal forests, alnus glutinosa, swamps, surface water, wetland soils, methylation, forestry operations, clear felling, water quality, landscape conservation, sweden
Keywords:mercury, methyl mercury, methylation, demethylation, boreal forest, soil, wetlands, forest harvest, water quality, restoration, Alnus glutinosa
Permanent URL:
ID Code:10994
Faculty:S - Faculty of Forest Sciences
Department:(S) > Dept. of Forest Ecology and Management
External funders:Centre for Environmental Research (CMF) and FORMAS
Deposited By: Rose-Marie Kronberg
Deposited On:05 Feb 2014 13:52
Metadata Last Modified:13 Dec 2014 21:46

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