Abstract
Mercury is a global pollutant of critical concern but its movement through terrestrial upland systems is poorly understood. We investigated whether forest harvesting practices and varying hydrological conditions resulted in different subsurface transport pathways, fluxes and proportions of recent vs. ambient mercury in runoff. In a multiyear field experiment, we measured subsurface lateral mercury fluxes at three adjacent hillslope plots, including one that was not harvested, one where all biomass was removed after harvest, and one where residual biomass was left after harvest. Two different enriched stable mercury isotopes (Hg tracer) were added to the hillslope plots (one pre-harvest and one post-harvest) to help differentiate between recently deposited mercury and ambient mercury in soils, as well as to identify changes between years. More Hg tracer was recovered in runoff post-harvest (16.2–54.0 μg) than pre-harvest (3.7–11.8 μg) from both harvested hillslopes, but differences between harvested hillslopes were not significant. The movement of Hg tracer was governed by periods of near surface preferential flow that was enhanced by forest harvesting. Runoff Hg tracer concentrations were significantly and inversely related to both event runoff magnitude (R2 = 0.85) and runoff ratio (R2 = 0.81). Insignificant relationships between Hg tracer concentrations and both DOC and precipitation pH suggest that for recently deposited mercury, the overarching controls on mobilization were hydrological rather than biogeochemical. The dependence of mercury mobilization on the routing of precipitation to different subsurface flow pathways suggests an important interaction between climate and the age of mercury pools for mercury transport from terrestrial landscapes.
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Data Availability
The datasets generated during and/or analysed during the current study are available in the U.S. Forest Service Research Data Archive, as McCarter et al. (2020b) https://doi.org/10.2737/RDS-2020-0049 and McCarter et al. (2021) https://doi.org/10.2737/RDS-2021-0007.
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Acknowledgements
We wish to thank the staff at the Marcell Experimental Forest Research Station for logistical support. In particular we thank Deacon Kyllander, Carrie Dorrance, Nate Aspelin, Josh Kragthorpe, Reid Peterson, Leigh Kastenson, Madeline Wiley, Mitchell Olds, Ben Munson, Paul Watson, Ross Bentson, Mike Palmer, Doris Nelson, Nicole King, Gerrard Graves, Donna Olson, and Anne Timm of the Northern Research Station (USDA Forest Service), and Jeromie Geroatte and Stephen Stalheim of the US Job Corps for their assistance with field work. Maxwell Mazur, Kristine Haynes and Haiyong (Planck) Huang provided important field and laboratory support. We thank Dwight Streblow, who harvested the plots in 2012. We would also like to thank three anonymous reviewers whose comments and suggestions improved the manuscript.
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We acknowledge the support of funding from the Great Lakes Air Deposition Program (GLAD 2010-7) and the Natural Sciences and Engineering Research Council of Canada (Funding Reference Number 355866-18) to CPJM as well as a Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship to CPRM. The Northern Research Station of the USDA Forest Service paid for forest harvesting, and funded the salaries of SDS, RKK, and SLE.
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Supplementary file1—The least-square means and 95 % confidence interval of the treatment ratios before and after harvest at the Biomass Left and Biomass Removed hillslopes. Bivariate regressions of ambient THg and DOC from all hillslopes pre- and post-harvest. Bivariate regressions between event mercury tracer and precipitation. A table presenting the results of the statistical analysis of the mixed effect model pair-wise comparison of the proportion of ambient mercury. A table presenting the results of the tracer and ambient Hg-DOC linear regressions for each hillslope, as well as the overall ambient Hg-DOC linear regression for all three hillslopes combined (DOCX 655 kb)
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McCarter, C.P.R., Sebestyen, S.D., Eggert, S.L. et al. Differential subsurface mobilization of ambient mercury and isotopically enriched mercury tracers in a harvested and residue harvested hardwood forest in northern Minnesota. Biogeochemistry 154, 119–138 (2021). https://doi.org/10.1007/s10533-021-00801-y
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DOI: https://doi.org/10.1007/s10533-021-00801-y