Forest harvesting leads to changes in soil moisture, temperature and incident solar radiation, all strong environmental drivers of soil-air mercury (Hg) fluxes. Whether different forest harvesting practices significantly alter Hg fluxes from forest soils is unknown. We conducted a field-scale experiment in a northern Minnesota deciduous forest wherein gaseous Hg emissions from the forest floor were monitored after two forest harvesting prescriptions, a traditional clear-cut and a clearcut followed by biomass harvest, and compared to an un-harvested reference plot. Gaseous Hg emissions were measured in quadruplicate at four different times between March and November 2012 using Teflon dynamic flux chambers. We also applied enriched Hg isotope tracers and separately monitored their emission in triplicate at the same times as ambient measurements. Clearcut followed by biomass harvesting increased ambient Hg emissions the most. While significant intra-site spatial variability was observed, Hg emissions from the biomass harvested plot (180±170ngm-2d-1) were significantly greater than both the traditional clearcut plot (-40±60ngm-2d-1) and the un-harvested reference plot (-180±115ngm-2d-1) during July. This difference was likely a result of enhanced Hg2+ photoreduction due to canopy removal and less shading from downed woody debris in the biomass harvested plot. Gaseous Hg emissions from more recently deposited Hg, as presumably representative of isotope tracer measurements, were not significantly influenced by harvesting. Most of the Hg tracer applied to the forest floor became sequestered within the ground vegetation and debris, leaf litter, and soil. We observed a dramatic lessening of tracer Hg emissions to near detection levels within 6months. As post-clearcutting residues are increasingly used as a fuel or fiber resource, our observations suggest that gaseous Hg emissions from forest soils will increase, although it is not yet clear for how long such an effect will persist.
Bibliographical noteFunding Information:
Funding for this project was provided through a NSERC Discovery grant and Great Lakes Air Deposition Program to CPJM, as well as a University of Toronto Graduate Fellowship to MM. We thank the members of the Mitchell research group for their assistance with all aspects of this experiment, especially the help of Planck Huang for sample analysis. We also wish to thank the staff at the Marcell Experimental Forest Research Station and the cooperation of the USDA Forest Service, without which the bulk of this research would not be possible. In particular we thank Nate Aspelin, Josh Kragthorpe, Reid Peterson, Leigh Kastenson, Paul Watson, Ross Bentson, Mike Palmer, Doris Nelson, Nicole King, Gerrard Graves, Donna Olson, Anne Timm, and Carrie Dorrance of the USFS Northern Research Station and Jeromie Geroatte and Stephen Stalheim of the US Job Corps for their assistance with field work. Finally, we thank Mr. Dwight Streblow for harvesting the plots in 2012. This research does not reflect the official positions and policies of the US EPA. Mention of products/trade names does not constitute recommendation for use by US EPA.
© 2014 Elsevier B.V.
Copyright 2014 Elsevier B.V., All rights reserved.
- Biomass harvesting
- Gaseous mercury emissions
- Mercury isotope
- Solar radiation