Gaseous mercury fluxes from forest soils in response to forest harvesting intensity: a field manipulation experiment

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 ± 170 ng m(-2)d(-1)) were significantly greater than both the traditional clearcut plot (-40 ± 60 ng m(-2)d(-1)) and the un-harvested reference plot (-180 ± 115 ng m(-2)d(-1)) during July. This difference was likely a result of enhanced Hg(2+) 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 6 months. 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.