Formation of mercury methylation hotspots as a consequence of forest stump harvest

Earlier studies have shown that boreal forest logging can increase the concentration and export of bioavailable methylmercury (MeHg) in stream runoff. Here we test whether forestry operations create soil environments of high MeHg net formation associated with distinct microbial communities. Furthermore, we test the hypothesis that stump harvest is more effective at creating Hg methylation hotspots than stem-only harvest because of the more severe soil compaction and soil disturbance. Concentrations of MeHg, percent MeHg of total Hg (THg) and bacterial community composition were analyzed from 200 sampling sites distributed across eight catchments. Each catchment was either stem-only harvested (n=3), stem and stump harvested (n=2) or left undisturbed (n=3). In support of our hypothesis, a higher percentage MeHg relative to THg was observed in one of the stump harvested catchments, particularly in water-filled cavities created by stump removal or driving damage. This catchment also featured the highest bacterial diversity and highest relative abundance of bacterial families known to include Hg methylating sulfate- and iron-reducers. We propose that water-logged and disturbed environments associated with stump harvest can favor methylating microorganisms, which also enhance MeHg formation.